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PRAISE FOR THE TANGLED WEB
“Thorough and comprehensive coverage from one of the foremost experts
in browser security.”
—TAVIS ORMANDY, GOOGLE INC.
“A must-read for anyone who values their security and privacy online.”
—COLLIN JACKSON, RESEARCHER AT THE CARNEGIE MELLON WEB
SECURITY GROUP
“Perhaps the most thorough and insightful treatise on the state of security
for web-driven technologies to date. A must have!”
—MARK DOWD, AZIMUTH SECURITY, AUTHOR OF THE ART OF SOFTWARE
SECURITY ASSESSMENT

PRAISE FOR SILENCE ON THE WIRE BY MICHAL ZALEWSKI
“One of the most innovative and original computing books available.”
—RICHARD BEJTLICH, TAOSECURITY
“For the pure information security specialist this book is pure gold.”
—MITCH TULLOCH, WINDOWS SECURITY
“Zalewski’s explanations make it clear that he’s tops in the industry.”
—COMPUTERWORLD
“The amount of detail is stunning for such a small volume and the examples
are amazing. . . . You will definitely think different after reading this title.”
—(IN)SECURE MAGAZINE
“Totally rises head and shoulders above other such security-related titles.”
—LINUX USER & DEVELOPER

THE TANGLED WEB
A Guide to Securing
Modern Web Applications

by Michal Zalewski

San Francisco

THE TANGLED WEB. Copyright © 2012 by Michal Zalewski.
All rights reserved. No part of this work may be reproduced or transmitted in any form or by any means, electronic or
mechanical, including photocopying, recording, or by any information storage or retrieval system, without the prior
written permission of the copyright owner and the publisher.
15 14 13 12 11

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ISBN-10: 1-59327-388-6
ISBN-13: 978-1-59327-388-0
Publisher: William Pollock
Production Editor: Serena Yang
Cover Illustration: Hugh D’Andrade
Interior Design: Octopod Studios
Developmental Editor: William Pollock
Technical Reviewer: Chris Evans
Copyeditor: Paula L. Fleming
Compositor: Serena Yang
Proofreader: Ward Webber
Indexer: Nancy Guenther
For information on book distributors or translations, please contact No Starch Press, Inc. directly:
No Starch Press, Inc.
38 Ringold Street, San Francisco, CA 94103
phone: 415.863.9900; fax: 415.863.9950; info@nostarch.com; www.nostarch.com
Library of Congress Cataloging-in-Publication Data
Zalewski, Michal.
The tangled Web : a guide to securing modern Web applications / Michal Zalewski.
p. cm.
Includes bibliographical references and index.
ISBN-13: 978-1-59327-388-0 (pbk.)
ISBN-10: 1-59327-388-6 (pbk.)
1. Computer networks--Security measures. 2. Browsers (Computer programs) 3. Computer security. I. Title.
TK5105.59.Z354 2011
005.8--dc23
2011039636

No Starch Press and the No Starch Press logo are registered trademarks of No Starch Press, Inc. “The Book of” is
a trademark of No Starch Press, Inc. Other product and company names mentioned herein may be the trademarks
of their respective owners. Rather than use a trademark symbol with every occurrence of a trademarked name, we
are using the names only in an editorial fashion and to the benefit of the trademark owner, with no intention of
infringement of the trademark.
The information in this book is distributed on an “As Is” basis, without warranty. While every precaution has been
taken in the preparation of this work, neither the author nor No Starch Press, Inc. shall have any liability to any
person or entity with respect to any loss or damage caused or alleged to be caused directly or indirectly by the
information contained in it.

For my son

BRIEF CONTENTS

Preface .......................................................................................................................xvii
Chapter 1: Security in the World of Web Applications ........................................................1

PART I: ANATOMY OF THE WEB ............................................................................ 21
Chapter 2: It Starts with a URL ........................................................................................23
Chapter 3: Hypertext Transfer Protocol ............................................................................41
Chapter 4: Hypertext Markup Language ......................................................................... 69
Chapter 5: Cascading Style Sheets .................................................................................87
Chapter 6: Browser-Side Scripts ......................................................................................95
Chapter 7: Non-HTML Document Types .........................................................................117
Chapter 8: Content Rendering with Browser Plug-ins........................................................127

PART II: BROWSER SECURITY FEATURES ............................................................... 139
Chapter 9: Content Isolation Logic ................................................................................141
Chapter 10: Origin Inheritance.....................................................................................165
Chapter 11: Life Outside Same-Origin Rules ...................................................................173
Chapter 12: Other Security Boundaries .........................................................................187

Chapter 13: Content Recognition Mechanisms................................................................197
Chapter 14: Dealing with Rogue Scripts ........................................................................213
Chapter 15: Extrinsic Site Privileges ..............................................................................225

PART III: A GLIMPSE OF THINGS TO COME ........................................................... 233
Chapter 16: New and Upcoming Security Features .........................................................235
Chapter 17: Other Browser Mechanisms of Note ............................................................255
Chapter 18: Common Web Vulnerabilities.....................................................................261
Epilogue ....................................................................................................................267
Notes ........................................................................................................................269
Index .........................................................................................................................283

viii

Brief Contents

CONTENTS IN DETAIL
PREFACE

xvii

Acknowledgments ................................................................................................... xix

1
S E C U R IT Y I N T H E W O R L D O F W E B A P P L IC A T I O N S

1

Information Security in a Nutshell ................................................................................ 1
Flirting with Formal Solutions ......................................................................... 2
Enter Risk Management................................................................................. 4
Enlightenment Through Taxonomy .................................................................. 6
Toward Practical Approaches ........................................................................ 7
A Brief History of the Web ......................................................................................... 8
Tales of the Stone Age: 1945 to 1994 ........................................................... 8
The First Browser Wars: 1995 to 1999 ........................................................ 10
The Boring Period: 2000 to 2003 ................................................................ 11
Web 2.0 and the Second Browser Wars: 2004 and Beyond .......................... 12
The Evolution of a Threat.......................................................................................... 14
The User as a Security Flaw......................................................................... 14
The Cloud, or the Joys of Communal Living.................................................... 15
Nonconvergence of Visions ......................................................................... 15
Cross-Browser Interactions: Synergy in Failure ............................................... 16
The Breakdown of the Client-Server Divide .................................................... 17

PART I: ANATOMY OF THE WEB
2
IT S TA R T S W I T H A U R L

21

23

Uniform Resource Locator Structure............................................................................ 24
Scheme Name ........................................................................................... 24
Indicator of a Hierarchical URL .................................................................... 25
Credentials to Access the Resource............................................................... 26
Server Address .......................................................................................... 26
Server Port ................................................................................................ 27
Hierarchical File Path.................................................................................. 27
Query String.............................................................................................. 28
Fragment ID............................................................................................... 28
Putting It All Together Again ........................................................................ 29
Reserved Characters and Percent Encoding ................................................................ 31
Handling of Non-US-ASCII Text.................................................................... 32
Common URL Schemes and Their Function.................................................................. 36
Browser-Supported, Document-Fetching Protocols ........................................... 36
Protocols Claimed by Third-Party Applications and Plug-ins.............................. 36
Nonencapsulating Pseudo-Protocols.............................................................. 37
Encapsulating Pseudo-Protocols .................................................................... 37
Closing Note on Scheme Detection .............................................................. 38

Resolution of Relative URLs ....................................................................................... 38
Security Engineering Cheat Sheet.............................................................................. 40
When Constructing Brand-New URLs Based on User Input ............................... 40
When Designing URL Input Filters ................................................................. 40
When Decoding Parameters Received Through URLs ...................................... 40

3
H YP E R T E X T T R A N S F E R P R O T O C O L

41

Basic Syntax of HTTP Traffic ..................................................................................... 42
The Consequences of Supporting HTTP/0.9 .................................................. 44
Newline Handling Quirks............................................................................ 45
Proxy Requests........................................................................................... 46
Resolution of Duplicate or Conflicting Headers............................................... 47
Semicolon-Delimited Header Values.............................................................. 48
Header Character Set and Encoding Schemes ............................................... 49
Referer Header Behavior ............................................................................. 51
HTTP Request Types ................................................................................................. 52
GET.......................................................................................................... 52
POST ........................................................................................................ 52
HEAD ....................................................................................................... 53
OPTIONS.................................................................................................. 53
PUT .......................................................................................................... 53
DELETE ..................................................................................................... 53
TRACE ...................................................................................................... 53
CONNECT ............................................................................................... 54
Other HTTP Methods .................................................................................. 54
Server Response Codes............................................................................................ 54
200–299: Success ..................................................................................... 54
300–399: Redirection and Other Status Messages......................................... 55
400–499: Client-Side Error ......................................................................... 55
500–599: Server-Side Error ........................................................................ 56
Consistency of HTTP Code Signaling ............................................................ 56
Keepalive Sessions .................................................................................................. 56
Chunked Data Transfers ........................................................................................... 57
Caching Behavior ................................................................................................... 58
HTTP Cookie Semantics............................................................................................ 60
HTTP Authentication................................................................................................. 62
Protocol-Level Encryption and Client Certificates .......................................................... 64
Extended Validation Certificates................................................................... 65
Error-Handling Rules ................................................................................... 65
Security Engineering Cheat Sheet.............................................................................. 67
When Handling User-Controlled Filenames in Content-Disposition Headers ....... 67
When Putting User Data in HTTP Cookies...................................................... 67
When Sending User-Controlled Location Headers .......................................... 67
When Sending User-Controlled Redirect Headers........................................... 67
When Constructing Other Types of User-Controlled Requests or Responses........ 67

x

Contents in D e ta i l

4
H YP E R T E X T M A RK U P L A N GU AG E

69

Basic Concepts Behind HTML Documents ................................................................... 70
Document Parsing Modes............................................................................ 71
The Battle over Semantics ............................................................................ 72
Understanding HTML Parser Behavior ........................................................................ 73
Interactions Between Multiple Tags ............................................................... 74
Explicit and Implicit Conditionals.................................................................. 75
HTML Parsing Survival Tips.......................................................................... 76
Entity Encoding ....................................................................................................... 76
HTTP/HTML Integration Semantics............................................................................. 78
Hyperlinking and Content Inclusion ........................................................................... 79
Plain Links ................................................................................................. 79
Forms and Form-Triggered Requests.............................................................. 80
Frames...................................................................................................... 82
Type-Specific Content Inclusion .................................................................... 82
A Note on Cross-Site Request Forgery........................................................... 84
Security Engineering Cheat Sheet.............................................................................. 85
Good Engineering Hygiene for All HTML Documents ...................................... 85
When Generating HTML Documents with Attacker-Controlled Bits .................... 85
When Converting HTML to Plaintext ............................................................. 85
When Writing a Markup Filter for User Content ............................................. 86

5
CASCADING STYLE SHEETS

87

Basic CSS Syntax .................................................................................................... 88
Property Definitions .................................................................................... 89
@ Directives and XBL Bindings ..................................................................... 89
Interactions with HTML ................................................................................ 90
Parser Resynchronization Risks.................................................................................. 90
Character Encoding................................................................................................. 91
Security Engineering Cheat Sheet.............................................................................. 93
When Loading Remote Stylesheets ............................................................... 93
When Putting Attacker-Controlled Values into CSS ......................................... 93
When Filtering User-Supplied CSS................................................................ 93
When Allowing User-Specified Class Values on HTML Markup ........................ 93

6
BROWSER-SIDE SCRIPTS

95

Basic Characteristics of JavaScript............................................................................. 96
Script Processing Model .............................................................................. 97
Execution Ordering Control ....................................................................... 100
Code and Object Inspection Capabilities .................................................... 101
Modifying the Runtime Environment ............................................................ 102
JavaScript Object Notation and Other Data Serializations ............................ 104
E4X and Other Syntax Extensions............................................................... 106

Contents in D etai l

xi

Standard Object Hierarchy .................................................................................... 107
The Document Object Model ..................................................................... 109
Access to Other Documents ....................................................................... 111
Script Character Encoding...................................................................................... 112
Code Inclusion Modes and Nesting Risks ................................................................. 113
The Living Dead: Visual Basic ................................................................................. 114
Security Engineering Cheat Sheet............................................................................ 115
When Loading Remote Scripts ................................................................... 115
When Parsing JSON Received from the Server ............................................ 115
When Putting User-Supplied Data Inside JavaScript Blocks ............................ 115
When Interacting with Browser Objects on the Client Side ............................ 115
If You Want to Allow User-Controlled Scripts on Your Page ........................... 116

7
N O N - H TM L D O C U M E N T T Y P E S

117

Plaintext Files ........................................................................................................ 117
Bitmap Images ...................................................................................................... 118
Audio and Video .................................................................................................. 119
XML-Based Documents ........................................................................................... 119
Generic XML View ................................................................................... 120
Scalable Vector Graphics.......................................................................... 121
Mathematical Markup Language................................................................ 122
XML User Interface Language..................................................................... 122
Wireless Markup Language....................................................................... 123
RSS and Atom Feeds ................................................................................ 123
A Note on Nonrenderable File Types ...................................................................... 124
Security Engineering Cheat Sheet............................................................................ 125
When Hosting XML-Based Document Formats .............................................. 125
On All Non-HTML Document Types............................................................. 125

8
C O N TE N T R E N D E R I N G WI T H BR O WS E R PL U G - IN S

127

Invoking a Plug-in.................................................................................................. 128
The Perils of Plug-in Content-Type Handling ................................................. 129
Document Rendering Helpers.................................................................................. 130
Plug-in-Based Application Frameworks ..................................................................... 131
Adobe Flash ............................................................................................ 132
Microsoft Silverlight .................................................................................. 134
Sun Java ................................................................................................. 134
XML Browser Applications (XBAP) .............................................................. 135
ActiveX Controls.................................................................................................... 136
Living with Other Plug-ins ....................................................................................... 137
Security Engineering Cheat Sheet............................................................................ 138
When Serving Plug-in-Handled Files ........................................................... 138
When Embedding Plug-in-Handled Files ...................................................... 138
If You Want to Write a New Browser Plug-in or ActiveX Component .............. 138

xii

C on t e n t s i n D e t a i l

PART II: BROWSER SECURITY FEATURES
9
C O N TE N T I S O L AT IO N L O G IC

139

141

Same-Origin Policy for the Document Object Model .................................................. 142
document.domain .................................................................................... 143
postMessage(...) ...................................................................................... 144
Interactions with Browser Credentials.......................................................... 145
Same-Origin Policy for XMLHttpRequest ................................................................... 146
Same-Origin Policy for Web Storage ....................................................................... 148
Security Policy for Cookies ..................................................................................... 149
Impact of Cookies on the Same-Origin Policy.............................................. 150
Problems with Domain Restrictions.............................................................. 151
The Unusual Danger of “localhost” ............................................................. 152
Cookies and “Legitimate” DNS Hijacking.................................................... 153
Plug-in Security Rules ............................................................................................. 153
Adobe Flash ............................................................................................ 154
Microsoft Silverlight .................................................................................. 157
Java ....................................................................................................... 157
Coping with Ambiguous or Unexpected Origins ....................................................... 158
IP Addresses ............................................................................................ 158
Hostnames with Extra Periods .................................................................... 159
Non–Fully Qualified Hostnames ................................................................. 159
Local Files ............................................................................................... 159
Pseudo-URLs ............................................................................................ 161
Browser Extensions and UI ........................................................................ 161
Other Uses of Origins ............................................................................................ 161
Security Engineering Cheat Sheet............................................................................ 162
Good Security Policy Hygiene for All Websites ............................................ 162
When Relying on HTTP Cookies for Authentication ....................................... 162
When Arranging Cross-Domain Communications in JavaScript ...................... 162
When Embedding Plug-in-Handled Active Content from Third Parties .............. 162
When Hosting Your Own Plug-in-Executed Content....................................... 163
When Writing Browser Extensions ............................................................. 163

10
O R IG I N I N H E R IT A N C E

165

Origin Inheritance for about:blank .......................................................................... 166
Inheritance for data: URLs....................................................................................... 167
Inheritance for javascript: and vbscript: URLs ............................................................ 169
A Note on Restricted Pseudo-URLs ........................................................................... 170
Security Engineering Cheat Sheet............................................................................ 172

11
L I F E O U T S I D E S A M E - O R I G IN R U L E S

173

Window and Frame Interactions ............................................................................. 174
Changing the Location of Existing Documents .............................................. 174
Unsolicited Framing.................................................................................. 178
Contents i n Detail

xiii

Cross-Domain Content Inclusion .............................................................................. 181
A Note on Cross-Origin Subresources......................................................... 183
Privacy-Related Side Channels ................................................................................ 184
Other SOP Loopholes and Their Uses ...................................................................... 185
Security Engineering Cheat Sheet............................................................................ 186
Good Security Hygiene for All Websites ..................................................... 186
When Including Cross-Domain Resources .................................................... 186
When Arranging Cross-Domain Communications in JavaScript ...................... 186

12
O T H E R S E C U R I T Y B O U N D A R IE S

187

Navigation to Sensitive Schemes............................................................................. 188
Access to Internal Networks.................................................................................... 189
Prohibited Ports ..................................................................................................... 190
Limitations on Third-Party Cookies............................................................................ 192
Security Engineering Cheat Sheet............................................................................ 195
When Building Web Applications on Internal Networks................................ 195
When Launching Non-HTTP Services, Particularly on Nonstandard Ports ......... 195
When Using Third-Party Cookies for Gadgets or Sandboxed Content ............. 195

13
C O N TE N T R E C O G N IT IO N M E CH A N I S M S

197

Document Type Detection Logic............................................................................... 198
Malformed MIME Types ............................................................................ 199
Special Content-Type Values...................................................................... 200
Unrecognized Content Type ...................................................................... 202
Defensive Uses of Content-Disposition ......................................................... 203
Content Directives on Subresources ............................................................ 204
Downloaded Files and Other Non-HTTP Content ......................................... 205
Character Set Handling ......................................................................................... 206
Byte Order Marks .................................................................................... 208
Character Set Inheritance and Override ...................................................... 209
Markup-Controlled Charset on Subresources................................................ 209
Detection for Non-HTTP Files...................................................................... 210
Security Engineering Cheat Sheet............................................................................ 212
Good Security Practices for All Websites..................................................... 212
When Generating Documents with Partly Attacker-Controlled Contents ........... 212
When Hosting User-Generated Files ........................................................... 212

14
D E A L I N G W IT H R O GU E S C R IP T S

213

Denial-of-Service Attacks ........................................................................................ 214
Execution Time and Memory Use Restrictions ............................................... 215
Connection Limits ..................................................................................... 216
Pop-Up Filtering ....................................................................................... 217
Dialog Use Restrictions.............................................................................. 218
Window-Positioning and Appearance Problems ........................................................ 219
Timing Attacks on User Interfaces ............................................................................ 222
xiv

Contents in D e ta i l

Security Engineering Cheat Sheet............................................................................ 224
When Permitting User-Created  Gadgets on Your Site ...................... 224
When Building Security-Sensitive UIs .......................................................... 224

15
EX TR I NSI C S IT E P R IV ILE G ES

225

Browser- and Plug-in-Managed Site Permissions ........................................................ 226
Hardcoded Domains ................................................................................ 227
Form-Based Password Managers............................................................................. 227
Internet Explorer’s Zone Model ............................................................................... 229
Mark of the Web and Zone.Identifier ......................................................... 231
Security Engineering Cheat Sheet............................................................................ 232
When Requesting Elevated Permissions from Within a Web Application ......... 232
When Writing Plug-ins or Extensions That Recognize Privileged Origins.......... 232

PART III: A GLIMPSE OF THINGS TO COME
16
N E W A N D U PC O M IN G S E C U R I TY FE A TU R E S

233

235

Security Model Extension Frameworks ..................................................................... 236
Cross-Domain Requests ............................................................................. 236
XDomainRequest ...................................................................................... 239
Other Uses of the Origin Header ............................................................... 240
Security Model Restriction Frameworks .................................................................... 241
Content Security Policy.............................................................................. 242
Sandboxed Frames .................................................................................. 245
Strict Transport Security............................................................................. 248
Private Browsing Modes............................................................................ 249
Other Developments .............................................................................................. 250
In-Browser HTML Sanitizers........................................................................ 250
XSS Filtering ............................................................................................ 251
Security Engineering Cheat Sheet............................................................................ 253

17
OTHER BROWSER MECHANISMS OF NOTE

255

URL- and Protocol-Level Proposals ............................................................................ 256
Content-Level Features............................................................................................ 258
I/O Interfaces ....................................................................................................... 259

18
C O M M O N W E B V U L N E RA B I L I T I E S

261

Vulnerabilities Specific to Web Applications............................................................. 262
Problems to Keep in Mind in Web Application Design............................................... 263
Common Problems Unique to Server-Side Code ........................................................ 265

C on t e n t s i n D e t a i l

xv

EPILOGUE

267

N O TE S

269

INDEX

273

xvi

Contents in D e ta i l

PREFACE

Just fifteen years ago, the Web was as simple as it
was unimportant: a quirky mechanism that allowed a
handful of students, plus a bunch of asocial, basementdwelling geeks, to visit each other’s home pages dedicated to science, pets, or poetry. Today, it is the platform
of choice for writing complex, interactive applications
(from mail clients to image editors to computer games)
and a medium reaching hundreds of millions of casual
users around the globe. It is also an essential tool of
commerce, important enough to be credited for causing a recession when the 1999 to 2001 dot-com bubble
burst.
This progression from obscurity to ubiquity was amazingly fast, even
by the standards we are accustomed to in today’s information age—and its
speed of ascent brought with it an unexpected problem. The design flaws

and implementation shortcomings of the World Wide Web are those of a
technology that never aspired to its current status and never had a chance
to pause and look back at previous mistakes. The resulting issues have quickly
emerged as some of the most significant and prevalent threats to data security today: As it turns out, the protocol design standards one would apply to
a black-on-gray home page full of dancing hamsters are not necessarily the
same for an online shop that processes millions of credit card transactions
every year.
When taking a look at the past decade, it is difficult not to be slightly
disappointed: Nearly every single noteworthy online application devised so
far has had to pay a price for the corners cut in the early days of the Web.
Heck, xssed.com, a site dedicated to tracking a narrow subset of web-related
security glitches, amassed some 50,000 entries in about three years of operation. Yet, browser vendors are largely unfazed, and the security community
itself has offered little insight or advice on how to cope with the widespread
misery. Instead, many security experts stick to building byzantine vulnerability taxonomies and engage in habitual but vague hand wringing about the
supposed causes of this mess.
Part of the problem is that said experts have long been dismissive of the
whole web security ruckus, unable to understand what it was all about. They
have been quick to label web security flaws as trivial manifestations of the
confused deputy problem* or of some other catchy label outlined in a trade journal three decades ago. And why should they care about web security, anyway?
What is the impact of an obscene comment injected onto a dull pet-themed
home page compared to the gravity of a traditional system-compromise flaw?
In retrospect, I’m pretty sure most of us are biting our tongues. Not only
has the Web turned out to matter a lot more than originally expected, but
we’ve failed to pay attention to some fundamental characteristics that put
it well outside our comfort zone. After all, even the best-designed and most
thoroughly audited web applications have far more issues, far more frequently,
than their nonweb counterparts.
We all messed up, and it is time to repent. In the interest of repentance,
The Tangled Web tries to take a small step toward much-needed normalcy, and
as such, it may be the first publication to provide a systematic and thorough
analysis of the current state of affairs in the world of web application security.
In the process of doing so, it aims to shed light on the uniqueness of the security challenges that we—security engineers, web developers, and users—have
to face every day.
The layout of this book is centered on exploring some of the most prominent, high-level browser building blocks and various security-relevant topics
derived from this narrative. I have taken this approach because it seems to be
more informative and intuitive than simply enumerating the issues using an
*

Confused deputy problem is a generic concept in information security used to refer to a broad
class of design or implementation flaws. The term describes any vector that allows the attacker
to trick a program into misusing some “authority” (access privileges) to manipulate a resource
in an unintended manner—presumably one that is beneficial to the attacker, however that
benefit is defined. The phrase “confused deputy” is regularly invoked by security researchers
in academia, but since virtually all real-world security problems could be placed in this bucket
when considered at some level of abstraction, this term is nearly meaningless.

xviii

P r ef a c e

arbitrarily chosen taxonomy (a practice seen in many other information
security books). I hope, too, that this approach will make The Tangled Web
a better read.
For readers looking for quick answers, I decided to include quick engineering cheat sheets at the end of many of the chapters. These cheat sheets
outline sensible approaches to some of the most commonly encountered
problems in web application design. In addition, the final part of the book
offers a quick glossary of the well-known implementation vulnerabilities that
one may come across.

Acknowledgments
Many parts of The Tangled Web have their roots in the research done for
Google’s Browser Security Handbook, a technical wiki I put together in 2008
and released publicly under a Creative Commons license. You can browse
the original document online at http://code.google.com/p/browsersec/.
I am fortunate to be with a company that allowed me to pursue this
project—and delighted to be working with a number of talented peers who
provided excellent input to make the Browser Security Handbook more useful
and accurate. In particular, thanks to Filipe Almeida, Drew Hintz, Marius
Schilder, and Parisa Tabriz for their assistance.
I am also proud to be standing on the shoulders of giants. This book owes
a lot to the research on browser security done by members of the information security community. Special credit goes to Adam Barth, Collin Jackson,
Chris Evans, Jesse Ruderman, Billy Rios, and Eduardo Vela Nava for the
advancement of our understanding of this field.
Thank you all—and keep up the good work.

P re f ace

xix

SECURITY IN THE WORLD
OF WEB APPLICATIONS

To provide proper context for the technical discussions later in the book, it seems prudent to first of all
explain what the field of security engineering tries to
achieve and then to outline why, in this otherwise wellstudied context, web applications deserve special treatment. So, shall we?
Information Security in a Nutshell
On the face of it, the field of information security appears to be a mature,
well-defined, and accomplished branch of computer science. Resident experts
eagerly assert the importance of their area of expertise by pointing to large
sets of neatly cataloged security flaws, invariably attributed to security-illiterate
developers, while their fellow theoreticians note how all these problems would
have been prevented by adhering to this year’s hottest security methodology.

A commercial industry thrives in the vicinity, offering various nonbinding
security assurances to everyone, from casual computer users to giant international corporations.
Yet, for several decades, we have in essence completely failed to come up
with even the most rudimentary usable frameworks for understanding and
assessing the security of modern software. Save for several brilliant treatises
and limited-scale experiments, we do not even have any real-world success
stories to share. The focus is almost exclusively on reactive, secondary security measures (such as vulnerability management, malware and attack detection, sandboxing, and so forth) and perhaps on selectively pointing out flaws
in somebody else’s code. The frustrating, jealously guarded secret is that when
it comes to enabling others to develop secure systems, we deliver far less value
than should be expected; the modern Web is no exception.
Let’s look at some of the most alluring approaches to ensuring information security and try to figure out why they have not made a difference so far.

Flirting with Formal Solutions
Perhaps the most obvious tool for building secure programs is to algorithmically prove they behave just the right way. This is a simple premise that intuitively should be within the realm of possibility—so why hasn’t this approach
netted us much?
Well, let’s start with the adjective secure itself: What is it supposed to convey,
precisely? Security seems like an intuitive concept, but in the world of computing, it escapes all attempts to usefully define it. Sure, we can restate the problem in catchy yet largely unhelpful ways, but you know there’s a problem
when one of the definitions most frequently cited by practitioners* is this:
A system is secure if it behaves precisely in the manner intended—
and does nothing more.

This definition is neat and vaguely outlines an abstract goal, but it tells
very little about how to achieve it. It’s computer science, but in terms of specificity, it bears a striking resemblance to a poem by Victor Hugo:
Love is a portion of the soul itself, and it is of the same nature as
the celestial breathing of the atmosphere of paradise.

One could argue that practitioners are not the ones to be asked for
nuanced definitions, but go ahead and pose the same question to a group of
academics and they’ll offer you roughly the same answer. For example, the
following common academic definition traces back to the Bell-La Padula security model, published in the 1960s. (This was one of about a dozen attempts
to formalize the requirements for secure systems, in this case in terms of a
finite state machine;1 it is also one of the most notable ones.)
A system is secure if and only if it starts in a secure state and cannot
enter an insecure state.
*
The quote is attributed originally to Ivan Arce, a renowned vulnerability hunter, circa 2000;
since then, it has been used by Crispin Cowan, Michael Howard, Anton Chuvakin, and scores
of other security experts.

2

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Definitions along these lines are fundamentally true, of course, and may
serve as the basis for dissertations or even a couple of government grants. But
in practice, models built on these foundations are bound to be nearly useless
for generalized, real-world software engineering for at least three reasons:


There is no way to define desirable behavior for a sufficiently complex
computer system. No single authority can define what the “intended
manner” or “secure states” should be for an operating system or a web
browser. The interests of users, system owners, data providers, business
process owners, and software and hardware vendors tend to differ significantly and shift rapidly—when the stakeholders are capable and willing to clearly and honestly disclose their interests to begin with. To add
insult to injury, sociology and game theory suggest that computing a simple sum of these particular interests may not actually result in a beneficial outcome. This dilemma, known as “the tragedy of the commons,” is
central to many disputes over the future of the Internet.



Wishful thinking does not automatically map to formal constraints.
Even if we can reach a perfect, high-level agreement about how the system should behave in a subset of cases, it is nearly impossible to formalize such expectations as a set of permissible inputs, program states, and
state transitions, which is a prerequisite for almost every type of formal
analysis. Quite simply, intuitive concepts such as “I do not want my mail
to be read by others,” do not translate to mathematical models particularly well. Several exotic approaches will allow such vague requirements
to be at least partly formalized, but they put heavy constraints on softwareengineering processes and often result in rulesets and models that are
far more complicated than the validated algorithms themselves. And,
in turn, they are likely to need their own correctness to be proven . . .
ad infinitum.



Software behavior is very hard to conclusively analyze. Static analysis of
computer programs with the intent to prove that they will always behave
according to a detailed specification is a task that no one has managed to
believably demonstrate in complex, real-world scenarios (though, as you
might expect, limited success in highly constrained settings or with very
narrow goals is possible). Many cases are likely to be impossible to solve
in practice (due to computational complexity) and may even turn out to
be completely undecidable due to the halting problem.*

Perhaps more frustrating than the vagueness and uselessness of the early
definitions is that as the decades have passed, little or no progress has been
made toward something better. In fact, an academic paper released in 2001
by the Naval Research Laboratory backtracks on some of the earlier work and
arrives at a much more casual, enumerative definition of software security—
one that explicitly disclaims its imperfection and incompleteness.2

*

In 1936, Alan Turing showed that (paraphrasing slightly) it is not possible to devise an algorithm
that can generally decide the outcome of other algorithms. Naturally, some algorithms are very
much decidable by conducting case-specific proofs, just not all of them.
Security in the World of Web Appli cati ons

3

A system is secure if it adequately protects information that it processes against unauthorized disclosure, unauthorized modification,
and unauthorized withholding (also called denial of service). We
say “adequately” because no practical system can achieve these
goals without qualification; security is inherently relative.

The paper also provides a retrospective assessment of earlier efforts
and the unacceptable sacrifices made to preserve the theoretical purity of
said models:
Experience has shown that, on one hand, the axioms of the BellLa Padula model are overly restrictive: they disallow operations that
users require in practical applications. On the other hand, trusted
subjects, which are the mechanism provided to overcome some
of these restrictions, are not restricted enough. . . . Consequently,
developers have had to develop ad hoc specifications for the desired
behavior of trusted processes in each individual system.

In the end, regardless of the number of elegant, competing models introduced, all attempts to understand and evaluate the security of real-world software using algorithmic foundations seem bound to fail. This leaves developers
and security experts with no method to make authoritative, future-looking
statements about the quality of produced code. So, what other options are on
the table?

Enter Risk Management
In the absence of formal assurances and provable metrics, and given the
frightening prevalence of security flaws in key software relied upon by modern societies, businesses flock to another catchy concept: risk management.
The idea of risk management, applied successfully to the insurance
business (with perhaps a bit less success in the financial world), simply states
that system owners should learn to live with vulnerabilities that cannot be
addressed in a cost-effective way and, in general, should scale efforts according to the following formula:
risk = probability of an event  maximum loss
For example, according to this doctrine, if having some unimportant
workstation compromised yearly won’t cost the company more than $1,000
in lost productivity, the organization should just budget for this loss and move
on, rather than spend say $100,000 on additional security measures or contingency and monitoring plans to prevent the loss. According to the doctrine
of risk management, the money would be better spent on isolating, securing,
and monitoring the mission-critical mainframe that churns out billing records
for all customers.

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Naturally, it’s prudent to prioritize security efforts. The problem is that
when risk management is done strictly by the numbers, it does little to help
us to understand, contain, and manage real-world problems. Instead, it introduces a dangerous fallacy: that structured inadequacy is almost as good as
adequacy and that underfunded security efforts plus risk management are
about as good as properly funded security work.
Guess what? No dice.


In interconnected systems, losses are not capped and are not tied to
an asset. Strict risk management depends on the ability to estimate typical and maximum cost associated with the compromise of a resource.
Unfortunately, the only way to do this is to overlook the fact that many
of the most spectacular security breaches—such as the attacks on TJX*
or Microsoft†—began at relatively unimportant and neglected entry
points. These initial intrusions soon escalated and eventually resulted
in the nearly complete compromise of critical infrastructure, bypassing
any superficial network compartmentalization on their way. In typical
by-the-numbers risk management, the initial entry point is assigned a
lower weight because it has a low value when compared to other nodes.
Likewise, the internal escalation path to more sensitive resources is
downplayed as having a low probability of ever being abused. Still,
neglecting them both proves to be an explosive mix.



The nonmonetary costs of intrusions are hard to offset with the value
contributed by healthy systems. Loss of user confidence and business
continuity, as well as the prospect of lawsuits and the risk of regulatory
scrutiny, are difficult to meaningfully insure against. These effects can, at
least in principle, make or break companies or even entire industries, and
any superficial valuations of such outcomes are almost purely speculative.



Existing data is probably not representative of future risks. Unlike the
participants in a fender bender, attackers will not step forward to helpfully report break-ins and will not exhaustively document the damage
caused. Unless the intrusion is painfully evident (due to the attacker’s
sloppiness or disruptive intent), it will often go unnoticed. Even though
industry-wide, self-reported data may be available, there is simply no reliable way of telling how complete it is or how much extra risk one’s current business practice may be contributing.

*
Sometime in 2006, several intruders, allegedly led by Albert Gonzalez, attacked an unsecured
wireless network at a retail location and subsequently made their way through the corporate
networks of the retail giant. They copied the credit card data of about 46 million customers and
the Social Security numbers, home addresses, and so forth of about 450,000 more. Eleven people
were charged in connection with the attack, one of whom committed suicide.
†
Microsoft’s formally unpublished and blandly titled presentation Threats Against and
Protection of Microsoft’s Internal Network outlines a 2003 attack that began with the compromise
of an engineer’s home workstation that enjoyed a long-lived VPN session to the inside of the
corporation. Methodical escalation attempts followed, culminating with the attacker gaining
access to, and leaking data from, internal source code repositories. At least to the general
public, the perpetrator remains unknown.

Security in the World of Web Appli cati ons

5



Statistical forecasting is not a robust predictor of individual outcomes.
Simply because on average people in cities are more likely to be hit by
lightning than mauled by a bear does not mean you should bolt a lightning rod to your hat and then bathe in honey. The likelihood that a
compromise will be associated with a particular component is, on an
individual scale, largely irrelevant: Security incidents are nearly certain,
but out of thousands of exposed nontrivial resources, any service can be
used as an attack vector—and no one service is likely to see a volume of
events that would make statistical forecasting meaningful within the
scope of a single enterprise.

Enlightenment Through Taxonomy
The two schools of thought discussed above share something in common:
Both assume that it is possible to define security as a set of computable goals
and that the resulting unified theory of a secure system or a model of acceptable risk would then elegantly trickle down, resulting in an optimal set of
low-level actions needed to achieve perfection in application design.
Some practitioners preach the opposite approach, which owes less to
philosophy and more to the natural sciences. These practitioners argue that,
much like Charles Darwin of the information age, by gathering sufficient
amounts of low-level, experimental data, we will be able to observe, reconstruct, and document increasingly more sophisticated laws in order to arrive
some sort of a unified model of secure computing.
This latter worldview brings us projects like the Department of Homeland Security–funded Common Weakness Enumeration (CWE), the goal of
which, in the organization’s own words, is to develop a unified “Vulnerability
Theory”; “improve the research, modeling, and classification of software flaws”;
and “provide a common language of discourse for discussing, finding and
dealing with the causes of software security vulnerabilities.” A typical, delightfully baroque example of the resulting taxonomy may be this:
Improper Enforcement of Message or Data Structure
Failure to Sanitize Data into a Different Plane
Improper Control of Resource Identifiers
Insufficient Filtering of File and Other Resource Names
for Executable Content

Today, there are about 800 names in the CWE dictionary, most of which
are as discourse-enabling as the one quoted here.
A slightly different school of naturalist thought is manifested in projects
such as the Common Vulnerability Scoring System (CVSS), a business-backed
collaboration that aims to strictly quantify known security problems in terms
of a set of basic, machine-readable parameters. A real-world example of the
resulting vulnerability descriptor may be this:
AV:LN / AC:L / Au:M / C:C / I:N / A:P / E:F / RL:T / RC:UR /
CDP:MH / TD:H / CR:M / IR:L / AR:M
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Organizations and researchers are expected to transform this 14 dimensional vector in a carefully chosen, use-specific way in order to arrive
at some sort of objective, verifiable, numerical conclusion about the significance of the underlying bug (say, “42”), precluding the need to judge the
nature of security flaws in any more subjective fashion.
Yes, I am poking gentle fun at the expense of these projects, but I do
not mean to belittle their effort. CWE, CVSS, and related projects serve noble
goals, such as bringing a more manageable dimension to certain security processes implemented by large organizations. Still, none has yielded a grand
theory of secure software, and I doubt such a framework is within sight.

Toward Practical Approaches
All signs point to security being largely a nonalgorithmic problem for now.
The industry is understandably reluctant to openly embrace this notion,
because it implies that there are no silver bullet solutions to preach (or better
yet, commercialize); still, when pressed hard enough, eventually everybody in
the security field falls back to a set of rudimentary, empirical recipes. These
recipes are deeply incompatible with many business management models,
but they are all that have really worked for us so far. They are as follows:


Learning from (preferably other people’s) mistakes. Systems should be
designed to prevent known classes of bugs. In the absence of automatic
(or even just elegant) solutions, this goal is best achieved by providing
ongoing design guidance, ensuring that developers know what could go
wrong, and giving them the tools to carry out otherwise error-prone tasks
in the simplest manner possible.



Developing tools to detect and correct problems. Security deficiencies
typically have no obvious side effects until they’re discovered by a malicious party: a pretty costly feedback loop. To counter this problem, we
create security quality assurance (QA) tools to validate implementations
and perform audits periodically to detect casual mistakes (or systemic
engineering deficiencies).



Planning to have everything compromised. History teaches us that major
incidents will occur despite our best efforts to prevent them. It is important to implement adequate component separation, access control, data
redundancy, monitoring, and response procedures so that service owners can react to incidents before an initially minor hiccup becomes a
disaster of biblical proportions.

In all cases, a substantial dose of patience, creativity, and real technical
expertise is required from all the information security staff.
Naturally, even such simple, commonsense rules—essentially basic engineering rigor—are often dressed up in catchphrases, sprinkled liberally with
a selection of acronyms (such as CIA: confidentiality, integrity, availability), and
then called “methodologies.” Frequently, these methodologies are thinly
veiled attempts to pass off one of the most frustrating failures of the security
industry as yet another success story and, in the end, sell another cure-all
Security in the World of Web Appli cati ons

7

product or certification to gullible customers. But despite claims to the contrary, such products are no substitute for street smarts and technical prowess—at least not today.
In any case, through the remainder of this book, I will shy away from
attempts to establish or reuse any of the aforementioned grand philosophical frameworks and settle for a healthy dose of anti-intellectualism instead. I
will review the exposed surface of modern browsers, discuss how to use the
available tools safely, which bits of the Web are commonly misunderstood,
and how to control collateral damage when things go boom.
And that is, pretty much, the best take on security engineering that I can
think of.

A Brief History of the Web
The Web has been plagued by a perplexing number, and a remarkable variety, of security issues. Certainly, some of these problems can be attributed to
one-off glitches in specific client or server implementations, but many are due
to capricious, often arbitrary design decisions that govern how the essential
mechanisms operate and mesh together on the browser end.
Our empire is built on shaky foundations—but why? Perhaps due to simple shortsightedness: After all, back in the innocent days, who could predict
the perils of contemporary networking and the economic incentives behind
today’s large-scale security attacks?
Unfortunately, while this explanation makes sense for truly ancient mechanisms such as SMTP or DNS, it does not quite hold water here: The Web is
relatively young and took its current shape in a setting not that different from
what we see today. Instead, the key to this riddle probably lies in the tumultuous and unusual way in which the associated technologies have evolved.
So, pardon me another brief detour as we return to the roots. The prehistory of the Web is fairly mundane but still worth a closer look.

Tales of the Stone Age: 1945 to 1994
Computer historians frequently cite a hypothetical desk-sized device called
the Memex as one of the earliest fossil records, postulated in 1945 by Vannevar
Bush.3 Memex was meant to make it possible to create, annotate, and follow
cross-document links in microfilm, using a technique that vaguely resembled
modern-day bookmarks and hyperlinks. Bush boldly speculated that this simple capability would revolutionize the field of knowledge management and
data retrieval (amusingly, a claim still occasionally ridiculed as uneducated
and naïve until the early 1990s). Alas, any useful implementation of the design
was out of reach at that time, so, beyond futuristic visions, nothing much
happened until transistor-based computers took center stage.
The next tangible milestone, in the 1960s, was the arrival of IBM’s
Generalized Markup Language (GML), which allowed for the annotation of
documents with machine-readable directives indicating the function of each
block of text, effectively saying “this is a header,” “this is a numbered list of
items,” and so on. Over the next 20 years or so, GML (originally used by only
8

Chapter 1

a handful of IBM text editors on bulky mainframe computers) became the
foundation for Standard Generalized Markup Language (SGML), a more
universal and flexible language that traded an awkward colon- and periodbased syntax for a familiar angle-bracketed one.
While GML was developing into SGML, computers were growing more
powerful and user friendly. Several researchers began experimenting with
Bush’s cross-link concept, applying it to computer-based document storage
and retrieval, in an effort to determine whether it would be possible to crossreference large sets of documents based on some sort of key. Adventurous
companies and universities pursued pioneering projects such as ENQUIRE,
NLS, and Xanadu, but most failed to make a lasting impact. Some common
complaints about the various projects revolved around their limited practical
usability, excess complexity, and poor scalability.
By the end of the decade, two researchers, Tim Berners-Lee and Dan
Connolly, had begun working on a new approach to the cross-domain reference challenge—one that focused on simplicity. They kicked off the project
by drafting HyperText Markup Language (HTML), a bare-bones descendant
of SGML, designed specifically for annotating documents with hyperlinks
and basic formatting. They followed their work on HTML with the development of HyperText Transfer Protocol (HTTP), an extremely basic, dedicated scheme for accessing HTML resources using the existing concepts of
Internet Protocol (IP) addresses, domain names, and file paths. The culmination of their work, sometime between 1991 and 1993, was Tim BernersLee’s World Wide Web (Figure 1-1), a rudimentary browser that parsed
HTML and allowed users to render the resulting data on the screen, and
then navigate from one page to another with a mouse click.

Figure 1-1: Tim Berners-Lee’s World Wide Web
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9

To many people, the design of HTTP and HTML must have seemed a
significant regression from the loftier goals of competing projects. After all,
many of the earlier efforts boasted database integration, security and digital
rights management, or cooperative editing and publishing; in fact, even
Berners-Lee’s own project, ENQUIRE, appeared more ambitious than his
current work. Yet, because of its low entry requirements, immediate usability,
and unconstrained scalability (which happened to coincide with the arrival
of powerful and affordable computers and the expansion of the Internet),
the unassuming WWW project turned out to be a sudden hit.
All right, all right, it turned out to be a “hit” by the standards of the mid1990s. Soon, there were no fewer than dozens of web servers running on the
Internet. By 1993, HTTP traffic accounted for 0.1 percent of all bandwidth
in the National Science Foundation backbone network. The same year also
witnessed the arrival of Mosaic, the first reasonably popular and sophisticated web browser, developed at the University of Illinois. Mosaic extended
the original World Wide Web code by adding features such as the ability to
embed images in HTML documents and submit user data through forms,
thus paving the way for the interactive, multimedia applications of today.
Mosaic made browsing prettier, helping drive consumer adoption of the
Web. And through the mid-1990s, it served as the foundation for two other
browsers: Mosaic Netscape (later renamed Netscape Navigator) and Spyglass
Mosaic (ultimately acquired by Microsoft and renamed Internet Explorer).
A handful of competing non-Mosaic engines emerged as well, including
Opera and several text-based browsers (such as Lynx and w3m). The first
search engines, online newspapers, and dating sites followed soon after.

The First Browser Wars: 1995 to 1999
By the mid-1990s, it was clear that the Web was here to stay and that users
were willing to ditch many older technologies in favor of the new contender.
Around that time, Microsoft, the desktop software behemoth that had been
slow to embrace the Internet before, became uncomfortable and began
to allocate substantial engineering resources to its own browser, eventually
bundling it with the Windows operating system in 1996.* Microsoft’s actions
sparked a period colloquially known as the “browser wars.”
The resulting arms race among browser vendors was characterized by the
remarkably rapid development and deployment of new features in the competing products, a trend that often defied all attempts to standardize or even properly document all the newly added code. Core HTML tweaks ranged from the
silly (the ability to make text blink, a Netscape invention that became the butt
of jokes and a telltale sign of misguided web design) to notable ones, such as
the ability to change typefaces or embed external documents in so-called frames.
Vendors released their products with embedded programming languages such
as JavaScript and Visual Basic, plug-ins to execute platform-independent Java
*
Interestingly, this decision turned out to be a very controversial one. On one hand, it could
be argued that in doing so, Microsoft contributed greatly to the popularization of the Internet.
On the other, it undermined the position of competing browsers and could be seen as anticompetitive. In the end, the strategy led to a series of protracted legal battles over the possible
abuse of monopoly by the company, such as United States v. Microsoft.

10

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or Flash applets on the user’s machine, and useful but tricky HTTP extensions
such as cookies. Only a limited degree of superficial compatibility, sometimes
hindered by patents and trademarks,* would be maintained.
As the Web grew larger and more diverse, a sneaky disease spread across
browser engines under the guise of fault tolerance. At first, the reasoning
seemed to make perfect sense: If browser A could display a poorly designed,
broken page but browser B refused to (for any reason), users would inevitably see browser B’s failure as a bug in that product and flock in droves to the
seemingly more capable client, browser A. To make sure that their browsers
could display almost any web page correctly, engineers developed increasingly complicated and undocumented heuristics designed to second-guess
the intent of sloppy webmasters, often sacrificing security and occasionally
even compatibility in the process. Unfortunately, each such change further
reinforced bad web design practices† and forced the remaining vendors to
catch up with the mess to stay afloat. Certainly, the absence of sufficiently
detailed, up-to-date standards did not help to curb the spread of this disease.
In 1994, in order to mitigate the spread of engineering anarchy and govern the expansion of HTML, Tim Berners-Lee and a handful of corporate
sponsors created the World Wide Web Consortium (W3C). Unfortunately
for this organization, for a long while it could only watch helplessly as the format was randomly extended and tweaked. Initial W3C work on HTML 2.0
and HTML 3.2 merely tried to catch up with the status quo, resulting in halfbaked specs that were largely out-of-date by the time they were released to
the public. The consortium also tried to work on some novel and fairly wellthought-out projects, such as Cascading Style Sheets, but had a hard time getting buy-in from the vendors.
Other efforts to standardize or improve already implemented mechanisms, most notably HTTP and JavaScript, were driven by other auspices such
as the European Computer Manufacturers Association (ECMA), the International Organization for Standardization (ISO), and the Internet Engineering
Task Force (IETF). Sadly, the whole of these efforts was seldom in sync, and
some discussions and design decisions were dominated by vendors or other
stakeholders who did not care much about the long-term prospects of the technology. The results were a number of dead standards, contradictory advice,
and several frightening examples of harmful cross-interactions between otherwise neatly designed protocols—a problem that will be particularly evident
when we discuss a variety of content isolation mechanisms in Chapter 9.

The Boring Period: 2000 to 2003
As the efforts to wrangle the Web floundered, Microsoft’s dominance grew
as a result of its operating system–bundling strategy. By the beginning of the
new decade, Netscape Navigator was on the way out, and Internet Explorer
*
For example, Microsoft did not want to deal with Sun to license a trademark for JavaScript
(a language so named for promotional reasons and not because it had anything to do with Java),
so it opted to name its almost-but-not-exactly-identical version “JScript.” Microsoft’s official
documentation still refers to the software by this name.
†
Prime examples of misguided and ultimately lethal browser features are content and character
set–sniffing mechanisms, both of which will be discussed in Chapter 13.

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11

held an impressive 80 percent market share—a number roughly comparable
to what Netscape had held just five years before. On both sides of the fence,
security and interoperability were the two most notable casualties of the feature war, but one could hope now that the fighting was over, developers
could put differences aside and work together to fix the mess.
Instead, dominance bred complacency: Having achieved its goals brilliantly, Microsoft had little incentive to invest heavily in its browser. Although
through version 5, major releases of Internet Explorer (IE) arrived yearly,
it took two years for version 6 to surface, then five full years for Internet
Explorer 6 to be updated to Internet Explorer 7. Without Microsoft’s interest, other vendors had very little leverage to make disruptive changes; most
sites were unwilling to make improvements that would work for only a small
fraction of their visitors.
On the upside, the slowdown in browser development allowed the
W3C to catch up and to carefully explore some new concepts for the future
of the Web. New initiatives finalized around the year 2000 included HTML 4
(a cleaned-up language that deprecated or banned many of the redundant or
politically incorrect features embraced by earlier versions) and XHTML 1.1 (a
strict and well-structured XML-based format that was easier to unambiguously
parse, with no proprietary heuristics allowed). The consortium also made significant improvements to JavaScript’s Document Object Model and to Cascading
Style Sheets. Regrettably, by the end of the century, the Web was too mature to
casually undo some of the sins of the old, yet too young for the security issues to
be pressing and evident enough for all to see. Syntax was improved, tags were
deprecated, validators were written, and deck chairs were rearranged, but the
browsers remained pretty much the same: bloated, quirky, and unpredictable.
But soon, something interesting happened: Microsoft gave the world a
seemingly unimportant, proprietary API, confusingly named XMLHttpRequest.
This trivial mechanism was meant to be of little significance, merely an
attempt to scratch an itch in the web-based version of Microsoft Outlook.
But XMLHttpRequest turned out to be far more, as it allowed for largely
unconstrained asynchronous HTTP communications between client-side
JavaScript and the server without the need for time-consuming and disruptive page transitions. In doing so, the API contributed to the emergence of
what would later be dubbed web 2.0—a range of complex, unusually responsive, browser-based applications that enabled users to operate on complex
data sets, collaborate and publish content, and so on, invading the sacred
domain of “real,” installable client software in the process. Understandably,
this caused quite a stir.

Web 2.0 and the Second Browser Wars: 2004 and Beyond
XMLHttpRequest, in conjunction with the popularity of the Internet and the
broad availability of web browsers, pushed the Web to some new, exciting
frontiers—and brought us a flurry of security bugs that impacted both individual users and businesses. By about 2002, worms and browser vulnerabilities had emerged as a frequently revisited theme in the media. Microsoft, by
virtue of its market dominance and a relatively dismissive security posture,
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Chapter 1

took much of the resulting PR heat. The company casually downplayed the
problem, but the trend eventually created an atmosphere conducive to a
small rebellion.
In 2004, a new contender in the browser wars emerged: Mozilla Firefox
(a community-supported descendant of Netscape Navigator) took the offensive, specifically targeting Internet Explorer’s poor security track record and
standards compliance. Praised by both IT journalists and security experts,
Firefox quickly secured a 20 percent market share. While the newcomer soon
proved to be nearly as plagued by security bugs as its counterpart from Redmond, its open source nature and the freedom from having to cater to stubborn corporate users allowed developers to fix issues much faster.
NOTE

Why would vendors compete so feverishly? Strictly speaking, there is no money to be
made by having a particular market share in the browser world. That said, pundits
have long speculated that it is a matter of power: By bundling, promoting, or demoting
certain online services (even as simple as the default search engine), whoever controls
the browser controls much of the Internet.
Firefox aside, Microsoft had other reasons to feel uneasy. Its flagship product, the Windows operating system, was increasingly being used as an (expendable?) launch pad for the browser, with more and more applications (from
document editors to games) moving to the Web. This could not be good.
These facts, combined with the sudden emergence of Apple’s Safari
browser and perhaps Opera’s advances in the world of smartphones, must
have had Microsoft executives scratching their heads. They had missed
the early signs of the importance of the Internet in the 1990s; surely they
couldn’t afford to repeat the mistake. Microsoft put some steam behind
Internet Explorer development again, releasing drastically improved and
somewhat more secure versions 7, 8, and 9 in rapid succession.
Competitors countered with new features and claims of even better (if still
superficial) standards compliance, safer browsing, and performance improvements. Caught off guard by the unexpected success of XMLHttpRequest and
quick to forget other lessons from the past, vendors also decided to experiment boldly with new ideas, sometimes unilaterally rolling out half-baked or
somewhat insecure designs like globalStorage in Firefox or httponly cookies in
Internet Explorer, just to try their luck.
To further complicate the picture, frustrated by creative differences with
W3C, a group of contributors created a wholly new standards body called the
Web Hypertext Application Technology Working Group (WHATWG). The
WHATWG has been instrumental in the development of HTML5, the first
holistic and security-conscious revision of existing standards, but it is reportedly shunned by Microsoft due to patent policy disputes.
Throughout much of its history, the Web has enjoyed a unique, highly
competitive, rapid, often overly political, and erratic development model
with no unifying vision and no one set of security principles. This state of
affairs has left a profound mark on how browsers operate today and how
secure the user data handled by browsers can be.
Chances are, this situation is not going to change anytime soon.
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13

The Evolution of a Threat
Clearly, web browsers, and their associated document formats and communication protocols, evolved in an unusual manner. This evolution may explain
the high number of security problems we see, but by itself it hardly proves
that these problems are unique or noteworthy. To wrap up this chapter, let’s
take a quick look at the very special characteristics behind the most prevalent
types of online security threats and explore why these threats had no particularly good equivalents in the years before the Web.

The User as a Security Flaw
Perhaps the most striking (and entirely nontechnical) property of web
browsers is that most people who use them are overwhelmingly unskilled.
Sure, nonproficient users have been an amusing, fringe problem since the
dawn of computing. But the popularity of the Web, combined with its remarkably low barrier to entry, means we are facing a new foe: Most users simply
don’t know enough to stay safe.
For a long time, engineers working on general-purpose software have
made seemingly arbitrary assumptions about the minimal level of computer
proficiency required of their users. Most of these assumptions have been without serious consequences; the incorrect use of a text editor, for instance, would
typically have little or no impact on system security. Incompetent users simply
would not be able to get their work done, a wonderfully self-correcting issue.
Web browsers do not work this way, however. Unlike certain complicated
software, they can be successfully used by people with virtually no computer
training, people who may not even know how to use a text editor. But at the
same time, browsers can be operated safely only by people with a pretty good
understanding of computer technology and its associated jargon, including
topics such as Public-Key Infrastructure. Needless to say, this prerequisite is
not met by most users of some of today’s most successful web applications.
Browsers still look and feel as if they were designed by geeks and for
geeks, complete with occasional cryptic and inconsistent error messages,
complex configuration settings, and a puzzling variety of security warnings
and prompts. A notable study by Berkeley and Harvard researchers in 2006
demonstrated that casual users are almost universally oblivious to signals that
surely make perfect sense to a developer, such as the presence or absence
of lock icons in the status bar.4 In another study, Stanford and Microsoft
researchers reached similar conclusions when they examined the impact of
the modern “green URL bar” security indicator. The mechanism, designed
to offer a more intuitive alternative to lock icons, actually made it easier to
trick users by teaching the audience to trust a particular shade of green, no
matter where this color appeared.5
Some experts argue that the ineptitude of the casual user is not the
fault of software vendors and hence not an engineering problem at all. Others
note that when creating software so easily accessible and so widely distributed,
it is irresponsible to force users to make security-critical decisions that depend
on technical prowess not required to operate the program in the first place.
14

Chapter 1

To blame browser vendors alone is just as unfair, however: The computing
industry as a whole has no robust answers in this area, and very little research
is available on how to design comparably complex user interfaces (UIs) in a
bulletproof way. After all, we barely get it right for ATMs.

The Cloud, or the Joys of Communal Living
Another peculiar characteristic of the Web is the dramatically understated
separation between unrelated applications and the data they process.
In the traditional model followed by virtually all personal computers
over the last 15 years or so, there are very clear boundaries between highlevel data objects (documents), user-level code (applications), and the operating system kernel that arbitrates all cross-application communications and
hardware input/output (I/O) and enforces configurable security rules should
an application go rogue. These boundaries are well studied and useful for
building practical security schemes. A file opened in your text editor is unlikely
to be able to steal your email, unless a really unfortunate conjunction of
implementation flaws subverts all these layers of separation at once.
In the browser world, this separation is virtually nonexistent: Documents
and code live as parts of the same intermingled blobs of HTML, isolation
between completely unrelated applications is partial at best (with all sites
nominally sharing a global JavaScript environment), and many types of interaction between sites are implicitly permitted with few, if any, flexible, browserlevel security arbitration frameworks.
In a sense, the model is reminiscent of CP/M, DOS, and other principally
nonmultitasking operating systems with no robust memory protection, CPU
preemption, or multiuser features. The obvious difference is that few users
depended on these early operating systems to simultaneously run multiple
untrusted, attacker-supplied applications, so there was no particular reason
for alarm.
In the end, the seemingly unlikely scenario of a text file stealing your
email is, in fact, a frustratingly common pattern on the Web. Virtually all web
applications must heavily compensate for unsolicited, malicious cross-domain
access and take cumbersome steps to maintain at least some separation of
code and the displayed data. And sooner or later, virtually all web applications
fail. Content-related security issues, such as cross-site scripting or cross-site
request forgery, are extremely common and have very few counterparts in
dedicated, compartmentalized client architectures.

Nonconvergence of Visions
Fortunately, the browser security landscape is not entirely hopeless, and
despite limited separation between web applications, several selective security mechanisms offer rudimentary protection against the most obvious attacks.
But this brings us to another characteristic that makes the Web such an interesting subject: There is no shared, holistic security model to grasp and live by.
We are not looking for a grand vision for world peace, mind you, but simply
a common set of flexible paradigms that would apply to most, if not all, of the
S ecur i t y i n t h e W or l d o f W e b Ap p l i c a t i o n s

15

relevant security logic. In the Unix world, for example, the rwx user/group permission model is one such strong unifying theme. But in the browser realm?
In the browser realm, a mechanism called same-origin policy could be
considered a candidate for a core security paradigm, but only until one realizes that it governs a woefully small subset of cross-domain interactions. That
detail aside, even within its scope, it has no fewer than seven distinct varieties,
each of which places security boundaries between applications in a slightly
different place.* Several dozen additional mechanisms, with no relation to
the same-origin model, control other key aspects of browser behavior (essentially implementing what each author considered to be the best approach to
security controls that day).
As it turns out, hundreds of small, clever hacks do not necessarily add up
to a competent security opus. The unusual lack of integrity makes it very difficult even to decide where a single application ends and a different one
begins. Given this reality, how does one assess attack surfaces, grant or take
away permissions, or accomplish just about any other security-minded task?
Too often, “by keeping your fingers crossed” is the best response we can give.
Curiously, many well-intentioned attempts to improve security by
defining new security controls only make the problem worse. Many of these
schemes create new security boundaries that, for the sake of elegance, do not
perfectly align with the hairy juxtaposition of the existing ones. When the
new controls are finer grained, they are likely to be rendered ineffective by
the legacy mechanisms, offering a false sense of security; when they are more
coarse grained, they may eliminate some of the subtle assurances that the
Web depends on right now. (Adam Barth and Collin Jackson explore the
topic of destructive interference between browser security policies in their
academic work.)6

Cross-Browser Interactions: Synergy in Failure
The overall susceptibility of an ecosystem composed of several different software products could be expected to be equal to a simple sum of the flaws
contributed by each of the applications. In some cases, the resulting exposure may be less (diversity improves resilience), but one would not expect it
to be more.
The Web is once again an exception to the rule. The security community
has discovered a substantial number of issues that cannot be attributed to any
particular piece of code but that emerge as a real threat when various browsers try to interact with each other. No particular product can be easily singled
out for blame: They are all doing their thing, and the only problem is that no
one has bothered to define a common etiquette for all of them to obey.
For example, one browser may assume that, in line with its own security
model, it is safe to pass certain URLs to external applications or to store or
read back certain types of data from disk. For each such assumption, there
likely exists at least one browser that strongly disagrees, expecting other
*
The primary seven varieties, as discussed throughout Part II of this book, include the security
policy for JavaScript DOM access; XMLHttpRequest API; HTTP cookies; local storage APIs; and
plug-ins such as Flash, Silverlight, or Java.

16

Chapter 1

parties to follow its rules instead. The exploitability of these issues is greatly
aggravated by vendors’ desire to get their foot in the door and try to allow
web pages to switch to their browser on the fly without the user’s informed
consent. For example, Firefox allows pages to be opened in its browser by
registering a firefoxurl: protocol; Microsoft installs its own .NET gateway plugin in Firefox; Chrome does the same to Internet Explorer via a protocol
named cf:.
NOTE

Especially in the case of such interactions, pinning the blame on any particular party
is a fool’s errand. In a recent case of a bug related to firefoxurl:, Microsoft and half of
the information security community blamed Mozilla, while Mozilla and the other half
of experts blamed Microsoft.7 It did not matter who was right: The result was still a
very real mess.
Another set of closely related problems (practically unheard of in the
days before the Web) are the incompatibilities in superficially similar security
mechanisms implemented in each browser. When the security models differ,
a sound web application–engineering practice in one product may be inadequate and misguided in another. In fact, several classes of rudimentary tasks,
such as serving a user-supplied plaintext file, cannot be safely implemented
in certain browsers at all. This fact, however, will not be obvious to developers unless they are working in one of the affected browsers—and even then,
they need to hit just the right spot.
In the end, all the characteristics outlined in this section contribute to
a whole new class of security vulnerabilities that a taxonomy buff might call a
failure to account for undocumented diversity. This class is very well populated
today.

The Breakdown of the Client-Server Divide
Information security researchers enjoy the world of static, clearly assigned
roles, which are a familiar point of reference when mapping security interactions in the otherwise complicated world. For example, we talk about Alice
and Bob, two wholesome, hardworking users who want to communicate, and
Mallory, a sneaky attacker who is out to get them. We then have client software
(essentially dumb, sometimes rogue I/O terminals that frivolously request
services) and humble servers, carefully fulfilling the clients’ whim. Developers learn these roles and play along, building fairly comprehensible and testable network-computing environments in the process.
The Web began as a classical example of a proper client-server architecture, but the functional boundaries between client and server responsibilities
were quickly eroded. The culprit is JavaScript, a language that offers the HTTP
servers a way to delegate application logic to the browser (“client”) side and
gives them two very compelling reasons to do so. First, such a shift often
results in more responsive user interfaces, as servers do not need to synchronously participate in each tiny UI state change imaginable. Second, serverside CPU and memory requirements (and hence service-provisioning costs)
can decrease drastically when individual workstations across the globe chip
in to help with the bulk of the work.
S ecur i t y i n t h e W or l d o f W e b Ap p l i c a t i o n s

17

The client-server diffusion process began innocently enough, but it
was only a matter of time before the first security mechanisms followed to the
client side too, along with all the other mundane functionality. For example,
what was the point of carefully scrubbing HTML on the server side when the
data was only dynamically rendered by JavaScript on the client machine?
In some applications, this trend was taken to extremes, eventually leaving the server as little more than a dumb storage device and moving almost
all the parsing, editing, display, and configuration tasks into the browser
itself. In such designs, the dependency on a server could even be fully severed by using offline web extensions such as HTML5 persistent storage.
A simple shift in where the entire application magic happens is not
necessarily a big deal, but not all security responsibilities can be delegated to
the client as easily. For example, even in the case of a server acting as dumb
storage, clients cannot be given indiscriminate access to all the data stored
on the server for other users, and they cannot be trusted to enforce access
controls. In the end, because it was not desirable to keep all the application
security logic on the server side, and it was impossible to migrate it fully to the
client, most applications ended up occupying some arbitrary middle ground
instead, with no easily discernible and logical separation of duties between
the client and server components. The resulting unfamiliar designs and
application behaviors simply had no useful equivalents in the elegant and
wholesome world of security role-play.
The situation has resulted in more than just a design-level mess; it has
led to irreducible complexity. In a traditional client-server model with wellspecified APIs, one can easily evaluate a server’s behavior without looking
at the client, and vice versa. Moreover, within each of these components, it
is possible to easily isolate smaller functional blocks and make assumptions
about their intended operation. With the new model, coupled with the
opaque, one-off application APIs common on the Web, these analytical
tools, and the resulting ease of reasoning about the security of a system,
have been brutally taken away.
The unexpected failure of standardized security modeling and testing
protocols is yet another problem that earns the Web a very special—and
scary—place in the universe of information security.

18

Chapter 1

Global browser market share, May 2011
Vendor

Browser Name

Microsoft

Internet Explorer 6

10%

Internet Explorer 7

7%

Internet Explorer 8

31%

Internet Explorer 9

4%

Firefox 3

12%

Firefox 4+

10%

Mozilla

Market Share

52%

22%

Google

Chrome

13%

Apple

Safari

7%

Opera Software

Opera

3%

Source: Data drawn from public Net Applications reports.1

PART I
ANATOMY OF THE WEB

The first part of this book focuses on the principal
concepts that govern the operation of web browsers,
namely, the protocols, document formats, and programming languages that make it all tick. Because all
the familiar, user-visible security mechanisms employed
in modern browsers are profoundly intertwined with
these inner workings, the bare internals deserve a fair
bit of attention before we wander off deeper into the
woods.

IT STARTS WITH A URL

The most recognizable hallmark of the Web is a simple
text string known as the Uniform Resource Locator (URL).
Each well-formed, fully qualified URL is meant to conclusively address and uniquely identify a single resource
on a remote server (and in doing so, implement a couple of related, auxiliary functions). The URL syntax is
the cornerstone of the address bar, the most important
user interface (UI) security indicator in every browser.
In addition to true URLs used for content retrieval, several classes of
pseudo-URLs use a similar syntax to provide convenient access to browser-level
features, including the integrated scripting engine, several special documentrendering modes, and so on. Perhaps unsurprisingly, these pseudo-URL
actions can have a significant impact on the security of any site that decides
to link to them.
The ability to figure out how a particular URL will be interpreted by the
browser, and the side effects it will have, is one of the most basic and common security tasks attempted by humans and web applications alike, but it can

be a problematic one. The generic URL syntax, the work of Tim Berners-Lee,
is codified primarily in RFC 3986;1 its practical uses on the Web are outlined
in RFCs 1738,2 2616,3 and a couple of other, less-significant standards. These
documents are remarkably detailed, resulting in a fairly complex parsing
model, but they are not precise enough to lead to harmonious, compatible
implementations in all client software. In addition, individual software vendors have chosen to deviate from the specifications for their own reasons.
Let’s have a closer look at how the humble URL works in practice.

Uniform Resource Locator Structure
Figure 2-1 shows the format of a fully qualified absolute URL, one that specifies
all information required to access a particular resource and that does not
depend in any way on where the navigation began. In contrast, a relative URL,
such as ../file.php?text=hello+world, omits some of this information and must
be interpreted in the context of a base URL associated with the current
browsing context.














scheme:// login.password@ address:port /path/to/resource ?query_string #fragment

 Scheme/protocol name
 Indicator of a hierarchical URL (constant)
 Credentials to access the resource (optional)
 Server to retrieve the data from

“Authority”

 Port number to connect to (optional)
 Hierarchical Unix path to a resource
 “Query string” parameters (optional)
“Fragment identifier” (optional)

Figure 2-1: Structure of an absolute URL

The segments of the absolute URL seem intuitive, but each comes with
a set of gotchas, so let’s review them now.

Scheme Name
The scheme name is a case-insensitive string that ends with a single colon,
indicating the protocol to be used to retrieve the resource. The official
registry of valid URL schemes is maintained by the Internet Assigned Numbers
Authority (IANA), a body more widely known for its management of the IP
address space.4 IANA’s current list of valid scheme names includes several
dozen entries such as http:, https:, and ftp:; in practice, a much broader set of
schemes is informally recognized by common browsers and third-party applications, some which have special security consequences. (Of particular interest are several types of pseudo-URLs, such as data: or javascript:, as discussed
later in this chapter and throughout the remainder of this book.)

24

Chapter 2

Before they can do any further parsing, browsers and web applications
need to distinguish fully qualified absolute URLs from relative ones. The
presence of a valid scheme in front of the address is meant to be the key
difference, as defined in RFC 1738: In a compliant absolute URL, only the
alphanumerics “+”, “-”, and “.” may appear before the required “:”. In practice, however, browsers deviate from this guidance a bit. All ignore leading
newlines and white spaces. Internet Explorer ignores the entire nonprintable
character range of ASCII codes 0x01 to 0x1F. Chrome additionally skips 0x00,
the NUL character. Most implementations also ignore newlines and tabs in the
middle of scheme names, and Opera accepts high-bit characters in the string.
Because of these incompatibilities, applications that depend on the ability to differentiate between relative and absolute URLs must conservatively
reject any anomalous syntax—but as we will soon find out, even this is not
enough.

Indicator of a Hierarchical URL
In order to comply with the generic syntax rules laid out in RFC 1738, every
absolute, hierarchical URL is required to contain the fixed string “//” right
before the authority section. If the string is missing, the format and function
of the remainder of the URL is undefined for the purpose of that specification and must be treated as an opaque, scheme-specific value.
NOTE

An example of a nonhierarchical URL is the mailto: protocol, used to specify
email addresses and possibly a subject line (mailto:user@example.com?subject=
Hello+world). Such URLs are passed down to the default mail client without making
any further attempt to parse them.
The concept of a generic, hierarchical URL syntax is, in theory, an elegant one. It ought to enable applications to extract some information about
the address without knowing how a particular scheme works. For example,
without a preconceived notion of the wacky-widget: protocol, and by applying
the concept of generic URL syntax alone, the browser could decide that
http://example.com/test1/ and wacky-widget://example.com/test2/ reference the
same, trusted remote host.
Regrettably, the specification has an interesting flaw: The aforementioned
RFC says nothing about what the implementer should do when encountering
URLs where the scheme is known to be nonhierarchical but where a “//”
prefix still appears, or vice versa. In fact, a reference parser implementation
provided in RFC 1630 contains an unintentional loophole that gives a counterintuitive meaning to the latter class of URLs. In RFC 3986, published some
years later, the authors sheepishly acknowledge this flaw and permit implementations to try to parse such URLs for compatibility reasons. As a consequence, many browsers interpret the following examples in unexpected ways:


http:example.com/ In Firefox, Chrome, and Safari, this address may be
treated identically to http://example.com/ when no fully qualified base
URL context exists and as a relative reference to a directory named
example.com when a valid base URL is available.
It Starts with a URL

25



javascript://example.com/%0Aalert(1) This string is interpreted as a valid
nonhierarchical pseudo-URL in all modern browsers, and the JavaScript
alert(1) code will execute, showing a simple dialog window.



mailto://user@example.com Internet Explorer accepts this URL as a valid
nonhierarchical reference to an email address; the “//” part is simply
skipped. Other browsers disagree.

Credentials to Access the Resource
The credentials portion of the URL is optional. This location can specify a
username, and perhaps a password, that may be required to retrieve the data
from the server. The method through which these credentials are exchanged
is not specified as a part of the abstract URL syntax, and it is always protocol
specific. For those protocols that do not support authentication, the behavior of a credential-bearing URL is simply undefined.
When no credentials are supplied, the browser will attempt to fetch the
resource anonymously. In the case of HTTP and several other protocols, this
means not sending any authentication data; for FTP, it involves logging into
a guest account named ftp with a bogus password.
Most browsers accept almost any characters, other than general URL
section delimiters, in this section with two exceptions: Safari, for unclear reasons, rejects a broader set of characters, including “<”, “>”, “{”, and “}”, while
Firefox also rejects newlines.*

Server Address
For all fully qualified hierarchical URLs, the server address section must specify a case-insensitive DNS name (such as example.com), a raw IPv4 address (such
as 127.0.0.1), or an IPv6 address in square brackets (such as [0:0:0:0:0:0:0:1]),
indicating the location of a server hosting the requested resource. Firefox
will also accept IPv4 addresses and hostnames in square brackets, but other
implementations reject them immediately.
Although the RFC permits only canonical notations for IP addresses, standard C libraries used by most applications are much more relaxed, accepting
noncanonical IPv4 addresses that mix octal, decimal, and hexadecimal notation or concatenate some or all of the octets into a single integer. As a result,
the following options are recognized as equivalent:
This is a canonical representation of an IPv4 address.



http://127.0.0.1/



http://0x7f.1/ This is a representation of the same address that uses a
hexadecimal number to represent the first octet and concatenates all the
remaining octets into a single decimal value.



http://017700000001/ The same address is denoted using a 0-prefixed
octal value, with all octets concatenated into a single 32-bit integer.

*
This is possibly out of the concern for FTP, which transmits user credentials without any
encoding; in this protocol, a newline transmitted as is would be misinterpreted by the server
as the beginning of a new FTP command. Other browsers may transmit FTP credentials in
noncompliant percent-encoded form or simply strip any problematic characters later on.

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Chapter 2

A similar laid-back approach can be seen with DNS names. Theoretically,
DNS labels need to conform to a very narrow character set (specifically, alphanumerics, “.”, and “-”, as defined in RFC 10355), but many browsers will happily
ask the underlying operating system resolver to look up almost anything, and
the operating system will usually also not make a fuss. The exact set of characters accepted in the hostname and passed to the resolver varies from client to
client. Safari is most rigorous, while Internet Explorer is the most permissive.
Perhaps of note, several control characters in the 0x0A–0x0D and 0xA0–0xAD
ranges are ignored by most browsers in this portion of the URL.
NOTE

One fascinating behavior of the URL parsers in all of the mainstream browsers is their
willingness to treat the character “ ” (ideographic full stop, Unicode point U+3002)
identically to a period in hostnames but not anywhere else in the URL. This is reportedly because certain Chinese keyboard mappings make it much easier to type this symbol
than the expected 7-bit ASCII value.

Server Port
This server port is an optional section that describes a nonstandard network
port to connect to on the previously specified server. Virtually all applicationlevel protocols supported by browsers and third-party applications use TCP
or UDP as the underlying transport method, and both TCP and UDP rely on
16-bit port numbers to separate traffic between unrelated services running
on a single machine. Each scheme is associated with a default port on which
servers for that protocol are customarily run (80 for HTTP, 21 for FTP, and
so on), but the default can be overridden at the URL level.
NOTE

An interesting and unintended side effect of this feature is that browsers can be tricked
into sending attacker-supplied data to random network services that do not speak the
protocol the browser expects them to. For example, one may point a browser to http://
mail.example.com:25/, where 25 is a port used by the Simple Mail Transfer Protocol
(SMTP) service rather than HTTP. This fact has caused a range of security problems
and prompted a number of imperfect workarounds, as discussed in more detail in
Part II of this book.

Hierarchical File Path
The next portion of the URL, the hierarchical file path, is envisioned as a
way to identify a specific resource to be retrieved from the server, such as
/documents/2009/my_diary.txt. The specification quite openly builds on top of
the Unix directory semantics, mandating the resolution of “/../” and “/./”
segments in the path and providing a directory-based method for sorting out
relative references in non–fully qualified URLs.
Using the filesystem model must have seemed like a natural choice in
the 1990s, when web servers acted as simple gateways to a collection of static
files and the occasional executable script. But since then, many contemporary web application frameworks have severed any remaining ties with the
filesystem, interfacing directly with database objects or registered locations in
resident program code. Mapping these data structures to well-behaved URL
It Starts with a URL

27

paths is possible but not always practiced or practiced carefully. All of this
makes automated content retrieval, indexing, and security testing more
complicated than it should be.

Query String
The query string is an optional section used to pass arbitrary, nonhierarchical parameters to the resource earlier identified by the path. One common
example is passing user-supplied terms to a server-side script that implements
the search functionality, such as:
http://example.com/search.php?query=Hello+world

Most web developers are accustomed to a particular layout of the query
string; this familiar format is generated by browsers when handling HTMLbased forms and follows this syntax:
name1=value1&name2=value2...

Surprisingly, such layout is not mandated in the URL RFCs. Instead, the
query string is treated as an opaque blob of data that may be interpreted by
the final recipient as it sees fit, and unlike the path, it is not encumbered
with specific parsing rules.
Hints of the commonly used format can be found in an informational
RFC 1630,6 in a mail-related RFC 2368,7 and in HTML specifications dealing
with forms.8 None of this is binding, and therefore, while it may be impolite,
it is not a mistake for web applications to employ arbitrary formats for whatever data they wish to put in that part of the URL.

Fragment ID
The fragment ID is an opaque value with a role similar to the query string
but that provides optional instructions for the client application rather than
the server. (In fact, the value is not supposed to be sent to the server at all.)
Neither the format nor function of the fragment ID is clearly specified in
the RFCs, but it is hinted that it may be used to address “subresources” in the
retrieved document or to provide other document-specific rendering cues.
In practice, fragment identifiers have only a single sanctioned use in
the browser: that of specifying the name of an anchor HTML element for
in-document navigation. The logic is simple. If an anchor name is supplied in
the URL and a matching HTML tag can be located, the document will be
scrolled to that location for viewing; otherwise, nothing happens. Because
the information is encoded in the URL, this particular view of a lengthy document could be easily shared with others or bookmarked. In this use, the
meaning of a fragment ID is limited to scrolling an existing document, so
there is no need to retrieve any new data from the server when only this portion of the URL is updated in response to user actions.

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Chapter 2

This interesting property has led to another, more recent and completely
ad hoc use of this value: to store miscellaneous state information needed by
client-side scripts. For example, consider a map-browsing application that
puts the currently viewed map coordinates in the fragment identifier so that
it will know to resume from that same location if the link is bookmarked or
shared. Unlike updating the query string, changing the fragment ID on-thefly will not trigger a time-consuming page reload, making this data-storage
trick a killer feature.

Putting It All Together Again
Each of the aforementioned URL segments is delimited by certain reserved
characters: slashes, colons, question marks, and so on. To make the whole
approach usable, these delimiting characters should not appear anywhere
in the URL for any other purpose. With this assumption in mind, imagine a
sample algorithm to split absolute URLs into the aforementioned functional
parts in a manner at least vaguely consistent with how browsers accomplish
this task. A reasonably decent example of such an algorithm could be:
STEP 1: Extract the scheme name.
Scan for the first “:” character. The part of the URL to its left is the
scheme name. Bail out if the scheme name does not conform to the
expected set of characters; the URL may need to be treated as a relative
one if so.
STEP 2: Consume the hierarchical URL identifier.
The string “//” should follow the scheme name. Skip it if found; bail out
if not.
NOTE

In some parsing contexts, implementations will be just as happy with zero, one, or even
three or more slashes instead of two, for usability reasons. In the same vein, from its
inception, Internet Explorer accepted backslashes (\) in lieu of slashes in any location
in the URL, presumably to assist inexperienced users.* All browsers other than Firefox
eventually followed this trend and recognize URLs such as http:\\ example.com\.
STEP 3: Grab the authority section.
Scan for the next “/”, “?”, or “#”, whichever comes first, to extract the
authority section from the URL. As mentioned above, most browsers will
also accept “ \” as a delimiter in place of a forward slash, which may need
to be accounted for. The semicolon (;) is another acceptable authority
delimiter in browsers other than Internet Explorer and Safari; the reason for this decision is unknown.

*
Unlike UNIX-derived operating systems, Microsoft Windows uses backslashes instead of slashes
to delimit file paths (say, c:\windows\system32\calc.exe). Microsoft probably tried to compensate for
the possibility that users would be confused by the need to type a different type of a slash on the
Web or hoped to resolve other possible inconsistencies with file: URLs and similar mechanisms
that would be interfacing directly with the local filesystem. Other Windows filesystem specifics
(such as case insensitivity) are not replicated, however.

It Starts with a URL

29

STEP 3A: Find the credentials, if any.
Once the authority section is extracted, locate the at symbol (@) in the
substring. If found, the leading snippet constitutes login credentials,
which should be further tokenized at the first occurrence of a colon (if
present) to split the login and password data.
STEP 3B: Extract the destination address.
The remainder of the authority section is the destination address. Look
for the first colon to separate the hostname from the port number. A
special case is needed for bracket-enclosed IPv6 addresses, too.
STEP 4: Identify the path (if present).
If the authority section is followed immediately by a forward slash—or
for some implementations, a backslash or semicolon, as noted earlier—
scan for the next “?”, “#”, or end-of-string, whichever comes first. The
text in between constitutes the path section, which should be normalized
according to Unix path semantics.
STEP 5: Extract the query string (if present).
If the last successfully parsed segment is followed by a question mark,
scan for the next “#” character or end-of-string, whichever comes first.
The text in between is the query string.
STEP 6: Extract the fragment identifier (if present).
If the last successfully parsed segment is followed by “#”, everything from
that character to the end-of-string is the fragment identifier. Either way,
you’re done!
This algorithm may seem mundane, but it reveals subtle details that even
seasoned programmers normally don’t think about. It also illustrates that it is
extremely difficult for casual users to understand how a particular URL may
be parsed. Let's start with this fairly simple case:
http://example.com&gibberish=1234@167772161/

The target of this URL—a concatenated IP address that decodes to
10.0.0.1—is not readily apparent to a nonexpert, and many users would
believe they are visiting example.com instead.* But all right, that was an easy
one! So let’s have a peek at this syntax instead:
http://example.com\@coredump.cx/

In Firefox, that URL will take the user to coredump.cx, because example.com\
will be interpreted as a valid value for the login field. In almost all other browsers, “\” will be interpreted as a path delimiter, and the user will land on example
.com instead.
*
This particular @-based trick was quickly embraced to facilitate all sorts of online fraud
targeted at casual users. Attempts to mitigate its impact ranged from the heavy-handed and
oddly specific (e.g., disabling URL-based authentication in Internet Explorer or crippling it
with warnings in Firefox) to the fairly sensible (e.g., hostname highlighting in the address bar
of several browsers).

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Chapter 2

An even more frustrating example exists for Internet Explorer.
Consider this:
http://example.com;.coredump.cx/

Microsoft’s browser permits “;” in the hostname and successfully
resolves this label, thanks to the appropriate configuration of the coredump.cx
domain. Most other browsers will autocorrect the URL to http://example.com/
;.coredump.cx and take the user to example.com instead (except for Safari, where
the syntax causes an error). If this looks messy, remember that we are just
getting started with how browsers work!

Reserved Characters and Percent Encoding
The URL-parsing algorithm outlined in the previous section relies on the
assumption that certain reserved, syntax-delimiting characters will not appear
literally in the URL in any other capacity (that is, they won’t be a part of the username, request path, and so on). These generic, syntax-disrupting delimiters are:
: / ? # [ ] @

The RFC also names a couple of lower-tier delimiters without giving
them any specific purpose, presumably to allow scheme- or applicationspecific features to be implemented within any of the top-level sections:
! $ & ' ( ) * + , ; =

All of the above characters are in principle off-limits, but there are legitimate cases where one would want to include them in the URL (for example,
to accommodate arbitrary search terms entered by the user and passed to the
server in the query string). Therefore, rather than ban them, the standard
provides a method to encode all spurious occurrences of these values. The
method, simply called percent encoding or URL encoding, substitutes characters
with a percent sign (%) followed by two hexadecimal digits representing a
matching ASCII value. For example, “/” will be encoded as %2F (uppercase
is customary but not enforced). It follows that to avoid ambiguity, the naked
percent sign itself must be encoded as %25. Any intermediaries that handle
existing URLs (browsers and web applications included) are further compelled never to attempt to decode or encode reserved characters in relayed
URLs, because the meaning of such a URL may suddenly change.
Regrettably, the immutability of reserved characters in existing URLs
is at odds with the need to respond to any URLs that are technically illegal
because they misuse these characters and that are encountered by the browser
in the wild. This topic is not covered by the specifications at all, which forces
browser vendors to improvise and causes cross-implementation inconsistencies. For example, should the URL http://a@b@c/ be translated to http://
a@b%40c/ or perhaps to http://a%40b@c/? Internet Explorer and Safari think
the former makes more sense; other browsers side with the latter view.
It Starts with a URL

31

The remaining characters not in the reserved set are not supposed to
have any particular significance within the URL syntax itself. However, some
(such as nonprintable ASCII control characters) are clearly incompatible
with the idea that URLs should be human readable and transport-safe. Therefore, the RFC outlines a confusingly named subset of unreserved characters
(consisting of alphanumerics, “-”, “.”, “_”, and “~”) and says that only this
subset and the reserved characters in their intended capacity are formally
allowed to appear in the URL as is.
NOTE

Curiously, these unreserved characters are only allowed to appear in an unescaped
form; they are not required to do so. User agents may encode or decode them at whim,
and doing so does not change the meaning of the URL at all. This property brings up
yet another way to confuse users: the use of noncanonical representations of unreserved
characters. Specifically, all of the following are equivalent:


http://example.com/



http://%65xample.%63om/



http://%65%78%61%6d%70%6c%65%2e%63%6f%6d/*

A number of otherwise nonreserved, printable characters are excluded
from the so-called unreserved set. Because of this, strictly speaking, the RFCs
require them to be unconditionally percent encoded. However, since browsers are not explicitly tasked with the enforcement of this rule, it is not taken
very seriously. In particular, all browsers allow “^”, “{”, “|”, and “}” to appear
in URLs without escaping and will send these characters to the server as is.
Internet Explorer further permits “<”, “>”, and “`” to go through; Internet
Explorer, Firefox, and Chrome all accept “\”; Chrome and Internet Explorer
will permit a double quote; and Opera and Internet Explorer both pass the
nonprintable character 0x7F (DEL) as is.
Lastly, contrary to the requirements spelled out in the RFC, most browsers also do not encode fragment identifiers at all. This poses an unexpected
challenge to client-side scripts that rely on this string and expect certain
potentially unsafe characters never to appear literally. We will revisit this
topic in Chapter 6.

Handling of Non-US-ASCII Text
Many languages used around the globe rely on characters outside the basic,
7-bit ASCII character set or the default 8-bit code page traditionally used by
all PC-compatible systems (CP437). Heck, some languages depend on alphabets that are not based on Latin at all.
In order to accommodate the needs of an often-ignored but formidable
non-English user base, various 8-bit code pages with an alternative set of highbit characters were devised long before the emergence of the Web: ISO 8859-1,
*
Similar noncanonical encodings were widely used for various types of social engineering attacks,
and consequently, various countermeasures have been deployed through the years. As usual,
some of these countermeasures are disruptive (for example, Firefox flat out rejects percentencoded text in hostnames), and some are fairly good (such as the forced “canonicalization”
of the address bar by decoding all the unnecessarily encoded text for display purposes).

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Chapter 2

CP850, and Windows 1252 for Western European languages; ISO 8859-2,
CP852, and Windows 1250 for Eastern and Central Europe; and KOI8-R and
Windows 1251 for Russia. And, because several alphabets could not be accommodated in the 256-character space, we saw the rise of complex variablewidth encodings, such as Shift JIS for katakana.
The incompatibility of these character maps made it difficult to exchange
documents between computers configured for different code pages. By the
early 1990s, this growing problem led to the creation of Unicode—a sort of
universal character set, too large to fit within 8 bits but meant to encompass
practically all regional scripts and specialty pictographs known to man. Unicode was followed by UTF-8, a relatively simple, variable-width representation
of these characters, which was theoretically safe for all applications capable of
handling traditional 8-bit formats. Unfortunately, UTF-8 required more bytes
to encode high-bit characters than did most of its competitors, and to many
users, this seemed wasteful and unnecessary. Because of this criticism, it took
well over a decade for UTF-8 to gain traction on the Web, and it only did so
long after all the relevant protocols had solidified.
This unfortunate delay had some bearing on the handling of URLs that
contain user input. Browsers needed to accommodate such use very early
on, but when the developers turned to the relevant standards, they found no
meaningful advice. Even years later, in 2005, the RFC 3986 had just this to say:
In local or regional contexts and with improving technology, users
might benefit from being able to use a wider range of characters;
such use is not defined by this specification.
Percent-encoded octets . . . may be used within a URI to represent
characters outside the range of the US-ASCII coded character set if
this representation is allowed by the scheme or by the protocol
element in which the URI is referenced. Such a definition should
specify the character encoding used to map those characters to
octets prior to being percent-encoded for the URI.

Alas, despite this wishful thinking, none of the remaining standards
addressed this topic. It was always possible to put raw high-bit characters in a
URL, but without knowing the code page they should be interpreted in, the
server would not be able to tell if that %B1 was supposed to mean “±”, “a”, or
some other squiggly character specific to the user’s native script.
Sadly, browser vendors have not taken the initiative and come up with a
consistent solution to this problem. Most browsers internally transcode URL
path segments to UTF-8 (or ISO 8859-1, if sufficient), but then they generate
the query string in the code page of the referring page instead. In certain
cases, when URLs are entered manually or passed to certain specialized APIs,
high-bit characters may be also downgraded to their 7-bit US-ASCII lookalikes, replaced with question marks, or even completely mangled due to
implementation flaws.

It Starts with a URL

33

Poorly implemented or not, the ability to pass non-English characters
in query strings and paths scratched an evident itch. The traditional percentencoding approach left just one URL segment completely out in the cold:
High-bit input could not be allowed as is when specifying the name of the
destination server, because at least in principle, the well-established DNS
standard permitted only period-delimited alphanumerics and dashes to
appear in domain names—and while nobody adhered to the rules, the set
of exceptions varied from one name server to another.
An astute reader might wonder why this limitation would matter; that is,
why was it important to have localized domain names in non-Latin alphabets,
too? That question may be difficult to answer now. Quite simply, several folks
thought a lack of these encodings would prevent businesses and individuals
around the world from fully embracing and enjoying the Web—and, rightly
or not, they were determined to make it happen.
This pursuit led to the formation of the Internationalized Domain Names
in Applications (IDNA). First, RFC 3490,9 which outlined a rather contrived
scheme to encode arbitrary Unicode strings using alphanumerics and dashes,
and then RFC 3492,10 which described a way to apply this encoding to DNS
labels using a format known as Punycode. Punycode looked roughly like this:
xn--[US-ASCII part]-[encoded Unicode data]

A compliant browser presented with a technically illegal URL that contained a literal non-US-ASCII character anywhere in the hostname was supposed to transform the name to Punycode before performing a DNS lookup.
Consequently, when presented with Punycode in an existing URL, it should
put a decoded, human-readable form of the string in the address bar.
NOTE

Combining all these incompatible encoding strategies can make for an amusing mix.
Consider this example URL of a made-up Polish-language towel shop:
Intent: http://www.ręczniki.pl/ręcznik?model=Jaś#Złóż_zamówienie
Actual URL: http://www.xn--rczniki-98a.pl/r%C4%99cznik?model=Ja%B6 #Złóż_zamówienie

Label converted
to Punycode

Path converted
to UTF-8

Query string
converted to
ISO 8859-2

Literal UTF-8

Of all the URL-based encoding approaches, IDNA soon proved to be the
most problematic. In essence, the domain name in the URL shown in the
browser’s address bar is one of the most important security indicators on the
Web, as it allows users to quickly differentiate sites they trust and have done
business with from the rest of the Internet. When the hostname shown by the
browser consists of 38 familiar and distinctive characters, only fairly careless
victims will be tricked into thinking that their favorite example.com domain
and an impostor examp1e.com site are the same thing. But IDNA casually and
indiscriminately extended these 38 characters to some 100,000 glyphs supported by Unicode, many of which look exactly alike and are separated from
each other based on functional differences alone.
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Chapter 2

How bad is it? Let’s consider Cyrillic, for example. This alphabet has a
number of homoglyphs that look practically identical to their Latin counterparts but that have completely different Unicode values and resolve to completely different Punycode DNS names:
Latin

Cyrillic

c

e

i

j

o

p

s

x

y

U+0061 U+0063

a

U+0065

U+0069

U+006A

U+006F

U+0070

U+0073

U+0078

U+0079

c

e

i

j

o

p

s

x

y

U+0430 U+0441

U+0435

U+0456

U+0458

U+043E

U+0440

U+0455

U+0445

U+0443

a

When IDNA was proposed and first implemented in browsers, nobody
seriously considered the consequences of this issue. Browser vendors apparently assumed that DNS registrars would prevent people from registering
look-alike names, and registrars figured it was the browser vendors’ problem
to have unambiguous visuals in the address bar.
In 2002 the significance of the problem was finally recognized by all
parties involved. That year, Evgeniy Gabrilovich and Alex Gontmakher published “The Homograph Attack,”11 a paper exploring the vulnerability in
great detail. They noted that any registrar-level work-arounds, even if implemented, would have a fatal flaw. An attacker could always purchase a wholesome top-level domain and then, on his own name server, set up a subdomain
record that, with the IDNA transformation applied, would decode to a string
visually identical to example.com/ (the last character being merely a nonfunctional look-alike of the actual ASCII slash). The result would be:
http://example.com/.wholesome-domain.com/

This only looks like a real slash.

There is nothing that a registrar can do to prevent this attack, and the
ball is in the browser vendors’ court. But what options do they have, exactly?
As it turns out, there aren’t many. We now realize that the poorly envisioned IDNA standard cannot be fixed in a simple and painless way. Browser
developers have responded to this risk by reverting to incomprehensible
Punycode when a user’s locale does not match the script seen in a particular
DNS label (which causes problems when browsing foreign sites or when using
imported or simply misconfigured computers); permitting IDNA only in certain country-specific, top-level domains (ruling out the use of internationalized domain names in .com and other high-profile TLDs); and blacklisting
certain “bad” characters that resemble slashes, periods, white spaces, and
so forth (a fool’s errand, given the number of typefaces used around the
world).
These measures are drastic enough to severely hinder the adoption of
internationalized domain names, probably to a point where the standard’s
lingering presence causes more security problems than it brings real usability
benefits to non-English users.
It Starts with a URL

35

Common URL Schemes and Their Function
Let’s leave the bizarre world of URL parsing behind us and go back to the
basics. Earlier in this chapter, we implied that certain schemes may have
unexpected security consequences and that because of this, any web application handling user-supplied URLs must be cautious. To explain this point a
bit better, it is useful to review all the URL schemes commonly supported in
a typical browser environment. These can be combined into four basic groups.

Browser-Supported, Document-Fetching Protocols
These schemes, handled internally by the browser, offer a way to retrieve
arbitrary content using a particular transport protocol and then display it
using common, browser-level rendering logic. This is the most rudimentary
and the most expected function of a URL.
The list of commonly supported schemes in this category is surprisingly
short: http: (RFC 2616), the primary transport mode used on the Web and
the focus of the next chapter of this book; https:, an encrypted version of HTTP
(RFC 281812); and ftp:, an older file transfer protocol (RFC 95913). All browsers also support file: (previously also known as local:), a system-specific method
for accessing the local filesystem or NFS and SMB shares. (This last scheme is
usually not directly accessible through Internet-originating pages, though.)
Two additional, obscure cases also deserve a brief mention: built-in
support for the gopher: scheme, one of the failed predecessors of the Web
(RFC 143614), which is still present in Firefox, and shttp:, an alternative,
failed take on HTTPS (RFC 266015), still recognized in Internet Explorer
(but today, simply aliased to HTTP).

Protocols Claimed by Third-Party Applications and Plug-ins
For these schemes, matching URLs are simply dispatched to external, specialized applications that implement functionality such as media playback,
document viewing, or IP telephony. At this point, the involvement of the
browser (mostly) ends.
Scores of external protocol handlers exist today, and it would take another
thick book to cover them all. Some of the most common examples include
the acrobat: scheme, predictably routed to Adobe Acrobat Reader; callto: and
sip: schemes claimed by all sorts of instant messengers and telephony software; daap:, itpc:, and itms: schemes used by Apple iTunes; mailto:, news:, and
nntp: protocols claimed by mail and Usenet clients; mmst:, mmsu:, msbd:, and
rtsp: protocols for streaming media players; and so on. Browsers are sometimes also included on the list. The previously mentioned firefoxurl: scheme
launches Firefox from within another browser, while cf: gives access to Chrome
from Internet Explorer.
For the most part, when these schemes appear in URLs, they usually
have no impact on the security of the web applications that allow them to
go through (although this is not guaranteed, especially in the case of plugin–supported content). It is worth noting that third-party protocol handlers
tend to be notoriously buggy and are sometimes abused to compromise the
36

Chapter 2

operating system. Therefore, restricting the ability to navigate to mystery protocols is a common courtesy to the user of any reasonably trustworthy website.

Nonencapsulating Pseudo-Protocols
An array of protocols is reserved to provide convenient access to the
browser’s scripting engine and other internal functions, without actually
retrieving any remote content and perhaps without establishing an isolated
document context to display the result. Many of these pseudo-protocols are
highly browser-specific and are either not directly accessible from the Internet or are incapable of doing harm. However, there are several important
exceptions to this rule.
Perhaps the best-known exception is the javascript: scheme (in earlier
years, also available under aliases such as livescript: or mocha: in Netscape browsers). This scheme gives access to the JavaScript-programming engine in the
context of the currently viewed website. In Internet Explorer, vbscript: offers
similar capabilities through the proprietary Visual Basic interface.
Another important case is the data: protocol (RFC 239716), which
permits short, inline documents to be created without any extra network
requests and sometimes inherits much of their operating context from the
referring page. An example of a data: URL is:
data:text/plain,Why,%20hello%20there!

These externally accessible pseudo-URLs are of acute significance to site
security. When navigated to, their payload may execute in the context of the
originating domain, possibly stealing sensitive data or altering the appearance of the page for the affected user. We’ll discuss the specific capabilities
of browser scripting languages in Chapter 6, but as you might expect, they
are substantial. (URL context inheritance rules, on the other hand, are the
focus of Chapter 10.)

Encapsulating Pseudo-Protocols
This special class of pseudo-protocols may be used to prefix any other URL
in order to force a special decoding or rendering mode for the retrieved
resource. Perhaps the best-known example is the view-source: scheme supported by Firefox and Chrome, used to display the pretty-printed source of
an HTML page. This scheme is used in the following way:
view-source:http://www.example.com/

Other protocols that function similarly include jar:, which allows content
to be extracted from ZIP files on the fly in Firefox; wyciwyg: and view-cache:,
which give access to cached pages in Firefox and Chrome respectively; an
oddball feed: scheme, which is meant to access news feeds in Safari;17 and a
host of poorly documented protocols associated with the Windows help subsystem and other components of Microsoft Windows (hcp:, its:, mhtml:, mk:,
ms-help:, ms-its:, and ms-itss:).
It Starts with a URL

37

The common property of many encapsulating protocols is that they allow
the attacker to hide the actual URL that will be ultimately interpreted by the
browser from naïve filters: view-source:javascript: (or even view-source:viewsource:javascript:) followed by malicious code is a simple way to accomplish
this. Some security restrictions may be present to limit such trickery, but they
should not be relied upon. Another significant problem, recurring especially
with Microsoft’s mhtml:, is that using the protocol may ignore some of the
content directives provided by the server on HTTP level, possibly leading
to widespread misery.18

Closing Note on Scheme Detection
The sheer number of pseudo-protocols is the primary reason why web applications need to carefully screen user-supplied URLs. The wonky and browserspecific URL-parsing patterns, coupled with the open-ended nature of the
list of supported schemes, means that it is unsafe to simply blacklist known
bad schemes; for example, a check for javascript: may be circumvented if this
keyword is spliced with a tab or a newline, replaced with vbscript:, or prefixed
with another encapsulating scheme.

Resolution of Relative URLs
Relative URLs have been mentioned on several occasions earlier in the chapter, and they deserve some additional attention at this point, too. The reason
for their existence is that on almost every web page on the Internet, a considerable number of URLs will reference resources hosted on that same server,
perhaps in the same directory. It would be inconvenient and wasteful to require
a fully qualified URL to appear in the document every time such a reference
is needed, so short, relative URLs (such as ../other_file.txt) are used instead.
The missing details are inferred from the URL of the referring document.
Because relative URLs are allowed to appear in exactly the same scenarios in which any absolute URL may appear, a method to distinguish between
the two is necessary within the browser. Web applications also benefit from the
ability to make the distinction, because most types of URL filters may want to
scrutinize absolute URLs only and allow local references through as is.
The specification may make this task seem very simple: If the URL string
does not begin with a valid scheme name followed by a semicolon and, preferably, a valid “//” sequence, it should be interpreted as a relative reference.
And if no context for parsing such a relative URL exists, it should be rejected.
Everything else is a safe relative link, right?
Predictably, it’s not as easy as it seems. First, as outlined in previous sections, the accepted set of characters in a valid scheme name, and the patterns
accepted in lieu of “//”, vary from one implementation to another. Perhaps
more interestingly, it is a common misconception that relative links can
point only to resources on the same server; quite a few other, less-obvious
variants of relative URLs exist.

38

Chapter 2

Let’s have a quick peek at the known classes of relative URLs to better
illustrate this possibility.
Scheme, but no authority present (http:foo.txt)
This infamous loophole is hinted at in RFC 3986 and attributed to an
oversight in one of the earlier specs. While said specs descriptively classified such URLs as (invalid) absolute references, they also provided a
promiscuous reference-parsing algorithm keen on interpreting them
incorrectly.
In the latter interpretation, these URLs would set a new protocol
and path, query, or fragment ID but have the authority section copied
over from the referring location. This syntax is accepted by several
browsers, but inconsistently. For example, in some cases, http:foo.txt
may be treated as a relative reference, while https:example.com may be
parsed as an absolute one!
No scheme, but authority present (//example.com)
This is another notoriously confusing but at least well-documented quirk.
While /example.com is areference to a local resource on the current server,
the standard compels browsers to treat //example.com as a very different
case: a reference to a different authority over the current protocol. In
this scenario, the scheme will be copied over from the referring location,
and all other URL details will be derived from the relative URL.
No scheme, no authority, but path present (../notes.txt)
This is the most common variant of a relative link. Protocol and authority information is copied over from the referring URL. If the relative
URL does not start with a slash, the path will also be copied over up to
the rightmost “/”. For example, if the base URL is http://www.example
.com/files/, the path is the same, but in http://www.example.com/files/index
.html, the filename is truncated. The new path is then appended, and
standard path normalization follows on the concatenated value. The
query string and fragment ID are derived only from the relative URL.
No scheme, no authority, no path, but query string present (?search=bunnies)
In this scenario, protocol, authority, and path information are copied
verbatim from the referring URL. The query string and fragment ID
are derived from the relative URL.
Only fragment ID present (#bunnies)
All information except for the fragment ID is copied verbatim from the
referring URL; only the fragment ID is substituted. Following this type of
relative URL does not cause the page to be reloaded under normal circumstances, as noted earlier.
Because of the risk of potential misunderstandings between applicationlevel URL filters and the browser when handling these types of relative references, it is a good design practice never to output user-supplied relative URLs
verbatim. Where feasible, they should be explicitly rewritten to absolute references, and all security checks should be carried out against the resulting
fully qualified address instead.
It Starts with a URL

39

Security Engineering Cheat Sheet
When Constructing Brand-New URLs Based on User Input
 If you allow user-supplied data in path, query, or fragment ID: If one of the section
delimiters manages to get through without proper escaping, the URL may have a different effect from what you intended (for example, linking one of the user-visible HTML
buttons to the wrong server-side action). It is okay to err on the side of caution: When
inserting an attacker-controlled field value, you can simply percent-escape everything
but alphanumerics.
 If you allow user-supplied scheme name or authority section: This is a major code injection and phishing risk! Apply the relevant input-validation rules outlined below.

When Designing URL Input Filters
 Relative URLs: Disallow or explicitly rewrite them to absolute references to avoid trouble.
Anything else is very likely unsafe.
 Scheme name: Permit only known prefixes, such as http://, https://, or ftp://. Do not use
blacklisting instead; it is extremely unsafe.
 Authority section: Hostname should contain only alphanumerics, “-”, and “.” and can only
be followed by “/”, “?”, “#”, or end-of-string. Allowing anything else will backfire. If you
need to examine the hostname, make sure to make a proper right-hand substring match.
In rare cases, you might need to account for IDNA, IPv6 bracket notation, port numbers, or HTTP credentials in the URL. If so, you must fully parse the URL, validate all sections and reject anomalous values, and reserialize them into a nonambiguous, canonical,
well-escaped representation.

When Decoding Parameters Received Through URLs
 Do not assume that any particular character will be escaped just because the standard says
so or because your browser does it. Before echoing back any URL-derived values or putting them inside database queries, new URLs, and so on, scrub them carefully for dangerous characters.

40

Chapter 2

HYPERTEXT TRANSFER
PROTOCOL

The next essential concept we need to discuss is the
Hypertext Transfer Protocol (HTTP): the core transfer mechanism of the Web and the preferred method
for exchanging URL-referenced documents between
servers and clients. Despite having hypertext in its
name, HTTP and the actual hypertext content (the
HTML language) often exist independent of each
other. That said, they are intertwined in sometimes
surprising ways.
The history of HTTP offers interesting insight into its authors’ ambitions
and the growing relevance of the Internet. Tim Berners-Lee’s earliest 1991
draft of the protocol (HTTP/0.91) was barely one and a half pages long, and
it failed to account for even the most intuitive future needs, such as extensibility needed to transmit non-HTML data.

Five years and several iterations of the specification later, the first
official HTTP/1.0 standard (RFC 19452) tried to rectify many of these shortcomings in about 50 densely packed pages of text. Fast-forward to 1999, and
in HTTP/1.1 (RFC 26163), the seven credited authors attempted to anticipate almost every possible use of the protocol, creating an opus over 150
pages long. That’s not all: As of this writing, the current work on HTTPbis,4
essentially a replacement for the HTTP/1.1 specification, comes to 360 pages
or so. While much of the gradually accumulated content is irrelevant to the
modern Web, this progression makes it clear that the desire to tack on new
features far outweighs the desire to prune failed ones.
Today, all clients and servers support a not-entirely-accurate superset of
HTTP/1.0, and most can speak a reasonably complete dialect of HTTP/1.1,
with a couple of extensions bolted on. Despite the fact that there is no practical need to do so, several web servers, and all common browsers, also maintain backward compatibility with HTTP/0.9.

Basic Syntax of HTTP Traffic
At a glance, HTTP is a fairly simple, text-based protocol built on top of
TCP/IP.* Every HTTP session is initiated by establishing a TCP connection
to the server, typically to port 80, and then issuing a request that outlines the
requested URL. In response, the server returns the requested file and, in the
most rudimentary use case, tears down the TCP connection immediately
thereafter.
The original HTTP/0.9 protocol provided no room for any additional
metadata to be exchanged between the participating parties. The client
request always consisted of a single line, starting with GET, followed by the
URL path and query string, and ending with a single CRLF newline (ASCII
characters 0x0D 0x0A; servers were also advised to accept a lone LF). A
sample HTTP/0.9 request might have looked like this:
GET /fuzzy_bunnies.txt

In response to this message, the server would have immediately returned
the appropriate HTML payload. (The specification required servers to wrap
lines of the returned document at 80 characters, but this advice wasn’t really
followed.)
The HTTP/0.9 approach has a number of substantial deficiencies. For
example, it offers no way for browsers to communicate users’ language preferences, supply a list of supported document types, and so on. It also gives
servers no way to tell a client that the requested file could not be found, that
it has moved to a different location, or that the returned file is not an HTML

*

Transmission Control Protocol (TCP) is one of the core communications protocols of the Internet,
providing the transport layer to any application protocols built on top of it. TCP offers reasonably reliable, peer-acknowledged, ordered, session-based connectivity between networked hosts.
In most cases, the protocol is also fairly resilient against blind packet spoofing attacks attempted
by other, nonlocal hosts on the Internet.

42

Chapter 3

document to begin with. Finally, the scheme is not kind to server administrators: When the transmitted URL information is limited to only the path
and query strings, it is impossible for a server to host multiple websites,
distinguished by their hostnames, under one IP address—and unlike DNS
records, IP addresses don’t come cheap.
In order to fix these shortcomings (and to make room for future
tweaks), HTTP/1.0 and HTTP/1.1 standards embrace a slightly different
conversation format: The first line of a request is modified to include protocol version information, and it is followed by zero or more name: value pairs
(also known as headers), each occupying a separate line. Common request
headers included in such requests are User-Agent (browser version information), Host (URL hostname), Accept (supported MIME document types*),
Accept-Language (supported language codes), and Referer (a misspelled field
indicating the originating page for the request, if known).
These headers are terminated with a single empty line, which may be
followed by any payload the client wishes to pass to the server (the length of
which must be explicitly specified with an additional Content-Length header).
The contents of the payload are opaque from the perspective of the protocol
itself; in HTML, this location is commonly used for submitting form data in
one of several possible formats, though this is in no way a requirement.
Overall, a simple HTTP/1.1 request may look like this:
POST /fuzzy_bunnies/bunny_dispenser.php HTTP/1.1
Host: www.fuzzybunnies.com
User-Agent: Bunny-Browser/1.7
Content-Type: text/plain
Content-Length: 17
Referer: http://www.fuzzybunnies.com/main.html
I REQUEST A BUNNY

The server is expected to respond to this query by opening with a line
that specifies the supported protocol version, a numerical status code (used
to indicate error conditions and other special circumstances), and an optional,
human-readable status message. A set of self-explanatory headers comes next,
ending with an empty line. The response continues with the contents of the
requested resource:
HTTP/1.1 200 OK
Server: Bunny-Server/0.9.2
Content-Type: text/plain
Connection: close
BUNNY WISH HAS BEEN GRANTED

*
MIME type (aka Internet media type) is a simple, two-component value identifying the class and
format of any given computer file. The concept originated in RFC 2045 and RFC 2046, where it
served as a way to describe email attachments. The registry of official values (such as text/plain or
audio/mpeg) is currently maintained by IANA, but ad hoc types are fairly common.

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RFC 2616 also permits the response to be compressed in transit using
one of three supported methods (gzip, compress, deflate), unless the client
explicitly opts out by providing a suitable Accept-Encoding header.

The Consequences of Supporting HTTP/0.9
Despite the improvements made in HTTP/1.0 and HTTP/1.1, the unwelcome
legacy of the “dumb” HTTP/0.9 protocol lives on, even if it is normally hidden from view. The specification for HTTP/1.0 is partly to blame for this,
because it requested that all future HTTP clients and servers support the
original, half-baked draft. Specifically, section 3.1 says:
HTTP/1.0 clients must . . . understand any valid response in the
format of HTTP/0.9 or HTTP/1.0.

In later years, RFC 2616 attempted to backtrack on this requirement
(section 19.6: “It is beyond the scope of a protocol specification to mandate
compliance with previous versions.”), but acting on the earlier advice, all
modern browsers continue to support the legacy protocol as well.
To understand why this pattern is dangerous, recall that HTTP/0.9 servers reply with nothing but the requested file. There is no indication that the
responding party actually understands HTTP and wishes to serve an HTML
document. With this in mind, let’s analyze what happens if the browser sends
an HTTP/1.1 request to an unsuspecting SMTP service running on port 25
of example.com:
GET /<html><body><h1>Hi! HTTP/1.1
Host: example.com:25
...

Because the SMTP server doesn’t understand what is going on, it’s likely
to respond this way:
220
500
500
...
421

example.com ESMTP
5.5.1 Invalid command: "GET /<html><body><h1>Hi! HTTP/1.1"
5.1.1 Invalid command: "Host: example.com:25"
4.4.1 Timeout

All browsers willing to follow the RFC are compelled to accept these
messages as the body of a valid HTTP/0.9 response and assume that the
returned document is, indeed, HTML. These browsers will interpret the
quoted attacker-controlled snippet appearing in one of the error messages
as if it comes from the owners of a legitimate website at example.com. This
profoundly interferes with the browser security model discussed in Part II
of this book and, therefore, is pretty bad.

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Newline Handling Quirks
Setting aside the radical changes between HTTP/0.9 and HTTP/1.0, several
other core syntax tweaks were made later in the game. Perhaps most notably,
contrary to the letter of earlier iterations, HTTP/1.1 asks clients not only to
honor newlines in the CRLF and LF format but also to recognize a lone CR
character. Although this recommendation is disregarded by the two most
popular web servers (IIS and Apache), it is followed on the client side by all
browsers except Firefox.
The resulting inconsistency makes it easier for application developers
to forget that not only LF but also CR characters must be stripped from any
attacker-controlled values that appear anywhere in HTTP headers. To illustrate the problem, consider the following server response, where a usersupplied and insufficiently sanitized value appears in one of the headers,
as highlighted in bold:
HTTP/1.1 200 OK[CR][LF]
Set-Cookie: last_search_term=[CR][CR]<html><body><h1>Hi![CR][LF]
[CR][LF]
Action completed.

To Internet Explorer, this response may appear as:
HTTP/1.1 200 OK
Set-Cookie: last_search_term=
<html><body><h1>Hi!
Action completed.

In fact, the class of vulnerabilities related to HTTP header newline
smuggling—be it due to this inconsistency or just due to a failure to filter any
type of a newline—is common enough to have its own name: header injection
or response splitting.
Another little-known and potentially security-relevant tweak is support
for multiline headers, a change introduced in HTTP/1.1. According to the
standard, any header line that begins with a whitespace is treated as a continuation of the previous one. For example:
X-Random-Comment: This is a very long string,
so why not wrap it neatly?

Multiline headers are recognized in client-issued requests by IIS and
Apache, but they are not supported by Internet Explorer, Safari, or Opera.
Therefore, any implementation that relies on or simply permits this syntax
in any attacker-influenced setting may be in trouble. Thankfully, this is rare.

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Proxy Requests
Proxies are used by many organizations and Internet service providers to
intercept, inspect, and forward HTTP requests on behalf of their users. This
may be done to improve performance (by allowing certain server responses
to be cached on a nearby system), to enforce network usage policies (for
example, to prevent access to porn), or to offer monitored and authenticated access to otherwise separated network environments.
Conventional HTTP proxies depend on explicit browser support: The
application needs to be configured to make a modified request to the proxy
system, instead of attempting to talk to the intended destination. To request
an HTTP resource through such a proxy, the browser will normally send a
request like this:
GET http://www.fuzzybunnies.com/ HTTP/1.1
User-Agent: Bunny-Browser/1.7
Host: www.fuzzybunnies.com
...

The key difference between the above example and the usual syntax is
the presence of a fully qualified URL in the first line of the request (http://
www.fuzzybunnies.com/), instructing the proxy where to connect to on behalf
of the user. This information is somewhat redundant, given that the Host
header already specifies the hostname; the only reason for this overlap is that
the mechanisms evolved independent of each other. To avoid being fooled
by co-conspiring clients and servers, proxies should either correct any mismatching Host headers to match the request URL or associate cached content with a particular URL-Host pair and not just one of these values.
Many HTTP proxies also allow browsers to request non-HTTP resources,
such as FTP files or directories. In these cases, the proxy will wrap the response
in HTTP, and perhaps convert it to HTML if appropriate, before returning it
to the user.* That said, if the proxy does not understand the requested protocol, or if it is simply inappropriate for it to peek into the exchanged data (for
example, inside encrypted sessions), a different approach must be used. A
special type of a request, CONNECT, is reserved for this purpose but is not
further explained in the HTTP/1.1 RFC. The relevant request syntax is instead
outlined in a separate, draft-only specification from 1998.5 It looks like this:
CONNECT www.fuzzybunnies.com:1234 HTTP/1.1
User-Agent: Bunny-Browser/1.7
...

*
In this case, some HTTP headers supplied by the client may be used internally by the proxy,
but they will not be transmitted to the non-HTTP endpoint, which creates some interesting, if
non-security-relevant, protocol ambiguities.

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If the proxy is willing and able to connect to the requested destination,
it acknowledges this request with a specific HTTP response code, and the role
of this protocol ends. At that point, the browser will begin sending and receiving raw binary data within the established TCP stream; the proxy, in turn, is
expected to forward the traffic between the two endpoints indiscriminately.
NOTE

Hilariously, due to a subtle omission in the draft spec, many browsers have incorrectly
processed the nonencrypted, proxy-originating error responses returned during an
attempt to establish an encrypted connection. The affected implementations interpreted
such plaintext responses as though they originated from the destination server over a
secure channel. This glitch effectively eliminated all assurances associated with the use
of encrypted communications on the Web. It took over a decade to spot and correct
the flaw.6
Several other classes of lower-level proxies do not use HTTP to communicate directly with the browser but nevertheless inspect the exchanged
HTTP messages to cache content or enforce certain rules. The canonical
example of this is a transparent proxy that silently intercepts traffic at the
TCP/IP level. The approach taken by transparent proxies is unusually dangerous: Any such proxy can look at the destination IP and the Host header
sent in the intercepted connection, but it has no way of immediately telling
if that destination IP is genuinely associated with the specified server name.
Unless an additional lookup and correlation is performed, co-conspiring clients and servers can have a field day with this behavior. Without these additional checks, the attacker simply needs to connect to his or her home server
and send a misleading Host: www.google.com header to have the response
cached for all other users as though genuinely coming from www.google.com.

Resolution of Duplicate or Conflicting Headers
Despite being relatively verbose, RFC 2616 does a poor job of explaining how
a compliant parser should resolve potential ambiguities and conflicts in the
request or response data. Section 19.2 of this RFC (“Tolerant Applications”)
recommends relaxed and error-tolerant parsing of certain fields in “unambiguous” cases, but the meaning of the term itself is, shall we say, not particularly unambiguous.
For example, because of a lack of specification-level advice, roughly half
of all browsers will favor the first occurrence of a particular HTTP header,
and the rest will favor the last one, ensuring that almost every header injection vulnerability, no matter how constrained, is exploitable for at least some
percentage of targeted users. On the server side, the situation is similarly random: Apache will honor the first Host header seen, while IIS will completely
reject a request with multiple instances of this field.

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On a related note, the relevant RFCs contain no explicit prohibition
on mixing potentially conflicting HTTP/1.0 and HTTP/1.1 headers and no
requirement for HTTP/1.0 servers or clients to ignore all HTTP/1.1 syntax.
Because of this design, it is difficult to predict the outcome of indirect conflicts between HTTP/1.0 and HTTP/1.1 directives that are responsible for
the same thing, such as Expires and Cache-Control.
Finally, in some rare cases, header conflict resolution is outlined in the
spec very clearly, but the purpose of permitting such conflicts to arise in the
first place is much harder to understand. For example, HTTP/1.1 clients are
required to send the Host header on all requests, but servers (not just proxies!) are also required to recognize absolute URLs in the first line of the
request, as opposed to the traditional path- and query-only method. This
rule permits a curiosity such as this:
GET http://www.fuzzybunnies.com/ HTTP/1.1
Host: www.bunnyoutlet.com

In this case, section 5.2 of RFC 2616 instructs clients to disregard the
nonfunctional (but still mandatory!) Host header, and many implementations follow this advice. The problem is that underlying applications are likely
to be unaware of this quirk and may instead make somewhat important decisions based on the inspected header value.
NOTE

When complaining about the omissions in the HTTP RFCs, it is important to recognize
that the alternatives can be just as problematic. In several scenarios outlined in that
RFC, the desire to explicitly mandate the handling of certain corner cases led to patently
absurd outcomes. One such example is the advice on parsing dates in certain HTTP
headers, at the request of section 3.3 in RFC 1945. The resulting implementation (the
prtime.c file in the Firefox codebase7) consists of close to 2,000 lines of extremely confusing and unreadable C code just to decipher the specified date, time, and time zone in
a sufficiently fault-tolerant way (for uses such as deciding cache content expiration).

Semicolon-Delimited Header Values
Several HTTP headers, such as Cache-Control or Content-Disposition, use a
semicolon-delimited syntax to cram several separate name=value pairs into a
single line. The reason for allowing this nested notation is unclear, but it is
probably driven by the belief that it will be a more efficient or a more intuitive
approach that using several separate headers that would always have to go
hand in hand.
Some use cases outlined in RFC 2616 permit quoted-string as the righthand parameter in such pairs. Quoted-string is a syntax in which a sequence of
arbitrary printable characters is surrounded by double quotes, which act as
delimiters. Naturally, the quote mark itself cannot appear inside the string,
but—importantly—a semicolon or a whitespace may, permitting many otherwise problematic values to be sent as is.

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Unfortunately for developers, Internet Explorer does not cope with
the quoted-string syntax particularly well, effectively rendering this encoding
scheme useless. The browser will parse the following line (which is meant to
indicate that the response is a downloadable file rather than an inline document) in an unexpected way:
Content-Disposition: attachment; filename="evil_file.exe;.txt"

In Microsoft’s implementation, the filename will be truncated at the
semicolon character and will appear to be evil_file.exe. This behavior creates a
potential hazard to any application that relies on examining or appending a
“safe” filename extension to an attacker-controlled filename and otherwise
correctly checks for the quote character and newlines in this string.
NOTE

An additional quoted-pair mechanism is provided to allow quotes (and any other characters) to be used safely in the string when prefixed by a backslash. This mechanism
appears to be specified incorrectly, however, and not supported by any major browser
except for Opera. For quoted-pair to work properly, stray “\” characters would need to
be banned from the quoted-string, which isn’t the case in RFC 2616. Quoted-pair
also permits any CHAR-type token to be quoted, including newlines, which is incompatible with other HTTP-parsing rules.
It is also worth noting that when duplicate semicolon-delimited fields are
found in a single HTTP header, their order of precedence is not defined by
the RFC. In the case of filename= in Content-Disposition, all mainstream browsers
use the first occurrence. But there is little consistency elsewhere. For example,
when extracting the URL= value from the Refresh header (used to force reloading the page after a specified amount of time), Internet Explorer 6 will fall
back to the last instance, yet all other browsers will prefer the first one. And
when handling Content-Type, Internet Explorer, Safari, and Opera will use the
first charset= value, while Firefox and Chrome will rely on the last.

NOTE

Food for thought: A fascinating but largely non-security-related survey of dozens
of inconsistencies associated with the handling of just a single HTTP header—
Content-Disposition—can be found on a page maintained by Julian Reschke:
http://greenbytes.de/tech/tc2231/.

Header Character Set and Encoding Schemes
Like the documents that laid the groundwork for URL handling, all subsequent HTTP specs have largely avoided the topic of dealing with non-USASCII characters inside header values. There are several plausible scenarios
where non-English text may legitimately appear in this context (for example,
the filename in Content-Disposition), but when it comes to this, the expected
browser behavior is essentially undefined.

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Originally, RFC 1945 permitted the TEXT token (a primitive broadly
used to define the syntax of other fields) to contain 8-bit characters, providing the following definition:
OCTET
CTL
TEXT

= <any 8-bit sequence of data>
= <any US-ASCII control character
(octets 0 - 31) and DEL (127)>
= <any OCTET except CTLs,
but including LWS>

The RFC followed up with cryptic advice: When non-US-ASCII characters are encountered in a TEXT field, clients and servers may interpret them
as ISO-8859-1, the standard Western European code page, but they don’t
have to. Later, RFC 2616 copied and pasted the same specification of TEXT
tokens but added a note that non-ISO-8859-1 strings must be encoded using
a format outlined in RFC 2047,8 originally created for email communications.
Fair enough; in this simple scheme, the encoded string opens with a “=?” prefix, followed by a character-set name, a “?q?” or “?b?” encoding-type indicator
(quoted-printable* or base64,† respectively), and lastly the encoded string itself.
The sequence ends with a “?=” terminator. An example of this may be:
Content-Disposition: attachment; filename="=?utf-8?q?Hi=21.txt?="

NOTE

The RFC should also have stated that any spurious “=?...?=” patterns must never be
allowed as is in the relevant headers, in order to avoid unintended decoding of values
that were not really encoded to begin with.
Sadly, the support for this RFC 2047 encoding is spotty. It is recognized
in some headers by Firefox and Chrome, but other browsers are less cooperative. Internet Explorer chooses to recognize URL-style percent encoding in
the Content-Disposition field instead (a habit also picked up by Chrome) and
defaults to UTF-8 in this case. Firefox and Opera, on the other hand, prefer
supporting a peculiar percent-encoded syntax proposed in RFC 2231,9 a
striking deviation from how HTTP syntax is supposed to look:
Content-Disposition: attachment; filename*=utf-8'en-us'Hi%21.txt

Astute readers may notice that there is no single encoding scheme supported by all browsers at once. This situation prompts some web application
developers to resort to using raw high-bit values in the HTTP headers, typically interpreted as UTF-8, but doing so is somewhat unsafe. In Firefox, for
example, a long-standing glitch causes UTF-8 text to be mangled when put
*

Quoted-printable is a simple encoding scheme that replaces any nonprintable or otherwise illegal
characters with the equal sign (=) followed by a 2-digit hexadecimal representation of the 8-bit
character value to be encoded. Any stray equal signs in the input text must be replaced with
“=3D” as well.

†
Base64 is a non-human-readable encoding that encodes arbitrary 8-bit input using a 6-bit alphabet of case-sensitive alphanumerics, “+”, and “/”. Every 3 bytes of input map to 4 bytes of output.
If the input does not end at a 3-byte boundary, this is indicated by appending one or two equal
signs at the end of the output string.

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in the Cookie header, permitting attacker-injected cookie delimiters to materialize in unexpected places.10 In other words, there are no easy and robust
solutions to this mess.
When discussing character encodings, the problem of handling of the
NUL character (0x00) probably deserves a mention. This character, used as a
string terminator in many programming languages, is technically prohibited
from appearing in HTTP headers (except for the aforementioned, dysfunctional quoted-pair syntax), but as you may recall, parsers are encouraged to be
tolerant. When this character is allowed to go through, it is likely to have
unexpected side effects. For example, Content-Disposition headers are truncated at NUL by Internet Explorer, Firefox, and Chrome but not by Opera
or Safari.

Referer Header Behavior
As mentioned earlier in this chapter, HTTP requests may include a Referer
header. This header contains the URL of a document that triggered the current navigation in some way. It is meant to help with certain troubleshooting
tasks and to promote the growth of the Web by emphasizing cross-references
between related web pages.
Unfortunately, the header may also reveal some information about user
browsing habits to certain unfriendly parties, and it may leak sensitive information that is encoded in the URL query parameters on the referring page.
Due to these concerns, and the subsequent poor advice on how to mitigate
them, the header is often misused for security or policy enforcement purposes, but it is not up to the task. The main problem is that there is no way
to differentiate between a client that is not providing the header because of
user privacy preferences, one that is not providing it because of the type of
navigation taking place, and one that is deliberately tricked into hiding this
information by a malicious referring site.
Normally, this header is included in most HTTP requests (and preserved
across HTTP-level redirects), except in the following scenarios:


After organically entering a new URL into the address bar or opening a
bookmarked page.



When the navigation originates from a pseudo-URL document, such as
data: or javascript:.



When the request is a result of redirection controlled by the Refresh
header (but not a Location-based one).



Whenever the referring site is encrypted but the requested page isn’t.
According to RFC 2616 section 15.1.2, this is done for privacy reasons, but
it does not make a lot of sense. The Referer string is still disclosed to third
parties when one navigates from one encrypted domain to an unrelated
encrypted one, and rest assured, the use of encryption is not synonymous
with trustworthiness.



If the user decides to block or spoof the header by tweaking browser settings or installing a privacy-oriented plug-in.
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As should be apparent, four out of five of these conditions can be purposefully induced by any rogue site.

HTTP Request Types
The original HTTP/0.9 draft provided a single method (or “verb”) for
requesting a document: GET. The subsequent proposals experimented
with an increasingly bizarre set of methods to permit interactions other
than retrieving a document or running a script, including such curiosities
as SHOWMETHOD, CHECKOUT, or—why not—SPACEJUMP.11
Most of these thought experiments have been abandoned in HTTP/1.1,
which settles on a more manageable set of eight methods. Only the first two
request types—GET and POST—are of any significance to most of the modern Web.

GET
The GET method is meant to signify information retrieval. In practice, it
is used for almost all client-server interactions in the course of a normal
browsing session. Regular GET requests carry no browser-supplied payloads,
although they are not strictly prohibited from doing so.
The expectation is that GET requests should not have, to quote the RFC,
“significance of taking an action other than retrieval” (that is, they should
make no persistent changes to the state of the application). This requirement
is increasingly meaningless in modern web applications, where the application state is often not even managed entirely on the server side; consequently,
the advice is widely ignored by application developers.*
NOTE

In HTTP/1.1, clients may ask the server for any set of possibly noncontiguous or overlapping fragments of the target document by specifying the Range header on GET
(and, less commonly, on some other types of requests). The server is not obliged to comply,
but where the mechanism is available, browsers may use it to resume aborted downloads.

POST
The POST method is meant for submitting information (chiefly HTML
forms) to the server for processing. Because POST actions may have persistent side effects, many browsers ask the user to confirm before reloading any
content retrieved with POST, but for the most part, GET and POST are used
in a quasi-interchangeable manner.
POST requests are commonly accompanied by a payload, the length of
which is indicated by the Content-Length header. In the case of plain HTML,
the payload may consist of URL-encoded or MIME-encoded form data (a format detailed in Chapter 4), although again, the syntax is not constrained at
the HTTP level in any special way.
*

There is an anecdotal (and perhaps even true) tale of an unfortunate webmaster by the name
of John Breckman. According to the story, John’s website has been accidentally deleted by a
search engine–indexing robot. The robot simply unwittingly discovered an unauthenticated,
GET-based administrative interface that John had built for his site . . . and happily followed every
“delete” link it could find.

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HEAD
HEAD is a rarely used request type that is essentially identical to GET but
that returns only the HTTP headers, and not the actual payload, for the
requested content. Browsers generally do not issue HEAD requests on their
own, but the method is sometimes employed by search engine bots and other
automated tools, for example, to probe for the existence of a file or to check
its modification time.

OPTIONS
OPTIONS is a metarequest that returns the set of supported methods for a
particular URL (or “*”, meaning the server in general) in a response header.
The OPTIONS method is almost never used in practice, except for server
fingerprinting; because of its limited value, the returned information may
not be very accurate.
NOTE

For the sake of completeness, we need to note that OPTIONS requests are also a cornerstone of a proposed cross-domain request authorization scheme, and as such, they may
gain some prominence soon. We will revisit this scheme, and explore many other upcoming browser security features, in Chapter 16.

PUT
A PUT request is meant to allow files to be uploaded to the server at the
specified target URL. Because browsers do not support PUT, intentional fileupload capabilities are almost always implemented through POST to a serverside script, rather than with this theoretically more elegant approach.
That said, some nonweb HTTP clients and servers may use PUT for their
own purposes. Just as interestingly, some web servers may be misconfigured
to process PUT requests indiscriminately, creating an obvious security risk.

DELETE
DELETE is a self-explanatory method that complements PUT (and that is
equally uncommon in practice).

TRACE
TRACE is a form of “ping” request that returns information about all the
proxy hops involved in processing a request and echoes the original request
as well. TRACE requests are not issued by web browsers and are seldom used
for legitimate purposes. TRACE’s primary use is for security testing, where it
may reveal interesting details about the internal architecture of HTTP servers in a remote network. Precisely for this reason, the method is often disabled by server administrators.

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CONNECT
The CONNECT method is reserved for establishing non-HTTP connections
through HTTP proxies. It is not meant to be issued directly to servers. If the
support for CONNECT request is enabled accidentally on a particular server,
it may pose a security risk by offering an attacker a way to tunnel TCP traffic
into an otherwise protected network.

Other HTTP Methods
A number of other request methods may be employed by other nonbrowser
applications or browser extensions; the most popular set of HTTP extensions
may be WebDAV, an authoring and version-control protocol described in
RFC 4918.12
Further, the XMLHttpRequest API nominally allows client-side JavaScript
to make requests with almost arbitrary methods to the originating server—
although this last functionality is heavily restricted in certain browsers (we
will look into this in Chapter 9).

Server Response Codes
Section 10 of RFC 2616 lists nearly 50 status codes that a server may choose
from when constructing a response. About 15 of these are used in real life,
and the rest are used to indicate increasingly bizarre or unlikely states, such
as “402 Payment Required” or “415 Unsupported Media Type.” Most of the
RFC-listed states do not map cleanly to the behavior of modern web applications; the only reason for their existence is that somebody hoped they eventually would.
A few codes are worth memorizing because they are common or carry
special meaning, as discussed below.

200–299: Success
This range of status codes is used to indicate a successful completion of a
request:
200 OK This is a normal response to a successful GET or POST. The
browser will display the subsequently returned payload to the user or
will process it in some other context-specific way.
204 No Content This code is sometimes used to indicate a successful
request to which no verbose response is expected. A 204 response aborts
navigation to the URL that triggered it and keeps the user on the originating page.
206 Partial Content This code is like 200, except that it is returned by
servers in response to range requests. The browser must already have a
portion of the document (or it would not have issued a range request)
and will normally inspect the Content-Range response header to reassemble the document before further processing it.
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300–399: Redirection and Other Status Messages
These codes are used to communicate a variety of states that do not indicate
an error but that require special handling on the browser end:
301 Moved Permanently, 302 Found, 303 See Other This response
instructs the browser to retry the request at a new location, specified in
the Location response header. Despite the distinctions made in the RFC,
when encountering any of these response codes, all modern browsers
replace POST with GET, remove the payload, and then resubmit the
request automatically.
NOTE

Redirect messages may contain a payload, but if they do, this message will
not be shown to the user unless the redirection is not possible (for example,
because of a missing or unsupported Location value). In fact, in some
browsers, display of the message may be suppressed even in that scenario.

304 Not Modified This nonredirect response instructs the client that
the requested document hasn’t been modified in relation to the copy the
client already has. This response is seen after conditional requests with
headers such as If-Modified-Since, which are issued to revalidate the browser
document cache. The response body is not shown to the user. (If the
server responds this way to an unconditional request, the result will be
browser-specific and may be hilarious; for example, Opera will pop up
a nonfunctional download prompt.)
307 Temporary Redirect Similar to 302, but unlike with other modes
of redirection, browsers will not downgrade POST to GET when following a 307 redirect. This code is not commonly used in web applications,
and some browsers do not behave very consistently when handling it.

400–499: Client-Side Error
This range of codes is used to indicate error conditions caused by the behavior of the client:
400 Bad Request (and related messages) The server is unable or unwilling to process the request for some unspecified reason. The response payload will usually explain the problem to some extent and will be typically
handled by the browser just like a 200 response.
More specific variants, such as “411 Length Required,” “405 Method
Not Allowed,” or “414 Request-URI Too Long,” also exist. It’s anyone’s
guess as to why not specifying Content-Length when required has a dedicated 411 response code but not specifying Host deserves only a generic
400 one.
401 Unauthorized This code means that the user needs to provide
protocol-level HTTP authentication credentials in order to access the
resource. The browser will usually prompt the user for login information
next, and it will present a response body only if the authentication process is unsuccessful. This mechanism will be explained in more detail
shortly, in “HTTP Authentication” on page 62.
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403 Forbidden The requested URL exists but can’t be accessed for
reasons other than incorrect HTTP authentication. Reasons may involve
insufficient filesystem permissions, a configuration rule that prevents
this request from being processed, or insufficient credentials of some
sort (e.g., invalid cookies or an unrecognized source IP address). The
response will usually be shown to the user.
404 Not Found The requested URL does not exist. The response body
is typically shown to the user.

500–599: Server-Side Error
This is a class of error messages returned in response to server-side problems:
500 Internal Server Error, 503 Service Unavailable, and so on The server
is experiencing a problem that prevents it from fulfilling the request. This
may be a transient condition, a result of misconfiguration, or simply the
effect of requesting an unexpected location. The response is normally
shown to the user.

Consistency of HTTP Code Signaling
Because there is no immediately observable difference between returning
most 2xx, 4xx, and 5xx codes, these values are not selected with any special
zeal. In particular, web applications are notorious for returning “200 OK”
even when an application error has occurred and is communicated on the
resulting page. (This is one of the many factors that make automated testing
of web applications much harder than it needs to be.)
On rare occasions, new and not necessarily appropriate HTTP codes are
invented for specific uses. Some of these are standardized, such as a couple
of messages introduced in the WebDAV RFC.13 Others, such as Microsoft’s
Microsoft Exchange “449 Retry With” status, are not.

Keepalive Sessions
Originally, HTTP sessions were meant to happen in one shot: Make one
request for each TCP connection, rinse, and repeat. The overhead of repeatedly completing a three-step TCP handshake (and forking off a new process
in the traditional Unix server design model) soon proved to be a bottleneck,
so HTTP/1.1 standardized the idea of keepalive sessions instead.
The existing protocol already gave the server an understanding of where
the client request ended (an empty line, optionally followed by Content-Length
bytes of data), but to continue using the existing connection, the client also
needed to know the same about the returned document; the termination of
a connection could no longer serve as an indicator. Therefore, keepalive sessions require the response to include a Content-Length header too, always specifying the amount of data to follow. Once this many payload bytes are received,
the client knows it is okay to send a second request and begin waiting for
another response.
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Although very beneficial from a performance standpoint, the way this
mechanism is designed exacerbates the impact of HTTP request and responsesplitting bugs. It is deceptively easy for the client and the server to get out of
sync on which response belongs to which request. To illustrate, let’s consider
a server that thinks it is sending a single HTTP response, structured as follows:
HTTP/1.1 200 OK[CR][LF]
Set-Cookie: term=[CR]Content-Length: 0[CR][CR]HTTP/1.1 200 OK[CR]Gotcha: Yup[CR][LF]
Content-Length: 17[CR][LF]
[CR][LF]
Action completed.

The client, on the other hand, may see two responses and associate the
first one with its most current request and the second one with the yet-to-beissued query* (which may even be addressed to a different hostname on the
same IP):
HTTP/1.1 200 OK
Set-Cookie: term=
Content-Length: 0
HTTP/1.1 200 OK
Gotcha: Yup
Content-Length: 17
Action completed.

If this response is seen by a caching HTTP proxy, the incorrect result
may also be cached globally and returned to other users, which is really bad
news. A much safer design for keepalive sessions would involve specifying the
length of both the headers and the payload up front or using a randomly generated and unpredictable boundary to delimit every response. Regrettably,
the design does neither.
Keepalive connections are the default in HTTP/1.1 unless they are
explicitly turned off (Connection: close) and are supported by many HTTP/1.0
servers when enabled with a Connection: keep-alive header. Both servers and
browsers can limit the number of concurrent requests serviced per connection and can specify the maximum amount of time an idle connection is kept
around.

Chunked Data Transfers
The significant limitation of Content-Length-based keepalive sessions is
the need for the server to know in advance the exact size of the returned
response. This is a pretty simple task when dealing with static files, as the
*
In principle, clients could be designed to sink any unsolicited server response data before
issuing any subsequent requests in a keepalive session, limiting the impact of the attack. This
proposal is undermined by the practice of HTTP pipelining, however; for performance reasons,
some clients are designed to dump multiple requests at once, without waiting for a complete
response in between.

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information is already available in the filesystem. When serving dynamically
generated data, the problem is more complicated, as the output must be
cached in its entirety before it is sent to the client. The challenge becomes
insurmountable if the payload is very large or is produced gradually (think
live video streaming). In these cases, precaching to compute payload size is
simply out of the question.
In response to this challenge, RFC 2616 section 3.6.1 gives servers the
ability to use Transfer-Encoding: chunked, a scheme in which the payload is sent
in portions as it becomes available. The length of every portion of the document is declared up front using a hexadecimal integer occupying a separate
line, but the total length of the document is indeterminate until a final zerolength chunk is seen.
A sample chunked response may look like this:
HTTP/1.1 200 OK
Transfer-Encoding: chunked
...
5
Hello
6
world!
0

There are no significant downsides to supporting chunked data transfers, other than the possibility of pathologically large chunks causing integer
overflows in the browser code or needing to resolve mismatches between
Content-Length and chunk length. (The specification gives precedence to
chunk length, although any attempts to handle this situation gracefully appear
to be ill-advised.) All the popular browsers deal with these conditions properly, but new implementations need to watch their backs.

Caching Behavior
For reasons of performance and bandwidth conservation, HTTP clients
and some intermediaries are eager to cache HTTP responses for later reuse.
This must have seemed like a simple task in the early days of the Web, but it
is increasingly fraught with peril as the Web encompasses ever more sensitive, user-specific information and as this information is updated more and
more frequently.
RFC 2616 section 13.4 states that GET requests responded to with a range
of HTTP codes (most notably, “200 OK” and “301 Moved Permanently”) may
be implicitly cached in the absence of any other server-provided directives.
Such a response may be stored in the cache indefinitely, and may be reused
for any future requests involving the same request method and destination
URL, even if other parameters (such as Cookie headers) differ. There is a prohibition against caching requests that use HTTP authentication (see “HTTP
Authentication” on page 62), but other authentication methods, such as
cookies, are not recognized in the spec.
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When a response is cached, the implementation may opt to revalidate it
before reuse, but doing so is not required most of the time. Revalidation is
achieved by request with a special conditional header, such as If-Modified-Since
(followed by a date recorded on the previously cached response) or If-NoneMatch (followed by an opaque ETag header value that the server returned
with an earlier copy). The server may respond with a “304 Not Modified”
code or return a newer copy of the resource.
NOTE

The Date/If-Modified-Since and ETag/If-None-Match header pairs, when coupled with Cache-Control: private, offer a convenient and entirely unintended way
for websites to store long-lived, unique tokens in the browser.14 The same can also be
achieved by depositing a unique token inside a cacheable JavaScript file and returning
“304 Not Modified” to all future conditional requests to the token-generating location.
Unlike purpose-built mechanisms such as HTTP cookies (discussed in the next section),
users have very little control over what information is stored in the browser cache,
under what circumstances, and for how long.
Implicit caching is highly problematic, and therefore, servers almost
always should resort to using explicit HTTP-caching directives. To assist with
this, HTTP/1.0 provides an Expires header that specifies the date by which
the cached copy should be discarded; if this value is equal to the Date header
provided by the server, the response is noncacheable. Beyond that simple
rule, the connection between Expires and Date is unspecified: It is not clear
whether Expires should be compared to the system clock on the caching system (which is problematic if the client and server clocks are not in sync) or
evaluated based on the Expires – Date delta (which is more robust, but which
may stop working if Date is accidentally omitted). Firefox and Opera use the
latter interpretation, while other browsers prefer the former one. In most
browsers, an invalid Expires value also inhibits caching, but depending on it
is a risky bet.
HTTP/1.0 clients can also include a Pragma: no-cache request header,
which may be interpreted by the proxy as an instruction to obtain a new
copy of the requested resource, instead of returning an existing one. Some
HTTP/1.0 proxies also recognize a nonstandard Pragma: no-cache response
header as an instruction not to make a copy of the document.
In contrast, HTTP/1.1 embraces a far more substantial approach to
caching directives, introducing a new Cache-Control header. The header takes
values such as public (the document is cacheable publicly), private (proxies
are not permitted to cache), no-cache (which is a bit confusing—the response
may be cached but should not be reused for future requests),* and no-store
(absolutely no caching at all). Public and private caching directives may be
accompanied with a qualifier such as max-age, specifying the maximum time
an old copy should be kept, or must-revalidate, requesting a conditional
request to be made before content reuse.
*
The RFC is a bit hazy in this regard, but it appears that the intent is to permit the cached
document to be used for purposes such as operating the “back” and “forward” navigation
buttons in a browser but not when a proper page load is requested. Firefox follows this
approach, while all other browsers consider no-cache and no-store to be roughly equivalent.

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Unfortunately, it is typically necessary for servers to return both HTTP/1.0
and HTTP/1.1 caching directives, because certain types of legacy commercial proxies do not understand Cache-Control correctly. In order to reliably
prevent caching over HTTP, it may be necessary to use the following set of
response headers:
Expires: [current date]
Date: [current date]
Pragma: no-cache
Cache-Control: no-cache, no-store

When these caching directives disagree, the behavior is difficult to predict: Some browsers will favor HTTP/1.1 directives and give precedence to
no-cache, even if it is mistakenly followed by public; others don’t.
Another risk of HTTP caching is associated with unsafe networks, such
as public Wi-Fi networks, which allow an attacker to intercept requests to certain URLs and return modified, long-cacheable contents on requests to the
victim. If such a poisoned browser cache is then reused on a trusted network,
the injected content will unexpectedly resurface. Perversely, the victim does
not even have to visit the targeted application: A reference to a carefully chosen sensitive domain can be injected by the attacker into some other context.
There are no good solutions to this problem yet; purging your browser cache
after visiting Starbucks may be a very good idea.

HTTP Cookie Semantics
HTTP cookies are not a part of RFC 2616, but they are one of the more
important protocol extensions used on the Web. The cookie mechanism
allows servers to store short, opaque name=value pairs in the browser by sending a Set-Cookie response header and to receive them back on future requests
via the client-supplied Cookie parameter. Cookies are by far the most popular
way to maintain sessions and authenticate user requests; they are one of the
four canonical forms of ambient authority* on the Web (the other forms being
built-in HTTP authentication, IP checking, and client certificates).
Originally implemented in Netscape by Lou Montulli around 1994,
and described in a brief four-page draft document,15 the mechanism has not
been outlined in a proper standard in the last 17 years. In 1997, RFC 210916
attempted to document the status quo, but somewhat inexplicably, it also proposed a number of sweeping changes that, to this day, make this specification
substantially incompatible with the actual behavior of any modern browser.
Another ambitious effort—Cookie2—made an appearance in RFC 2965,17 but
a decade later, it still has virtually no browser-level support, a situation that is

*

Ambient authority is a form of access control based on a global and persistent property of the
requesting entity, rather than any explicit form of authorization that would be valid only for a
specific action. A user-identifying cookie included indiscriminately on every outgoing request to
a remote site, without any consideration for why this request is being made, falls into that
category.

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unlikely to change. A new effort to write a reasonably accurate cookie specification—RFC 626518—was wrapped up shortly before the publication of this
book, finally ending this specification-related misery.
Because of the prolonged absence of any real standards, the actual
implementations evolved in very interesting and sometimes incompatible
ways. In practice, new cookies can be set using Set-Cookie headers followed
by a single name=value pair and a number of optional semicolon-delimited
parameters defining the scope and lifetime of the cookie.
Expires Specifies the expiration date for a cookie in a format similar to
that used for Date or Expires HTTP headers. If a cookie is served without
an explicit expiration date, it is typically kept in memory for the duration
of a browser session (which, especially on portable computers with suspend functionality, can easily span several weeks). Definite-expiry cookies may be routinely saved to disk and persist across sessions, unless a
user’s privacy settings explicitly prevent this possibility.
Max-age This alternative, RFC-suggested expiration mechanism is not
supported in Internet Explorer and therefore is not used in practice.
Domain This parameter allows the cookie to be scoped to a domain
broader than the hostname that returned the Set-Cookie header. The
exact rules and security consequences of this scoping mechanism are
explored in Chapter 9.
NOTE

Contrary to what is implied in RFC 2109, it is not possible to scope
cookies to a specific hostname when using this parameter. For example,
domain=example.com will always match www.example.com as well.
Omitting domain is the only way to create host-scoped cookies, but even
this approach is not working as expected in Internet Explorer.

Path Allows the cookie to be scoped to a particular request path prefix.
This is not a viable security mechanism for the reasons explained in
Chapter 9, but it may be used for convenience, to prevent identically
named cookies used in various parts of the application from colliding
with each other.
Secure attribute Prevents the resulting cookie from being sent over
nonencrypted connections.
HttpOnly attribute Removes the ability to read the cookie through the
document.cookie API in JavaScript. This is a Microsoft extension, although
it is now supported by all mainstream browsers.
When making future requests to a domain for which valid cookies are
found in the cookie jar, browsers will combine all applicable name=value pairs
into a single, semicolon-delimited Cookie header, without any additional metadata, and return them to the server. If too many cookies need to be sent on a
particular request, server-enforced header size limits will be exceeded, and
the request may fail; there is no method for recovering from this condition,
other than manually purging the cookie jar.

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Curiously, there is no explicit method for HTTP servers to delete unneeded
cookies. However, every cookie is uniquely identified by a name-domain-path
tuple (the secure and httponly attributes are ignored), which permits an old
cookie of a known scope to be simply overwritten. Furthermore, if the overwriting cookie has an expires date in the past, it will be immediately dropped,
effectively giving a contrived way to purge the data.
Although RFC 2109 requires multiple comma-separated cookies to be
accepted within a single Set-Cookie header, this approach is dangerous and is
no longer supported by any browser. Firefox allows multiple cookies to be
set in a single step via the document.cookie JavaScript API, but inexplicably, it
requires newlines as delimiters instead. No browser uses commas as Cookie
delimiters, and recognizing them on the server side should be considered
unsafe.
Another important difference between the spec and reality is that cookie
values are supposed to use the quoted-string format outlined in HTTP specs
(see “Semicolon-Delimited Header Values” on page 48), but only Firefox
and Opera recognize this syntax in practice. Reliance on quoted-string values
is therefore unsafe, and so is allowing stray quote characters in attackercontrolled cookies.
Cookies are not guaranteed to be particularly reliable. User agents enforce
modest settings on the number and size of cookies permitted per domain
and, as a misguided privacy feature, may also restrict their lifetime. Because
equally reliable user tracking may be achieved by other means, such as the
ETag/If-None-Match behavior outlined in the previous section, the efforts to
restrict cookie-based tracking probably do more harm than good.

HTTP Authentication
HTTP authentication, as specified in RFC 2617,19 is the original credentialhandling mechanism envisioned for web applications, one that is now almost
completely extinct. The reasons for this outcome might have been the inflexibility of the associated browser-level UIs, the difficulty of accommodating
more sophisticated non-password-based authentication schemes, or perhaps
the inability to exercise control over how long credentials are cached and
what other domains they are shared with.
In any case, the basic scheme is fairly simple. It begins with the browser
making an unauthenticated request, to which the server responds with a “401
Unauthorized” code.* The server must also include a WWW-Authenticate
HTTP header, specifying the requested authentication method, the realm
string (an arbitrary identifier to which the entered credentials should be
bound), and other method-specific parameters, if applicable.

*
The terms authentication and authorization appear to be used interchangeably in this RFC, but
they have a distinctive meaning elsewhere in information security. Authentication is commonly
used to refer to the process of proving your identity, whereas authorization is the process of
determining whether your previously established credentials permit you to carry out a specific
privileged action.

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The client is expected to obtain the credentials in one way or the other,
encode them in the Authorization header, and retry the original request with
this header included. According to the specification, for performance reasons, the same Authorization header may also be included on subsequent
requests to the same server path prefix without the need for a second WWWAuthenticate challenge. It is also permissible to reuse the same credentials in
response to any WWW-Authenticate challenges elsewhere on the server, if the
realm string and the authentication method match.
In practice, this advice is not followed very closely: Other than Safari and
Chrome, most browsers ignore the realm string or take a relaxed approach to
path matching. On the flip side, all browsers scope cached credentials not
only to the destination server but also to a specific protocol and port, a practice that offers some security benefits.
The two credential-passing methods specified in the original RFC are
known as basic and digest. The first one essentially sends the passwords in
plaintext, encoded as base64. The other computes a one-time cryptographic
hash that protects the password from being viewed in plaintext and prevents
the Authorization header from being replayed later. Unfortunately, modern
browsers support both methods and do not distinguish between them in any
clear way. As a result, attackers can simply replace the word digest with basic in
the initial request to obtain a clean, plaintext password as soon as the user
completes the authentication dialog. Surprisingly, section 4.8 of the RFC predicted this risk and offered some helpful yet ultimately ignored advice:
User agents should consider measures such as presenting a visual
indication at the time of the credentials request of what authentication scheme is to be used, or remembering the strongest authentication scheme ever requested by a server and produce a warning
message before using a weaker one. It might also be a good idea
for the user agent to be configured to demand Digest authentication in general, or from specific sites.

In addition to these two RFC-specified authentication schemes, some
browsers also support less-common methods, such as Microsoft’s NTLM and
Negotiate, used for seamless authentication with Windows domain credentials.20
Although HTTP authentication is seldom encountered on the Internet,
it still casts a long shadow over certain types of web applications. For example,
when an external, attacker-supplied image is included in a thread on a message board, and the server hosting that image suddenly decides to return
“401 Unauthorized” on some requests, users viewing the thread will be presented out of the blue with a somewhat cryptic password prompt. After doublechecking the address bar, many will probably confuse the prompt for a request
to enter their forum credentials, and these will be immediately relayed to the
attacker’s image-hosting server. Oops.

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Protocol-Level Encryption and Client Certificates
As should now be evident, all information in HTTP sessions is exchanged in
plaintext over the network. In the 1990s, this would not have been a big deal:
Sure, plaintext exposed your browsing choices to nosy ISPs, and perhaps to
another naughty user on your office network or an overzealous government
agency, but that seemed no worse than the behavior of SMTP, DNS, or any
other commonly used application protocol. Alas, the growing popularity of
the Web as a commerce platform has aggravated the risk, and substantial network security regression caused by the emergence of inherently unsafe public wireless networks put another nail in that coffin.
After several less successful hacks, a straightforward solution to this
problem was proposed in RFC 2818:21 Why not encapsulate normal HTTP
requests within an existing, multipurpose Transport Layer Security (TLS, aka
SSL) mechanism developed several years earlier? This transport method leverages public key cryptography* to establish a confidential, authenticated communication channel between the two endpoints, without requiring any
HTTP-level tweaks.
In order to allow web servers to prove their identity, every HTTPS-enabled
web browser ships with a hefty set of public keys belonging to a variety of
certificate authorities. Certificate authorities are organizations that are trusted
by browser vendors to cryptographically attest that a particular public key
belongs to a particular site, hopefully after validating the identity of the person who requests such attestation and after verifying his claim to the domain
in question.
The set of trusted organizations is diverse, arbitrary, and not particularly
well documented, which often prompts valid criticisms. But in the end, the
system usually does the job reasonably well. Only a handful of bloopers have
been documented so far (including a recent high-profile compromise of a
company named Comodo22), and no cases of widespread abuse of CA privileges are on the record.
As to the actual implementation, when establishing a new HTTPS connection, the browser receives a signed public key from the server, verifies the
signature (which can’t be forged without having access to the CA’s private
key), checks that the signed cn (common name) or subjectAltName fields in
the certificate indicate that this certificate is issued for the server the browser
wants to talk to, and confirms that the key is not listed on a public revocation
list (for example, due to being compromised or obtained fraudulently). If
everything checks out, the browser can proceed by encrypting messages to
the server with that public key and be certain that only that specific party will
be able to decrypt them.
Normally, the client remains anonymous: It generates a temporary encryption key, but that process does not prove the client’s identity. Such a proof
can be arranged, though. Client certificates are embraced internally by certain organizations and are adopted on a national level in several countries
*

Public key cryptography relies on asymmetrical encryption algorithms to create a pair of keys: a
private one, kept secret by the owner and required to decrypt messages, and a public one,
broadcast to the world and useful only to encrypt traffic to that recipient, not to decrypt it.

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around the world (e.g., for e-government services). Since the usual purpose
of a client certificate is to provide some information about the real-world
identity of the user, browsers usually prompt before sending them to newly
encountered sites, for privacy reasons; beyond that, the certificate may act as
yet another form of ambient authority.
It is worth noting that although HTTPS as such is a sound scheme that
resists both passive and active attackers, it does very little to hide the evidence
of access to a priori public information. It does not mask the rough HTTP
request and response sizes, traffic directions, and timing patterns in a typical
browsing session, thus making it possible for unsophisticated, passive attackers to figure out, for example, which embarrassing page on Wikipedia is being
viewed by the victim over an encrypted channel. In fact, in one extreme case,
Microsoft researchers illustrated the use of such packet profiling to reconstruct user keystrokes in an online application.23

Extended Validation Certificates
In the early days of HTTPS, many public certificate authorities relied on
fairly pedantic and cumbersome user identity and domain ownership checks
before they would sign a certificate. Unfortunately, in pursuit of convenience
and in the interest of lowering prices, some now require little more than a
valid credit card and the ability to put a file on the destination server in order
to complete the verification process. This approach renders most of the certificate fields other than cn and subjectAltName untrustworthy.
To address this problem, a new type of certificate, tagged using a special
flag, is being marketed today at a significantly higher price: Extended Validation
SSL (EV SSL). These certificates are expected not only to prove domain ownership but also more reliably attest to the identity of the requesting party,
following a manual verification process. EV SSL is recognized by all modern
browsers by making portion of the address bar blue or green. Although having this tier of certificates is valuable, the idea of coupling a higher-priced
certificate with an indicator that vaguely implies a “higher level of security”
is often criticized as a cleverly disguised money-making scheme.

Error-Handling Rules
In an ideal world, HTTPS connections that involve a suspicious certificate
error, such as a grossly mismatched hostname or an unrecognized certification authority, should simply result in a failure to establish the connection.
Less-suspicious errors, such as a recently expired certificate or a hostname
mismatch, perhaps could be accompanied by just a gentle warning.
Unfortunately, most browsers have indiscriminately delegated the
responsibility for understanding the problem to the user, trying hard (and
ultimately failing) to explain cryptography in layman’s terms and requiring
the user to make a binary decision: Do you actually want to see this page or
not? (Figure 3-1 shows one such prompt.)

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Figure 3-1: An example certificate warning dialog
in the still-popular Internet Explorer 6

The language and appearance of SSL warnings has evolved through the
years toward increasingly dumbed-down (but still problematic) explanations
of the problem and more complicated actions required to bypass the warning. This trend may be misguided: Studies show that over 50 percent of even
the most frightening and disruptive warnings are clicked through.24 It is easy
to blame the users, but ultimately, we may be asking them the wrong questions
and offering exactly the wrong choices. Simply, if it is believed that clicking
through the warning is advantageous in some cases, offering to open the
page in a clearly labeled “sandbox” mode, where the harm is limited, would
be a more sensible solution. And if there is no such belief, any override capabilities should be eliminated entirely (a goal sought by Strict Transport Security,
an experimental mechanism that will be discussed in Chapter 16).

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Security Engineering Cheat Sheet
When Handling User-Controlled Filenames in Content-Disposition Headers
 If you do not need non-Latin characters: Strip or substitute any characters except for alphanumerics, “.”, “-”, and “_”. To protect your users against potentially harmful or deceptive
filenames, you may also want to confirm that at least the first character is alphanumeric
and substitute all but the rightmost period with something else (e.g., an underscore).
Keep in mind that allowing quotes, semicolons, backslashes, and control characters
(0x00–0x1F) will introduce vulnerabilities.
 If you need non-Latin names: You must use RFC 2047, RFC 2231, or URL-style percent
encoding in a browser-dependent manner. Make sure to filter out control characters
(0x00–0x1F) and escape any semicolons, backslashes, and quotes.

When Putting User Data in HTTP Cookies
 Percent-encode everything except for alphanumerics. Better yet, use base64. Stray quote
characters, control characters (0x00–0x1F), high-bit characters (0x80–0xFF), commas,
semicolons, and backslashes may allow new cookie values to be injected or the meaning
and scope of existing cookies to be altered.

When Sending User-Controlled Location Headers
 Consult the cheat sheet in Chapter 2. Parse and normalize the URL, and confirm that the
scheme is on a whitelist of permissible values and that you are comfortable redirecting
to the specified host.
Make sure that any control and high-bit characters are escaped properly. Use Punycode for hostnames and percent-encoding for the remainder of the URL.

When Sending User-Controlled Redirect Headers
 Follow the advice provided for Location. Note that semicolons are unsafe in this header
and cannot be escaped reliably, but they also happen to have a special meaning in some
URLs. Your choice is to reject such URLs altogether or to percent-encode the “;” character, thereby violating the RFC-mandated syntax rules.

When Constructing Other Types of User-Controlled Requests or Responses
 Examine the syntax and potential side effects of the header in question. In general, be
mindful of control and high-bit characters, commas, quotes, backslashes, and semicolons;
other characters or strings may be of concern on a case-by-case basis. Escape or substitute
these values as appropriate.
 When building a new HTTP client, server, or proxy: Do not create a new implementation
unless you absolutely have to. If you can’t help it, read this chapter thoroughly and aim to
mimic an existing mainstream implementation closely. If possible, ignore the RFC-provided
advice about fault tolerance and bail out if you encounter any syntax ambiguities.
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HYPERTEXT MARKUP
LANGUAGE

The Hypertext Markup Language (HTML) is the primary method of authoring online documents. One of
the earliest written accounts of this language is a brief
summary posted on the Internet by Tim Berners-Lee
in 1991.1 His proposal outlines an SGML-derived syntax that allows text documents to be annotated with
inline hyperlinks and several types of layout aids. In the following years,
this specification evolved gradually under the direction of Sir Berners-Lee
and Dan Connolly, but it wasn’t until 1995, at the onset of the First Browser
Wars, that a reasonably serious and exhaustive specification of the language
(HTML 2.0) made it to RFC 1866.2
From that point on, all hell broke loose: For the next few years, competing browser vendors kept introducing all sorts of flashy, presentation-oriented
features and tweaked the language to their liking. Several attempts to amend
the original RFC have been undertaken, but ultimately the IETF-managed

standardization approach proved to be too inflexible. The newly formed
World Wide Web Consortium took over the maintenance of the language
and eventually published the HTML 3.2 specification in 1997.3
The new specification tried to reconcile the differences in browser
implementations while embracing many of the bells and whistles that
appealed to the public, such as customizable text colors and variable typefaces. Ultimately, though, HTML 3.2 proved to be a step back for the clarity
of the language and had only limited success in catching up with the facts.
In the following years, the work on HTML 4 and 4.014 focused on pruning HTML of all accumulated excess and on better explaining how document
elements should be interpreted and rendered. It also defined an alternative,
strict XHTML syntax derived from XML, which was much easier to consistently parse but more punishing to write. Despite all this work, however, only
a small fraction of all websites on the Internet could genuinely claim compliance with any of these standards, and little or no consistency in parsing modes
and error recovery could be seen on the client end. Consequently, some of
the work on improving the core language fizzled out, and the W3C turned
its attention to stylesheets, the Document Object Model, and other more
abstract or forward-looking challenges.
In the late 2000s, some of the low-level work has been revived under the
banner of HTML5,5 an ambitious project to normalize almost every aspect
of the language syntax and parsing, define all the related APIs, and more
closely police browser behavior in general. Time will tell if it will be successful; until then, the language itself, and each of the four leading parsing
engines,* come with their own set of frustrating quirks.

Basic Concepts Behind HTML Documents
From a purely theoretical standpoint, HTML relies on a fairly simple syntax:
a hierarchical structure of tags, name=value tag parameters, and text nodes
(forming the actual document body) in between. For example, a simple document with a title, a heading, and a hyperlink may look like this:
<html>
<head>
<title>Hello world</title>
</head>
<body>
<h1>Welcome to our example page</h1>
<a href="http://www.example.com/">Click me!</a>
</body>
</html>

*
To process HTML documents, Internet Explorer uses the Trident engine (aka MSHTML);
Firefox and some derived products use Gecko; Safari, Chrome, and several other browsers use
WebKit; and Opera relies on Presto. With the exception of WebKit, a collaborative open source
effort maintained by several vendors, these engines are developed largely in-house by their
respective browser teams.

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This syntax puts some constraints on what may appear inside a parameter value or inside the document body. Five characters—angle brackets, single and double quotes, and an ampersand—are reserved as the building
blocks of the HTML markup, and these need to be avoided or escaped in
some way when used outside of their intended function. The most important
rules are:


Stray ampersands (&) should never appear in most sections of an HTML
document.



Both types of angle brackets are obviously problematic inside a tag,
unless properly quoted.



The left angle bracket (<) is a hazard inside a text node.



Quote characters appearing inside a tag can have undesirable effects,
depending on their exact location, but are harmless in text nodes.

To allow these characters to appear in problematic locations without
causing side effects, an ampersand-based encoding scheme, discussed in
“Entity Encoding” on page 76, is provided.
NOTE

Of course, the availability of such an encoding scheme is not a guarantee of its use.
The failure to properly filter out or escape reserved characters when displaying usercontrolled data is the cause of a range of extremely common and deadly web application
security flaws. A particularly well-known example of this is cross-site scripting (XSS),
an attack in which malicious, attacker-provided JavaScript code is unintentionally
echoed back somewhere in the HTML markup, effectively giving the attacker full control over the appearance and operation of the targeted site.

Document Parsing Modes
For any HTML document, a top-level <!DOCTYPE> directive may be used to
instruct the browser to parse the file in a manner that at least superficially
conforms to one of the officially defined standards; to a more limited extent,
the same signal can be conveyed by the Content-Type header, too. Of all the
available parsing modes, the most striking difference exists between XHTML
and traditional HTML. In the traditional mode, parsers will attempt to recover
from most types of syntax errors, including unmatched opening and closing
tags. In addition, tag and parameter names will be considered case insensitive, parameter values will not always need to be quoted, and certain types of
tags, such as <img>, will be closed implicitly. In other words, the following
input will be grudgingly tolerated:
<hTmL>
<BODY>
<IMG src="/hello_world.jpg">
<a HREF=http://www.example.com/>
Click me!
</oops>
</html>

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The XML mode, on the other hand, is strict: All tags need to be balanced
carefully, named using the proper case, and closed explicitly. (The XMLspecific self-closing tag syntax, such as <img />, is permitted.) In addition,
most syntax mistakes, even trivial ones, will result in an error and prevent the
document from being displayed at all.
Unlike the regular flavor of HTML, XML-based documents may also elegantly incorporate sections using other XML-compliant markup formats,
such as MathML, a mathematical formula markup language. This is done by
specifying a different xmlns namespace setting for a particular tag, with no
need for one-off, language-level hacks.
The last important difference worth mentioning here is that traditional
HTML parsing strategies feature a selection of special modes, entered into
after certain tags are encountered and exited only when a specific terminator
string is seen; everything in between is interpreted as non-HTML text. Some
examples of such special tags include <style>, <script>, <textarea>, or <xmp>. In
practical implementations, these modes are exited only when a literal, caseinsensitive match on </style, </script, or a similar matching value, is made; any
other markup inside such a block will not be interpreted as HTML. (Interestingly, there is one officially obsolete tag, <plaintext>, that cannot be exited at
all; it stays in effect for the remainder of the document.)
In comparison, the XML mode is more predictable. It generally forbids
stray “<” and “&” characters inside the document, but it provides a special
syntax, starting with “<![CDATA[” and ending with “]]>”, as a way to encapsulate any raw text inside an arbitrary tag. For example:
<script>
<![CDATA[
alert('>>> Hello world! <<<');
]]>
</script>

The other notable special parsing mode available in both XHTML and
normal HTML is a comment block. In XML, it quite simply begins with “<!--”
and ends with “-->”. In the traditional HTML parser in Firefox versions prior
to 4, any occurrence of “--”, later followed by “>”, is also considered good
enough.

The Battle over Semantics
The low-level syntax of the language aside, HTML is also the subject of a fascinating conceptual struggle: a clash between the ideology and the reality of
the online world. Tim Berners-Lee always championed the vision of a semantic
web, an interconnected system of documents in which every functional block,
such as a citation, a snippet of code, a mailing address, or a heading, has its
meaning explained by an appropriate machine-readable tag (say, <cite>, <code>,
<address>, or <h1> to <h6>).

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This approach, he and other proponents argued, would make it easier
for machines to crawl, analyze, and index the content in a meaningful way,
and in the near future, it would enable computers to reason using the sum
of human knowledge. According to this philosophy, the markup language
should provide a way to stylize the appearance of a document, but only as
an afterthought.
Sir Berners-Lee has never given up on this dream, but in this one regard,
the actual usage of HTML proved to be very different from what he wished for.
Web developers were quick to pragmatically distill the essence of HTML 3.2
into a handful of presentation-altering but semantically neutral tags, such as
<font>, <b>, and <pre>, and saw no reason to explain further the structure of
their documents to the browser. W3C attempted to combat this trend but with
limited success. Although tags such as <font> have been successfully obsoleted and largely abandoned in favor of CSS, this is only because stylesheets
offered more powerful and consistent visual controls. With the help of CSS,
the developers simply started relying on a soup of semantically agnostic <span>
and <div> tags to build everything from headings to user-clickable buttons, all
in a manner completely opaque to any automated content extraction tools.
Despite having had a lasting impact on the design of the language, in
some ways, the idea of a semantic web may be becoming obsolete: Online
content less frequently maps to the concept of a single, viewable document,
and HTML is often reduced to providing a convenient drawing surface and
graphic primitives for JavaScript applications to build their interfaces with.

Understanding HTML Parser Behavior
The fundamentals of HTML syntax outlined in the previous sections are usually enough to understand the meaning of well-formed HTML and XHTML
documents. When the XHTML dialect is used, there is little more to the
story: The minimal fault-tolerance of the parser means that anomalous syntax almost always leads simply to a parsing error. Alas, the picture is very different with traditional, laid-back HTML parsers, which aggressively secondguess the intent of the page developer even in very ambiguous or potentially
harmful situations.
Since an accurate understanding of user-supplied markup is essential to
designing many types of security filters, let’s have a quick look at some of these
behaviors and quirks. To begin, consider the following reference snippet:
<img src=image.jpg title="Hello world" class=examples>
 









Web developers are usually surprised to learn that this syntax can be drastically altered without changing its significance to the browser. For example,
Internet Explorer will allow an NUL character (0x00) to be inserted in the
location marked at , a change that is likely to throw all naïve HTML filters
off the trail. It is also not widely known that the whitespaces at  and  can
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be substituted with uncommon vertical tab (0x0B) or form feed (0x0C) characters in all browsers and with a nonbreaking UTF-8 space (0xA0) in Opera.*
Oh, and here's a really surprising bit: In Firefox, the whitespace at  can also
be replaced with a single, regular slash—yet the one at  can’t.
Moving on, the location marked  is also of note. In this spot, NUL
characters are ignored by most parsers, as are many types of whitespaces. Not
long ago, WebKit browsers accepted a slash in this location, but recent parser
improvements have eliminated this quirk.
Quote characters are a yet another topic of interest. Website developers
know that single and double quotes can be used to put a string containing
whitespaces or angle brackets in an HTML parameter, but it usually comes as
a surprise that Internet Explorer also honors backticks (`) instead of real
quotes in the location marked . Similarly, few people realize that in any
browser, an implicit whitespace is inserted after a quoted parameter, and
that the explicit whitespace at  can therefore be skipped without changing
the meaning of the tag.
The security impact of these patterns is not always easy to appreciate, but
consider an HTML filter tasked with scrubbing an <img> tag with an attackercontrolled title parameter. Let’s say that in the input markup, this parameter
is not quoted if it contains no whitespaces and angle brackets—a design that
can be seen on a popular blogging site. This practice may appear safe at first,
but in the following two cases, a malicious, injected onerror parameter will
materialize inside a tag:
<img ... title=""onerror="alert(1)">

and
<img ... title=``onerror=`alert(1)`>

Yet another wonderful quote-related quirk in Internet Explorer makes
this job even more complicated. While most browsers recognize quoting only
when it is used at the beginning of a parameter value, Internet Explorer simply checks for any occurrence of an equal sign (=) followed by a quote and
will parse this syntax in a rather unexpected way:
<img src=test.jpg?value=">Yes, we are still inside a tag!">

Interactions Between Multiple Tags
Parsing a single tag can be a daunting task, but as you might imagine, anomalous arrangements of multiple HTML tags will be even less predictable.
Consider the following trivial example:
<i <b>
*
The behavior exhibited by Opera is particularly sneaky: The Unicode whitespace is not
recognized by many standard library functions used in server-side HTML sanitizers, such as
isspace(...) in libc. This increases the risk of implementation glitches.

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When presented with such syntax, most browsers only interpret <i> and
treat the “<b” string as an invalid tag parameter. Firefox versions before 4,
however, would automatically close the <i> tag first when encountering an
angle bracket and, in the end, will interpret both <i> and <b>. In the spirit of
fault tolerance, until recently WebKit followed that model, too.
A similar behavior can be observed in previous versions of Firefox when
dealing with tag names that contain invalid characters (in this case, the equal
sign). Instead of doing its best to ignore the entire block, the parser would
simply reset and interpret the quoted tag:
<i="<b>">

The handling of tags that are not closed before the end of the file is
equally fascinating. For example, the following snippet will prompt most
browsers to interpret the <i> tag or ignore the entire string, but Internet
Explorer and Opera use a different backtracking approach and will see <b>
instead:
<i foo="<b>" [EOF]

In fact, Firefox versions prior to version 4 engaged in far-fetched reparsing whenever particular special tags, such as <title>, were not closed before
the end of the document:
<title>This text will be interpreted as a title
<i>This text will be shown as document body!
[EOF]

The last two parsing quirks have interesting security consequences in any
scenario where the attacker may be able to interrupt page load prematurely.
Even if the markup is otherwise fairly well sanitized, the meaning of the document may change in a very unexpected way.

Explicit and Implicit Conditionals
To further complicate the job of HTML parsing, some browsers exhibit behaviors that can be used to conditionally skip some of the markup in a document.
For example, in an attempt to help novice users of Microsoft’s Active Server
Pages development platform, Internet Explorer treats <% … %> blocks as a
completely nonstandard comment, hiding any markup between these two
character sequences. Another Internet Explorer–specific feature is explicit
conditional expressions interpreted by the parser and smuggled inside standard HTML comment blocks:
<!--[if IE 6]>
Markup that will be parsed only for Internet Explorer 6
<![endif]-->

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Many other quirks of this type are related to the idiosyncrasies of SGML
and XML. For example, due to the comment-handling behavior mentioned
earlier in an aside, browsers disagree on how to parse !- and ?-directives (such
as <!DOCTYPE> or <?xml>), whether to allow XML-style CDATA blocks in
non-XHTML modes, and on what precedence to give to overlapping special
parsing mode tags (such as “<style><!-- </style> -->”).

HTML Parsing Survival Tips
The set of parsing behaviors discussed in the previous sections is by no means
exhaustive. In fact, an entire book has been written on this topic: Inquisitive
readers are advised to grab Web Application Obfuscation (Syngress, 2011) by
Mario Heiderich, Eduardo Alberto Vela Nava, Gareth Heyes, and David
Lindsay—and then weep about the fate of humanity. The bottom line is
that building HTML filters that try to block known dangerous patterns,
and allow the remaining markup as is, is simply not feasible.
The only reasonable approach to tag sanitization is to employ a realistic
parser to translate the input document into a hierarchical in-memory document tree, and then scrub this representation for all unrecognized tags and
parameters, as well as any undesirable tag/parameter/value configurations.
At that point, the tree can be carefully reserialized into a well-formed, wellescaped HTML that will not flex any of the error correction muscles in the
browser itself. Many developers think that a simpler design should be possible, but eventually they discover the reality the hard way.

Entity Encoding
Let’s talk about character encoding again. As noted on the first pages of this
chapter, certain reserved characters are generally unsafe inside text nodes
and tag parameter values, and they will often lead to outright syntax errors
in XHTML. In order to allow such characters to be used safely (and to allow
a convenient way to embed high-bit text), a simple ampersand-prefixed,
semicolon-terminated encoding scheme, known as entity encoding, is available to developers.
The most familiar use of this encoding method is the inclusion of certain
predefined, named entities. Only a handful of these are specified for XML,
but several hundred more are scattered in HTML specifications and supported by all modern browsers. In this approach, &lt; is used to insert a left
angle bracket; &gt; substitutes a right angle bracket; &amp; replaces the
ampersand itself; while, say, &rarr; is a nice Unicode arrow.
NOTE

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In XHTML documents, additional named entities can be defined using the <!ENTITY>
directive and made to resolve to internally defined strings or to the contents of an external file URL. (This last option is obviously unsafe if allowed when processing untrusted
content; the resulting attack is sometimes called External XML Entity, or XXE for
short.)

In addition to the named entities, it is also possible to insert an arbitrary
ASCII or Unicode character using a decimal &#number; notation. In this
case, &#60; maps to a left angle bracket; &#62; substitutes a right one; and
&#128569; is, I kid you not, a Unicode 6.0 character named “smiling cat face
with tears of joy.” Hexadecimal notation can also be used if the number is
prefixed with “x”. In this variant, the left angle bracket becomes &#x3c;, etc.
The HTML parser recognizes entity encoding inside text nodes and
parameter values and decodes it transparently when building an in-memory
representation of the document tree. Therefore, the following two cases are
functionally identical:
<img src="http://www.example.com">

and
<img src="ht&#x74;p&#x3a;//www.example.com">

The following two examples, on the other hand, will not work as
expected, as the encoding interferes with the structure of the tag itself:
<img src&#x3d;"http://www.example.com">

and
<img s&#x72;c="http://www.example.com">

The largely transparent behavior of entity encoding makes it important
to correctly resolve it prior to making any security decisions about the contents of a document and, if applicable, to properly restore it in the sanitized
output later on. To illustrate, the following syntax must be recognized as an
absolute reference to a javascript: pseudo-URL and not to a cryptic fragment
ID inside a relative resource named “./javascript&”:
<a href="javascript&#x3a;alert(1)">

Unfortunately, even the simple task of recognizing and parsing HTML
entities can be tricky. In traditional parsing, for example, entities may often
be accepted even if the trailing semicolon is omitted, as long as the next
character is not an alphanumeric. (In Firefox, dashes and periods are also
accepted in entity names.) Numeric entities are even more problematic, as
they may have an overlong notation with an arbitrary number of trailing
zeros. Moreover, if the numerical value is higher than 232, the standard size
of an integer on many computer architectures, the corresponding character
may be computed incorrectly.

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Developers working with XHTML should be aware of a potential pitfall
in that dialect, too. Although HTML entities are not recognized in most of
the special parsing modes, XHTML differs from traditional HTML in that
tags such as <script> and <style> do not automatically toggle a special parsing
mode on their own. Instead, an explicit <![CDATA[…]]> block around any
scripts or stylesheets is required to achieve a comparable effect. Therefore,
the following snippet with an attacker-controlled string (otherwise scrubbed
for angle brackets, quotes, backslashes, and newlines) is perfectly safe in
HTML, but not in XHTML:
<script>
var tmp = 'I am harmless! &#x27;+alert(1);// Or am I?';
...
</script>

HTTP/HTML Integration Semantics
From Chapter 3, we recall that HTTP headers may give new meaning to the
entire response (Location, Transfer-Encoding, and so on), change the way the
payload is presented (Content-Type, Content-Disposition), or affect the clientside environment in other, auxiliary ways (Refresh, Set-Cookie, Cache-Control,
Expires, etc.).
But what if an HTML document is delivered through a non-HTTP protocol or loaded from a local file? Clearly, in this case, there is no simple way to
express or preserve this information. We can part with some of it easily, but
parameters such as the MIME type or the character set are essential, and losing them forces browsers to improvise later on. (Consider, for example, that
charsets such as UTF-7, UTF-16, and UTF-32 are not ASCII-compatible and,
therefore, HTML documents can’t even be parsed without determining
which of these transformations needs to be used.)
The security consequences of the browser-level heuristics used to detect
character sets and document types will be explored in detail in Chapter 13.
Meanwhile, the problem of preserving protocol-level information within a
document is somewhat awkwardly addressed by a special HTML directive,
<meta http-equiv=...>. By the time the browser examines the markup, many
content-handling decisions must have already been made, but some tweaks
are still on the table; for example, it may be possible to adjust the charset to
a generally compatible value or to specify Refresh, Set-Cookie, and caching
directives.
As an illustration of permissible syntax, consider the following directive
that, when appearing in an 8-bit ASCII document, will clarify for the browser
that the charset of the document is UTF-8 and not, say, ISO-8859-1:
<meta http-equiv="Content-Type" content="text/html;charset=utf-8">

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On the flip side, all of the following directives will fail, because at this point
it is too late to switch to an incompatible UTF-32 encoding, change the document type to a video format, or execute a redirect instead of parsing the file:
<meta http-equiv="Content-Type" content="text/html;charset=utf-32">
<meta http-equiv="Content-Type" content="video/mpeg">
<meta http-equiv="Location" content="http://www.example.com">

Be mindful that when http-equiv values conflict with each other, or contradict the HTTP headers received from the server earlier on, their behavior
is not consistent and should not be relied upon. For example, the first supported charset= value usually prevails (and HTTP headers have precedence
over <meta> in this case), but with several conflicting Refresh values, the behavior is highly browser-specific.
NOTE

Some browsers will attempt to speculatively extract <meta http-equiv> information
before actually parsing the document, which may lead to embarrassing mistakes. For
example, a security bug recently fixed in Firefox 4 caused the browser to interpret the
following statement as a character set declaration: <meta http-equiv="Refresh"
content="10;http://www.example.com/charset=utf-7">.6

Hyperlinking and Content Inclusion
One of the most important and security-relevant features of HTML is,
predictably, the ability to link to and embed external content. HTTP-level
features such as Location and Refresh aside, this can be accomplished in a
couple of straightforward ways.

Plain Links
The following markup demonstrates the most familiar and most basic
method for referencing external content from within a document:
<a href="http://www.example.com/">Click me!</a>

This hyperlink may point to any of the browser-recognized schemes,
including pseudo-URLs (data:, javascript:, and so on) and protocols handled
by external applications (such as mailto:). Clicking on the text (or any HTML
elements) nested inside such a <a href=...> block will typically prompt the
browser to navigate away from the linking document and go to the specified
location, if meaningfully possible for the protocol used.
An optional target parameter may be used to target other windows or
document views for navigation. The parameter must specify the name of the
target view. If the name cannot be found, or if access is denied, the default
behavior is typically to open a new window instead. The conditions in which
access may be denied are the topic of Chapter 11.

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Four special target names can be used, too (as shown on the left of Figure 4-1): _blank always opens a brand-new window, _parent navigates a higherlevel view that embeds the link-bearing document (if any), and _top always navigates the top-level browser window, no matter how many document embedding levels are in between. Oh, right, the fourth special target, _self, is identical
to not specifying a value at all and exists for no reason whatsoever.
Bunny Browser 2000

Bunny Browser 2000

http://fuzzybunnies.com
_top

_blank

_parent
_self (default)

<a href=...target=...>

Figure 4-1: Predefined targets for hyperlinks

Forms and Form-Triggered Requests
An HTML form can be thought of as an information-gathering hyperlink:
When the “submit” button is clicked, a dynamic request is constructed on the
fly from the data collected via any number of input fields. Forms allow user
input and files to be uploaded to the server, but in almost every other way,
the result of submitting a form is similar to following a normal link.
A simple form markup may look like this:
<form method=GET action="/process_form.cgi">
Given name: <input type=text name=given>
Family name: <input type=text name=family>
...
<input type=submit value="Click here when done!">
</form>

The action parameter works like the href value used for normal links, with
one minor difference: If the value is absent, the form will be submitted to the
location of the current document, whereas any destination-free <a> links will
simply not work at all. An optional target parameter may also be specified and
will behave as outlined in the previous section.
NOTE

Unusually, unlike <a> tags, forms cannot be nested inside each other, and only the toplevel <form> tag will remain operational in such a case.
When the method value is set to GET or is simply not present at all, all the
nested field names and their current values will be escaped using the familiar
percent-encoding scheme outlined in Chapter 2, but with two rather arbitrary
differences. First, the space character (0x20) will be substituted with the plus

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sign, rather than encoded as “%20”. Second, following from this, any existing
plus signs need to be encoded as “%2B”, or else they will be misinterpreted
as spaces.
Encoded name=value pairs are then delimited with ampersands and combined into a single string, such as this:
given=Erwin+Rudolf+Josef+Alexander&family=Schr%C3%B6dinger

The resulting value is inserted into the query part of the destination URL
(replacing any existing contents of that section) and submitted to the server.
The received response is then shown to the user in the targeted viewport.
The situation is a bit more complicated if the method parameter is set to
POST. For that type of HTTP request, three data submission formats are available. In the default mode (referred to as application/x-www-form-urlencoded),
the message is constructed the same way as for GET but is transmitted in the
request payload instead, leaving the query string and all other parts of the
destination URL intact.*
The existence of the second POST submission mode, triggered by specifying enctype="text/plain" on the <form> tag, is difficult to justify. In this mode,
field names and values will not be percent encoded at all (but, depending on
the browser, plus signs may be used to substitute for spaces), and a newline
delimiter will be used in place of an ampersand. The resulting format is essentially useless, as it can’t be parsed unambiguously: Form-originating newlines
and equal signs are indistinguishable from browser inserted ones.
The last mode is triggered with enctype="multipart/form-data" and must be
used whenever submitting user-selected files through a form (which is possible with a special <input type="file"> tag). The resulting request body consists
of a series of short MIME messages corresponding to every submitted field.†
These messages are delimited with a client-selected random, unique boundary token that should otherwise not appear in the encapsulated data:
POST /process_form.cgi HTTP/1.1
…
Content-Type: multipart/form-data; boundary=random1234
--random1234
Content-Disposition: form-data; name="given"
Erwin Rudolf Josef Alexander
--random1234
Content-Disposition: form-data; name="family"

*
This has the potential for confusion, as the same parameter may appear both in the query string
and in the POST payload. There is no consistency in how various server-side web applications
frameworks resolve this conflict.
†
MIME (Multipurpose Internet Mail Extensions) is a data format intended for encapsulating
and safely transmitting various types of documents in email messages. The format makes several
unexpected appearances in the browser world. For example, Content-Type file format identifiers
also have unambiguous MIME roots.

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Schrödinger
--random1234
Content-Disposition: form-data; name="file"; filename="cat_names.txt"
Content-Type: text/plain
(File contents follow)
--random1234--

Despite the seemingly open-ended syntax of the tag, other request
methods and submission formats are not supported by any browser, and
this is unlikely to change. For a short while, the HTML5 standard tried to
introduce PUT and DELETE methods in forms, but this proposal was quickly
shot down.

Frames
Frames are a form of markup that allows the contents of one HTML document to be displayed in a rectangular region of another, embedding page.
Several framing tags are supported by modern browsers, but the most common way of achieving this goal is with a hassle-free and flexible inline frame:
<iframe src="http://www.example.com/">

In traditional HTML documents, this tag puts the parser in one of the
special parsing modes, and all text between the opening and the closing tag
will simply be ignored in frame-aware browsers. In legacy browsers that do
not understand , the markup between the opening and closing tags
is processed normally, however, offering a decidedly low-budget, conditional
rendering directive. This conditional behavior is commonly used to provide
insightful advice such as “This page must be viewed in a browser that supports frames.”
The frame is a completely separate document view that in many aspects
is identical to a new browser window. (It even enjoys its own JavaScript execution context.) Like browser windows, frames can be equipped with a name
parameter and then targeted from <a> and <form> tags.
The constraints on the src URL for framed content are roughly similar to
the rules enforced on regular links. This includes the ability to point frames
to javascript: or to load externally handled protocols that leave the frame
empty and open the target application in a new process.
Frames are of special interest to web security, as they allow almost unconstrained types of content originating from unrelated websites to be combined onto a single page. We will have a second look at the problems
associated with this behavior in Chapter 11.

Type-Specific Content Inclusion
In addition to content-agnostic link navigation and document framing, HTML
also provides multiple ways for a more lightweight inclusion of several predefined types of external content.
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Images
Image files can be retrieved and displayed on a page using <img> tags, via
stylesheets, and through a legacy background= parameter on markup such
as <body> or <table>.
The most popular image type on the Internet is a lossy but very efficient JPEG file, followed by lossless and more featured (but slower) PNG.
An increasingly obsolete lossless GIF format is also supported by every
browser, and so is the rarely encountered and usually uncompressed Windows bitmap file (BMP). An increasing number of rendering engines
support SVG, an XML-based vector graphics and animation format, too,
but the inclusion of such images through the <img> tag is subject to additional restrictions.
The list of recognized image types can be wrapped up with odds and
ends such as Windows metafiles (WMF and EMF), Windows Media Photo
(WDP and HDP), Windows icons (ICO), animated PNG (APNG), TIFF
images, and—more recently—WebP. Browser support for these is far
from universal, however.
Cascading stylesheets
These text-based files can be loaded with a <link rel=stylesheet href=...>
tag—even though <style src=...> would be a more intuitive choice—and
may redefine the visual aspects of almost any other HTML tag within their
parent document (and in some cases, even include embedded JavaScript).
The syntax and function of CSS are the subject of Chapter 5.
In the absence of the appropriate charset value in the Content-Type
header for the downloaded stylesheet, the encoding according to which
this subresource will be interpreted can be specified by the including
party through the charset parameter of the <link> tag.
Scripts
Scripts are text-based programs included with <script> tags and are executed in a manner that gives them full control over the host document.
The primary scripting language for the Web is JavaScript, although an
embedded version of Visual Basic is also supported in Internet Explorer
and can be used at will. Chapter 6 takes an in-depth look at client-side
scripts and their capabilities.
As with CSS, in the absence of valid Content-Type data, the charset
according to which the script is interpreted may be controlled by the
including party.
Plug-in content
This category spans miscellaneous binary files included with <embed> or
<object> tags or via an obsolete, Java-specific <applet> tag. Browser plug-in
content follows its own security rules, which are explored to some extent in
Chapters 8 and 9. In many cases, it is safe to consider plug-in-supported
content as equivalent to or more powerful than JavaScript.

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83

NOTE

The standard permits certain types of browser-supported documents, such as text/html
or text/plain, to be loaded through <object> tags, in which case they form a close
equivalent of <iframe>. This functionality is not used in practice, and the rationale
behind it is difficult to grasp.
Other supplementary content
This category includes various rendering cues that may or may not be
honored by the browser; they are most commonly provided through
<link> directives. Examples include website icons (known as “favicons”),
alternative versions of a page, and chapter navigation links.
Several other once-supported content inclusion methods, such as the
<bgsound> tag for background music, were commonplace in the past but have
fallen out of grace. On the other hand, as a part of HTML5, new tags such as
<video> and <audio> are expected to gain popularity soon.
There is relatively little consistency in what URL schemes are accepted
for type-specific content retrieval. It should be expected that protocols routed
to external applications will be rejected, as they do not have a sensible meaning in this context, but beyond this, not many assumptions should be made.
As a security precaution, most browsers will also reject scripting-related schemes
when loading images and stylesheets, although Internet Explorer 6 and Opera
do not follow this practice. As of this writing, javascript: URLs are also permitted on <embed> and <applet> tags in Firefox but not, for example, on <img>.
For almost all of the type-specific content inclusion methods, Content-Type
and Content-Disposition headers provided by the server will typically be ignored
(perhaps except for the charset= value), as may be the HTTP response code
itself. It is best to assume that whenever the body of any server-provided
resource is even vaguely recognizable as one of the data formats enumerated
in this section, it may be interpreted as such.

A Note on Cross-Site Request Forgery
On all types of cross-domain navigation, the browser will transparently include
any ambient credentials; consequently, to the server, a request legitimately
originating from its own client-side code will appear roughly the same as a
request originating from a rogue third-party site, and it may be granted the
same privileges.
Applications that fail to account for this possibility when processing any
sensitive, state-changing requests are said to be vulnerable to cross-site request
forgery (XSRF or CSRF). This vulnerability can be mitigated in a number of
ways, the most common of which is to include a secret user- and sessionspecific value on such requests (as an additional query parameter or a hidden form field). The attacker will not be able to obtain this value, as read
access to cross-domain documents is restricted by the same-origin policy
(see Chapter 9).

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Security Engineering Cheat Sheet
Good Engineering Hygiene for All HTML Documents
 Always output consistent, valid, and browser-supported Content-Type and charset information to prevent the document from being interpreted contrary to your original intent.

When Generating HTML Documents with Attacker-Controlled Bits
This task is difficult to perform consistently across the entire web application, and it is one of
the most significant sources of web application security flaws. Consider using context-sensitive
auto-escaping frameworks, such as JSilver or CTemplate, to automate it. If that is not possible,
read on.
 User-supplied content in text body: Always entity-encode “<”, “>”, and “&”. Note that certain other patterns may be dangerous in certain non-ASCII-compatible output encodings.
If applicable, consult Chapter 13.
Keep in mind that some Unicode metacharacters (e.g., U+202E) alter the direction
or flow of the subsequent text. It may be desirable to remove them in particularly sensitive uses.
 Tag-specific style and on* parameters: Multiple levels of escaping are required. This practice is extremely error prone, meaning not really something to attempt. If it is absolutely
unavoidable, review the cheat sheets in Chapters 5 and 6.
 All other HTML parameter values: Always use quotes around attacker-controlled input.
Entity-encode “<”, “>”, “&”, and any stray quotes. Remember that some parameters
require additional validation. For URLs, see the cheat sheet in Chapter 2.
Never attempt to blacklist known bad values in URLs or any other parameters; doing
so will backfire and may lead to script execution flaws.
 Special parsing modes (e.g., <script> and <style> blocks): For values appearing inside
quoted strings, replace quote characters, backslash, “<”, “>”, and all nonprintable characters with language-appropriate escape codes. For values appearing outside strings, exercise extreme caution and allow only carefully validated, known, alphanumeric values.
In XHTML mode, remember to wrap the entire script section in a CDATA block.
Avoid cases that require multiple levels of encoding, such as building parameters to the
JavaScript eval(...) function using attacker-supplied strings. Never place user-controlled
data inside HTML comments, !-type or ?-type tags, and other nonessential or unusually
parsed blocks.

When Converting HTML to Plaintext
 A common mistake is to strip only well-formed tags. Remember that all left-angle brackets
must be removed, even if no matching right-angle bracket is found. To minimize the risk
of errors, always entity-escape angle brackets and ampersands in the generated output, too.

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When Writing a Markup Filter for User Content
 Read this chapter carefully. Use a reasonably robust HTML parser to build an in-memory
document tree. Walk the tree, removing any unrecognized or unnecessary tags and
parameters and scrubbing any undesirable tags/parameters/value combinations.
When done, reserialize the document, making sure to apply proper escaping rules to
parameter values and text content. (See the first tip on this cheat sheet.) Be aware of the
impact of special parsing modes.
 Because of the somewhat counterintuitive namespace interactions with JavaScript, do
not allow name and id parameters on user-supplied markup—at least not without reading
Chapter 6 first.
 Do not attempt to sanitize an existing, serialized document in place. Doing so inevitably
leads to security problems.

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CASCADING STYLE SHEETS

As the Web matured through the 1990s, website developers increasingly needed a consistent and flexible way
to control the appearance of HTML documents; the
collection of random, vendor-specific tag parameters
available at the time simply would not do. After reviewing several competing proposals, W3C eventually settled on Cascading Style Sheets (CSS), a fairly simple textbased page appearance description language proposed
by Håkon Wium Lie.
The initial CSS level 1 specification saw the light of day by the end of
1996,1 but further revisions of this document continued until 2008. The initial draft of CSS level 2 followed in December 1998 and has yet to be finalized
as of 2011. The work on the most recent iteration, level 3, started in 2005 and
also continues to this day. Although most of the individual features envisioned
for CSS2 and CSS3 have been adopted by all modern browsers after years of
trial and error, many subtle details vary significantly from one implementation
to another, and the absence of a finalized standard likely contributes to this.

Despite the differences from one browser to another, CSS is a very powerful tool. With only a couple of constraints, stylesheets permit almost every
HTML tag to be scaled, positioned, and decorated nearly arbitrarily, thereby
overcoming the constraints originally placed on it by the underlying markup
language; in some implementations, JavaScript programs can be embedded
in the CSS presentation directives as well. The job of placing user-controlled
values inside stylesheets, or recoding any externally provided CSS, is therefore of great interest to web application security.

Basic CSS Syntax
Stylesheets can be placed in an HTML document in three ways: inlined globally for the entire document with a <style> block, retrieved from an external
URL via the <link rel=stylesheet> directive, or attached to a specific tag using
the style parameter. In addition, XML-based documents (including XHTML)
may also leverage a little-known <?xml-stylesheet href=... ?> directive to achieve
the same goal.
The first two methods of inclusion require a fully qualified stylesheet
consisting of any number of selectors (directives describing which HTML
tags the following ruleset will apply to) followed by semicolon-delimited
name: value rules between curly brackets. Here is a simple example of such
syntax, defining the appearance of <img>, <span>, and <div> tags:
img {
border-size: 1px;
border-style: solid;
}
span, div {
color: red;
}

Selectors can reference a particular type of a tag (such as img), a
period-prefixed name of a class of tags (for example, .photos, which will apply
to all tags with an inline class=photos parameter), or a combination of both
(img.company_logo). Selector suffixes such as :hover or :visited may also be used
to make the selector match only under certain circumstances, such as when
the mouse hovers over the content or when a particular displayed hyperlink
has already been visited before.
So-called complex selectors 2 are an interesting feature introduced in CSS2
and extended in CSS3. They allow any given ruleset to apply only to tags with
particular strings appearing in parameter values or that are positioned in a particular relation to other markup. One example of such a selector is this:
a[href^="ftp:"] {
/* Styling applicable only to FTP links. */
}

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NOTE

Oh, while we are at it: As evident in this example, C-style /*...*/ comment blocks are
permitted in CSS syntax anywhere outside a quoted string. On the flip side, //-style
comments are not recognized at all.

Property Definitions
Inside the { … } block that follows a selector, as well as inside the style parameter attached to a specific tag, any number of name: value rules can be used to
redefine almost every aspect of how the affected markup is displayed. Visibility, shape, color, screen position, rendering order, local or remote typeface,
and even any additional text (content property supported on certain pseudoclasses) and mouse cursor shape are all up for grabs.* Simple types of automation, such as counters for numbered lists, are available through CSS rules
as well.
Property values can be formatted as the following:


Raw text This method is used chiefly to specify numerical values (with
optional units), RGB vectors and named colors, and other predefined
keywords (“absolute,” “left,” “center,” etc.).



Quoted strings Single or double quotes should be placed around
any nonkeyword values, but there is little consistency in how this rule is
enforced. For example, quoting is not required around typeface names
or certain uses of URLs, but it is necessary for the aforementioned content
property.



Functional notation Two parameter-related pseudo-functions are
mentioned in the original CSS specification: rgb(...), for converting individual RGB color values into a single color code, and url(...), required for
URLs in most but not all contexts. On top of this, several more pseudofunctions have been rolled out in recent years, including scale(...),
rotate(...), or skew(...).
A proprietary expression(...) function is also available in Internet
Explorer; it permits JavaScript statements to be inserted within CSS. This
function is one of the most important reasons why attacker-controlled
stylesheets can be a grave security risk.

@ Directives and XBL Bindings
In addition to selectors and properties, several @-prefixed directives are recognized in stand-alone stylesheets. All of them modify the meaning of the
stylesheet; for example, by specifying the namespace or the display media that
the stylesheet should be applied to. But two special directives also affect the
behavior of the parsing process. The first of these is @charset, which sets the
charset of the current CSS block; the other is @import, which inserts an external file into the stylesheet.
*

The ability to redefine mouse cursors using an arbitrary bitmap has predictably resulted in
some security bugs. An oversized cursor combined with script-based mouse position tracking
could be used to obscure or replace important elements of the browser UI and trick the user
into doing something dangerous.
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89

The @import directive itself serves as a good example of the idiosyncrasies
of CSS parsing; the parser views all of the following examples as equivalent:
@import "foo.css";
@import url('foo.css');
@import'foo.css';

In Firefox, external content directives, including JavaScript code, may be
also loaded from an external source using the -moz-binding property, a vendorspecific way to weave XML Binding Language3 files (an obscure method of
providing automation to XML content) into the document. There is some talk
of supporting XBL in other browsers, too, at which point the name of the property would change and the XSS risk may or may not be addressed in some way.
NOTE

As can be expected, the handling of pseudo-URLs in @import, url(...) and other CSSbased content inclusion schemes is a potential security risk. While most current browsers
do not accept scripting-related schemes in these contexts, Internet Explorer 6 allows them
without reservations, thereby creating a code injection vector if the URL is not validated carefully enough.

Interactions with HTML
It follows from the discussion in the previous chapter that for any stylesheets
inlined in HTML documents, HTML parsing is performed first and is completely independent of CSS syntax rules. Therefore, it is unsafe to place certain
HTML syntax characters inside CSS properties, as in the following example,
even when quoted properly. A common mistake is permitting this:
<style>
some_descriptor {
background: url('http://www.example.com/</style><h1>Gotcha!');
}
</style>

We’ll discuss a way to encode problematic characters in stylesheets shortly,
but first, let’s have a quick look at another very distinctive property of CSS.

Parser Resynchronization Risks
An undoubtedly HTML-inspired behavior that sets CSS apart from most
other languages is that compliant parsers are expected to continue after
encountering a syntax error and restart at the next matching curly bracket
(some superficial nesting-level tracking is mandated by the spec). In particular, the following stylesheet snippet, despite being obviously malformed, will
still apply the specified border style to all <img> tags:
a {
$$$ This syntax makes absolutely no sense $$$
!(@*#)!!@ 123
}

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img {
border: 1px solid red;
}

This unusual behavior creates an opportunity to exploit parser incompatibilities in an interesting way: If there is any way to derail a particular CSS
implementation with inputs that seem valid to other parsers, the resynchronization logic may cause the attacked browser to resume parsing at an incorrect location, such as in the middle of an attacker-supplied string.
A naïve illustration of this issue may be Internet Explorer’s support for
multiline string literals. In this browser, it is seemingly safe not to scrub CR
and LF characters in user-supplied CSS strings, so some webmasters may
allow it. Unfortunately, the same pattern will cause any other browser to
resume at an unexpected offset and interpret the evil_rule ruleset:
some_benign_selector {
content: 'Attacker-controlled text...
} evil_rule { margin-left: -1000px; }';
}

The support for multiline strings is a Microsoft-specific extension, and
the aforementioned problem is easily fixed by avoiding such noncompliant
syntax to begin with. Unfortunately, other desynchronization risks are introduced by the standard itself. For example, recall complex selectors: This
CSS3 syntax makes no sense to pre-CSS3 parsers. In the following example,
an older implementation may bail out after encountering an unexpected
angle bracket and resume parsing from the attacker-supplied evil_rule
instead:
a[href^='} evil_rule { margin-left: -1000px; }'] {
/* Harmless, validated rules here. */
}

The still-popular browser Internet Explorer 6 would be vulnerable to this
trick.

Character Encoding
To make it possible to quote reserved or otherwise problematic characters
inside strings, CSS offers an unorthodox escaping scheme: a backslash (\)
followed by one to six hexadecimal digits. For example, according to this
scheme, the letter e may be encoded as “\65”, “\065”, or “\000065”. Alas, only
the last syntax, “\000065”, will be unambiguous if the next character happens
to be a valid hexadecimal digit; encoding “teak” as “t\65ak” would not work
as expected, because the escape sequence would be interpreted as “\65A”, an
Arabic sign in the Unicode character map.

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91

To avoid this problem, the specification embraces an awkward compromise: A whitespace can follow an escape sequence and will be interpreted as
a terminator, and then removed from the string (e.g., “t\65 ak”). Regrettably,
more familiar and predictable fixed-length C-style escape sequences such as
\x65 cannot be used instead.
In addition to the numerical escaping scheme, it is also possible to place
a backslash in front of a character that is not a valid hexadecimal digit. In this
case, the subsequent character will be treated as a literal. This mechanism is
useful for encoding quote characters and the backslash itself, but it should
not be used to escape HTML control characters such as angle brackets. The
aforementioned precedence of HTML parsing over CSS parsing renders this
approach inadequate.
In a bizarre twist, due to somewhat ambiguous guidance in the W3C drafts,
many CSS parsers recognize arbitrary escape sequences in locations other than
quote-enclosed strings. To add insult to injury, in Internet Explorer, the substitution of these sequences apparently takes place before the pseudo-function
syntax is parsed, effectively making the following two examples equivalent:
color: expression(alert(1))
color: expression\028 alert \028 1 \029 \029

Even more confusingly, in a misguided bid to maintain fault tolerance,
Microsoft’s implementation does not recognize backslash escape codes inside
url(...) values; this is, once more, to avoid hurting the feelings of users who
type the wrong type of a slash when specifying a URL.
These and similar quirks make the detection of known dangerous CSS
syntax extremely error prone.

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Security Engineering Cheat Sheet
When Loading Remote Stylesheets
 You are linking the security of your site to the originating domain of the stylesheet. Even
in browsers that do not support JavaScript expressions inside stylesheets, features such as
conditional selectors and url(...) references can be used to exfiltrate portions of your site.4
 When in doubt, make a local copy of the data instead.
 On HTTPS sites, require stylesheets to be served over HTTPS as well.

When Putting Attacker-Controlled Values into CSS
 Strings and URLs inside stand-alone blocks. Always use quotes. Backslash-escape all control characters (0x00–0x1F), “\”, “<”, “>”, “{“, “}”, and quotes using numerical codes. It is
also preferable to escape high-bit characters. For URLs, consult the cheat sheet in Chapter 2 to avoid code injection vulnerabilities.
 Strings in style parameters. Multiple levels of escaping are involved. The process is error
prone, so do not attempt it unless absolutely necessary. If it is unavoidable, apply the above
CSS escaping rules first and then apply HTML parameter encoding to the resulting string.
 Nonstring attributes. Allow only whitelisted alphanumeric keywords and carefully validated numerical values. Do not attempt to reject known bad patterns instead.

When Filtering User-Supplied CSS
 Remove all content outside of functional rulesets. Do not preserve or generate usercontrolled comment blocks, @-directives, and so on.
 Carefully validate selector syntax, permitting only alphanumerics; underscores; whitespaces; and correctly positioned colons, periods, and commas before “{”. Do not permit
complex text-matching selectors; they are unsafe.
 Parse and validate every rule in the { … } block. Permit only whitelisted properties with
well-understood consequences and confirm that they take expected, known safe values.
Note that strings passed to certain properties may sometimes be interpreted as URLs even
in the absence of a url(...) wrapper.
 Encode every parameter value using the rules outlined earlier in this section. Bail out on
any syntax abnormalities.
 Keep in mind that unless specifically prevented from doing so, CSS may position user
content outside the intended drawing area or redefine the appearance of any part of the
UI of your application. The safest way to avoid this problem is to display the untrusted
content inside a separate frame.

When Allowing User-Specified Class Values on HTML Markup
 Ensure that user-supplied content can’t reuse class names that are used for any part of the
application UI. If a separate frame is not being used, it’s advisable to maintain separate
namespace prefixes.
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93

BROWSER-SIDE SCRIPTS

The first browser scripting engine debuted in Netscape
Navigator around 1995, thanks to the work of Brendan
Eich. The integrated Mocha language, as it was originally called, gave web developers the ability to manipulate HTML documents, display simple, system-level
dialogs, open and reposition browser windows, and use
other basic types of client-side automation in a hasslefree way.
While iterating through beta releases, Netscape eventually renamed
Mocha LiveScript, and after an awkward branding deal was struck with Sun
Microsystems, JavaScript was chosen as the final name. The similarities
between Brendan’s Mocha and Sun’s Java were few, but the Netscape Corporation bet that this odd marketing-driven marriage would secure JavaScript’s
dominance in the more lucrative server world. It made this sentiment clear

in a famously confusing 1995 press release that introduced the language to
the world and immediately tried to tie it to an impressive range of random
commercial products:1
Netscape and Sun Announce JavaScript, the Open, CrossPlatform Object Scripting Language for Enterprise Networks
and the Internet
[...]
Netscape Navigator Gold 2.0 enables developers to create and edit
JavaScript scripts, while Netscape LiveWire enables JavaScript programs to be installed, run and managed on Netscape servers, both
within the enterprise and across the Internet. Netscape LiveWire
Pro adds support for JavaScript connectivity to high-performance
relational databases from Illustra, Informix, Microsoft, Oracle and
Sybase. Java and JavaScript support are being built into all Netscape
products to provide a unified, front-to-back, client/server/tool
environment for building and deploying live online applications.

Despite Netscape’s misplaced affection for Java, the value of JavaScript
for client-side programming seemed clear, including to the competition. In
1996 Microsoft responded by shipping a near-verbatim copy of JavaScript in
Internet Explorer 3.0 along with a counterproposal of its own: a Visual Basic–
derived language dubbed VBScript. Perhaps because it was late to the party,
and perhaps because of VBScript’s clunkier syntax, Microsoft’s alternative
failed to gain prominence or even any cross-browser support. In the end,
JavaScript secured its position in the market, and in part due to Microsoft’s
failure, no new scripting languages have been attempted in mainstream
browsers since.
Encouraged by the popularity of the JavaScript language, Netscape
handed over some of the responsibility for maintaining it to an independent
body, the European Computer Manufacturers Association (ECMA). The new
overseers successfully released ECMAScript, 3rd edition in 19992 but had
substantially more difficulty moving forward from there. The 4th edition, an
ambitious overhaul of the language, was eventually abandoned after several
years of bickering between the vendors, and a scaled-down 5th edition,3 published in 2009, still enjoys only limited (albeit steadily improving) browser
support. The work on a new iteration, called “Harmony,” begun in 2008, still
has not been finalized. Absent an evolving and widely embraced standard,
vendor-specific extensions of the language are common, but they usually
cause only pain.

Basic Characteristics of JavaScript
JavaScript is a fairly simple language meant to be interpreted at runtime. It has
vaguely C-influenced syntax (save for pointer arithmetic); a straightforward
classless object model, said to be inspired by a little-known programming language named Self; automatic garbage collection; and weak, dynamic typing.
JavaScript as such has no built-in I/O mechanisms. In the browser, limited abilities to interact with the host environment are offered through a set
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of predefined methods and properties that map to native code inside the
browser, but unlike what can be seen in many other programming languages,
these interfaces are fairly limited and purpose built.
Most of the core features of JavaScript are fairly unremarkable and
should be familiar to developers already experience with C, C++, or, to a
lesser extent, Java. A simple JavaScript program might look like this:
var text = "Hi mom!";
function display_string(str) {
alert(str);
return 0;
}
// This will display "Hi mom!".
display_str(text);

Because it is beyond the scope of this book to provide a more detailed
overview of the semantics of JavaScript, we’ll summarize only some of its more
unique and security-relevant properties later in this chapter. For readers looking for a more systematic introduction to the language, Marijn Haverbeke’s
Eloquent JavaScript (No Starch Press, 2011) is a good choice.

Script Processing Model
Every HTML document displayed in a browser—be it in a separate window
or in a frame—is given a separate instance of the JavaScript execution environment, complete with an individual namespace for all global variables and
functions created by the loaded scripts. All scripts executing in the context
of a particular document share this common sandbox and can also interact
with other contexts through browser-supplied APIs. Such cross-document
interactions must be done in a very explicit way; accidental interference is
unlikely. Superficially, script-isolation rules are reminiscent of the processcompartmentalization model in modern multitasking operating systems but
a lot less inclusive.
Within a particular execution context, all encountered JavaScript blocks
are processed individually and almost always in a well-defined order. Each
code block must consist of any number of self-contained, well-formed syntax
units and will be processed in three distinct, consequent steps: parsing, function resolution, and code execution.
Parsing
The parsing stage validates the syntax of the script block and, usually, converts it to an intermediate binary representation, which can be subsequently
executed at a more reasonable speed. The code has no global effects until
this step completes successfully. In case of syntax errors, the entire problematic block is abandoned, and the parser proceeds to the next available chunk
of code.

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97

To illustrate the behavior of a compliant JavaScript parser, consider the
following HTML snippet:
block #1:

<script>
var my_variable1 = 1;
var my_variable2 =
</script>

block #2:

<script>
2;
</script>

Contrary to what developers schooled in C may be accustomed to, the
above sequence is not equivalent to the following snippet:
<script>
var my_variable1 = 1;
var my_variable2 = 2;
</script>

This is because <script> blocks are not concatenated before parsing.
Instead, the first script segment will simply cause a syntax error (an assignment with a missing right-hand value), resulting in the entire block being
ignored and not reaching execution stage. The fact that the whole segment
is abandoned before it can have any global side effects also means that the
original example is not equivalent to this:
<script>
var my_variable1 = 1;
</script>
<script>
2;
</script>

This sets JavaScript apart from many other scripting languages such as
Bash, where the parsing stage is not separated from execution in such a
strong way.
What will happen in the original example provided earlier in this section
is that the first block will be ignored but the second one (<script>2;</script>)
will be parsed properly. That second block will amount to a no-op when executed, however, because it uses a pure, numerical expression as a code
statement.
Function Resolution
Once the parsing stage is completed successfully, the next step involves registering every named, global function that the parser found within the currently processed block. Past this point, each function found will be reachable

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from the subsequently executed code. Because of this extra pre-execution
step, the following syntax will work flawlessly (contrary to what programmers
may be accustomed to in C or C++, hello_world() will be registered before the
first code statement—a call to said function—is executed):
<script>
hello_world();
function hello_world() {
alert('Hi mom!');
}
</script>

On the other hand, the modified example below will not have the
desired effect:
<script>
hello_world();
</script>
<script>
function hello_world() {
alert('Hi mom!');
}
</script>

This modified case will fail with a runtime error because individual
blocks of code are not processed simultaneously but, rather, are looked at
based on the order in which they are made available to the JavaScript engine.
The block that defines hello_world() will not yet be parsed when the first block
is already executing.
To further complicate the picture, the mildly awkward global name resolution model outlined here applies only to functions, not to variable declarations. Variables are registered sequentially at execution time, in a way similar
to other interpreted scripting languages. Consequently, the following code
sample, which merely replaces our global hello_world() with an unnamed
function assigned to a global variable, will not work as planned:
<script>
hello_world();
var hello_world = function() {
alert('Hi mom!');
}
</script>

In this case, the assignment to the hello_world variable will not be done by
the time the hello_world() call is attempted.

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99

Code Execution
Once function resolution is completed, the JavaScript engine normally
proceeds with the ordered execution of all statements outside of function
blocks. The execution of a script may fail at this point due to an unhandled
exception or for a couple of other, more esoteric reasons. If such an error is
encountered, however, any resolved functions within the offending code
block will remain callable, and any effects of the already executed code will
persist in the current scripting context.
Exception recovery and several other JavaScript execution characteristics
are illustrated by the following lengthy but interesting code snippet:
<script>
function not_called() {
return 42;
}

This function will not execute, because
it’s not called from anywhere.

function hello_world() {
alert("With this program, anything is possible!");
do_stuff();
}

alert("Welcome to our demo application.");

This function will execute only
when called. It will show a dialog,
but then will throw an exception
due to an unresolved reference to
a function named do_stuff().
The execution of the program
will start from this statement.

hello_world();

The “With this...” message will be displayed next.

alert("Thank you, come again.");
</script>

This code will not be reached due to an unhandled
exception triggered inside hello_world().
The previous exception will not
prevent this independent block
from executing next.

<script>
alert("Now that you are done, how about a nice game of chess?");
</script>

Try to follow this example on your own and see if you agree with the
annotations provided on the right.
As should be evident from this exercise, any unexpected and unhandled
exceptions have an unusual consequence: They may leave the application in
an inconsistent but still potentially executable state. Because exceptions are
meant to prevent error propagation caused by unanticipated errors, this
design is odd—especially given that on many other fronts (such as the ban
on goto statements), JavaScript exhibits a more fundamentalist stance.

Execution Ordering Control
In order to properly analyze the security properties of certain common web
application design patterns, it is important to understand the JavaScript
engine’s execution ordering and timing model. Thankfully, this model is
remarkably sane.
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Virtually all JavaScript living within a particular execution context is executed synchronously. The code can’t be reentered due to an external event
while it is still executing, and there is no support for threads that would be able
to simultaneously modify any shared memory. While the execution engine is
busy, the processing of events, timers, page navigation requests, and so on, is
postponed; in most cases, the entire browser, or at least the HTML renderer,
will also remain largely unresponsive. Only once the execution stops and the
scripting engine enters an idle state will the processing of queued events
resume. At this point, the JavaScript code may be entered again.
Further, JavaScript offers no sleep(...) or pause(...) function to temporarily
release the CPU and later resume execution from the same location. Instead,
if a programmer desires to postpone the execution of a script, it is necessary to
register a timer to initiate a new execution flow later on. This flow will need
to start at the beginning of a specified handler function (or at the beginning
of an ad hoc, self-contained snippet of code provided when setting up a timer).
Although these design decisions can be annoying, they substantially reduce
the risk of race conditions in the resulting code.
NOTE

There are several probably unintentional loopholes in this synchronous execution model.
One of them is the possibility of code execution while the execution of another piece of
JavaScript is temporarily suspended after calling alert(...) or showModalDialog(...).
Such corner cases do not come into play very often, though.
The disruptive, browser-blocking behavior of busy JavaScript loops requires
the implementation of some mitigation on the browser level. We will explore
these mitigations in detail in Chapter 14. For now, suffice it to say that they
have another highly unusual consequence: Any endless loop may, in fact, terminate, in a fashion similar to throwing an unhandled exception. The engine
will then return to the idle state but will remain operational, the offending
code will remain callable, and all timers and event handlers will stay in place.
When triggered on purpose by the attacker, the ability to unexpectedly
terminate the execution of CPU-intensive code may put the application in an
inconsistent state by aborting an operation that the author expects to always
complete successfully. And that’s not all: Another, closely related consequence of these semantics should become evident in “JavaScript Object
Notation and Other Data Serializations” on page 104.

Code and Object Inspection Capabilities
The JavaScript language has a rudimentary provision for inspecting the
decompiled source code of any nonnative functions, simply by invoking the
toString() or toSource() method on any function that the developer wishes to
examine. Beyond that capability, opportunities to inspect the flow of programs
are limited. Applications may leverage access to the in-memory representation of their host document and look up all inlined <script> blocks, but there
is no direct visibility into any remotely loaded or dynamically generated code.
Some insight into the call stack may also be gained through a nonstandard
caller property, but there is also no way to tell which line of code is being currently executed or which one is coming up next.
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The ability to dynamically create new JavaScript code is a more prominent part of the language. It is possible to instruct the engine to synchronously interpret strings passed to the built-in eval(...) function. For example,
this will display an alert dialog:
eval("alert(\"Hi mom!\")")

Syntax errors in any input text provided to eval(...) will cause this function to throw an exception. Similarly, if parsing succeeds, any unhandled
exceptions thrown by the interpreted code will be passed down to the caller.
Finally, in the absence of syntax errors or runtime problems, the value of the
last statement evaluated by the engine while executing the supplied code will
be used as the return value of eval(...) itself.
In addition to this function, other browser-level mechanisms can be
leveraged to schedule deferred parsing and execution of new JavaScript
blocks once the execution engine returns to the idle state. Examples of such
mechanisms include timers (setTimeout, setInterval), event handlers (onclick,
onload, and so on), and interfaces to the HTML parser itself (innerHTML,
document.write(...), and such).
Whereas the ability to inspect the code is somewhat underhanded, runtime object introspection capabilities are well developed in JavaScript. Applications are permitted to enumerate almost any object method or property
using simple for ... in or for each ... in iterators and can leverage operators
such as typeof, instanceof, or “strictly equals” (===) and properties such as
length to gain additional insight into the identity of every discovered item.
All of the foregoing features make it largely impossible for scripts running in the same context to keep secrets from each other. The functionality
also makes it more difficult to keep secrets across document contexts, a problem that browser vendors had to combat for a very long time—and that, as
you’ll learn in Chapter 11, is still not completely a thing of the past.

Modifying the Runtime Environment
Despite the relative simplicity of the JavaScript language, executed scripts
have many unusual ways of profoundly manipulating the behavior of their
own JavaScript sandbox. In some rare cases, these behaviors can impact
other documents, as well.
Overriding Built-Ins
One of the more unusual tools at the disposal of a rogue script is the ability
to delete, overwrite, or shadow most of the built-in JavaScript functions and
virtually all browser-supplied I/O methods. For example, consider the behavior of the following code:
// This assignment will not trigger an error.
eval = alert;
// This call will unexpectedly open a dialog prompt.
eval("Hi mom!");

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And this is just where the fun begins. In Chrome, Safari, and Opera, it is
possible to subsequently remove the eval(...) function altogether, using the
delete operator. Confusingly, attempting the same in Firefox will restore the
original built-in function, undoing the effect of the original override. Finally,
in Internet Explorer, the deletion attempt will generate a belated exception
that seems to serve no meaningful purpose at that point.
Further along these lines, almost every object, including built-ins such as
String or Array, has a freely modifiable prototype. This prototype is a master
object from which all existing and future object instances derive their methods and properties (forming a crude equivalent of class inheritance present
in more fully featured programming languages). The ability to tamper with
object prototypes can cause rather counterintuitive behavior of newly created objects, as illustrated here:
Number.prototype.toString = function() {
return "Gotcha!";
};
// This will display "Gotcha!" instead of "42":
alert(new Number(42));

Setters and Getters
More interesting features of the object model available in contemporary dialects of JavaScript are setters and getters: ways to supply custom code that handles reading or setting properties of the host object. Although not as powerful
as operator overloading in C++, these can be used to make existing objects or
object prototypes behave in even more confusing ways. In the following snippet, the acts of setting the object property and reading it back later on are
both subverted easily:
var evil_object = {
set foo() { alert("Gotcha!"); },
get foo() { return 2; }
};
// This will display "Gotcha!" and have no other effect.
evil_object.foo = 1;
// This comparison will fail.
if (evil_object.foo != 1) alert("What's going on?!");

NOTE

Setters and getters were initially developed as a vendor extension but are now standardized under ECMAScript edition 5. The feature is available in all modern browsers but
not in Internet Explorer 6 or 7.
Impact on Potential Uses of the Language
As a result of the techniques discussed in the previous two sections, a script
executing inside a context once tainted by any other untrusted content has
no reliable way to examine its operating environment or take corrective
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action; even the behavior of simple conditional expressions or loops can’t
necessarily be relied upon. The proposed enhancements to the language are
likely to make the picture even more complicated. For example, the failed
proposal for ECMAScript edition 4 featured full-fledged operator overloading, and this idea may return.
Even more interestingly, these design decisions also make it difficult to
inspect any execution context from outside the per-page sandbox. For example,
blind reliance on the reliability of the location object of a potentially hostile document has led to a fair number of security vulnerabilities in browser plug-ins,
JavaScript-based extensions, and several classes of client-side web application
security features. These vulnerabilities eventually resulted in the development
of browser-level workarounds designed to partially protect this specific object
against sabotage, but most of the remaining object hierarchy is up for grabs.
NOTE

The ability to tamper with one’s own execution context is limited in the “strict” mode of
ECMAScript edition 5. This mode is not fully supported in any browser as of this writing, however, and is meant to be an opt-in, discretionary mechanism.

JavaScript Object Notation and Other Data Serializations
A very important syntax structure in JavaScript is its very compact and convenient in-place object serialization, known as JavaScript Object Notation, or
JSON (RFC 46274). This data format relies on overloading the meaning of
the curly bracket symbol ({). When such a brace is used to open a fully qualified statement, it is treated in a familiar way, as the start of a nested code block.
In an expression, however, it is assumed to be the beginning of a serialized
object. The following example illustrates a correct use of this syntax and will
display a simple prompt:
var impromptu_object = {
"given_name"
: "John",
"family_name"
: "Smith",
"lucky_numbers" : [ 11630, 12067, 12407, 12887 ]
};
// This will display "John".
alert(impromptu_object.given_name);

In contrast to the unambiguous serializations of numbers, strings, or
arrays, the overloading of the curly bracket means that JSON blocks will not
be recognized properly when used as a standalone statement. This may seem
insignificant, but it is an advantage: It prevents any server-supplied responses
that comply with this syntax from being meaningfully included across domains
via <script src=...>.* The listing that follows will cause a syntax error, ostensibly

*
Unlike most other content inclusion schemes available to scripts (such as XMLHttpRequest),
<script src=...> is not subject to the cross-domain security restrictions outlined in Chapter 9.
Therefore, the mechanism is a security risk whenever ambient authority credentials, such as
cookies, are used by the server to dynamically generate user-specific JavaScript code. This class
of vulnerabilities is unimaginatively referred to as cross-site script inclusion, or XSSI.

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due to an illegal quote () in what the interpreter attempts to treat as a code
label,* and will have no measurable side effects:
<script>
{

"given_name"
: "John",
"family_name"
: "Smith",
"lucky_numbers" : [ 11630, 12067, 12407, 12887 ]
};
</script>

NOTE

The inability to include JSON via <script src=...> is an interesting property, but it is
also a fragile one. In particular, wrapping the response in parentheses or square brackets, or removing quotes around the labels, will render the syntax readily executable in a
standalone block, which may have observable side effects. Given the rapidly evolving
syntax of JavaScript, it is not wise to bank on this particular code layout always causing a parsing error in the years to come. That said, in many noncritical uses, this level
of assurance will be good enough to rely on as a simple security mechanism.
Once retrieved through a channel such as XMLHttpRequest, the JSON
serialization can be quickly and effortlessly converted to an in-memory object
using the JSON.parse(...) function in all common browsers, other than Internet
Explorer. Unfortunately, for purposes of compatibility with Internet Explorer,
and sometimes just out of custom, many developers resort to an equally fast
yet far more dangerous hack:
var parsed_object = eval("(" + json_text + ")");

The problem with this syntax is that the eval(...) function used to compute the “value” of a JSON expression permits not only pure JSON inputs but
any other well-formed JavaScript syntax to appear in the string. This can have
undesirable, global side effects. For example, the function call embedded in
this faux JSON response will execute:
{ "given_name": alert("Hi mom!") }

This behavior creates an additional burden on web developers to accept
JSON payloads only from trusted sources and always to correctly escape feeds
produced by their own server-side code. Predictably, failure to do so has contributed a fair number of application-level security bugs.
NOTE

The difficulty of getting eval(...) right is embodied by the JSON specification (RFC
4627) itself: The allegedly secure parser implementation included in that document
unintentionally permits rogue JSON responses to freely increment or decrement any program variables that happen to consist solely of the letters “a”, “e”, “f”, “l”, “n”, “r”,
*
Somewhat unexpectedly, JavaScript supports C-style labeled statements, such as my_label:
alert(“Hi mom!”). This is interesting because for philosophical reasons, the language has no
support for goto and, therefore, such a label can’t be meaningfully referenced in most cases.

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“s”, “t”, “u”, plus digits; that’s enough to spell “unsafe” and about 1,000 other common English words. The faulty regular expression legitimized in this RFC appears all
over the Internet and will continue to do so.
Thanks to their ease of use, JSON serializations are ubiquitous in serverto-client communications across all modern web applications. The format is
rivaled only by other, less secure string or array serializations and by JSONP.*
All of these schemes are incompatible with JSON.parse(...), however, and must
rely on unsafe eval(...) to be converted to in-memory data. The other property of these formats is that, unlike proper JSON, they will parse properly
when loaded with <script src=...> on a third-party page. This property is advantageous in some rare cases, but mostly it just constitutes an unobvious risk. For
example, consider that even though loading an array serialization via a <script>
tag normally has no measurable side effects, an attacker could, at least until
recent improvements, modify the setters on an Array prototype to retrieve the
supplied data. A common but often insufficient practice of prefixing a response
with a while(1); loop to prevent this attack can backfire in interesting ways if
you recall the possibility of endless loops terminating in JavaScript.

E4X and Other Syntax Extensions
Like HTML, JavaScript is quickly evolving. Some of the changes made to it
over the years have been fairly radical and may end up turning text formats
that were previously rejected by the parser into a valid JavaScript code. This,
in turn, may lead to unexpected data disclosure, especially in conjunction
with the extensive code and object inspection and modification capabilities
discussed earlier in this chapter—and the ability to use <script src=...> to load
cross-domain code.
One of the more notable examples of this trend is ECMAScript for XML
(E4X),5 a completely unnecessary but elegant plan to incorporate XML syntax directly into JavaScript as an alternative to JSON-style serializations. In
any E4X-compatible engine, such as Firefox, the following two snippets of
code would be roughly equivalent:
// Normal object serialization
var my_object = { "user": {
"given_name": "John",
"family_name": "Smith",
"id": make_up_value()
} };
// E4X serialization
var my_object = <user>
<given_name>John</given_name>
<family_name>Smith</family_name>
<id>{ make_up_value() }</id>
</user>;
*
JSONP literally means “JSON with padding” and stands for JSON serialization wrapped in some
supplementary code that turns it into a valid, standalone JavaScript statement for convenience.
Common examples may include a function call (e.g., callback_function({ ...JSON data... })) or a
variable assignment (var return_value = { ...JSON data... }).

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The unexpected consequence of E4X is that, under this regime, any wellformed XML document suddenly becomes a valid <script src=...> target that
will parse as an expression-as-statement block. Moreover, if an attacker can
strategically place “{” and “}” characters on an included page, or alter the setters for the right object prototype, the attacker may be able to extract userspecific text displayed in an unrelated document. The following example
illustrates the risk:
<html xmlns="http://www.w3.org/1999/xhtml">
...
{ steal_stuff(
...
<span>User-specific secrets here</span>
...
) }
...
</html>

attacker-supplied string

attacker-supplied string

To their credit, after several years of living with the flaw, Firefox developers decided to disallow any E4X statements that span the entirety of any
parsed script, partly closing this loophole. Nevertheless, the fluidity of the
language is evident, and it casts some doubt on the robustness of using of
JSON responses as a defense against cross-domain script inclusion. The
moment a third meaning is given to the “{” symbol or quotes-as-labels start
having a purpose, the security of this server-to-client data exchange format
will be substantially degraded. Be sure to plan ahead.

Standard Object Hierarchy
The JavaScript execution environment is structured around an implicit root
object, which is used as the default namespace for all global variables and functions created by the program. In addition to a handful of language-mandated
built-ins, this namespace is prepopulated with a hierarchy of functions that
implement input and output capabilities in the browser environment. These
capabilities include manipulating browser windows (open(...), close(), moveTo(...),
resizeTo(...), focus(), blur(), and such); configuring JavaScript timers (setTimeout(...),
setInterval(...), and so on); displaying various UI prompts (alert(...), prompt(...),
print(...)); and performing a variety of other vendor-specific and frequently
risky functions, such as accessing the system clipboard, creating bookmarks,
or changing the home page.
The top-level object also provides JavaScript references to root objects
belonging to related contexts, including the parent frame (parent), the toplevel document in the current browser window (top), the window that created
the current one (opener), and all subframes of the current document (frames[]).
Several circular references to the current root object itself are also included—
say, window and self. In browsers other than Firefox, elements with specified
id or name parameters will be automatically registered in this namespace, too,
permitting syntax such as this:
<img id="hello" src="http://www.example.com/">
...
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<script>
alert(hello.src);
</script>

Thankfully, in case of any name conflicts with JavaScript variables or builtins, id data will not be given precedence, largely avoiding any possible interference between otherwise sanitized, user-supplied markup and in-document
scripts.
The remainder of the top-level hierarchy consists primarily of a couple
of distinguished children objects that group browser API features by theme:
location object
This is a collection of properties and methods that allow the program to
read the URL of the current document or initiate navigation to a new
one. This last action, in most cases, is lethal to the caller: The current
scripting context will be destroyed and replaced with a new one shortly
thereafter. Updating just the fragment identifier (location.hash) is an
exception to this rule, as explained in Chapter 2.
Note that when using location.* data to construct new strings (HTML
and JavaScript code in particular), it is unsafe to assume that it is escaped
in any specific way. Internet Explorer will keep angle brackets as is in
the location.search property (which corresponds to the URL query string).
Chrome, on the other hand, will escape them, but it will glance over double quotes (") or backslashes. Most browsers also do not apply any escaping to the fragment ID.
history object
This hierarchy provides several infrequently used methods for moving
through the per-window browsing history, in a manner similar to clicking
the “back” and “forward” buttons in the browser UI. It is not possible to
directly examine any of the previously visited URLs; the only option is to
navigate to the history blindly by providing numerical offsets, such as
history.go(-2). (Some recent additions to this hierarchy will be discussed in
Chapter 17.)
screen object
A basic API for examining the dimensions of the screen and the browser
window, monitor DPI, color depth, and so on. This is offered to help websites optimize the presentation of a page for a particular display device.
navigator object
An interface for querying the browser version, the underlying operating
system, and the list of installed plug-ins.
document object
By far the most complex of the hierarchies, this is a doorway to the Document Object Model6 of the current page; we will have a look at this model
in the following section. A couple of functions not related to document
structure also appear under the document hierarchy, usually due to arbitrary design decisions. Examples include document.cookie for manipulating
cookies, document.write(...) for appending HTML to the current page, and
document.execCommand(...) for performing certain WYSIWYG editing tasks.
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NOTE

Interestingly, the information available through the navigator and screen objects is
sufficient to uniquely fingerprint many users with a high degree of confidence. This
long-known property is emphatically demonstrated by Panopticlick, a project of the
Electronic Frontier Foundation: https://panopticlick.eff.org/.
Several other language-mandated objects offer simple string-processing
or arithmetic capabilities. For example, Math.random() implements an
unsafe, predictable pseudo-random number generator (a safe PRNG alternative is unfortunately not available at this time in most browsers*), while
String.fromCharCode() can be used to convert numerical values into Unicode
strings. In privileged execution contexts, which are not reachable by normal
web applications, a fair number of other task-specific objects will also appear.

NOTE

When accessing any of the browser-supplied objects, it is important to remember that
while JavaScript does not use NUL-terminated ASCIZ strings, the underlying browser
(written in C or C++) sometimes will. Therefore, the outcomes of assigning NULcontaining strings to various DOM properties, or supplying them to native functions,
may be unpredictable and inconsistent. Almost all browsers truncate assignments to
location.* at NUL, but only some engines will do the same when dealing with DOM
*.innerHTML.

The Document Object Model
The Document Object Model, accessible through the document hierarchy,
provides a structured, in-memory representation of the current document as
mapped out by the HTML parser. The resulting object tree exposes all HTML
elements on the page, their tag-specific methods and properties, and the associated CSS data. This representation, not the original HTML source, is used
by the browser to render and update the currently displayed document.
JavaScript can access the DOM in a very straightforward way, similarly to
any normal objects. For example, the following snippet will go to the fifth tag
within the document’s <body> block, look up the first nested subtag, and set
that element’s CSS color to red:
document.body.children[4].children[0].style.color = "red";

To avoid having to waddle through the DOM tree in order to get to a
particular deeply nested element, the browser provides several documentwide lookup functions, such as getElementById(...) and getElementsByTagName(...),
as well as partly redundant grouping mechanisms such as frames[], images[],
or forms[]. These features permit syntax such as the following two lines of
code, both of which directly reference an element no matter where in the
document hierarchy it happens to appear:
document.getElementsByTagName("input")[2].value = "Hi mom!";
document.images[7].src = "/example.jpg";

*
There are a recently added window.crypto.getRandomValues(...) API in Chrome and a currently
nonoperational window.crypto.random(...) API in Firefox.

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For legacy reasons, the names of certain HTML elements (<img>, <form>,
<embed>, <object>, and <applet>) are also directly mapped to the document
namespace, as illustrated in the following snippet:
<img name="hello" src="http://www.example.com/">
<script>
alert(document.hello.src);
</script>

Unlike in the more reasonable case of name and id mapping in the global
namespace (see previous section), such document entries may clobber built-in
functions and objects such as getElementById or body. Therefore, permitting
user-specified tag names, for example for the purpose of constructing forms,
can be unsafe.
In addition to providing access to an abstract representation of the
document, many DOM nodes may expose properties such as innerHTML and
outerHTML, which permit a portion of the document tree to be read back as a
well-formed, serialized HTML string. Interestingly, the same property can be
written to in order to replace any portion of the DOM tree with the result of
parsing a script-supplied snippet of HTML. One example of that last use is this:
document.getElementById("output").innerHTML = "<b>Hi mom!</b>";

Every assignment to innerHTML must involve a well-formed and selfcontained block of HTML that does not alter the document hierarchy outside
the substituted fragment. If this condition is not met, the input will be coerced
to a well-formed syntax before the substitution takes place. Therefore, the
following example will not work as expected; that is, it will not display “Hi
mom!” in bold and will not put the remainder of the document in italics:
some_element.innerHTML = "<b>Hi";
some_element.innerHTML += " mom!</b><i>";

Instead, each of these two assignments will be processed and corrected
individually, resulting in a behavior equivalent to this:
some_element.innerHTML = "<b>Hi</b> mom!<i></i>";

It is important to note that the innerHTML mechanism should be used
with extreme caution. In addition to being inherently prone to markup injection if proper HTML escaping is not observed, browser implementations of
the DOM-to-HTML serialization algorithms are often imperfect. A recent
(now fixed) example of such a problem in WebKit7 is illustrated here:
<textarea>
&lt;/textarea&gt;&lt;script&gt;alert(1)&lt;/script&gt;
</textarea>

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Because of the confusion over the semantics of <textarea>, this seemingly
unambiguous input markup, when parsed to a DOM tree and then accessed
through innerHTML, would be incorrectly read back as:
<textarea>
</textarea><script>alert(1)</script>
</textarea>

In such a situation, even performing a no-op assignment of this serialization (such as some_element.innerHTML += "") would lead to unexpected script
injection. Similar problems tend to plague other browsers, too. For example,
Internet Explorer developers working on the innerHTML code were unaware
that MSHTML recognizes backticks (`) as quote characters and so ended up
handling them incorrectly. In their implementation, the following markup:
<img src="test.jpg" alt="``onload=alert(1)">

would be reserialized as this:
<img src=test.jpg alt=``onload=alert(1)>

Individual bugs aside, the situation with innerHTML is pretty dire: Section 10.3 of the current draft of HTML5 simply acknowledges that certain
script-created DOM structures are completely impossible to serialize to
HTML and does not require browsers to behave sensibly in such a case.
Caveat emptor!

Access to Other Documents
Scripts may come into possession of object handles that point to the root
hierarchy of another scripting context. For example, by default, every context can readily reference parent, top, opener, and frames[], all supplied to it in
the top-level object. Calling the window.open(...) function to create a new window will also return a reference, and so will an attempt to look up an existing
named window using this syntax:
var window_handle = window.open("", "window_name");

Once the program holds a handle pointing to another scripting context,
it may attempt to interact with that context, subject to security checks discussed in Chapter 9. An example of a simple interaction might be as follows:
top.location.path = "/new_path.html";

or
frames[2].document.getElementById("output").innerHTML = "Hi mom!";

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In the absence of a valid handle, JavaScript-level interaction with an
unrelated document should not be possible. In particular, there is no way
to look up unnamed windows opened in completely separate navigation
flows, at least until their name is explicitly set by one of the visited pages
(the window.name property permits this).

Script Character Encoding
JavaScript engines support several familiar, backslash-based string-encoding
methods that can be employed to escape quote characters, HTML markup,
and other problematic bits in the embedded text. These methods are as follows:

NOTE



C-style shorthand notation for certain control characters: \b for backspace, \t for horizontal tab, \v for vertical tab, \f for form feed, \r for CR,
and \n for LF. This exact set of escape codes is recognized by both
ECMAScript and the JSON RFC.



Three-digit, zero-padded, 8-bit octal character codes with no prefix
(such as “\145” instead of “e”). This C-inspired syntax is not a part of
ECMAScript but is in practice supported by all scripting engines, both
in normal code and in JSON.parse(...).



Two-digit, zero-padded, 8-bit hexadecimal character codes, prefixed
with “x” (“e” becomes “\x65”). Again, this scheme is not endorsed by
ECMAScript or RFC 4627, but having its roots in the C language, it is
widely supported in practice.



Four-digit, zero-padded, 16-bit hexadecimal Unicode values, prefixed
with “u” (“e” turns into “\u0065”). This format is sanctioned by ECMAScript and RFC 4627 and is supported by all modern browsers.



A backslash followed by any character other than an octal digit; “b”, “t”,
“v”, “f”, “r,” or “n” characters used for other predefined escape sequences;
and “x” or “u”. In this scheme, the subsequent character will be treated
as a literal. ECMAScript permits this scheme to be used to escape only
quotes and the backslash character itself, but in practice, any other value
is accepted as well.
This approach is somewhat error prone, and as in the case of CSS,
it should not be used to escape angle brackets and other HTML syntax
delimiters. This is because JavaScript parsing takes place after HTML
parsing, and the backslash prefix will be not treated in any special way
by the HTML parser itself.

Somewhat inexplicably, Internet Explorer does not recognize the vertical tab (“\v”)
shorthand, thereby creating one of the more convenient (but very naughty!) ways to
test for that particular browser:
if ("\v" == "v") alert("Looks like Internet Explorer!");

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Surprisingly, the Unicode-based escaping method (but not the other
ones) is also recognized outside strings. Although the idea seems arbitrary, the
behavior is a bit more sensible than with CSS: Escape codes can be used only
in identifiers, and they will not work as a substitute for any syntax-sensitive
symbols. Therefore, the following is possible:
\u0061lert("This displays a message!");

On the other hand, any attempt to substitute the parentheses or quotes
in a similar fashion would fail.
Unlike in some C or C++ implementations, stray multiline string literals
are not tolerated by any JavaScript engine. That said, despite a strongly worded
prohibition in ECMAScript specs, there is one exception: A lone backslash at
the end of a line may be used to join multiline literals seamlessly. This behavior is illustrated below:
var text = 'This syntax
is invalid.';
var text = 'This syntax, on the other hand, \
is OK in all browsers.';

Code Inclusion Modes and Nesting Risks
As should be evident from the earlier discussions in this chapter, there are
several ways to execute scripts in the context of the current page. It is probably useful to enumerate some of the most common ones:


Inline <script> blocks



Remote scripts loaded with <script src=...>*



javascript: URLs in various HTML parameters and in CSS



CSS expression(...) syntax and XBL bindings in certain browsers



Event handlers (onload, onerror, onclick, etc.)



Timers (setTimeout, setInterval)



eval(...) calls

Combining these methods often seems natural, but doing so can create
very unexpected and dangerous parsing chains. For example, consider the
transformation that would need to be applied to the value inserted by the
server in place of user_string in this code:
<div onclick="setTimeout('do_stuff(\'user_string\')', 1000)">

*
On both types of <script> blocks, Microsoft supports a pseudo-dialect called JScript.Encode. This
mode can be selected by specifying a language parameter on the <script> tag and simply permits
the actual script to be encoded using a trivial alphabet substitution cipher to make it unreadable
to casual users. The mechanism is completely worthless from the security standpoint, as the
“encryption” can be reverted easily.

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It is often difficult to notice that the value will go through no fewer
than three rounds of parsing! First, the HTML parser will extract the onclick
parameter and put it into DOM; next, when the button is clicked, the first
round of JavaScript parsing will extract the setTimeout(...) syntax; and finally,
one second after the initial click, the actual do_stuff(...) sequence will be
parsed and executed.
Therefore, in the example above, in order to survive the process, user_string
needs to be double-encoded using JavaScript backslash sequences, and then
encoded again using HTML entities, in that exact order. Any different approach
will likely lead to code injection.
Another tricky escaping situation is illustrated here:
<script>
var some_value = "user_string";
...
setTimeout("do_stuff('" + some_value + "')", 1000);
</script>

Even though the initial assignment of some_value requires user_string to
be escaped just once, the subsequent ad hoc construction of a second-order
script in the setTimeout(...) parameter introduces a vulnerability if no additional escaping is applied beforehand.
Such coding patterns happen frequently in JavaScript programs, and
they are very easy to miss. It is much better to consistently discourage them
than to audit the resulting code.

The Living Dead: Visual Basic
Having covered most of the needed ground related to JavaScript, it’s time
for an honorable mention of the long-forgotten contender for the scripting
throne. Despite 15 years of lingering in almost complete obscurity, browserside VBScript is still supported in Internet Explorer. In most aspects, Microsoft’s language is supposed to be functionally equivalent to JavaScript, and it
has access to exactly the same Document Object Model APIs and other builtin functions as JavaScript. But, as one might expect, some tweaks and extensions are present—for example, a couple of VB-specific functions in place of
the JavaScript built-ins.
There is virtually no research into the security properties of VBScript,
the robustness of the parser, or its potential incompatibilities with the modern DOM. Anecdotal evidence suggests that the language receives no consistent scrutiny on Microsoft’s end, either. For example, the built-in MsgBox8
can be used to display modal, always-on-top prompts with a degree of flexibility completely unheard of in the JavaScript world, leaving alert(...) in the dust.
It is difficult to predict how long VBScript will continue to be supported
in this browser and what unexpected consequences for user and web application security it is yet to have. Only time will tell.

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Security Engineering Cheat Sheet
When Loading Remote Scripts
As with CSS, you are linking the security of your site to the originating domain of the script.
When in doubt, make a local copy of the data instead. On HTTPS sites, require all scripts to
be served over HTTPS.

When Parsing JSON Received from the Server
Rely on JSON.parse(...) where supported. Do not use eval(...) or the eval-based implementation
provided in RFC 4627. Both are unsafe, especially when processing data from third parties. A
later implementation from the author of RFC 4627, json2.js,9 is probably okay.

When Putting User-Supplied Data Inside JavaScript Blocks
 Stand-alone strings in <script> blocks: Backslash-escape all control characters (0x00–0x1F),
“\”, “<”, “>”, and quotes using numerical codes. It is also preferable to escape high-bit
characters.
Do not rely on user-supplied strings to construct dynamic HTML. Always use safe
DOM features such as innerText or createTextNode(...) instead. Do not use user-supplied
strings to construct second-order scripts; avoid eval(...), setTimeout(...), and so on.
 Stand-alone strings in separately served scripts: Follow the same rules as for <script>
blocks. If your scripts contain any sensitive, user-specific information, be sure to account
for cross-site script inclusion risks; use reliable parser-busting prefixes, such as “)}]'\n”,
near the beginning of a file or, at the very minimum, use a proper JSON serialization with
no padding or other tweaks. Additionally, consult Chapter 13 for tips on how to prevent
cross-site scripting in non-HTML content.
 Strings in inlined event handlers, javascript: URLs, and so on: Multiple levels of escaping
are involved. Do not attempt this because it is error prone. If unavoidable, apply the above
JS escaping rules first and then apply HTML or URL parameter encoding, as applicable,
to the resulting string. Never use in conjunction with eval(...), setTimeout(...), innerHTML,
and such.
 Nonstring content: Allow only whitelisted alphanumeric keywords and carefully validated
numerical values. Do not attempt to reject known bad patterns instead.

When Interacting with Browser Objects on the Client Side
 Generating HTML content on the client side: Do not resort to innerHTML, document.write(...),
and similar tools because they are prone to introducing cross-site scripting flaws, often in
unexpected ways. Use safe methods such as createElement(...) and appendChild(...) and
properties such as innerText or textContent to construct the document instead.
 Relying on user-controlled data: Make no assumptions about the escaping rules applied
to any values read back from the browser and, in particular, to location properties and
other external sources of URLs, which are inconsistent and vary from one implementation to another. Always do your own escaping.
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If You Want to Allow User-Controlled Scripts on Your Page
It is virtually impossible to do this safely. Experimental JavaScript rewriting frameworks,
such as Caja (http://code.google.com/p/google-caja/ ), are the only portable option. Also see
Chapter 16 for information on sandboxed frames, an upcoming alternative for embedding
untrusted gadgets on web pages.

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NON-HTML
DOCUMENT TYPES

In addition to HTML documents, about a dozen other
file formats are recognized and displayed by the rendering engines of modern web browsers; a list that is
likely to grow over time.
Because of the powerful scripting capabilities available in some of these
formats, and because of the antics of browser-content handling, the set of
natively supported non-HTML inputs deserves a closer examination at this
point, even if a detailed discussion of some of their less-obvious security
consequences—such as content sniffing—will have to wait until Part II of
this book.

Plaintext Files
Perhaps the most prosaic type of non-HTML document recognized by every
single browser is a plaintext file. In this rendering mode, the input is simply
displayed as is, typically using a nonproportional typeface, and save for
optional character set transcoding, the data is not altered in any way.

All browsers recognize plaintext files served with Content-Type: text/plain
in the HTTP headers. In all implementations but Internet Explorer, plaintext is also the fallback display method for headerless HTTP/0.9 responses
and HTTP/1.x data with Content-Type missing; in both these cases, plaintext
is used when all other content detection heuristics fail. (Internet Explorer
unconditionally falls back to HTML rendering, true to the letter of Tim
Berners-Lee’s original protocol drafts.)
For the convenience of developers, most browsers also automatically
map several other MIME types, including application/javascript and friends*
or text/css, to plaintext. Interestingly, application/json, the value mandated for
JSON responses in RFC 4627, is not on the list (perhaps because it is seldom
used in practice).
Plaintext rendering has no specific security consequences. That said,
due to a range of poor design decisions in other browser components and in
third-party code, even seemingly harmless non-HTML formats are at a risk
of being misidentified as, for example, HTML. Attacker-controlled plaintext
documents are of special concern because their layout is often fairly unconstrained and therefore particularly conducive to being misidentified. Chapter 13 dissects these threats and provides advice on how to mitigate the risk.

Bitmap Images
Browser-rendering engines recognize direct navigation to the same set of bitmap image formats that are normally supported in HTML documents when
loaded via the <img> tag, including JPEG, PNG, GIF, BMP, and a couple more.
When the user navigates directly to such a resource, the decoded bitmap is
shown in the document window, allowing the user little more than the ability
to scroll, zoom in and out, and save the file to disk.
In the absence of Content-Type information, images are detected based on
file header checks. When a Content-Type value is present, it is compared with
about a dozen predefined image types, and the user is routed accordingly.
But if an attempt to decode the image fails, file headers are used to make a
second guess. It is therefore possible (but, for the reasons explored in Chapter 13, often unwise) to serve a GIF file as image/jpeg.
As with text files, bitmap images are a passive resource and carry no
unusual security risks.† However, whenever serving user-supplied images,
remember that attackers will have a degree of control over the data, even if
the format is carefully validated and scaled or recompressed. Therefore, the
concerns about such a document format being misinterpreted by a browser
or a plug-in still remain.

*
The official MIME type for JavaScript is application/javascript, as per RFC 4329, but about a
dozen other values have been used in the past (e.g., text/javascript, application/x-javascript,
application/ecmascript).
†
Naturally, exploitable coding errors occasionally happen in all programs that deal with
complex data formats, and image parsers are no exception.

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Audio and Video
For a very long time, browsers had no built-in support for playing audio and
video content, save for an obscure and oft-ridiculed <bgsound> tag in Internet
Explorer, which to this day can be used to play simple MID or WAV files. In
the absence of real, cross-browser multimedia playback functionality, audio
and video were almost exclusively the domain of browser plug-ins, whether
purpose-built (such as Windows Media Player or Apple QuickTime) or generic
(Adobe Flash, Microsoft Silverlight, and so on).
The ongoing work on HTML5 seeks to change this through support for
<audio> and <video> tags: convenient, scriptable methods to interface with
built-in media decoders. Unfortunately, there is substantial vendor-level disagreement as to which video formats to support and what patent consequences
this decision may have. For example, while many browsers already support
Ogg Theora (a free, open source, but somewhat niche codec), spirited arguments surrounding the merits of supporting the very popular but patent- and
royalty-encumbered H.264 format and the prospects of a new, Google-backed
WebM alternative will probably continue for the foreseeable future.
As with other passive media formats (and unlike some types of plug-inrendered content!), neither <bgsound> nor HTML5 multimedia are expected
to have any unusual implications for web application security, as long as the
possibility of content misidentification is mitigated appropriately.*

XML-Based Documents
Readers who found the handling of the formats discussed so far to be too
sane for their tastes are in for a well-deserved treat. The largest and definitely
most interesting family of browser-supported non-HTML document types
relies on the common XML syntax and provides more than a fair share of
interesting surprises.
Several of the formats belonging to this category are forwarded to
specialized, single-purpose XML analyzers, usually based on the received
Content-Type value or other simple heuristics. But more commonly, the payload is routed to the same parser that is relied upon to render XHTML documents and then displayed using this common pipeline.
In the latter case, the actual meaning of the document is determined by
the URL-like xmlns namespace directives present in the markup itself, and
the namespace parameter may have nothing to do with the value originally
supplied in Content-Type. Quite simply, there is no mechanism that would prevent a document served as application/mathml+xml from containing nothing
but XHTML markup and beginning with <html xmlns="http://www.w3.org/
1999/xhtml">.

*

But some far-fetched interactions between various technologies are a distinct possibility. For
example, what if the <audio> tag supports raw, uncompressed audio and is pointed to a sensitive
nonaudio document, and then the proposed HTML5 microphone API is used by another
website to capture the resulting waveform and reconstruct the contents of the file?
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In the most common scenario, the namespace for the entire XML file is
defined only once and is attached to the top-level tag. In principle, however,
any number of different xmlns directives may appear in a single file, giving
different meanings to each section of the document. For example:
<html xmlns="http://www.w3.org/1999/xhtml">
<u>Hello world!</u>
<svg xmlns="http://www.w3.org/2000/svg">
<line x1="0" y1="0" x2="100" y2="100" style="stroke: red" />
</svg>
</html>

Faced with such input, the general-purpose renderer will usually do
its best to make sense of all the recognized namespaces and assemble the
markup into a single, consistent document with a normal Document Object
Model representation. And, if any one of the recognized namespaces happens to support scripting, any embedded scripts will execute, too.
Because of the somewhat counterintuitive xmlns handling behavior,
Content-Type is not a suitable way to control how a particular XML document
will be parsed; the presence of a particular top-level xmlns directive is also not
a guarantee that no other data formats will be honored later on. Any attackercontrolled XML-based formats must therefore be handled with care and sanitized very thoroughly.

Generic XML View
In most browsers, a valid XML document with no renderer-recognized
namespaces present anywhere in the markup will be shown as an interactive,
pretty-printed representation of the document tree, as shown in Figure 7-1.
This mode is not particularly useful to end users, but it can aid debugging.
That said, when any of the namespaces in the document is known to the
browser (even when the top-level one is not recognized at all!), the document
will be rendered differently: All recognized markup will work as intended, all
unsupported tags will simply have no effect, and any text between them will
be shown as is.
To illustrate this rendering strategy, consider the following input:
<foo xmlns="http://www.example.com/nonexistent">
<u>Hello</u>
<html xmlns="http://www.w3.org/1999/xhtml">
<u>world!</u>
</html>
</foo>

The above example will be rendered as “Hello world!” The first <u> tag,
with no semantics-defining namespace associated with it, will have no visible
effect. The second one will be understood as an XHTML tag that triggers
underlining.

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Figure 7-1: Firefox displaying an XML document with no recognized namespaces

The consequences of this fault-tolerant approach to the rendering of
unknown XML documents and unrecognized namespaces are subtle but
fairly important. For example, it will not be safe to proxy an unsanitized RSS
feed, even though this format is typically routed to a specialized renderer
and thus not subject to XSS risks. Any browser with no built-in RSS reader
may fall back to generic rendering and then find HTML buried deep inside
the feed.

Scalable Vector Graphics
Scalable Vector Graphics (SVG)1 is a quickly evolving, XML-based vector
graphics format. First published in 2001 by W3C, it is noteworthy for its integrated animation capabilities and direct JavaScript scripting features. The
following example of a vector image draws a circle and displays a message
when this circle is clicked:
<svg xmlns="http://www.w3.org/2000/svg">
<script><![CDATA[
function clicked() { alert("Hi mom!"); }
]]></script>
<circle onclick="clicked()" cx="50" cy="50"
r="50" fill="pink" />
</svg>

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The SVG file format is recognized all modern browsers except for
Internet Explorer prior to 9, and it is handled by the general-purpose XML
renderer. SVG images can be embedded into XHTML with an appropriate
xmlns directive or inlined in non-XML HTML5 documents using a predefined <svg> tag.
Interestingly, in several browsers the format can also be placed in a standalone XML document and then viewed directly, or it can be loaded on thirdparty pages via the <img> markup. While it is safe to load SVG images via <img>
(scripting should be disabled in this scenario), it is fairly dangerous to host
user-supplied SVG data because in cases of direct navigation, all embedded
scripts will execute in the context of the hosting domain. This unexpected
problem means that serving any externally originating SVG images will require
very careful syntax sanitization to eliminate non-SVG xmlns content from the
XML container and to permit only certain types of markup in the remainder
of the document.
NOTE

The Content-Disposition header on the relevant HTTP responses is a potential
workaround that permits SVG to be included via <img> but not accessed directly. This
approach is not perfect, but it limits the risk. Using a throwaway domain to host such
images is another possibility.

Mathematical Markup Language
Mathematical Markup Language (MathML)2 is a fairly straightforward means
to facilitate the semantic, if a bit verbose, representation of mathematical
equations. The standard was originally proposed by the W3C in 1998, and it
has been substantially refined through the years. Because of its somewhat
niche application, MathML needed over a decade to gain partial support in
Opera and Firefox browsers, but it is slowly gaining acceptance today. In the
browsers that support the language, it may be placed in a standalone file or
inline in XHTML and HTML5 documents.
Unlike SVG, MathML has no additional security considerations beyond
those associated with generically handled XML.

XML User Interface Language
The XML User Interface Language (XUL)3 is a presentation markup language created by Mozilla specifically for building browser-based applications,
rather than documents. XUL exists because although modern HTML is often
powerful enough to build basic graphical user interfaces, it is not particularly
convenient for certain specialized tasks that desktop applications excel in,
such as implementing common dialog windows or system menus.
XUL is not currently supported by any browser other than Firefox and
appears to be disabled in the recent release, Firefox 6. In Firefox, it is handled
by the general-purpose renderer, based on the appropriate xmlns namespace.
Firefox uses XUL for much of its internal UI, but otherwise the language is
seldom encountered on the Internet.
From the standpoint of web application security, Internet-originating
XUL documents can be considered roughly equivalent to HTML documents.
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Essentially, the language has JavaScript scripting capabilities and allows broad
control over the appearance of the rendered page. Other than that property,
it has no unusual quirks.

Wireless Markup Language
Wireless Markup Language (WML)4 is a largely obsolete “optimized” HTML
syntax developed in the 1990s by a consortium of mobile handset manufacturers and cellular network operators. This XML-based language, a part of
the Wireless Application Protocol suite (WAP), offered a simplified weblike
browsing experience for pre-smartphone devices with limited bandwidth and
CPU resources.* A simple WML page might have looked like this:
<wml>
<card title="Hello world!">
<a href="elsewhere.wml">Click here!</a>
</card>
</wml>

Because WAP services needed to be engineered independently of normal HTML content and had to deal with closed and underspecified client
architectures and other carrier-imposed restrictions, WML never became as
popular as its proponents hoped. In almost all developed markets, WML has
been displaced by fast, Internet-enabled smartphones with fully featured
HTML browsers. Nevertheless, the legacy of the language lives on, and it is
still routed to specialized renderers in Opera and in Internet Explorer Mobile.
In the browsers that support the format, it is often possible to use WMLbased scripts. There are two methods to achieve this. The canonical way is to
use WMLScript (WMLS), a JavaScript-derived execution environment that
depends on stand-alone script files, coupled with an extremely inconsiderate
abuse of fragment IDs for an equivalent of possibly attacker-controlled
eval(...) statements:
<a href="scriptfile.wmls#some_function()">Click here!</a>

The other method of executing scripts, available in more featured browsers, is to simply embed normal javascript: URLs or insert <script> blocks into
the WML file.

RSS and Atom Feeds
Feeds are a standardized way for clients to periodically poll sites of interest
to users (such as their favorite blogs) for machine-readable updates to said
sites’ content. Really Simple Syndication (RSS)5 and Atom6 are two superficially similar but fiercely competing XML-based feed formats. The first (RSS)
is popular; the second (Atom) is said to be good.
*
Astute readers will note that XML is not a particularly good way to conserve bandwidth or CPU
resources. To that effect, the WAP suite provides an alternative, binary-only serialization of
XML, known as WBXML.

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Built-in, specialized RSS and Atom renderers are available in Firefox,
Safari, and Opera. The determination to route an XML document to these
modules is based on simple, browser-specific heuristics, such as the top-level
tag being named <rss> or <feed> (and not having any conflicting xmlns directives). In Firefox, RSS parsing may kick in even if Content-Type is image/svg+xml
or text/html. Safari will happily recognize feeds in even more unrelated MIME
types.
One interesting feature of both feed formats is that they permit a subset
of HTML, including CSS, to be embedded in a document in a rather peculiar, indirect way: as an entity-escaped text. Here is an example of this syntax:
<rss>
...
<description type="html">
&lt;u&gt; Underlined text! &lt;/u&gt;
</description>
...
</rss>

The subset of HTML permitted in RSS and Atom feeds is not well defined,
and some feed renderers have previously permitted direct scripting or navigation to potentially dangerous pseudo-URLs. Perhaps more importantly,
however, any browser that does not have built-in feed previews may render
the file using the generic XML parsing approach; if such feeds are not sanitized carefully, script execution will ensue.

A Note on Nonrenderable File Types
For the sake of completeness, it should be noted that all modern browsers
support a number of specialized file formats that remain completely opaque
to the renderer or to the web application layer but that are nevertheless recognized by a variety of in-browser subsystems.
A detailed investigation of these formats is beyond the scope of this
book, but some notable examples include plug-in and extension installation
manifests, automatic HTTP proxy autoconfiguration files (PAC), installable
visual skins, Certificate Revocation Lists (CRLs), antimalware site blacklists,
and downloadable TrueType and OpenType fonts.
The security properties of these mechanisms should be studied individually before deciding to allow any of these formats to be served to the user.
Save for the generic content-hosting considerations outlined in Chapter 13,
they are unlikely to harm the hosting web application directly, but they may
cause problems for users.

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Security Engineering Cheat Sheet
When Hosting XML-Based Document Formats
Assume that the payload may be interpreted as XHTML or some other script-enabled document type, regardless of the Content-Type and the top-level xmlns directive. Do not allow unconstrained attacker-controlled markup anywhere inside the file. Use the Content-Disposition:
attachment if data is not meant to be viewed directly; <img> and feeds will still work.

On All Non-HTML Document Types
Use correct, browser-recognized Content-Type and charset values. Specify the Content-Disposition:
attachment where possible. Verify and constrain output syntax. Consult the cheat sheet in
Chapter 13 to avoid security problems related to content-sniffing flaws.

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CONTENT RENDERING WITH
BROWSER PLUG-INS

Browser plug-ins come in many forms and shapes, but
the most common variety give the ability to display new
file formats in the browser, as if they were HTML. The
browser simply hands over the retrieved file, provides
the helper application with a rectangular drawing surface in the document window, and essentially backs away
from the scene. Such content-rendering plug-ins are clearly distinguished from
browser extensions, a far more numerous bunch that commonly relies on
JavaScript code to tweak how the already-supported, in-browser content is
presented to the user.
Browser plug-ins have a long and colorful history of security flaws. In
fact, according to some analysts, 12 out of the 15 most frequently exploited
client-side vulnerabilities in 2010 could be attributed to the quality of plug-in
software.1 Many of these problems are because the underlying parsers were
originally not meant to handle malicious inputs gracefully and have not benefited from the intense scrutiny that the remainder of the Web has been subject to. Other problems stem from the unusual security models devised by

plug-in developers and the interference between these permissions, the traditional design of web browsers, and the commonsense expectations of application developers.
We will review some of the security mechanisms used by popular plug-ins
in the next chapter of this book. Before taking this dive, it makes sense to
look at the ways plug-ins integrate with other online content and the common functionality they offer.

Invoking a Plug-in
Content-rendering plug-ins can be activated in a couple of ways. The most
popular explicit method is to use <embed src=...> or <object data=...> markup
in a “host” HTML document, with the src or data parameter pointing to the
URL from which the actual plug-in-recognized document is to be retrieved.
The dimensions and position of the drawable area allocated for the plug-in
can be controlled with CSS (or with legacy HTML parameters).
In this scenario, every <embed> or <object> tag should be accompanied by
an additional type parameter. The MIME type specified there will be compared to the list of MIME types registered by all the active plug-ins, and the
retrieved file will be routed to the appropriate handler. If no match is found,
a warning asking the user to download a plug-in should be theoretically displayed instead, although most browsers look at other signals before resorting
to this unthinkable possibility; examining Content-Type or the apparent file
extension spotted in the URL are two common choices.
NOTE

An obsolete <applet> tag, used to load Java programs (roughly equivalent to
<object type="application/x-java-applet">), works in a comparable way but
unconditionally disregards these auxiliary signals.
Additional input to the plug-in is commonly passed using <param> tags
nested inside the <object> block or through nonstandard additional parameters attached to the <embed> markup itself. The former, more modern
approach may look like this:
<object data="app.swf" type="application/x-shockwave-flash">
<param name="some_param1" value="some_value1">
<param name="some_param2" value="some_value2">
...
</object>

In this content-inclusion mode, the Content-Type header returned by the
server when retrieving the subresource is typically ignored, unless the type
parameter is unknown to the browser. This is an unfortunate design, for reasons that will be explained shortly.
The other method for displaying plug-in content involves navigating
directly to a suitable file. In this case, and in the case of <embed> or <object>
with a missing type parameter, the Content-Type value obtained from the server
is honored, and it will be compared with the list of plug-in-recognized MIME
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types. If a match is found, the content is routed to the appropriate component.
If the Content-Type lookup fails or the header is missing, some browsers will
examine the response body for known content signatures; others just give up.
NOTE

The aforementioned content-focused methods aside, several types of plug-ins can be
loaded directly from within JavaScript or VBScript programs without the need to explicitly create any HTML markup or retrieve any external data. Such is the case for ActiveX,
an infamous script-to-system integration bridge available in Internet Explorer. (We will
devote some time to ActiveX later in this chapter, but first things first.)

The Perils of Plug-in Content-Type Handling
As noted in the previous section, in certain scenarios the Content-Type parameter on a retrieved plug-in-handled file is ignored, and the type parameter in
the corresponding markup on the embedding page is used instead. While
this decision is somewhat similar to the behavior of other type-specific content-inclusion tags (say, <img>), as discussed in “Type-Specific Content Inclusion” on page 82, it has some unique and ultimately disastrous consequences
in the plug-in world.
The big problem is that several types of plug-ins are essentially fullfledged code execution environments and give the executed applications
(applets) a range of special privileges to interact with the originating domain.
For example, a Flash file retrieved from fuzzybunnies.com would be granted
access to its originating domain (complete with a user’s cookies) when
embedded on the decidedly rogue bunnyoutlet.com.
In such a scenario, it would seem to be important for fuzzybunnies.com
to be able to clearly communicate that a particular type of a document is
indeed meant to be interpreted by a plug-in—and, consequently, that some
documents aren’t meant to be used this way. Unfortunately, there is no way
for this to happen: The handling of a retrieved file is fully controlled by
the embedding site (in our example, by the mean-spirited bullies who own
bunnyoutlet.com). Therefore, if the originating domain hosts any type of usercontrolled content, even in a nominally harmless format (such as text/plain
or image/jpeg), the owners of bunnyoutlet.com may instruct the browser to disregard the existing metadata and route that document to a plug-in of their
choice. A simple markup to achieve this sinister goal may be
<object data="http://fuzzybunnies.com/avatars/user11630.jpg"
type="application/x-shockwave-flash">

If this turn of events seems wrong, that’s because it is. Security researchers
have repeatedly demonstrated that it is quite easy to construct documents that
are, for example, simultaneously a valid image and a valid plug-in-recognized
executable. The well-known “GIFAR” vulnerability, discovered in 2008 by
Billy Rios,2 exploited that very trick: It smuggled a Java applet inside a perfectly kosher GIF image. In response, Sun Microsystems reportedly tightened
down the Java JAR file parser to mitigate the risk, but the general threat of
such mistakes is still very real and will likely rear its ugly head once more.
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129

Interestingly, the decision by some developers to rely on Content-Type and
other signals if the type parameter is unrecognized is almost as bad. This decision makes it impossible for the well-intentioned fuzzybunnies.com to safely
embed a harmless video from the rogues at bunnyoutlet.com by simply specifying
type="video/x-ms-wmv", because if any of the visitors do not have a plug-in for
that specific media type, bunnyoutlet.com will suddenly have a say in what type
of plug-in should be loaded on the embedding site instead. Some browsers,
such as Internet Explorer, Chrome, or Opera, may also resort to looking for
apparent file extensions present in the URL, which can lead to an interesting
situation where neither the embedding nor the hosting party has real control
over how a document is displayed—and quite often only the attacker is in
charge.
A much safer design would require the embedder-controlled type parameter and the host-controlled Content-Type header to match (at least superficially). Unfortunately, there is currently no way to make this happen. Several
individual plug-ins try to play nice (for example, following a 2008 overhaul,
Adobe Flash rejects applets served with Content-Disposition: attachment, as does
the built-in PDF reader in Chrome), but these improvements are few and far
between.

Document Rendering Helpers
A significant portion of the plug-in landscape belongs to programs that allow
certain very traditional, “nonweb” document formats to be shown directly in
the browser. Some of these programs are genuinely useful: Windows Media
Player, RealNetworks RealPlayer, and Apple QuickTime have been the backbone of online multimedia playback for about a decade, at least until their
displacement by Adobe Flash. The merits of others are more questionable,
however. For example, Adobe Reader and Microsoft Office both install inbrowser document viewers, increasing the user’s attack surface appreciably,
though it is unclear whether these viewers offer a real benefit over opening
the same document in a separate application with one extra click.
Of course, in a perfect world, hosting or embedding a PDF or a Word
document should have no direct consequences for the security of the participating websites. Yet, predictably, the reality begs to differ. In 2009, a researcher
noted that PDF-based forms that submit to javascript: URLs can apparently lead
to client-side code execution on the embedding site.3 Perhaps even more troubling than this report alone, according to that researcher’s account, Adobe initially dismissed the report with the following note: “Our position is that, like
an HTML page, a PDF file is active content.”
It is regrettable that the hosting party does not have full control of when
this active content is detected and executed and that otherwise reasonable
webmasters may think of PDFs or Word documents as just a fancy way to present text. In reality, despite their harmless appearance, in a bid to look cool,
many such document formats come equipped with their own hyperlinking
capabilities or even scripting languages. For example, JavaScript code can
be embedded in PDF documents, and Visual Basic macros are possible in
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Microsoft Office files. When a script-bearing document is displayed on an
HTML page, some form of a programmatic plug-in-to-browser bridge usually
permits a degree of interaction with the embedding site, and the design of
such bridges can vary from vaguely questionable to outright preposterous.
In one 2007 case, Petko D. Petkov noticed that a site that hosts any
PDF documents can be attacked simply by providing completely arbitrary
JavaScript code in the fragment identifier. This string will be executed on
the hosting page through the plug-in bridge:4
http://example.com/random_document.pdf#foo=javascript:alert(1)

The two vulnerabilities outlined here are now fixed, but the lesson is
that special care should be exercised when hosting or embedding any usersupplied documents in sensitive domains. The consequences of doing so are
not well documented and can be difficult to predict.

Plug-in-Based Application Frameworks
The boring job of rendering documents is a well-established role for browser
plug-ins, but several ambitious vendors go well beyond this paradigm. The
aim of some plug-ins is simply to displace HTML and JavaScript by providing
alternative, more featured platforms for building interactive web applications.
That reasoning is not completely without merit: Browsers have long lacked
in performance, in graphics capabilities, and in multimedia codecs, stifling
some potential uses of the Web. Reliance on plug-ins is a reasonable shortterm way to make a difference. On the flip side, when proprietary, patentand copyright-encumbered plug-ins are promoted as the ultimate way to build
an online ecosystem, without any intent to improve the browsers themselves,
the openness of the Web inevitably suffers. Some critics, notably Steve Jobs,
think that creating a tightly controlled ecosystem is exactly what several plugin vendors, most notably Adobe, aspire to.5
In response to this perceived threat of a hostile takeover of the Web,
many of the shortcomings that led to the proliferation of alternative application frameworks are now being hastily addressed under the vaguely defined
umbrella of HTML5; <video> tags and WebGL* are the prime examples of this
work. That said, some of the features available in plug-ins will probably not be
captured as a part of any browser standard in the immediate future. For example, there is currently no serious plan to add inherently dangerous elevated
privilege programs supported by Java or security-by-obscurity content protection schemes (euphemistically called Digital Rights Management, or DRM).
Therefore, while the landscape will change dramatically in the coming
years, we can expect that in one form or another, proprietary web application frameworks are here to stay.

*
WebGL is a fairly recent attempt to bring OpenGL-based 3D graphics to JavaScript applications. The first specification of the standard appeared in March 2011, and wide browser-level
support is expected to follow.

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Adobe Flash
Adobe Flash is a web application framework introduced in 1996, in the heat
of the First Browser Wars. Before its acquisition by Adobe in 2005, the Flash
platform was known as Macromedia Flash or Shockwave Flash (hence the .swf
file extension used for Flash files), and it is still sometimes referred to as such.
Flash is a fairly down-to-earth platform built on top of a JavaScript-based
language dubbed ActionScript.7 It includes a 2-D vector and bitmap graphicsrendering engine and built-in support for several image, video, and audio
formats, such as the popular and efficient H.264 codec (which is used for
much of today’s online multimedia).
By most estimates, Flash is installed on around 95 to 99 percent of all
desktop systems.8, 9 This user base is substantially higher than that of any
other media player plug-in. (Support for the Windows Media Player and
QuickTime plug-ins is available on only about 60 percent of PCs, despite
aggressive bundling strategies, while the increasingly unpopular RealPlayer
is still clinging to 25 percent.) The market position contributes to the product’s most significant and unexpected use: the replacement of all multimedia
playback plug-ins previously relied upon for streaming video on the Web.
Although the plug-in is also used for a variety of other jobs (including implementing online games, interactive advertisements, and so on), simple multimedia constitutes a disproportionately large slice of the pie.
NOTE

Confusingly, a separate plug-in called Adobe Shockwave Player (without the word
“Flash”) is also available, which can be used to play back content created with Adobe
Director. This plug-in is sometimes mistakenly installed in place of or alongside Adobe
Flash, contributing to an approximately 20 percent install base,6 but it is almost always
unnecessary. The security properties of this plug-in are not particularly well studied.
Properties of ActionScript
The capabilities of ActionScript in SWF files are generally analogous to those
of JavaScript code embedded on HTML pages with some minor, yet interesting, differences. For example, Flash programs are free to enumerate all fonts
installed on a system and collect other useful system fingerprinting signals
not available to normal scripts. Flash programs can also use full screen rendering, facilitating UI spoofing attacks, and they can request access to input
devices such as a camera or a microphone (this requires the user’s consent).
Flash also tends to ignore browser security and privacy settings and uses its
own configuration for mechanisms such as in-plug-in persistent data storage
(although some improvements in this area were announced in May 2011).
The remaining features are less surprising. We’ll discuss the network
and DOM access permissions of Flash applications in more detail in the next
chapter, but in short, by default, every Flash applet can use the browser HTTP
stack (and any ambient credentials managed therein) to talk back to its originating server, request a limited range of subresources from other sites, and
navigate the current browser window or open a new one. ActionScript programs may also negotiate browser-level access to other currently running

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Flash applications and, in some cases, access the DOM of the embedding
page. This last functionality is implemented by injecting eval(...)-like statements into the target JavaScript context.
ActionScript offers fertile ground for web application vulnerabilities.
For example, the getURL(...) and navigateToURL(...) functions, used to navigate the browser or open new windows, are sometimes invoked with attackercontrolled inputs. Such a use is dangerous. Even though javascript: URLs do
not have a special meaning to Flash, the function will pass such strings to the
browser, in some cases resulting in script injection on the embedding site.
Until recently, a related problem was present with other URL-handling
APIs, such as loadMovie(...). Even though the function did not rely on the
browser to load the document, it would recognize an internal asfunction:
scheme, which works similarly to eval(...) and could be trivially leveraged to
perform a call to getURL(...):
asfunction:getURL,javascript:alert('Hi mom!')

The issue with loading scripts from untrusted sources, discussed in
Chapter 6, also has an equivalent in the plug-in word. In Flash, it is very unsafe
to invoke certain functions that affect the state of the ActionScript execution
environment (such as the LoadVars.load(...)) with attacker-controlled URLs,
even if the scheme from which the resource is loaded is http: or https:.
Another commonly overlooked attack surface is the internal, simplified HTML parser offered by the Flash plug-in: Basic HTML markup can be
assigned to properties such as TextField.htmlText and TextArea.htmlText. It is easy
to forget that user-supplied content must be escaped correctly in this setting.
Failure to do so may permit attackers to modify the appearance of the application UI or to inject potentially problematic scripting-oriented links.
Yet another class of Flash-related security bugs may arise due to design
or implementation problems in the plug-in itself. For example, take the
ExternalInterface.call(...) API. It is meant to allow ActionScript to call existing
JavaScript functions on the embedding page and takes two parameters: the
name of the JavaScript function to call and an optional string to be passed to
this routine. While it is understood that the first parameter should not be
attacker controlled, it appears to be safe to put user data in the second one.
In fact, the documentation provides the following code snippet outlining this
specific use case:10
ExternalInterface.call("sendToJavaScript", input.text);

This call will result in the following eval(...) statement being injected on
the embedding page:
try {
__flash__toXML(sendToJavaScript, "value of input.text"));
} catch (e) {
"<undefined/>";
}

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When writing the code behind this call, the authors of the plug-in
remembered to use backslash escaping when outputting the second parameter: hello"world becomes hello\"world. Unfortunately, they overlooked the need
to escape any stray backslash characters, too. Because of this, if the value of
input.text is set to the following string, the embedded script will unexpectedly
execute:
Hello world!\"+alert(1)); } catch(e) {} //

I contacted Adobe about this particular problem in March 2010. Over a
year later, its response was this: “We have not made any change to this behavior for backwards compatibility reasons.”
That seems unfortunate.

Microsoft Silverlight
Microsoft Silverlight is a versatile development platform built on the Windows
Presentation Foundation, a GUI framework that is a part of Microsoft’s .NET
stack. It debuted in 2007 and combines an Extensible Application Markup
Language (XAML)11 (Microsoft’s alternative to Mozilla’s XUL) with code written in one of several managed .NET languages,* such as C# or Visual Basic.
Despite substantial design differences and a more ambitious (and confusing) architecture, this plug-in is primarily meant to compete with Adobe
Flash. Many of the features available to Silverlight applications mirror those
implemented in its competitor, including a nearly identical security model
and a similar eval(...)-based bridge to the embedding page. To Microsoft’s
credit, Silverlight does not come with an equivalent of the asfunction: scheme
or with a built-in HTML renderer, however.
Silverlight is marketed by Microsoft fairly aggressively, and it is bundled
with some editions of Internet Explorer. As a result, depending on the source,
it is believed to have about a 60 to 75 percent desktop penetration.12 Despite
its prevalence, Silverlight is used fairly infrequently to develop actual web
applications, perhaps because it usually offers no compelling advantages over
its more established counterpart or because its architecture is seen as more
contrived and platform-specific. (Netflix, a popular video streaming and
rental service, is one of the very few high-profile websites that actually relies
on Silverlight for playback on some devices.)

Sun Java
Java is a programming language coupled with a platform-independent,
managed-code execution platform. Developed in the early to mid-1990s by
James Gosling for Sun Microsystems, Java has a well-established role as a serverside programming language and a very robust presence in many other niches,
*

Managed code is not executed directly by the CPU (which would be inherently unsafe, because
CPUs are not designed to enforce web security rules). Rather, it is compiled to an intermediate
binary form and then interpreted at runtime by a specialized virtual machine. This approach is
faster than interpreting scripts at runtime and permits custom security policy enforcement as
the program is being executed.

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including mobile devices. Yet, from the beginning, Sun hoped that Java
would also occupy a prominent place on the browser end.
Java in the browser predated Flash and most similar plug-ins, and the
now-obsolete <applet> tag is a testament to how important and unique and
novel this addition must have seemed back in its day. Yet, despite this head
start, the Java language is nearly extinct as an in-browser development platform, and even in its heyday it never enjoyed real prominence. It retains a
remarkable 80 percent installed base, but this high percentage is attributed
largely to the fact that the Java plug-in is bundled with Java Runtime Environment (JRE), a more practically useful and commonly preinstalled component that is required to run normal, desktop Java applications on the system
without any involvement on the browser end.
The reasons for the failure of Java as a browser technology are difficult
to pinpoint. Perhaps it’s due to the plug-in’s poor startup performance, the
clunky UI libraries that made it difficult to develop snappy and user-friendly
web applications, or the history of vicious litigation between Sun and Microsoft
that cast a long shadow over the future of the language on Microsoft’s operating systems.* Whatever the reasons may be, the high install base of Java
coupled with its marginal use means that the risks it creates far outweigh any
potential benefits to the users. (The plug-in had close to 80 security vulnerabilities in 2010,13 and the vendor is commonly criticized for patching such
bugs very slowly.)
Java’s security policies are somewhat similar to those of other plug-ins,
but in some aspects, such as its understanding of the same-origin policy or
its ability to restrict access to the embedding page, it compares unfavorably.
(The next chapter provides an overview of this.) It is also worth noting that
unlike with Flash or Silverlight, certain types of cryptographically signed
applets may request access to potentially dangerous OS features, such as
unconstrained networking or file access, and only a user’s easily coaxed
consent stands in the way.

XML Browser Applications (XBAP)
XML Browser Applications (XBAP)14 is Microsoft’s heavy-handed foray into
the world of web application frameworks, attempted in the years during
which the battle over Java started going sour and before the company
released Silverlight.
XBAP is reminiscent of Silverlight in that it leverages the same Windows
Presentation Foundation and .NET architecture. However, instead of being a
self-contained and snappy browser plug-in, it depends on the large and unwieldy
.NET runtime, in a manner similar to the Java plug-in’s dependence on JRE.
It executes the managed code in a separate process called PresentationHost.exe,
often loading extensive dependencies at initialization time. By Microsoft’s own
admission, the load time of a medium-size previously uncached application
*
The legal battles started in 1997, when Microsoft decided to roll out its own (and in some
ways, superior) version of the Java virtual machine. Sun Microsystems sued, hoping to win an
injunction that would force Microsoft to bundle Sun’s version instead. The two companies initially settled in 2001, but shortly thereafter they headed back to court. In the final settlement in
2004, Sun walked away with $1.6 billion in cash, but Windows users were not getting any Java
runtime at all.

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could easily reach 10 seconds or more. When the technology premiered in
2002, most users were already expecting Internet applications to be far more
responsive than that.
The security model of XBAP applications is poorly documented and has
not been researched to date, perhaps due to XBAP’s negligible real-world
use and obtuse, multilayer architecture. One would reasonably expect that
XBAP’s security properties would parallel the model eventually embraced for
Silverlight, but with broader access to certain .NET libraries and UI widgets.
And, apparently as a result of copying from Sun, XBAP programs can also be
given elevated privileges when loaded from the local filesystem or signed
with a cryptographic certificate.
Microsoft bundled XBAP plug-ins with its .NET framework to the point of
silently installing nonremovable Windows Presentation Foundation plug-ins—
not only in Internet Explorer but also in the competing Firefox and Chrome.
This move stirred some well-deserved controversy, especially once the first
vulnerability reports started pouring in. (Mozilla even temporarily disabled
the plug-in through an automated update to protect its users.) Still, despite
such bold and questionable moves to popularize it, nobody actually wanted
to write XBAP applets, and inch by inch, the technology followed Java into
the dustbin of history.
Eventually, Microsoft appeared to acknowledge this failure and chose to
focus on Silverlight instead. Beginning with Internet Explorer 9, XBAP is disabled by default for Internet-originating content, and the dubious Firefox
and Chrome plug-ins are no longer automatically pushed to users. Nevertheless, it seems reasonable to assume that at least 10 percent of all Internet
users may be still browsing with a complex, partly abandoned, and largely
unnecessary plug-in installed on their machines and will continue to do so
for the next couple of years.

ActiveX Controls
At its core, ActiveX is the successor to Object Linking and Embedding
(OLE), a 1990 technology that made it possible for programs to reuse components of other applications in a standardized, language-independent way.
A simple use case for ActiveX would be a spreadsheet application wishing to
embed an editable vector image from a graphics-editing program or a simple
game that wants to embed a video player.
The idea is not controversial, but by the mid-1990s Microsoft had decided
that ActiveX made sense in the browser, too. After all, wouldn’t websites want
to benefit from the same Windows components that desktop applications could
rely on? The approach violates the idea of nurturing an open, OS-independent
web, but it’s otherwise impressive, as illustrated by the following JavaScript
example that casually creates, edits, and saves an Excel spreadsheet:
var sheet = new ActiveXObject("Excel.Sheet");
sheet.ActiveSheet.Cells(42,42).Value = "Hi mom!";
sheet.SaveAs("c:\\spreadsheet.xls");
sheet.Application.Quit();

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Standards compliance aside, Microsoft’s move to ActiveX proved disastrous from a security standpoint. Many of the exposed ActiveX components
were completely unprepared to behave properly when interacting with
untrusted environments, and over the next 15 years, researchers discovered
several hundred significant security vulnerabilities in web-accessible ActiveX
controls. Heck, the simple observation that Firefox does not support this
technology helped bolster its security image at the onset of the Second
Browser Wars.
Despite this fiasco, Microsoft stood by ActiveX defiantly, investing in gradually limiting the number of controls that could be accessed from the Internet and fixing the bugs in those it considered essential. Not until Internet
Explorer 9 did Microsoft finally decide to let go: Internet Explorer 9 disables
all ActiveX access by default, requiring several extra clicks to use it when needed.
NOTE

The wisdom of delegating the choice to the user is unclear, especially since the permission
granted to a site extends not only to legitimate content on that website but also to any
payloads injected due to application bugs such as XSS. Still, Internet Explorer 9 is
some improvement.

Living with Other Plug-ins
So far, we have covered almost all general-purpose browser plug-ins in use
today. Although there is a long tail of specialized or experimental plug-ins,
their use is fairly insignificant and not something that we need to take into
account when surveying the overall health of the online ecosystem.
Well, with one exception. An unspecified but probably significant
percentage of online users can be expected to have an assortment of webexposed browser plug-ins or ActiveX controls that they never knowingly
installed, or that they were forced to install even though it’s doubtful that
they would ever benefit from the introduced functionality.
This inexcusable practice is sometimes embraced by otherwise reputable
and trusted companies. For example, Adobe forces users who wish to download Adobe Flash to also install GetRight, a completely unnecessary thirdparty download utility. Microsoft does the same with Akamai Download Manager on its developer-oriented website, complete with a hilarious justification
(emphasis mine):15
What is the Akamai Download Manager and why do I have to use it?
To help you download large files with reduced chance of interruption, some downloads require the use of the Akamai Download
Manager.

The primary concern with software installed this way and exposed
directly to malicious input from anywhere on the Internet is that unless it
is designed with extreme care, it is likely to have vulnerabilities (and sure
enough, both GetRight and Akamai Download Manager had some). Therefore, the risks of browsing with a completely unnecessary plug-in that only
served a particular purpose once or twice far outweigh the purported (and
usually unwanted) benefits.
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Security Engineering Cheat Sheet
When Serving Plug-in-Handled Files
 Data from trusted sources: Data from trusted sources is generally safe to host, but remember that security vulnerabilities in Flash, Java, or Silverlight applets, or in the Adobe Reader
JavaScript engine, may impact the security of your domain. Avoid processing user-supplied
URLs and generating or modifying user-controlled HTML from within plug-in-executed
applets. Exercise caution when using the JavaScript bridge.
 User-controlled simple multimedia: User-controlled multimedia is relatively safe to host,
but be sure to validate and constrain the format, use the correct Content-Type, and consult
the cheat sheet in Chapter 13 to avoid security problems caused by content-sniffing flaws.
 User-controlled document formats: These are not inherently unsafe, but they have an
increased risk of contributing security problems due to plug-in design flaws. Consider hosting from a dedicated domain when possible. If you need to authenticate the request to an
isolated domain, do so with a single-use request token instead of by relying on cookies.
 User-controlled active applications: These are unsafe to host in sensitive domains.

When Embedding Plug-in-Handled Files
Always make sure that plug-in content on HTTPS sites is also loaded over HTTPS,* and always
explicitly specify the type parameter on <object> or <embed>. Note that because of the nonauthoritative handling of type parameters, restraint must be exercised when embedding plugin content from untrusted sources, especially on highly sensitive sites.
 Simple multimedia: It is generally safe to load simple multimedia from third-party sources,
with the caveats outlined above.
 Document formats: These are usually safe, but they carry a greater potential for plug-in
and browser content-handling issues than simple multimedia. Exercise caution.
 Flash and Silverlight: In principle, Flash and Silverlight apps can be embedded safely
from external sources if the appropriate security flags are present in the markup. If the
flags are not specified correctly, you may end up tying the security of your site to that of
the provider of the content. Consult the cheat sheet in Chapter 9 for advice.
 Java: Java always ties the security of your service to that of the provider of the content,
because DOM access to the embedding page can’t be reliably restricted. See Chapter 9.
Do not load Java apps from untrusted sites.

If You Want to Write a New Browser Plug-in or ActiveX Component
Unless you are addressing an important, common-use case that will benefit a significant
fraction of the Internet, please reconsider. If you are scratching an important itch, consider
doing it in a peer-reviewed, standardized manner as a part of HTML5.
*
If loading an HTTP-delivered applet on an HTTPS page is absolutely unavoidable, it is safer to place it inside an
intermediate HTTP frame rather than directly inside the HTTPS document, as this prevents the applet-to-JavaScript
bridge from being leveraged for attacks.

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PART II
BROWSER SECURITY
FEATURES

Having reviewed the basic building blocks of the Web,
we can now comfortably examine all the security features that keep rogue web applications at bay. Part II
of this book takes a look at everything from the wellknown but often misunderstood same-origin policy to
the obscure and proprietary zone settings of Internet
Explorer. It explains what these mechanisms can do
for you—and when they tend to fall apart.

CONTENT ISOLATION LOGIC

Most of the security assurances provided by web browsers are meant to isolate documents based on their origin. The premise is simple: Two pages from different
sources should not be allowed to interfere with each
other. Actual practice can be more complicated, however, as no universal agreement exists about where a
single document begins and ends or what constitutes a single origin. The
result is a sometimes unpredictable patchwork of contradictory policies that
don’t quite work well together but that can’t be tweaked without profoundly
affecting all current legitimate uses of the Web.
These problems aside, there is also little clarity about what actions should
be subject to security checks in the first place. It seems clear that some interactions, such as following a link, should be permitted without special restrictions as they are essential to the health of the entire ecosystem, and that others,
such as modifying the contents of a page loaded in a separate window,
should require a security check. But a large gray area exists between these
extremes, and that middle ground often feels as if it’s governed more by a
roll of the dice than by any unified plan. In these murky waters, vulnerabilities such as cross-site request forgery (see Chapter 4) abound.

It’s time to start exploring. Let’s roll a die of our own and kick off the
journey with JavaScript.

Same-Origin Policy for the Document Object Model
The same-origin policy (SOP) is a concept introduced by Netscape in 1995
alongside JavaScript and the Document Object Model (DOM), just one year
after the creation of HTTP cookies. The basic rule behind this policy is
straightforward: Given any two separate JavaScript execution contexts, one
should be able to access the DOM of the other only if the protocols, DNS
names,* and port numbers associated with their host documents match
exactly. All other cross-document JavaScript DOM access should fail.
The protocol-host-port tuple introduced by this algorithm is commonly
referred to as origin. As a basis for a security policy, this is pretty robust: SOP
is implemented across all modern browsers with a good degree of consistency and with only occasional bugs.† In fact, only Internet Explorer stands
out, as it ignores the port number for the purpose of origin checks. This
practice is somewhat less secure, particularly given the risk of having nonHTTP services running on a remote host for HTTP/0.9 web servers (see
Chapter 3). But usually it makes no appreciable difference.
Table 9-1 illustrates the outcome of SOP checks in a variety of situations.
Table 9-1: Outcomes of SOP Checks

NOTE

Originating document

Accessed document

Non–IE browser

Internet Explorer

http://example.com/a/

http://example.com/b/

Access okay

Access okay

http://example.com/

http://www.example.com/

Host mismatch

Host mismatch

http://example.com/

https://example.com/

Protocol mismatch

Protocol mismatch

http://example.com:81/

http://example.com/

Port mismatch

Access okay

This same-origin policy was originally meant to govern access only to the DOM ; that is,
the methods and properties related to the contents of the actual displayed document. The
policy has been gradually extended to protect other obviously sensitive areas of the root
JavaScript object, but it is not all-inclusive. For example, non-same-origin scripts can usually still call location.assign() or location.replace(...) on an arbitrary window or a
frame. The extent and the consequences of these exemptions are the subject of Chapter 11.
*
This and most other browser security mechanisms are based on DNS labels, not on examining the underlying IP addresses. This has a curious consequence: If the IP of a particular host
changes, the attacker may be able to talk to the new destination through the user’s browser, possibly engaging in abusive behaviors while hiding the true origin of the attack (unfortunate, not
very interesting) or interacting with the victim's internal network, which normally would not be
accessible due to the presence of a firewall (a much more problematic case). Intentional change
of an IP for this purpose is known as DNS rebinding. Browsers try to mitigate DNS rebinding to
some extent by, for example, caching DNS lookup results for a certain time (DNS pinning), but
these defenses are imperfect.
†

One significant source of same-origin policy bugs is having several separate URL-parsing
routines in the browser code. If the parsing approach used in the HTTP stack differs from that
used for determining JavaScript origins, problems may arise. Safari, in particular, combated a
significant number of SOP bypass flaws caused by pathological URLs, including many of the
inputs discussed in Chapter 2.

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The simplicity of SOP is both a blessing and a curse. The mechanism
is fairly easy to understand and not too hard to implement correctly, but its
inflexibility can be a burden to web developers. In some contexts, the policy
is too broad, making it impossible to, say, isolate home pages belonging to
separate users (short of giving each a separate domain). In other cases, the
opposite is true: The policy makes it difficult for legitimately cooperating sites
(say, login.example.com and payments.example.com) to seamlessly exchange data.
Attempts to fix the first problem—to narrow down the concept of an
origin—are usually bound to fail because of interactions with other explicit
and hidden security controls in the browser. Attempts to broaden origins or
facilitate cross-domain interactions are more common. The two broadly supported ways of achieving these goals are document.domain and postMessage(...),
as discussed below.

document.domain
This JavaScript property permits any two cooperating websites that share a
common top-level domain (such as example.com, or even just .com) to agree
that for the purpose of future same-origin checks, they want to be considered
equivalent. For example, both login.example.com and payments.example.com may
perform the following assignment:
document.domain = "example.com"

Setting this property overrides the usual hostname matching logic during
same-origin policy checks. The protocols and port numbers still have to match,
though; if they don’t, tweaking document.domain will not have the desired effect.
Both parties must explicitly opt in for this feature. Simply because
login.example.com has set its document.domain to example.com does not mean
that it will be allowed to access content originating from the website hosted
at http://example.com/. That website needs to perform such an assignment,
too, even if common sense would indicate that it is a no-op. This effect is symmetrical. Just as a page that sets document.domain will not be able to access
pages that did not, the action of setting the property also renders the caller
mostly (but not fully!)* out of reach of normal documents that previously
would have been considered same-origin with it. Table 9-2 shows the effects
of various values of document.domain.
Despite displaying a degree of complexity that hints at some special sort
of cleverness, document.domain is not particularly safe. Its most significant
weakness is that it invites unwelcome guests. After two parties mutually set
this property to example.com, it is not simply the case that login.example.com
and payments.example.com will be able to communicate; funny-cat-videos.example
.com will be able to jump on the bandwagon as well. And because of the degree
*
For example, in Internet Explorer, it will still be possible for one page to navigate any other documents that were nominally same-origin but that became “isolated” after setting document.domain,
to javascript: URLs. Doing so permits any JavaScript to execute in the context of such as a pseudoisolated domain. On top of this, obviously nothing stops the originating page from simply setting
its own document.domain to a value identical with that of the target in order to eliminate the boundary. In other words, the ability to make a document non-same-origin with other pages through
document.domain should not be relied upon for anything even remotely serious or security relevant.

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of access permitted between the pages, the integrity of any of the participating JavaScript contexts simply cannot be guaranteed to any realistic extent.
In other words, touching document.domain inevitably entails tying the security
of your page to the security of the weakest link in the entire domain. An
extreme case of setting the value to *.com is essentially equivalent to assisted
suicide.
Table 9-2: Outcomes of document.domain Checks
Originating document

Accessed document

Outcome

URL

document
.domain

URL

document
.domain

http://www.example.com/

example.com

http://payments.example.com/

example.com

http://www.example.com/

example.com

https://payments.example.com/ example.com

http://payments.example.com/

example.com

http://example.com/

(not set)

Access denied

http://www.example.com/

(not set)

http://www.example.com/

example.com

Access denied

Access okay
Protocol
mismatch

postMessage(...)
The postMessage(...) API is an HTML5 extension that permits slightly less
convenient but remarkably more secure communications between non-sameorigin sites without automatically giving up the integrity of any of the parties
involved. Today it is supported in all up-to-date browsers, although because it
is fairly new, it is not found in Internet Explorer 6 or 7.
The mechanism permits a text message of any length to be sent to any
window for which the sender holds a valid JavaScript handle (see Chapter 6).
Although the same-origin policy has a number of gaps that permit similar
functionality to be implemented by other means,* this one is actually safe to
use. It allows the sender to specify what origins are permitted to receive the
message in the first place (in case the URL of the target window has changed),
and it provides the recipient with the identity of the sender so that the integrity of the channel can be ascertained easily. In contrast, legacy methods that
rely on SOP loopholes usually don’t come with such assurances; if a particular action is permitted without robust security checks, it can usually also be
triggered by a rogue third party and not just by the intended participants.
To illustrate the proper use of postMessage(...), consider a case in which a
top-level document located at payments.example.com needs to obtain user login
information for display purposes. To accomplish this, it loads a frame pointing to login.example.com. This frame can simply issue the following command:
parent.postMessage("user=bob", "https://payments.example.com");

*
More about this in Chapter 11, but the most notable example is that of encoding data in URL
fragment identifiers. This is possible because navigating frames to a new URL is not subject to
security restrictions in most cases, and navigation to a URL where only the fragment identifier
changes does not actually trigger a page reload. Framed JavaScipt can simply poll location.hash
and detect incoming messages this way.

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The browser will deliver the message only if the embedding site indeed
matches the specified, trusted origin. In order to securely process this response,
the top-level document needs to use the following code:
// Register the intent to process incoming messages:
addEventListener("message", user_info, false);
// Handle actual data when it arrives:
function user_info(msg) {
if (msg.origin == "https://login.example.com") {
// Use msg.data as planned
}
}

PostMessage(...) is a very robust mechanism that offers significant benefits
over document.domain and over virtually all other guerrilla approaches that
predate it; therefore, it should be used as often as possible. That said, it can
still be misused. Consider the following check that looks for a substring in
the domain name:
if (msg.origin.indexOf(".example.com") != -1) { ... }

As should be evident, this comparison will not only match sites within
example.com but will also happily accept messages from www.example.com
.bunnyoutlet.com. In all likelihood, you will stumble upon code like this more
than once in your journeys. Such is life!
NOTE

Recent tweaks to HTML5 extended the postMessage(...) API to incorporate somewhat
overengineered “ports” and “channels,” which are meant to facilitate stream-oriented
communications between websites. Browser support for these features is currently very
limited and their practical utility is unclear, but from the security standpoint, they do
not appear to be of any special concern.

Interactions with Browser Credentials
As we are wrapping up the overview of the DOM-based same-origin policy, it
is important to note that it is in no way synchronized with ambient credentials, SSL state, network context, or many other potentially security-relevant
parameters tracked by the browser. Any two windows or frames opened in a
browser will remain same-origin with each other even if the user logs out
from one account and logs into another, if the page switches from using a
good HTTPS certificate to a bad one, and so on.
This lack of synchronization can contribute to the exploitability of other
security bugs. For example, several sites do not protect their login forms against
cross-site request forgery, permitting any third-party site to simply submit a
username and a password and log the user into an attacker-controlled account.
This may seem harmless at first, but when the content loaded in the browser
before and after this operation is considered same-origin, the impact of normally ignored “self-inflicted” cross-site scripting vulnerabilities (i.e., ones
where the owner of a particular account can target only himself) is suddenly
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much greater than it would previously appear. In the most basic scenario, the
attacker may first open and keep a frame pointing to a sensitive page on the
targeted site (e.g., http://www.fuzzybunnies.com/address_book.php) and then log
the victim into the attacker-controlled account to execute self-XSS in an
unrelated component of fuzzybunnies.com. Despite the change of HTTP credentials, the code injected in that latter step will have unconstrained access
to the previously loaded frame, permitting data theft.

Same-Origin Policy for XMLHttpRequest
The XMLHttpRequest API, mentioned in this book on several prior occasions,
gives JavaScript programs the ability to issue almost unconstrained HTTP
requests to the server from which the host document originated, and read
back response headers and the document body. The ability to do so would
not be particularly significant were it not for the fact that the mechanism
leverages the existing browser HTTP stack and its amenities, including ambient credentials, caching mechanisms, keep-alive sessions, and so on.
A simple and fairly self-explanatory use of a synchronous XMLHttpRequest
could be as follows:
var x = new XMLHttpRequest();
x.open("POST", "/some_script.cgi", false);
x.setRequestHeader("X-Random-Header", "Hi mom!");
x.send("...POST payload here...");
alert(x.responseText);

Asynchronous requests are very similar but are executed without blocking the JavaScript engine or the browser. The request is issued in the background, and an event handler is called upon completion instead.
As originally envisioned, the ability to issue HTTP requests via this API
and to read back the data is governed by a near-verbatim copy of the sameorigin policy with two minor and seemingly random tweaks. First, the document
.domain setting has no effect on this mechanism, and the destination URL
specified for XMLHttpRequest.open(...) must always match the true origin of the
document. Second, in this context, port number is taken into account in Internet Explorer versions prior to 9, even though this browser ignores it elsewhere.
The fact that XMLHttpRequest gives the user an unprecedented level of
control over the HTTP headers in a request can actually be advantageous to
security. For example, inserting a custom HTTP header, such as X-ComingFrom: same-origin, is a very simple way to verify that a particular request is not
coming from a third-party domain, because no other site should be able to
insert a custom header into a browser-issued request. This assurance is not
very strong, because no specification says that the implicit restriction on crossdomain headers can’t change;* nevertheless, when it comes to web security,
such assumptions are often just something you have to learn to live with.
Control over the structure of an HTTP request can also be a burden,
though, because inserting certain types of headers may change the meaning
of a request to the destination server, or to the proxies, without the browser
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realizing it. For example, specifying an incorrect Content-Length value may
allow an attacker to smuggle a second request into a keep-alive HTTP session
maintained by the browser, as shown here.
var x = new XMLHttpRequest();
x.open("POST", "http://www.example.com/", false);
// This overrides the browser-computed Content-Length header:
x.setRequestHeader("Content-Length", "7");
// The server will assume that this payload ends after the first
// seven characters, and that the remaining part is a separate
// HTTP request.
x.send(
"Gotcha!\n" +
"GET /evil_response.html HTTP/1.1\n" +
"Host: www.bunnyoutlet.com\n\n"
);

If this happens, the response to that second, injected request may be misinterpreted by the browser later, possibly poisoning the cache or injecting content into another website. This problem is especially pronounced if an HTTP
proxy is in use and all HTTP requests are sent through a shared channel.
Because of this risk, and following a lot of trial and error, modern browsers blacklist a selection of HTTP headers and request methods. This is done
with relatively little consistency: While Referer, Content-Length, and Host are
universally banned, the handling of headers such as User-Agent, Cookie, Origin,
or If-Modified-Since varies from one browser to another. Similarly, the TRACE
method is blocked everywhere, because of the unanticipated risk it posed to
httponly cookies—but the CONNECT method is permitted in Firefox, despite
carrying a vague risk of messing with HTTP proxies.
Naturally, implementing these blacklists has proven to be an entertaining exercise on its own. Strictly for your amusement, consider the following
cases that worked in some browsers as little as three years ago:1
XMLHttpRequest.setRequestHeader("X-Harmless", "1\nOwned: Gotcha");

or
XMLHttpRequest.setRequestHeader("Content-Length: 123 ", "");

or simply
XMLHttpRequest.open("GET\thttp://evil.com\tHTTP/1.0\n\n", "/", false);

*
In fact, many plug-ins had problems in this area in the past. Most notably, Adobe Flash permitted
arbitrary cross-domain HTTP headers until 2008, at which point its security model underwent a
substantial overhaul. Until 2011, the same plug-in suffered from a long-lived implementation
bug that caused it to resend any custom headers to an unrelated server following an attackersupplied HTTP 307 redirect code. Both of these problems are fixed now, but discovery-to-patch
time proved troubling.

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NOTE

Cross-Origin Resource Sharing2 (CORS) is a proposed extension to
XMLHttpRequest that permits HTTP requests to be issued across domains and
then read back if a particular response header appears in the returned data. The mechanism changes the semantics of the API discussed in this session by allowing certain
“vanilla” cross-domain requests, meant to be no different from regular navigation, to be
issued via XMLHttpRequest.open(...) with no additional checks; more elaborate
requests require an OPTIONS-based preflight request first. CORS is already available
in some browsers, but it is opposed by Microsoft engineers, who pursued a competing
XDomainRequest approach in Internet Explorer 8 and 9. Because the outcome of
this conflict is unclear, a detailed discussion of CORS is reserved for Chapter 16, which
provides a more systematic overview of upcoming and experimental mechanisms.

Same-Origin Policy for Web Storage
Web storage is a simple database solution first implemented by Mozilla engineers in Firefox 1.5 and eventually embraced by the HTML5 specification.3 It
is available in all current browsers but not in Internet Explorer 6 or 7.
Following several dubious iterations, the current design relies on two
simple JavaScript objects: localStorage and sessionStorage. Both objects offer an
identical, simple API for creating, retrieving, and deleting name-value pairs
in a browser-managed database. For example:
localStorage.setItem("message", "Hi mom!");
alert(localStorage.getItem("message"));
localstorage.removeItem("message");

The localStorage object implements a persistent, origin-specific storage that
survives browser shutdowns, while sessionStorage is expected to be bound to the
current browser window and provide a temporary caching mechanism that is
destroyed at the end of a browsing session. While the specification says that
both localStorage and sessionStorage should be associated with an SOP-like origin (the protocol-host-port tuple), implementations in some browsers do not
follow this advice, introducing potential security bugs. Most notably, in Internet Explorer 8, the protocol is not taken into account when computing the
origin, putting HTTP and HTTPS pages within a shared context. This design
makes it very unsafe for HTTPS sites to store or read back sensitive data
through this API. (This problem is corrected in Internet Explorer 9, but
there appears to be no plan to backport the fix.)
In Firefox, on the other hand, the localStorage behaves correctly, but the
sessionStorage interface does not. HTTP and HTTPS use a shared storage context, and although a check is implemented to prevent HTTP content from
reading keys created by HTTPS scripts, there is a serious loophole: Any key
first created over HTTP, and then updated over HTTPS, will remain visible
to nonencrypted pages. This bug, originally reported in 2009,4 will eventually
be resolved, but when is not clear.

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Security Policy for Cookies
We discussed the semantics of HTTP cookies in Chapter 3, but that discussion left out one important detail: the security rules that must be implemented to protect cookies belonging to one site from being tampered with
by unrelated pages. This topic is particularly interesting because the approach
taken here predates the same-origin policy and interacts with it in a number
of unexpected ways.
Cookies are meant to be scoped to domains, and they can’t be limited
easily to just a single hostname value. The domain parameter provided with
a cookie may simply match the current hostname (such as foo.example.com),
but this will not prevent the cookie from being sent to any eventual subdomains, such as bar.foo.example.com. A qualified right-hand fragment of the
hostname, such as example.com, can be specified to request a broader scope,
however.
Amusingly, the original RFCs imply that Netscape engineers wanted to
allow exact host-scoped cookies, but they did not follow their own advice.
The syntax devised for this purpose was not recognized by the descendants
of Netscape Navigator (or by any other implementation for that matter). To
a limited extent, setting host-scoped cookies is possible in some browsers by
completely omitting the domain parameter, but this method will have no
effect in Internet Explorer.
Table 9-3 illustrates cookie-setting behavior in some distinctive cases.
Table 9-3: A Sample of Cookie-Setting Behaviors
Cookie set at foo.example.com,
domain parameter is:

Scope of the resulting cookie
Non–IE browsers

Internet Explorer

(value omitted)

foo.example.com (exact)

*.foo.example.com

bar.foo.example.com

Cookie not set: domain more specific than origin

foo.example.com

*.foo.example.com

baz.example.com

Cookie not set: domain mismatch

example.com

*.example.com

ample.com

Cookie not set: domain mismatch

.com

Cookie not set: domain too broad, security risk

The only other true cookie-scoping parameter is the path prefix: Any
cookie can be set with a specified path value. This instructs the browser to send
the cookie back only on requests to matching directories; a cookie scoped to
domain of example.com and path of /some/path/ will be included on a request to
http://foo.example.com/some/path/subdirectory/hello_world.txt

This mechanism can be deceptive. URL paths are not taken into account
during same-origin policy checks and, therefore, do not form a useful security boundary. Regardless of how cookies work, JavaScript code can simply hop
between any URLs on a single host at will and inject malicious payloads into
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such targets, abusing any functionality protected with path-bound cookies.
(Several security books and white papers recommend path scoping as a security measure to this day. In most cases, this advice is dead wrong.)
Other than the true scoping features (which, along with cookie name,
constitute a tuple that uniquely identifies every cookie), web servers can also
output cookies with two special, independently operated flags: httponly and
secure. The first, httponly, prevents access to the cookie via the document.cookie
API in the hope of making it more difficult to simply copy a user’s credentials
after successfully injecting a malicious script on a page. The second, secure,
stops the cookie from being submitted on requests over unencrypted protocols, which makes it possible to build HTTPS services that are resistant to
active attacks.*
The pitfall of these mechanisms is that they protect data only against
reading and not against overwriting. For example, it is still possible for JavaScript code delivered over HTTP to simply overflow the per-domain cookie
jar and then set a new cookie without the secure flag.† Because the Cookie
header sent by the browser provides no metadata about the origin of a particular cookie or its scope, such a trick is very difficult to detect. A prominent
consequence of this behavior is that the common “stateless” way of preventing cross-site request forgery vulnerabilities by simultaneously storing a secret
token in a client-side cookie and in a hidden form field, and then comparing
the two, is not particularly safe for HTTPS websites. See if you can figure
out why!
NOTE

Speaking of destructive interference, until 2010, httponly cookies also clashed with
XMLHttpRequest. The authors of that API simply have not given any special
thought to whether the XMLHttpRequest.getResponseHeader(...) function
should be able to inspect server-supplied Set-Cookie values flagged as httponly—
with predictable results.

Impact of Cookies on the Same-Origin Policy
The same-origin policy has some undesirable impact on the security of cookies
(specifically, on the path-scoping mechanism), but the opposite interaction
is more common and more problematic. The difficulty is that HTTP cookies
often function as credentials, and in such cases, the ability to obtain them is
roughly equivalent to finding a way to bypass SOP. Quite simply, with the right
set of cookies, an attacker could use her own browser to interact with the target site on behalf of the victim; same-origin policy is taken out of the picture,
and all bets are off.
*
It does not matter that https://webmail.example.com/ is offered only over HTTPS. If it uses a cookie
that is not locked to encrypted protocols, the attacker may simply wait until the victim navigates
to http://www.fuzzybunnies.com/, silently inject a frame pointing to http://webmail.example.com/ on
that page, and then intercept the resulting TCP handshake. The browser will then send all the
webmail.example.com cookies over an unencrypted channel, and at this point the game is essentially over.
†
Even if this possibility is prevented by separating the jars for httponly and normal cookies,
multiple identically named but differently scoped cookies must be allowed to coexist, and they
will be sent together on any matching requests. They will be not accompanied by any useful
metadata, and their ordering will be undefined and browser specific.

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Because of this property, any discrepancies between the two security mechanisms can lead to trouble for the more restrictive one. For example, the relatively promiscuous domain-scoping rules used by HTTP cookies mean that it is
not possible to isolate fully the sensitive content hosted on webmail.example.com
from the less trusted HTML present on blog.example.com. Even if the owners of
the webmail application scope their cookies tightly (usually at the expense of
complicating the sign-on process), any attacker who finds a script injection
vulnerability on the blogging site can simply overflow the per-domain cookie
jar, drop the current credentials, and set his own *.example.com cookies. These
injected cookies will be sent to webmail.example.com on all subsequent requests
and will be largely indistinguishable from the real ones.
This trick may seem harmless until you realize that such an action may
effectively log the victim into a bogus account and that, as a result, certain
actions (such as sending email) may be unintentionally recorded within that
account and leaked to the attacker before any foul play is noticed. If webmail
sounds too exotic, consider doing the same on Amazon or Netflix: Your casual
product searches may be revealed to the attacker before you notice anything
unusual about the site. (On top of this, many websites are simply not prepared
to handle malicious payloads in injected cookies, and unexpected inputs may
lead to XSS or similar bugs.)
The antics of HTTP cookies also make it very difficult to secure encrypted
traffic against network-level attackers. A secure cookie set by https://webmail
.example.com/ can still be clobbered and replaced by a made-up value set by a
spoofed page at http://webmail.example.com/, even if there is no actual web service listening on port 80 on the target host.

Problems with Domain Restrictions
The misguided notion of allowing domain-level cookies also poses problems
for browser vendors and is a continuing source of misery. The key question is
how to reliably prevent example.com from setting a cookie for *.com and avoid
having this cookie unexpectedly sent to every other destination on the Internet.
Several simple solutions come to mind, but they fall apart when you have
to account for country-level TLDs: example.com.pl must be prevented from setting a *.com.pl cookie, too. Realizing this, the original Netscape cookie specification provided the following advice:
Only hosts within the specified domain can set a cookie for a domain
and domains must have at least two (2) or three (3) periods in them
to prevent domains of the form: “.com”, “.edu”, and “va.us”.
Any domain that fails within one of the seven special top level
domains listed below only requires two periods. Any other domain
requires at least three. The seven special top level domains are:
“COM”, “EDU”, “NET”, “ORG”, “GOV”, “MIL”, and “INT”.

Alas, the three-period rule makes sense only for country-level registrars
that mirror the top-level hierarchy (example.co.uk) but not for the just as populous group of countries that accept direct registrations (example.fr). In fact,
there are places where both approaches are allowed; for example, both
example.jp and example.co.jp are perfectly fine.
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Because of the out-of-touch nature of this advice, most browsers disregarded it and instead implemented a patchwork of conditional expressions
that only led to more trouble. (In one case, for over a decade, you could actually set cookies for *.com.pl.) Comprehensive fixes to country-code top-level
domain handling have shipped in all modern browsers in the past four years,
but as of this writing they have not been backported to Internet Explorer 6
and 7, and they probably never will be.
NOTE

To add insult to injury, the Internet Assigned Numbers Authority added a fair number
of top-level domains in recent years (for example, .int and .biz), and it is contemplating a proposal to allow arbitrary generic top-level domain registrations. If it comes to
this, cookies will probably have to be redesigned from scratch.

The Unusual Danger of “localhost”
One immediately evident consequence of the existence of domain-level scoping of cookies is that it is fairly unsafe to delegate any hostnames within a sensitive domain to any untrusted (or simply vulnerable) party; doing so may
affect the confidentiality, and invariably the integrity, of any cookie-stored
credentials—and, consequently, of any other information handled by the targeted application.
So much is obvious, but in 2008, Tavis Ormandy spotted something far less
intuitive and far more hilarious:5 that because of the port-agnostic behavior of
HTTP cookies, an additional danger lies in the fairly popular and convenient
administrative practice of adding a “localhost” entry to a domain and having
it point to 127.0.0.1.* When Ormandy first published his advisory, he asserted
that this practice is widespread—not a controversial claim to make—and
included the following resolver tool output to illustrate his point:
localhost.microsoft.com has address 127.0.0.1
localhost.ebay.com has address 127.0.0.1
localhost.yahoo.com has address 127.0.0.1
localhost.fbi.gov has address 127.0.0.1
localhost.citibank.com has address 127.0.0.1
localhost.cisco.com has address 127.0.0.1

Why would this be a security risk? Quite simply, it puts the HTTP services
on the user’s own machine within the same domain as the remainder of the
site, and more importantly, it puts all the services that only look like HTTP in
the very same bucket. These services are typically not exposed to the Internet,
so there is no perceived need to design them carefully or keep them up-todate. Tavis’s case in point is a printer-management service provided by CUPS
(Common UNIX Printing System), which would execute attacker-supplied
JavaScript in the context of example.com if invoked in the following way:
http://localhost.example.com:631/jobs/?[...]
&job_printer_uri=javascript:alert("Hi mom!")
*
This IP address is reserved for loopback interfaces; any attempt to connect to it will route you
back to the services running on your own machine.

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The vulnerability in CUPS can be fixed, but there are likely many other
dodgy local services on all operating systems—everything from disk management tools to antivirus status dashboards. Introducing entries pointing back
to 127.0.0.1, or any other destinations you have no control over, ties the security of cookies within your domain to the security of random third-party software. That is a good thing to avoid.

Cookies and “Legitimate” DNS Hijacking
The perils of the domain-scoping policy for cookies don’t end with localhost.
Another unintended interaction is related to the common, widely criticized
practice of some ISPs and other DNS service providers of hijacking domain
lookups for nonexistent (typically mistyped) hosts. In this scheme, instead of
returning the standard-mandated NXDOMAIN response from an upstream
name server (which would subsequently trigger an error message in the
browser or other networked application), the provider will falsify a record to
imply that this name resolves to its site. Its site, in turn, will examine the Host
header supplied by the browser and provide the user with unsolicited, paid
contextual advertising that appears to be vaguely related to her browsing
interests. The usual justification offered for this practice is that of offering a
more user-friendly browsing experience; the real incentive, of course, is to
make more money.
Internet service providers that have relied on this practice include
Cablevision, Charter, Earthlink, Time Warner, Verizon, and many more.
Unfortunately, their approach is not only morally questionable, but it also
creates a substantial security risk. If the advertising site contains any scriptinjection vulnerabilities, the attacker can exploit them in the context of any
other domain simply by accessing the vulnerable functionality through an
address such as nonexistent.example.com. When coupled with the design of
HTTP cookies, this practice undermines the security of any arbitrarily targeted services on the Internet.
Predictably, script-injection vulnerabilities can be found in such hastily
designed advertising traps without much effort. For example, in 2008, Dan
Kaminsky spotted and publicized a cross-site scripting vulnerability on the
pages operated by Earthlink.6
All right, all right: It’s time to stop obsessing over cookies and move on.

Plug-in Security Rules
Browsers do not provide plug-in developers with a uniform and extensible
API for enforcing security policies; instead, each plug-in decides what rules
should be applied to executed content and how to put them into action. Consequently, even though plug-in security models are to some extent inspired
by the same-origin policy, they diverge from it in a number of ways.
This disconnect can be dangerous. In Chapter 6, we discussed the tendency for plug-ins to rely on inspecting the JavaScript location object to determine the origin of their hosting page. This misguided practice forced browser
developers to restrict the ability of JavaScript programs to tamper with some
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portions of their runtime environment to save the day. Another related, common source of incompatibilities is the interpretation of URLs. For example,
in the middle of 2010, one researcher discovered that Adobe Flash had trouble with the following URL:7
http://example.com:80@bunnyoutlet.com/

The plug-in decided that the origin of any code retrieved through this
URL should be set to example.com, but the browser, when presented with such
a URL, would naturally retrieve the data from bunnyoutlet.com instead and then
hand it over to the confused plug-in for execution.
While this particular bug is now fixed, other vulnerabilities of this type
can probably be expected in the future. Replicating some of the URL-parsing
quirks discussed in Chapters 2 and 3 can be a fool’s errand and, ideally,
should not be attempted at all.
It would not be polite to end this chapter on such a gloomy note!
Systemic problems aside, let’s see how some of the most popular plug-ins
approach the job of security policy enforcement.

Adobe Flash
The Flash security model underwent a major overhaul in 2008,8 and since
then, it has been reasonably robust. Every loaded Flash applet is now assigned
an SOP-like origin derived from its originating URL* and is granted nominal
origin-related permissions roughly comparable to those of JavaScript. In
particular, each applet can load cookie-authenticated content from its originating site, load some constrained datatypes from other origins, and make
same-origin XMLHttpRequest-like HTTP calls through the URLRequest API.
The set of permissible methods and request headers for this last API is managed fairly reasonably and, as of this writing, is more restrictive than most of
the browser-level blacklists for XMLHttpRequest itself.9
On top of this sensible baseline, three flexible but easily misused mechanisms permit this behavior to be modified to some extent, as discussed next.
Markup-Level Security Controls
The embedding page can specify three special parameters provided through
<embed> or <object> tags to control how an applet will interact with its host
page and the browser itself:


AllowScriptAccess parameter This setting controls an applet’s ability to
use the JavaScript ExternalInterface.call(...) bridge (see Chapter 8) to execute JavaScript statements in the context of the embedding site. Possible
values are always, never, and sameorigin; the last setting gives access to the
page only if the page is same-origin with the applet itself. (Prior to the
2008 security overhaul, the plug-in defaulted to always; the current
default is the much safer sameorigin.)

*
In some contexts, Flash may implicitly permit access from HTTPS origins to HTTP ones but
not the other way round. This is usually harmless, and as such, it is not given special attention
throughout the remainder of this section.

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

AllowNetworking parameter This poorly named setting restricts an
applet’s permission to open or navigate browser windows and to make
HTTP requests to its originating server. When set to all (the default), the
applet can interfere with the browser; when set to internal, it can perform
only nondisruptive, internal communications through the Flash plug-in.
Setting this parameter to none disables most network-related APIs altogether.* (Prior to recent security improvements, allowNetworking=all
opened up several ways to bypass allowScriptAccess=none, for example, by
calling getURL(...) on a javascript: URL. As of this writing, however, all
scripting URLs should be blacklisted in this scenario.)



AllowFullScreen parameter This parameter controls whether an applet
should be permitted to go into full-screen rendering mode. The possible
values are true and false, with false being the default. As noted in Chapter 8,
the decision to give this capability to Flash applets is problematic due to
UI spoofing risks; it should be not enabled unless genuinely necessary.

Security.allowDomain(...)
The Security.allowDomain(...) method10 allows Flash applets to grant access to
their variables and functions to any JavaScript code or to other applets coming
from a different origin. Buyer beware: Once such access is granted, there is
no reliable way to maintain the integrity of the original Flash execution context. The decision to grant such permissions should not be taken lightly, and
the practice of calling allowDomain("*") should usually be punished severely.
Note that a weirdly named allowInsecureDomain(...) method is also available. The existence of this method does not indicate that allowDomain(...)
is particularly secure; rather, the “insecure” variant is provided for compatibility with ancient, pre-2003 semantics that completely ignored the HTTP/
HTTPS divide.
Cross-Domain Policy Files
Through the use of loadPolicyFile(...), any Flash applet can instruct its runtime
environment to retrieve a security policy file from an almost arbitrary URL.
This XML-based document, usually named crossdomain.xml, will be interpreted as an expression of consent to cross-domain, server-level access to the
origin determined by examining the policy URL.11 The syntax of a policy file
is fairly self-explanatory and may look like this:
<cross-domain-policy>
<allow-access-from domain="foo.example.com"/>
<allow-http-request-headers-from domain="*.example.com"
headers="X-Some-Header" />
</cross-domain-policy>

*
It should not be assumed that this setting prevents any sensitive data available to a rogue applet
from being relayed to third parties. There are many side channels that any Flash applet could
leverage to leak information to a cooperating party without directly issuing network requests. In
the simplest and most universal case, CPU loads can be manipulated to send out individual bits of
information to any simultaneously loaded applet that continuously samples the responsiveness of
its runtime environment.

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The policy may permit actions such as loading cross-origin resources
or issuing arbitrary URLRequest calls with whitelisted headers, through the
browser HTTP stack. Flash developers do attempt to enforce a degree of
path separation: A policy loaded from a particular subdirectory can in principle permit access only to files within that path. In practice, however, the
interactions with SOP and with various path-mapping semantics of modern
browsers and web application frameworks make it unwise to depend on this
boundary.
NOTE

Making raw TCP connections via XMLSocket is also possible and controlled by an
XML policy, but following Flash’s 2008 overhaul, XMLSocket requires that a separate policy file be delivered on TCP port 843 of the destination server. This is fairly safe,
because no other common services run on this port and, on many operating systems,
only privileged users can launch services on any port below 1024. Because of the interactions with certain firewall-level mechanisms, such as FTP protocol helpers, this design
may still cause some network-level interference,12 but this topic is firmly beyond the
scope of this book
As expected, poorly configured crossdomain.xml policies are an appreciable security risk. In particular, it is a very bad idea to specify allow-access-from
rules that point to any domain you do not have full confidence in. Further,
specifying “*” as a value for this parameter is roughly equivalent to executing
document.domain = “com”. That is, it’s a death wish.
Policy File Spoofing Risks
Other than the possibility of configuration mistakes, another security risk
with Adobe’s policy-based security model is that random user-controlled
documents may be interpreted as cross-domain policies, contrary to the site
owner’s intent.
Prior to 2008, Flash used a notoriously lax policy parser, which when
processing loadPolicyFile(...) files would skip arbitrary leading garbage in
search of the opening <cross-domain-policy> tag. It would simply ignore the
MIME type returned by the server when downloading the resource, too. As
a result, merely hosting a valid, user-supplied JPEG image could become a
grave security risk. The plug-in also skipped over any HTTP redirects, making it dangerous to do something as simple as issuing an HTTP redirect to a
location you did not control (an otherwise harmless act).
Following the much-needed revamp of the loadPolicyFile behavior, many
of the gross mistakes have been corrected, but the defaults are still not perfect. On the one hand, redirects now work intuitively, and the file must be a
well-formed XML document. On the other, permissible MIME types include
text/*, application/xml, and application/xhtml+xml, which feels a bit too broad.
text/plain or text/csv may be misinterpreted as a policy file, and that should
not be the case.
Thankfully, to mitigate the problem, Adobe engineers decided to roll
out meta-policies, policies that are hosted at a predefined, top-level location
(/crossdomain.xml) that the attacker can’t override. A meta-policy can specify
sitewide restrictions for all the remaining policies loaded from attacker-supplied

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URLs. The most important of these restrictions is <site-control permitted-crossdomain-policies="...">. This parameter, when set to master-only, simply instructs
the plug-in to disregard subpolicies altogether. Another, less radical value,
by-content-type, permits additional policies to be loaded but requires them to
have a nonambiguous Content-Type header set to text/x-cross-domain-policy.
Needless to say, it’s highly advisable to use a meta-policy that specifies
one of these two directives.

Microsoft Silverlight
If the transition from Flash to Silverlight seems abrupt, it’s because the
two are easy to confuse. The Silverlight plug-in borrows from Flash with
remarkable zeal; in fact, it is safe to say that most of the differences between
their security models are due solely to nomenclature. Microsoft’s platform
uses the same-origin-determination approach, substitutes allowScriptAccess
with enableHtmlAccess, replaces crossdomain.xml with the slightly different
clientaccesspolicy.xml syntax, provides a System.Net.Sockets API instead of
XMLSocket, uses HttpWebRequest in place of URLRequest, rearranges the
flowers, and changes the curtains in the living room.
The similarities are striking, down to the list of blocked request headers
for the HttpWebRequest API, which even includes X-Flash-Version from the Adobe
spec.13 Such consistency is not a problem, though: In fact, it is preferable to
having a brand-new security model to take into account. Plus, to its credit,
Microsoft did make a couple of welcome improvements, including ditching
the insecure allowDomain logic in favor of RegisterScriptableObject, an approach
that allows only explicitly specified callbacks to be exposed to third-party
domains.

Java
Sun’s Java (now officially belonging to Oracle) is a very curious case. Java
is a plug-in that has fallen into disuse, and its security architecture has not
received much scrutiny in the past decade or so. Yet, because of its large
installed base, it is difficult to simply ignore it and move on.
Unfortunately, the closer you look, the more evident it is that the ideas
embraced by Java tend to be incompatible with the modern Web. For example, a class called java.net.HttpURLConnection14 permits credential-bearing
HTTP requests to be made to an applet’s originating website, but the “originating website” is understood as any website hosted at a particular IP address,
as sanctioned by the java.net.URL.equals(...) check. This model essentially
undoes any isolation between HTTP/1.1 virtual hosts—an isolation strongly
enforced by the same-origin policy, HTTP cookies, and virtually all other
browser security mechanisms in use today.
Further along these lines, the java.net.URLConnection class15 allows arbitrary request headers, including Host, to be set by the applet, and another
class, Socket,16 permits unconstrained TCP connections to arbitrary ports
on the originating server. All of these behaviors are frowned upon in the
browser and in any other contemporary plug-in.
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Origin-agnostic access from the applet to the embedding page is provided through the JSObject mechanism and is expected to be controlled by
the embedding party through the mayscript attribute specified in the <applet>,
<embed>, or <object> tags.17 The documentation suggests that this is a security
feature:
Due to security reasons, JSObject support is not enabled in Java
Plug-in by default. To enable JSObject support in Java Plug-in,
a new attribute called MAYSCRIPT needs to be present in the
EMBED/OBJECT tag.

Unfortunately, the documentation neglects to mention that another
closely related mechanism, DOMService,18 ignores this setting and gives applets
largely unconstrained access to the embedding page. While DOMService is
not supported in Firefox and Opera, it is available in other browsers, which
makes any attempt to load third-party Java content equivalent to granting full
access to the embedding site.
Whoops.
NOTE

Interesting fact: Recent versions of Java attempt to copy the crossdomain.xml support
available in Flash.

Coping with Ambiguous or Unexpected Origins
This concludes our overview of the basic security policies and consent isolation mechanisms. If there is one observation to be made, it’s that most of
these mechanisms depend on the availability of a well-formed, canonical
hostname from which to derive the context for all the subsequent operations. But what if this information is not available or is not presented in the
expected form?
Well, that’s when things get funny. Let’s have a look at some of the common corner cases, even if just for fleeting amusement.

IP Addresses
Due to the failure to account for IP addresses when designing HTTP cookies
and the same-origin policy, almost all browsers have historically permitted
documents loaded from, say, http://1.2.3.4/ to set cookies for a “domain”
named *.3.4. Adjusting document.domain in a similar manner would work as
well. In fact, some of these behaviors are still present in older versions of
Internet Explorer.
This behavior is unlikely to have an impact on mainstream web applications, because such applications are not meant to be accessed through an IPbased URL and will often simply fail to function properly. But a handful of
systems, used primarily by technical staff, are meant to be accessed by their IP
addresses; these systems may simply not have DNS records configured at all.
In these cases, the ability for http://1.2.3.4/ to inject cookies for http://123
.234.3.4/ may be an issue. The IP-reachable administrative interfaces of home
routers are of some interest, too.
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Hostnames with Extra Periods
At their core, cookie-setting algorithms still depend on counting the number
of periods in a URL to determine whether a particular domain parameter is
acceptable. In order to make the call, the count is typically correlated with a
list of several hundred entries on the vendor-maintained Public Suffix List
(http://publicsuffix.org/).
Unfortunately for this algorithm, it is often possible to put extra periods
in a hostname and still have it resolve correctly. Noncanonical hostname representations with excess periods are usually honored by OS-level resolvers
and, if honored, will confuse the browser. Although said browser would not
automatically consider a domain such as www.example.com.pl. (with an extra
trailing period) to be the same as the real www.example.com.pl, the subtle and
seemingly harmless difference in the URL could escape even the most attentive users.
In such a case, interacting with the URL with trailing period can be
unsafe, as other documents sharing the *.com.pl. domain may be able to
inject cross-domain cookies with relative ease.
This period-counting problem was first noticed around 1998.19 About a
decade later, many browser vendors decided to roll out basic mitigations by
adding a yet another special case to the relevant code; as of this writing,
Opera is still susceptible to this trick.

Non–Fully Qualified Hostnames
Many users browse the Web with their DNS resolvers configured to append
local suffixes to all found hostnames, often without knowing. Such settings
are usually sanctioned by ISPs or employers through automatic network configuration data (Dynamic Host Configuration Protocol, DHCP).
For any user browsing with such a setting, the resolution of DNS labels
is ambiguous. For example, if the DNS search path includes coredump.cx,
then www.example.com may resolve to the real www.example.com website or to
www.example.com.coredump.cx if such a record exists. The outcomes are partly
controlled by configuration settings and, to some extent, can be influenced
by an attacker.
To the browser, both locations appear to be the same, which may have
some interesting side effects. Consider one particularly perverse case: Should
http://com, which actually resolves to http://com.coredump.cx/, be able to set
*.com cookies by simply omitting the domain parameter?

Local Files
Because local resources loaded through the file: protocol do not have an
explicit hostname associated with them, it’s impossible for the browser to
compute a normal origin. For a very long time, the vendors simply decided
that the best course of action in such a case would be to simply ditch the sameorigin policy. Thus, any HTML document saved to disk would automatically

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be granted access to any other local files via XMLHttpRequest or DOM and, even
more inexplicably, would be able to access any Internet-originating content
in the same way.
This proved to be a horrible design decision. No one expected that the
mere act of downloading an HTML document would put all of the user’s local
files, and his online credentials, in jeopardy. After all, accessing that same
document over the Web would be perfectly safe.
Many browsers have tried to close this loophole in recent years, with varying degrees of success:
Chrome (and, by extension, other WebKit browsers)
The Chrome browser completely disallows any cross-document DOM or
XMLHttpRequest access from file: origins, and it ignores document.cookie
calls or <meta http-equiv="Set-Cookie" ...> directives in this setting. Access to
a localStorage container shared by all file: documents is permitted, but this
may change soon.
Firefox
Mozilla’s browser permits access only to files within the directory of the
original document, as well as nearby subdirectories. This policy is pretty
good, but it still poses some risk to documents stored or previously downloaded to that location. Access to cookies via document.cookie or <meta httpequiv="Set-Cookie" ...> is possible, and all file: cookies are visible to any
other local JavaScript code.* The same holds true for access to storage
mechanisms.
Internet Explorer 7 and above
Unconstrained access to local and Internet content from file: origins is
permitted, but it requires the user to click through a nonspecific warning to execute JavaScript first. The consequences of this action are not
explained clearly (the help subsystem cryptically states that “Internet
Explorer restricts this content because occasionally these programs can malfunction
or give you content you don’t want”), and many users may well be tricked
into clicking through the prompt.
Internet Explorer’s cookie semantics are similar to those of Firefox.
Web storage is not supported in this origin, however.
Opera and Internet Explorer 6
Both of these browsers permit unconstrained DOM or XMLHttpRequest
access without further checks. Noncompartmentalized file: cookies are
permitted, too.
NOTE

Plug-ins live by their own rules in file: land: Flash uses a local-with-filesystem sandbox model,20 which gives largely unconstrained access to the local filesystem, regardless
of the policy enforced by the browser itself, while executing Java or Windows Presentation Framework applets from the local filesystem may in some cases be roughly equivalent to running an untrusted binary.
*

Because there is no compartmentalization between file: cookies, it is unsafe to rely on them for
legitimate purposes. Some locally installed HTML applications ignore this advice, and consequently, their cookies can be easily tampered with by any downloaded, possibly malicious, HTML
document viewed by the user.

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Pseudo-URLs
The behavior of pseudo-URLs such as about:, data:, or javascript: originally
constituted a significant loophole in the implementations of the same-origin
policy. All such URLs would be considered same-origin and would permit
unconstrained cross-domain access from any other resource loaded over the
same scheme. The current behavior, which is very different, will be the topic
of the next chapter of this book; in a nutshell, the status quo reflects several
rounds of hastily implemented improvements and is a complex mix of
browser-specific special cases and origin-inheritance rules.

Browser Extensions and UI
Several browsers permit JavaScript-based UI elements or certain user-installed
browser extensions to run with elevated privileges. These privileges may entail
circumventing specific SOP checks or calling normally unavailable APIs in
order to write files, modify configuration settings, and so on.
Privileged JavaScript is a prominent feature of Firefox, where it is used
with XUL to build large portions of the browser user interface. Chrome also
relies on privileged JavaScript to a smaller but still notable degree.
The same-origin policy does not support privileged contexts in any
specific way. The actual mechanism by which extra privileges are granted
may involve loading the document over a special and normally unreachable
URL scheme, such as chrome: or res:, and then adding special cases for that
scheme in other portions of the browser code. Another option is simply to
toggle a binary flag for a JavaScript context, regardless of its actual origin,
and examine that flag later. In all cases, the behavior of standard APIs such
as localStorage, document.domain, or document.cookie may be difficult to predict
and should not be relied upon: Some browsers attempt to maintain isolation
between the contexts belonging to different extensions, but most don’t.
NOTE

Whenever writing browser extensions, any interaction with nonprivileged contexts must
be performed with extreme caution. Examining untrusted contexts can be difficult,
and the use of mechanisms such as eval(...) or innerHMTL may open up privilegeescalation paths.

Other Uses of Origins
Well, that’s all to be said about browser-level content isolation logic for now.
It is perhaps worth noting that the concept of origins and host- or domainbased security mechanisms is not limited to that particular task and makes
many other appearances in the browser world. Other quasi-origin-based privacy or security features include preferences and cached information related
to per-site cookie handling, pop-up blocking, geolocation sharing, password
management, camera and microphone access (in Flash), and much, much
more. These features tend to interact with the security features described in
this chapter at least to some extent; we explore this topic in more detail soon.

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Security Engineering Cheat Sheet
Good Security Policy Hygiene for All Websites
To protect your users, include a top-level crossdomain.xml file with the permitted-cross-domainpolicies parameter set to master-only or by-content-type, even if you do not use Flash anywhere
on your site. Doing so will prevent unrelated attacker-controlled content from being misinterpreted as a secondary crossdomain.xml file, effectively undermining the assurances of
the same-origin policy in Flash-enabled browsers.

When Relying on HTTP Cookies for Authentication
 Use the httponly flag; design the application so that there is no need for JavaScript to
access authentication cookies directly. Sensitive cookies should be scoped as tightly as
possible, preferably by not specifying domain at all.
 If the application is meant to be HTTPS only, cookies must be marked as secure, and you
must be prepared to handle cookie injection gracefully. (HTTP contexts may overwrite
secure cookies, even though they can’t read them.) Cryptographic cookie signing may
help protect against unconstrained modification, but it does not defend against replacing
a victim’s cookies with another set of legitimately obtained credentials.

When Arranging Cross-Domain Communications in JavaScript
 Do not use document.domain. Rely on postMessage(...) where possible and be sure to
specify the destination origin correctly; then verify the sender’s origin when receiving
the data on the other end. Beware of naïve substring matches for domain names:
msg.origin.indexOf(".example.com") is very insecure.
 Note that various pre-postMessage SOP bypass tricks, such as relying on window.name, are
not tamper-proof and should not be used for exchanging sensitive data.

When Embedding Plug-in-Handled Active Content from Third Parties
Consult the cheat sheet in Chapter 8 first for general advice.
 Flash: Do not specify allowScriptAccess=always unless you fully trust the owner of the originating domain and the security of its site. Do not use this setting when embedding HTTP
applets on HTTPS pages. Also, consider restricting allowFullScreen and allowNetworking as
appropriate.
 Silverlight: Do not specify enableHtmlAccess=true unless you trust the originating domain,
as above.
 Java: Java applets can’t be safely embedded from untrusted sources. Omitting mayscript
does not fully prevent access to the embedding page, so do not attempt to do so.

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When Hosting Your Own Plug-in-Executed Content
 Note that many cross-domain communication mechanisms provided by browser plug-ins
may have unintended consequences. In particular, avoid crossdomain.xml, clientaccesspolicy
.xml, or allowDomain(...) rules that point to domains you do not fully trust.

When Writing Browser Extensions
 Avoid relying on innerHTML, document.write(...), eval(...), and other error-prone coding
patterns, which can cause code injection on third-party pages or in a privileged JavaScript
context.
 Do not make security-critical decisions by inspecting untrusted JavaScript security contexts, as their behavior can be deceptive.

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ORIGIN INHERITANCE

Some web applications rely on pseudo-URLs such as
about:, javascript:, or data: to create HTML documents
that do not contain any server-supplied content and that
are instead populated with the data constructed entirely
on the client side. This approach eliminates the delay
associated with the usual HTTP requests to the server
and results in far more responsive user interfaces.
Unfortunately, the original vision of the same-origin policy did not
account for such a use case. Specifically, a literal application of the protocol-,
host-, and port-matching rules discussed in Chapter 9 would cause every
about:blank document created on the client side to have a different origin
from its parent page, preventing it from being meaningfully manipulated.
Further, all about:blank windows created by completely unrelated websites
would belong to the same origin and, under the right circumstances, would
be able to interfere with each other with no supervision at all.

To address this incompatibility of client-side documents with the sameorigin policy, browsers gradually developed incompatible and sometimes
counterintuitive approaches to computing a synthetic origin and access permissions for pseudo-URLs. An understanding of these rules is important on
its own merit, and it will lay the groundwork for the discussion of certain
other SOP exceptions in Chapter 11.

Origin Inheritance for about:blank
The about: scheme is used in modern browsers for a variety of purposes, most
of which are not directly visible to normal web pages. The about:blank document is an interesting special case, however: This URL can be used to create
a minimal DOM hierarchy (essentially a valid but empty document) to which
the parent document may write arbitrary data later on.
Here is an example of a typical use of this scheme:
<iframe src="about:blank" name="test">


NOTE

In the HTML markup provided in this example, and when creating new windows or
frames in general, about:blank can be omitted. The value is defaulted to when no
other URL is specified by the creator of the parent document.
In every browser, most types of navigation to about:blank result in the creation of a new document that inherits its SOP origin from the page that initiated the navigation. The inherited origin is reflected in the document.domain
property of the new JavaScript execution context, and DOM access to or
from any other origins is not permitted.
This simple formula holds true for navigation actions such as clicking a
link, submitting a form, creating a new frame or a window from a script, or
programmatically navigating an existing document. That said, there are exceptions, the most notable of which are several special, user-controlled navigation
methods. These include manually entering about:blank in the address bar, following a bookmark, or performing a gesture reserved for opening a link in a
new window or a tab.* These actions will result in a document that occupies a
unique synthetic origin and that can’t be accessed by any other page.
Another special case is the loading of a normal server-supplied document that subsequently redirects to about:blank using Location or Refresh. In
Firefox and WebKit-based browsers, such redirection results in a unique, nonaccessible origin, similar to the scenario outlined in the previous paragraph.
In Internet Explorer, on the other hand, the resulting document will be
*
This is usually accomplished by holding CTRL or SHIFT while clicking on a link, or by rightclicking the mouse to access a contextual menu, and then selecting the appropriate option.

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accessible by the parent page if the redirection occurs inside an  but
not if it took place in a separate window. Opera’s behavior is the most difficult to understand: Refresh results in a document that can be accessed by the
parent page, but the Location redirect will give the resulting page the origin
of the site that performed the redirect.
Further, it is possible for a parent document to navigate an existing
document frame to an about:blank URL, even if the existing document shown
in that container has a different origin than the caller.* The newly created
blank document will inherit the origin from the caller in all browsers other
than Internet Explorer. In the case of Internet Explorer, such navigation will
succeed but will result in an inaccessible document. (This behavior is most
likely not intentional.)
If this description makes your head spin, the handling of about:blank documents is summarized in Table 10-1.
Table 10-1: Origin Inheritance for about:blank URLs
Type of navigation

Internet
Explorer

New page

Existing nonsame-origin
page

Location redirect

Refresh redirect

URL entry or
gesture

Inherited
from caller

Unique origin

(Denied)

Frame: Inherited
from parent

Unique
origin

Window:
Unique origin
Firefox

Inherited from caller

Unique origin

All WebKit

Inherited from caller

(Denied)

Unique origin

Opera

Inherited from caller

Inherited from
redirecting party

Inherited from
parent

Inheritance for data: URLs
The data: scheme,1 first outlined in Chapter 2, was designed to permit small
documents, such as icons, to be conveniently encoded and then directly
inlined in an HTML document, saving time on HTTP round-trips. For
example:
<img src="data:image/jpeg;base64,/9j/4AAQSkZJRgABAQEBLAEsAAD...">

When the data: scheme is used in conjunction with type-specific subresources, the only unusual security consideration is that it poses a challenge
for plug-ins that wish to derive permissions for an applet from its originating

*
The exact circumstances that make this possible will be the focus of Chapter 11. For now,
suffice it to say that this can be accomplished in many settings in a browser-specific way. For
example, in Firefox, you call window.open(..., 'target'), while in Internet Explorer, calling
target.location.assign(...) is the way to go.

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167

URL. The origin can’t be computed by looking at the URL alone, and the
behavior is somewhat unpredictable and highly plug-in specific (for example, Adobe Flash currently rejects any attempts to use data: documents).
More important than the case of type-specific content is the use of data:
as a destination for windows and frames. In all browsers but Internet Explorer,
the scheme can be used as an improved variant of about:blank, as in this
example:
<iframe src="data:text/html;charset=utf-8,<h1>Hi mom!</h1>">


In this scenario, there is no compelling reason for a data: URL to behave
differently than about:blank. In reality, however, it will behave differently in
some browsers and therefore must be used with care.


WebKit browsers In Chrome and Safari, all data: documents are given a
unique, nonaccessible origin and do not inherit from the parent at all.



Firefox In Firefox, the origin for data: documents is inherited from the
navigating context, similar to about:blank. However, unlike with about:blank,
manually entering data: URLs or opening bookmarked ones results in
the new document inheriting origin from the page on which the navigation occurred.



Opera As of this writing, a shared “empty” origin is used for all data:
URLs, which is accessible by the parent document. This approach is
unsafe, as it may allow cross-domain access to frames created by unrelated pages, as shown in Figure 10-1. (I reported this behavior to Opera,
and it likely will be amended soon.)



Internet Explorer data: URLs are not supported in Internet Explorer
versions prior to 8. The scheme is supported only for select types of subresources in Internet Explorer 8 and 9 and can’t be used for navigation.
Table 10-2 summarizes the current behavior of data: URLs.

Table 10-2: Origin Inheritance for data: URLs
Type of navigation
New page

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Existing non-sameorigin page

Location
redirect

Refresh
redirect

URL entry or
gesture

Internet
Explorer 6/7

(Not supported)

Internet
Explorer 8/9

(Not supported for navigation)

Firefox

Inherited from caller

Unique origin

Inherited from
previous page

All WebKit

Unique origin

(Denied)

Unique
origin

Unique origin

Opera

Shared origin (This is a bug!)

(Denied)

Inherited
from
parent

Opera
Top-level document: fuzzybunnies.com
frame: data:text/html,...

frame: bunnyoutlet.com

Cross-domain DOM
access possible

frame: data:text/html,...


Figure 10-1: Access between data: URLs in Opera

Inheritance for javascript: and vbscript: URLs
Scripting-related pseudo-URLs, such as javscript:, are a very curious mechanism. Using them to load some types of subresources will lead to code execution in the context of the document that attempts to load such an operation
(subject to some inconsistent restrictions, as discussed in Chapter 4). An
example of this may be


More interestingly (and far less obviously) than the creation of new
subresources, navigating existing windows or frames to javascript: URLs will
cause the inlined JavaScript code to execute in the context of the navigated
page (and not the navigating document!)—even if the URL is entered manually or loaded from a bookmark.
Given this behavior, it is obviously very unsafe to allow one document
to navigate any other non-same-origin context to a javascript: URL, as it
would enable the circumvention of all other content-isolation mechanisms: Just load fuzzybunnies.com in a frame, and then navigate that frame
to javascript:do_evil_stuff() and call it a day. Consequently, such navigation
is prohibited in all browsers except for Firefox. Firefox appears to permit it
for some reason, but it changes the semantics in a sneaky way. When the
origin of the caller and the navigation target do not match, it executes the
javascript: payload in a special null origin, which lacks its own DOM or any of
the browser-supplied I/O functions registered (thus permitting only purely
algorithmic operations to occur).
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169

The cross-origin case is dangerous, but its same-origin equivalent is not:
Within a single origin, any content is free to navigate itself or its peers to
javascript: URLs on its own volition. In this case, the javascript: scheme is honored when following links, submitting forms, calling location.assign(...), and
so on. In WebKit and Opera, Refresh redirection to javascript: will work as well;
other browsers reject such navigation due to vague and probably misplaced
script-injection concerns.
The handling of scripting URLs is outlined in Table 10-3.
Table 10-3: Origin Inheritance for Scripting URLs
Type of navigation

Internet
Explorer
Firefox
All WebKit

Opera

New page

Existing
same-origin
page

Existing
non-sameorigin page

Location
redirect

Refresh
redirect

URL entry
or gesture

Inherited
from caller

Inherited
from
navigated
page

(Denied)

(Denied)

(Denied)

Inherited
from
navigated
page

Null context

(Denied)

(Denied)

Inherited from
navigated
page

(Denied)

Inherited from
navigated
page

On top of these fascinating semantics, there is a yet another twist unique
to the javascript: scheme: In some cases, the handling of such script-containing
URLs involves a second step. Specifically, if the supplied code evaluates properly, and the value of the last statement is nonvoid and can be converted to a
string, this string will be interpreted as an HTML document and will replace
the navigated page (inheriting origin from the caller). The logic governing
this curious behavior is very similar to that influencing the behavior of data:
URLs. An example of such a document-replacing expression is this:
javascript:"2 + 2 = " + (2+2) + ""

A Note on Restricted Pseudo-URLs
The somewhat quirky behavior of the three aforementioned classes of
URLs—about:blank, javascript:, and data:—are all that most websites need to
be concerned with. Nevertheless, browsers use a range of other documents
with no inherent, clearly defined origin (e.g., about:config in Firefox, a privileged JavaScript page that can be used to tweak the browser’s various underthe-hood settings, or chrome://downloads in Chrome, which lists the recently
downloaded documents with links to open any of them). These documents
are a continued source of security problems, even if they are not reachable
directly from the Internet.
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Because of the incompatibility of these URLs with the boundaries controlled by the same-origin policy, special care must be taken to make sure
that these URLs are sufficiently isolated from other content whenever they
are loaded in the browser as a result of user action or some other indirect
browser-level process. An interesting case illustrating the risk is a 2010 bug
in the way Firefox handled about:neterror.2 Whenever Firefox can’t correctly
retrieve a document from a remote server (a condition that is usually easy
to trigger with a carefully crafted link), it puts the destination URL in the
address bar but loads about:neterror in place of the document body. Unfortunately, due to a minor oversight, this special error page would be same-origin
with any about:blank document opened by any Internet-originating content,
thereby permitting the attacker to inject arbitrary content into the
about:neterror window while preserving the displayed destination URL.
The moral of this story? Avoid the urge to gamble with the same-origin
policy; instead, play along with it. Note that making about:neterror a hierarchical URL, instead of trying to keep track of synthetic origins, would have prevented the bug.

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Security Engineering Cheat Sheet
Because of their incompatibility with the same-origin policy, data:, javascript:, and implicit
or explicit about:blank URLs should be used with care. When performance is not critical, it is
preferable to seed new frames and windows by pointing them to a server-supplied blank document with a definite origin first.
Keep in mind that data: and javascript: URLs are not a drop-in replacement for about:blank,
and they should be used only when absolutely necessary. In particular, it is currently unsafe to
assume that data: windows can’t be accessed across domains.

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LIFE OUTSIDE
SAME-ORIGIN RULES

The same-origin policy is the most important mechanism we have to keep hostile web applications at bay,
but it’s also an imperfect one. Although it is meant to
offer a robust degree of separation between any two
different and clearly identifiable content sources, it
often fails at this task.
To understand this disconnect, recall that contrary to what common
sense may imply, the same-origin policy was never meant to be all-inclusive.
Its initial focus, the DOM hierarchy (that is, just the document object exposed
to JavaScript code) left many of the peripheral JavaScript features completely
exposed to cross-domain manipulation, necessitating ad hoc fixes. For example, a few years after the inception of SOP, vendors realized that allowing thirdparty documents to tweak the location.host property of an unrelated window is
a bad idea and that such an operation could send potentially sensitive data
present in other URL segments to an attacker-specified site. The policy has

subsequently been extended to at least partly protect this and a couple of
other sensitive objects, but in some less clear-cut cases, awkward loopholes
remain.
The other problem is that many cross-domain interactions happen
completely outside of JavaScript and its object hierarchy. Actions such as
loading third-party images or stylesheets are deeply rooted in the design of
HTML and do not depend on scripting in any meaningful way. (In principle,
it would be possible to retrofit them with origin-based security controls, but
doing so would interfere with existing websites. Plus, some think that such a
decision would go against the design principles that made the Web what it is;
they believe that the ability to freely cross-reference content should not be
infringed upon.)
In light of this, it seems prudent to explore the boundaries of the sameorigin policy and learn about the rich life that web applications can lead outside its confines. We begin with document navigation—a mechanism that at
first seems strikingly simple but that is really anything but.

Window and Frame Interactions
On the Web, the ability to steer the browser from one website to another
is taken for granted. Some of the common methods of achieving such navigation are discussed throughout Part I of this book; the most notable of
these are HTML links, forms, and frames; HTTP redirects; and JavaScript
window.open(...) and location.* calls.
Actions such as pointing a newly opened window to an off-domain URL
or specifying the src parameter of a frame are intuitive and require no further review. But when we look at the ability of one page to navigate another,
existing document—well, the reign of intuition comes to a sudden end.

Changing the Location of Existing Documents
In the simple days before the advent of HTML frames, only one document
could occupy a given browser window, and only that single window would be
under the document’s control. Frames changed this paradigm, however, permitting several different and completely separate documents to be spliced
into a single logical view, coexisting within a common region of the screen.
The introduction of the mechanism also necessitated another step: To sanely
implement certain frame-based websites, any of the component documents
displayed in a window needed the ability to navigate its neighboring frames
or perhaps the top-level document itself. (For example, imagine a two-frame
page with a table of contents on the left and the actual chapter on the right.
Clicking a chapter name in the left pane should navigate the chapter in the
right pane, and nothing else.)
The mechanism devised for this last purpose is fairly simple: One can
specify the target parameter on  links or forms, or provide the
name of a window to the JavaScript method known as window.open(...), in

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order to navigate any other, previously named document view. In the mid1990s, when this functionality first debuted, there seemed to be no need to
incorporate any particular security checks into this logic; any page could navigate any other named window or a frame displayed by the browser to a new
location at will.
To understand the consequences of this design, it is important to pause
for a moment and examine the circumstances under which a particular document may obtain a name to begin with. For frames, the story is simple: In
order to reference a frame easily on the embedding page, virtually all frames
have a name attribute (and some browsers, such as Chrome, also look at id).
Browser windows, on the other hand, are typically anonymous (that is, their
window.name property is an empty string), unless created programmatically;
in the latter case, the name is specified by whoever creates the view. Anonymous windows do not necessarily stay anonymous, however. If a rogue application is displayed in such a window even briefly, it may set the window.name
property to any value, and this effect will persist.
The aforementioned ability to target windows and frames by name is not
the only way to navigate them; JavaScript programs that hold window handles
pointing to other documents may directly invoke certain DOM methods without knowing the name of their target at all. Attacker-supplied code will not
normally hold handles to completely unrelated windows, but it can traverse
properties such as opener, top, parent, or frames[] in order to locate even distant
relatives within the same navigation flow. An example of such a far-reaching
lookup (and subsequently, navigation) is
opener.opener.frames[2].location.assign("http://www.bunnyoutlet.com/");

These two lookup techniques are not mutually exclusive: JavaScript
programs can first obtain the handle of an unrelated but named window
through window.open(...) and then traverse the opener or frames[] properties
of that context in order to reach its interesting relatives nearby.
Once a suitable handle is looked up in any fashion, the originating context can leverage one of several DOM methods and properties in order to
change the address of the document displayed in that view. In every contemporary browser, calling the .location.replace(...) method, or assigning a
value to .location or .location.href properties, should do the
trick. Amusingly, due to random implementation quirks, other theoretically
equivalent approaches (such as invoking .location.assign(...) or
.window.open(..., "_self")) may be hit-and-miss.
Okay, so it may be possible to navigate unrelated documents to new
locations—but let’s see what could possibly go wrong.
Frame Hijacking Risks
The ability for one domain to navigate windows created by other sites, or
ones that are simply no longer same-origin with their creator, is usually not
a grave concern. This laid-back design may be an annoyance and may pose

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some minor, speculative phishing risk,* but in the grand scheme of things, it
is neither a very pronounced issue nor a particularly distinctive one. This is,
perhaps, the reason why the original authors of the relevant APIs have not
given the entire mechanism too much thought.
Alas, the concept of HTML frames alters the picture profoundly: Any
application that relies on frames to build a trusted user interface is at an obvious risk if an unrelated site is permitted to hijack such UI elements without
leaving any trace of the attack in the address bar! Figure 11-1 shows one such
plausible attack scenario.
Bunny Browser 2000
https://fuzzybunnies.com
Welcome to Fuzzy Bunnies
Online Banking and BBQ!

Bunny Browser 2000
http://bunnyoutlet.com


frame: login.fuzzybunnies.com
Login:
Password:

Login frame can be navigated
to an attacker-supplied URL.

Figure 11-1: A historically permitted, dangerous frame navigation scenario: The window
on the right is opened at the same time as a banking website and is actively subverting it.

Georgi Guninski, one of the pioneering browser security researchers,
realized as early as 1999 that by permitting unconstrained frame navigation,
we were headed for some serious trouble. Following his reports, vendors
attempted to roll out frame navigation restrictions mid-2000.1 Their implementation constrained all cross-frame navigation to the scope of a single
window, preventing malicious web pages from interfering with any other
simultaneously opened browser sessions.
Surprisingly, even this simple policy proved difficult to implement
correctly. It was only in 2008 that Firefox eliminated this class of problems,2
while Microsoft essentially ignored the problem until 2006. Still, these setbacks aside, we should be fine—right?
Frame Descendant Policy and Cross-Domain Communications
The simple security restriction discussed in the previous session was not,
in fact, enough. The reason was a new class of web applications, sometimes
known as mashups, that combined data from various sources to enable users
to personalize their working environment and process data in innovative ways.
Unfortunately for browser vendors, such web applications frequently relied
on third-party gadgets loaded through  tags, and their developers
*
One potential attack is this: Open a legitimate website (say, http://trusted-bank.com/) in a new
window, wait for the user to inspect the address bar, and then quickly change the location to an
attacker-controlled but similarly named site (e.g., http://trustea-bank.com/). The likelihood of
successfully phishing the victim may be higher than when the user is navigating to the bad URL
right away.

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could not reasonably expect that loading a single frame from a rogue source
would put all other frames on the page at risk. Yet, the simple and elegant
window-level navigation policy amounted to permitting exactly that.
Around 2006, Microsoft agreed that the current approach was not sustainable and developed a more secure descendant policy for frame navigation in
Internet Explorer 7. Under this policy, navigation of non-same-origin frames
is permitted only if the party requesting the navigation shares the origin with
one of the ancestors of the targeted view. Figure 11-2 shows the navigation
scenario permitted by this new policy.
Bunny Browser 2000
http://bunnyoutlet.com

frame: bunnyoutlet.com
<script>
window.open("http://bunnyoutlet.com/fakeframe", "private");
</script>

frame: fuzzybunnies.com

Nested frame
navigation
possible

frame “private”: fuzzybunnies.com

Figure 11-2: A complex but permissible navigation between non-same-origin frames.
This attempt succeeds only because the originating frame has the same origin as one
of the ancestors of the targeted document—here, it’s the top-level page itself.

As with many other security improvements, Microsoft never backported
this policy to the still popular Internet Explorer 6, and it never convincingly
pressured users to abandon the older and increasingly insecure (but still
superficially supported) version of its browser. On a more positive note, by
2009, three security researchers (Adam Barth, Collin Jackson, and John C.
Mitchell) convinced Mozilla, Opera, and WebKit to roll out a similar policy
in their browsers,3 finally closing the mashup loophole for a good majority
of the users of the Internet.
Well, almost closing it. Even the new, robust policy has a subtle flaw.
Notice in Figure 11-2 that a rogue site, http://bunnyoutlet.com/, can interfere
with a private frame that http://fuzzybunnies.com/ has created for its own use.
At first glance, there is no harm here: The attacker’s domain is shown in the
address bar, so the victim, in theory, should not be fooled into interacting
with the subverted UI of http://fuzzybunnies.com/ in any meaningful way. Sadly,
there is a catch: Some web applications have learned to use frames not to
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create user interfaces but to relay programmatic messages between origins.
For applications that need to support Internet Explorer 6 and 7, where
postMessage(...) is not available, the tricks similar to the approach shown
in Figure 11-3 are commonplace.
Bunny Browser 2000
http://www.fuzzybunnies.com
// Step 1: send message to login.fuzzybunnies.com
// This is permitted because the send_to_child frame is a descendant of this document.
frames["send_to_child"].src = "http://login.fuzzybunnies.com/login_handler#" + message_to_send;

frame “send_to_child”: login.fuzzybunnies.com/login_handler#
// Step 2: read message sent in step 1.
// It is always possible to examine your own fragment ID.
response_text = process_message_from_parent(location.hash);
// Step 3: send response to www.fuzzybunnies.com.
// This is permitted because send_to_parent is a descendant of this document.
frames["send_to_parent"].location = "http://www.fuzzywunnies.com/blank#" + response_text

frame “send_to_parent”: www.fuzzybunnies.com/blank#

// Step 4: read back data from login.fuzzybunnies.com.
// This is permitted because the send_to_parent frame is same-origin with this document.
process_message_from_child(frames["send_to_parent"].location.hash);

Figure 11-3: A potential cross-domain communication scheme, where the top-level
page encodes messages addressed to the embedded gadget in the fragment identifier of the gadget frame and the gadget responds by navigating a subframe that is
same-origin with the top-level document. If this application is framed on a rogue site,
the top-level document controlled by the attacker will be able to inject messages
between the two parties by freely navigating send_to_parent and send_to_child.

If an application that relies on a similar hack is embedded by a rogue
site, the integrity of the communication frames may be compromised, and
the attacker will be able to inject messages into the stream. Even the uses of
postMessage(...) may be at risk: If the party sending the message does not specify a destination origin or if the recipient does not examine the originating
location, hijacking a frame will benefit the attacker in exactly the same way.

Unsolicited Framing
The previous discussion of cross-frame navigation highlights one of the more
interesting weaknesses in the browser security model, as well as the disconnect between the design goals of HTML and the aim of the same-origin policy. But that’s not all: The concept of cross-domain framing is, by itself, fairly
risky. Why? Well, any malicious page may embed a third-party application without a user’s knowledge, let alone consent. Further, it may obfuscate this fact by
overlaying other visual elements on top of the frame, leaving visible just a small
chunk of the original site, such as a button that performs a state-changing
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action. In such a setting, any user logged into the targeted application with
ambient credentials may be easily tricked into interacting with the disguised
UI control and performing an undesirable and unintended action, such as
changing sharing settings for a social network profile or deleting data.
This attack can be improved by the rogue site leveraging a CSS2 property
called opacity to make the targeted frame completely invisible without affecting
its actual behavior. Any click in the area occupied by such a see-through frame
will be delivered to the UI controls contained therein (see Figure 11-4). Too,
by combining CSS opacity with JavaScript code to make the frame follow the
mouse pointer, it is possible to carry out the attack fairly reliably in almost
any setting: Convincing the user to click anywhere in the document window
is not particularly hard.

Figure 11-4: A simplified example of a UI-splicing attack that
uses CSS opacity to hide the document the user will actually
interact with

Researchers have recognized the possibility of such trickery to some
extent since the early 2000s, but a sufficiently convincing attack wasn’t demonstrated until 2008, when Robert Hansen and Jeremiah Grossman publicized the issue broadly.4 Thus, the term clickjacking was born.
The high profile of Hansen and Grossman’s report, and their interesting
proof-of-concept example, piqued vendors’ interest. This interest proved to
be short-lived, however, and there appears to be no easy way to solve this
problem without taking some serious risks. The only even remotely plausible
way to mitigate the impact would be to add renderer-level heuristics to disallow event delivery to cross-domain frames that are partly obstructed or that
have not been displayed long enough. But this solution is complicated and
hairy enough to be unpopular.5 Instead, the problem has been slapped with
a Band-Aid. A new HTTP header, X-Frame-Options, permits concerned sites to
opt out of being framed altogether (X-Frame-Options: deny) or consent only to
framing within a single origin (X-Frame-Options: same-origin).6 This header

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is supported in all modern browsers (in Internet Explorer, beginning with
version 8),* but it actually does little to address the vulnerability.
Firstly, the opt-in nature of the defense means that most websites will
not adopt it or will not adopt it soon enough; in fact, a 2011 survey of the top
10,000 destinations on the Internet found that barely 0.5 percent used this
feature.7
To add insult to injury, the proposed mechanism is useless for applications that want to be embedded on third-party sites but that wish to preserve
the integrity of their UIs. Various mashups and gadgets, those syndicated
“like” buttons provided by social networking sites, and managed online discussion interfaces are all at risk.
Beyond the Threat of a Single Click
As the name implies, the clickjacking attack outlined by Grossman and
Hansen targets simple, single-click UI actions. In reality, however, the problem with deceptive framing is more complicated than the early reporting
would imply. One example of a more complex interaction is the act of selecting, dragging, and dropping a snippet of text. In 2010, Paul Stone proposed
a number of ways in which such an action could be disguised as a plausible
interaction with an attacker’s site,8 the most notable of which is the similarity
between drag-and-drop and the use of a humble document-level scrollbar.
The same click-drag-release action may be used to interact with a legitimate
UI control or to unwittingly drag a portion of preselected text out of a sensitive document and drop it into an attacker-controlled frame. (Cross-domain
drag-and-drop is no longer permitted in WebKit, but as of this writing other
browser vendors are still debating the right way to address this risk.)
An even more challenging problem is keystroke redirection. Sometime
in 2010, I noticed that it was possible to selectively redirect keystrokes across
domains by examining the code of a pressed key using the onkeydown event in
JavaScript. If the pressed key matched what a rogue site wanted to enter into
a targeted application, HTML element focus could be changed momentarily
to a hidden <iframe>, thereby ensuring the delivery of the actual keystrokes to
the targeted web application rather than the harmless text field the user seems
to be interacting with.9 Using this method, an attacker can synthesize arbitrarily complex text in another domain on the user’s behalf—for example,
inviting the attacker as an administrator of the victim’s blog.
Browser vendors addressed the selective keystroke redirection issue by
disallowing element focus changes in the middle of a keypress, but doing so
did not close the loophole completely. After all, in some cases, an attacker
can predict what key will be pressed next and roughly at what time, thereby
permitting a preemptive, blindly executed focus switch. The two most obvious cases are a web-based action game or a typing-speed test, since both typically involve rapid pressing of attacker-influenced keys.
*
In older versions of Internet Explorer, web application developers sometimes resort to JavaScript in an attempt to determine whether the window object is the same as parent, a condition
that should be satisfied if no higher-level frame is present. Unfortunately, due to the flexibility of
JavaScript DOM, such checks, as well as many types of possible corrective actions, are notoriously
unreliable.

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In fact, it gets better: Even if a malicious application only relies on freeform text entry—for example, by offering the user a comment-submission
form—it’s often possible to guess which character will be pressed next based
on the previous few keystrokes alone. English text (and text in most other
human languages) is highly redundant, and in many cases, a considerable
amount of input can be predicted ahead of time: You can bet that a-a-r-d-v
will be followed by a-r-k, and almost always you will be right.

Cross-Domain Content Inclusion
Framing and navigation are a distinct source of trouble, but these mechanisms aside, HTML supports a number of other ways to interact with nonsame-origin data. The usual design pattern for these features is simple and
seemingly safe: A constrained data format that will affect the appearance of
the document is retrieved and parsed without being directly shown to the origin that referenced it. Examples of mechanisms that follow this rule include
markup such as <script src=...>, <link rel=stylesheet href=...>, <img src=...>, and
several related cases discussed throughout Part I of this book.
Regrettably, the devil is in the details. When these mechanisms were first
proposed, nobody asked several extremely pressing questions:






Should these subresources be requested with ambient credentials associated with their origin? If so, there is a danger that the response may contain sensitive data not intended for the requesting party. It would probably
be better to require some explicit form of authentication or to notify the
server about the origin of the requesting page.
Should the relevant parsers be designed to minimize the risk of mistaking one document type for another? And should the servers have
control over how their responses are interpreted (for example through
the Content-Type header)? If not, what are the consequences of, say, interpreting a user’s private JPEG image as a script?
Should the requesting page have no way to infer anything about the
contents of the retrieved payloads? If yes, then this goal needs to be
taken into account with utmost care when designing all the associated
APIs. (If such separation is not a goal, the importance of the previous
questions is even more pronounced.)

The developers acted with conflicting assumptions about these topics, or
perhaps had not given them any thought at all, leading to a number of profound security risks. For example, in most browsers, it used to be possible to
read arbitrary, cookie-authenticated text by registering an onerror handler on
cross-domain <script> loads: The verbose “syntax error” message generated by
the browser would include a snippet of the retrieved file. Still, no problem in
this category is more interesting than a glitch discovered by Chris Evans in
2009.10 He noticed that the hallmark fault tolerance of CSS parsers (which,
as you may recall, recover from syntax errors by attempting to resynchronize
at the nearest curly bracket) is also a fatal security flaw.
In order to understand the issue, consider the following simple HTML
document. This document contains two occurrences of an attacker-controlled
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string, and—sandwiched in between—a sensitive, user-specific value (in this
case, a user’s name):
<head>
<title>Page not found: ');} gotcha { background-image: url('/</title>
</head>
<body>
...
<span class="header">You are logged in as: John Doe</span>
...
<div class="error_message">
Page not found: ');} gotcha { background-image: url('/
</div>
...
</body>

Let’s assume that the attacker lured the victim to his own page and, on
this page, used <link rel=stylesheet> to load the aforementioned cross-domain
HTML document in place of a stylesheet. The victim’s browser will happily
comply: It will request the document using the victim’s cookies, will ignore
Content-Type on the subsequent response, and will hand the retrieved content
over to the CSS parser. The parser will cheerfully ignore all syntax errors
leading up to what appears to be a CSS rule named gotcha. It will then process
the url('... pseudo-function, consuming all subsequent HTML (including the
secret user name!), until it reaches a matching quote and a closing parenthesis. When this faux stylesheet is later applied to a class=gotcha element on the
attacker’s website, the browser will attempt to load the resulting URL and will
leak the secret value to the attacker’s server in the process.
Astute readers may note that the CSS standard does not support multiline string literals, and as such, this trick would not work as specified. That’s
partly true: In most browsers, the attempt will succeed only if the critical segment of the page contains no stray newlines. Some web applications are optimized to avoid unnecessary whitespaces and therefore will be vulnerable, but
most web developers use newlines liberally, thwarting the attack. Alas, as noted
in Chapter 5, one browser behaves differently: Internet Explorer accepts
multiline strings in stylesheets and many other egregious syntax violations,
accidentally amplifying the impact of this flaw.
NOTE

Since identifying this problem, Chris Evans has pushed for fixes in all mainstream browsers, and as of this writing, most implementations reject cross-domain stylesheets that don’t
begin right away with a valid CSS rule or that are served with an incompatible ContentType header (same-origin stylesheets are treated less restrictively). The only vendor to
resist was Microsoft, which changed its mind only after a demonstration of a successful
proof-of-concept attack against Twitter.11 Following this revelation, Microsoft agreed not
only to address the problem in Internet Explorer 8 but also—uncharacteristically—to
backport this particular fix to Internet Explorer 6 and 7 as well.
Thanks to Chris’s efforts, stylesheets are a solved problem, but similar
problems are bound to recur for other types of cross-domain subresources.
In such cases, not all transgressions can be blamed on the sins of the old. For

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example, when browser vendors rolled out <canvas>, a simple HTML5 mechanism that enables JavaScript to create vector and bitmap graphics,12 many
implementations put no restrictions on loading cross-domain images onto
the canvas and then reading them back pixel by pixel. As of this writing, this
issue, too, has been resolved: A canvas once touched by a cross-domain image
becomes “tainted” and can only be written to, not read. But when we need
to fix each such case individually, something is very wrong.

A Note on Cross-Origin Subresources
So far, we have focused on the risks of malicious websites navigating or
including content that belongs to trusted parties. That said, the ability to
load certain types of subresources from other origins has significant consequences, even if not actively subverted by a third-party site.
In Part I of the book, we hinted that loading a script or a stylesheet
from another origin effectively equates the security of the document that
performs the load to the security of the origin of the loaded subresource; in
particular, loading an HTTP script on an HTTPS page undoes most of the
benefits of encryption. Similarly, loading a script from a provider whose
infrastructure is vulnerable to attack can be nearly as problematic as not
properly maintaining your own servers.
In addition to scripts and stylesheets, other content types that may lead
to serious trouble include remote fonts (a recent addition to CSS) and plugins with access to the embedding page (such as allowScriptAccess=always for
Flash). It is also somewhat dangerous to load images, icons, cursors, or HTML
frames from untrusted sources, although the impact of doing so is contained
to some extent and will be use specific.
Contemporary browsers attempt to detect cases where HTTPS documents
load HTTP resources—a condition known as mixed content. They do so fairly
inconsistently, however: Internet Explorer is the only browser that blocks most
types of mixed content by default (and Chrome is expected to follow suit), but
neither Internet Explorer nor Firefox nor Opera consistently detects mixed
content on <embed>, <object>, or <applet> tags. In browsers other than Internet
Explorer, the default action is a subtle warning (for example, an exclamation
mark next to the lock icon) or a cryptic dialog, which does very little to protect the user but which may alert a sufficiently attentive web developer.
As to the other flavor of mixed content—loading subresources across
domains that offer different levels of trust—browsers have no way to detect
this. The decision to include content from dubious sources is often made too
lightly and such mistakes can be difficult to spot until too late.
NOTE

Another interesting problem with cross-domain subresources is that they may request
certain additional permissions or credentials from the browser. The associated browser
security prompts are usually not designed with such scenarios with mind, and they do
not always make sufficiently clear which origin is requesting the permission and based
on what sort of relationship with the top-level site. We discussed one such problem in
Chapter 3: the authentication prompt shown in response to HTTP code 401. Several
other, related cases will appear in Chapter 15.
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Privacy-Related Side Channels
Another unfortunate and noteworthy consequence of the gaps in the sameorigin policy is the ability to collect information about a user’s interaction
with unrelated sites. Some of the most rudimentary examples, most of them
known for well over a decade,13 include the following:


Using onload handlers to measure the time it takes to load certain documents, an indication of whether they have been previously visited and
cached by the browser or not.14



Using onload and onerror on <img> tags to see if an authentication-requiring image on a third-party site can be loaded, thus disclosing whether
the user is logged into that site or not. (Bonus: Sometimes, the error
message disclosed to the onerror handler will include snippets of the targeted page, too.)



Loading an unrelated web application in a hidden frame and examining
properties such as the number and names of subframes created on that
page (available through the <handle>.frames[] array) or the set of global
variables (sometimes leaked through the semantics of the delete operator) in order to detect the same. Naturally, the set of sites the user visits
or is logged into can be fairly sensitive.

In addition to these tricks, a particularly frightening class of privacy problems is associated with two APIs created several years ago to help websites understand the style applied to any document element (the sum of browser-specific
defaults, CSS rules, and any runtime tweaks made automatically by the browser
or performed via JavaScript). The two APIs in question are getComputedStyle,
mandated by CSS Level 2,15 and currentStyle, proprietary to Internet Explorer.16
Their functionality, together with the ability to assign distinctive styling to
visited links (using the :visited pseudo-class), means that any rogue JavaScript
can rapidly display and examine thousands of URLs to see which ones are
shaded differently (due to being present in a user’s browsing history), thereby
building a reliable, extensive, and possibly incriminating overview of a user’s
online habits with unprecedented efficiency and reliability.
This problem has been known since at least since 2002, when Andrew
Clover posted a brief note about it to the popular BUGTRAQ mailing list.17
The issue received little scrutiny in the following years, until a series of
layperson-targeted demonstrations and a subsequent public outcry around
2006. A few years later, Firefox and WebKit browsers rolled out security
improvements to limit the extent of styling possible in :visited selectors
and to limit the ability to inspect the resulting composite CSS data.
That said, such fixes will never be perfect. Even though they make
automated data collection impossible, smaller quantities of data can be
obtained with a user’s help. Case in point: Collin Jackson and several other
researchers proposed a simple scheme that involved presenting a faux

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CAPTCHA* consisting of seven-segment, LCD-like digits.18 Rather than being
an actual, working challenge, the number the user would see depended on
the :visited-based styling applied to superimposed links (see Figure 11-5); by
typing that number back onto the page, the user would unwittingly tell the
author of the site what exact styling had been applied and, therefore, what
sites appeared in the victim’s browsing history.
Vertical pipe character (|) linked
to www.fuzzybunnies.com
(white if visited)

Segment linked to
www.bunnyoutlet.com
(white if visited)

Figure 11-5: A fake seven-segment display can be used to read
back link styling when the displayed number is entered into the
browser in an attempt to solve a CAPTCHA. The user will see
5, 6, 9, or 8, depending on prior browsing history.

Other SOP Loopholes and Their Uses
Although this chapter has focused on areas where the limitations of the
same-origin policy have a clear, negative impact on the security or privacy of
online browsing, there are several accidental gaps in the scheme that in most
cases seem to be of no special consequence. For example, in many versions
of Internet Explorer, it was possible to manipulate the value of window.opener
or window.name of an unrelated window. Meanwhile in Firefox, there are currently no constraints on setting location.hash across domains, even though all
other partial location properties are restricted.
The primary significance of these mechanisms is that they are often
repurposed to build cross-domain communication channels in browsers that
do not support the postMessage(...) API. Such mechanisms are often built on
shaky ground: The lack of SOP enforcement is typically uniform and means
that any website, not just the “authorized” parties, will be able to interfere with
the data. The ability for rogue parties to navigate nested frames, as discussed
in “Frame Hijacking Risks” on page 175, further complicates the picture.
*
CAPTCHA (sometimes expanded as Completely Automated Public Turing test to tell Computers and Humans Apart) is a term for a security challenge that is believed to be difficult to
solve using computer algorithms but that should be easy for a human being. It is usually implemented by showing an image of several randomly selected, heavily distorted characters and asking the user to type them back. CAPTCHA may be used to discourage the automation of certain
tasks, such as opening new accounts or sending significant volumes of email. (Needless to say,
due to advances in computer image processing, robust CAPTCHAs are increasingly difficult for
humans to solve, too.)

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Security Engineering Cheat Sheet
Good Security Hygiene for All Websites
 Serve all content for your site with X-Frame-Options: sameorigin. Make case-by-case exceptions only for specific, well-understood locations that require cross-domain embedding.
Try not to depend on JavaScript “framebusting” code to prevent framing because it’s very
tricky to get that code right.
 Return user-specific, sensitive data that is not meant to be loaded across domains using
well-constrained formats that are unlikely to be misinterpreted as standalone scripts,
stylesheets, and so on. Always use the right Content-Type.

When Including Cross-Domain Resources
 In many scenarios (especially when dealing with scripts, stylesheets, fonts, and certain
types of plug-in-handled content), you are linking the security of your site to the originating domain of the subresource. When in doubt, make a local copy of the data instead. On
HTTPS sites, require all subresources to be served over HTTPS.

When Arranging Cross-Domain Communications in JavaScript
 Consult the cheat sheet in Chapter 9. Do not use cross-frame communication schemes
based on location.hash, window.name, frameElements, and similar ephemeral hacks, unless
you are prepared to deal with injected content.
 Do not expect subframes on your page to sit still, especially if you are not using X-FrameOptions to limit the ability of other sites to frame your application. In certain cases, an
attacker may be able to navigate such frames to a different location without your knowledge or consent.

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OTHER SECURITY
BOUNDARIES

All previously described origin-level content-isolation
policies, and the accompanying context inheritance
and document navigation logic, work hand in hand to
form the bulk of the browser security model. Impenetrable and fragile, that model is also incomplete: A
handful of interesting corner cases completely escape
any origin-based frameworks.
The security risks associated with these corner cases can’t be addressed
simply by fine-tuning the mechanisms discussed earlier in this book. Instead,
additional, sometimes hopelessly imperfect security boundaries need to be
created from scratch. These new boundaries may, for example, further
restrict the ability of rogue web pages to navigate to certain URLs.
This chapter offers a quick look at some of the most significant examples
of the loopholes in the origin-based model and the ways that vendors have
dealt with them.

Navigation to Sensitive Schemes
In the past, browser vendors reasoned that there was no harm in allowing
any page on the Internet to navigate to a document stored on a user’s hard
drive using the file: protocol or to open a new window pointing to a privileged resource, such as the about:config page in Firefox. After all, they thought,
the originating document and the destination would not be same-origin,
and, therefore, any direct access to the sensitive data would be prevented.
For many years, based on this rationale, browsers permitted such navigation to take place. Alas, this decision proved to be not only extremely confusing* but also dangerous. The danger comes from the fact that many programs,
browsers included, tend to store various types of Internet-originating content
in the filesystem; temporary files and cached documents are a common example. In many cases, an attacker could have some control over the creation
and contents of such files, and, if the resources are created at a predictable
location, subsequent navigation to the right file: URL could allow the attacker
to execute his own payload in this coveted origin, with access to any other
file on the disk and, perhaps, any other website on the Internet.
Comparably disastrous consequences have been observed with a variety
of privileged, internally handled URLs. The ability to navigate directly to
locations such as about:config (Firefox) not only made it possible to exploit
potential vulnerabilities in the privileged scripts (a transgression to which
browser vendors are not immune) but also led to system compromise if,
through a literal application of the same-origin policy, the browser naïvely
deemed about:config and about:blank to come from the same origin.
Having learned from a history of painful mishaps, modern browsers typically police navigation based on three tiers of URL schemes:


Unrestricted This category includes virtually all true network protocols,
such as HTTP, HTTPS, FTP; most encapsulating pseudo-protocols such
as mhtml: or jar:; and all schemes registered to plug-ins and external applications. Navigation to these URLs is not constrained in any specific way.



Partly restricted This category includes several security-sensitive schemes
such as file: and special pseudo-URLs such as javascript: or vbscript:. Navigation to them is not completely denied, but it is subject to additional,
scheme-specific security checks. For example, access to file: is usually
permitted only from other file: documents, requiring the first one to be
opened manually. (The rules for navigation to javascript: URLs were discussed in Chapter 10.)



Fully restricted This category includes privileged pages in about:, res:,
chrome:, and similar browser-specific namespaces. Normal, unprivileged
HTML documents are not permitted to navigate to them under any
circumstance.

*
For example, on Windows systems, a common prank was to use a seamlessly embedded <iframe>
pointing to file:///c:/ in order to display the contents of a victim’s hard drive, leading some users
to believe that the page doing so has somehow gained access to their files.

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Access to Internal Networks
The trouble with accessing sensitive protocols is merely a prelude to a far
more serious issue that somehow escaped the creators of the same-origin
policy. The problem is that DNS-derived origins may have nothing to do with
actual network-level boundaries—or with how these boundaries change over
time. A malicious script may be granted same-origin access to intranet sites
on the victim’s local network, even if a firewall prevents the attacker from
interacting with these destinations directly.
There are at least three distinctive venues for such attacks.
Origin Infiltration
When a user visits a rogue network—such as an open wireless network at
an airport or in a café—an attacker on that network may trick the victim’s
browser into opening a URL such as http://us-payroll/. When this happens,
the attacker may provide his own, spoofed content for that site. Frighteningly, if the user then brings the same browser to a corporate network,
the previously injected content will have same-origin access to the real
version of http://us-payroll/, complete with the user’s ambient credentials.
The persistence of injected content may be achieved in a couple of
ways. The most basic method is for an attacker simply to inject a hidden
http://us-payroll/ frame onto every visited page in the hope that the user
will suspend a portable computer with the browser still running and then
take it to another network. Another technique is cache poisoning : creating
long-lived, cached objects that the browser will use instead of retrieving
a fresh copy from the destination site. Several other, more obscure
approaches also exist.
DNS Rebinding
This arguably less serious but more easily exploitable problem was mentioned in footnote 1 in Chapter 9. In short, since the same-origin policy
looks just at the DNS name of a host, not at the IP address, an attacker
who owns bunnyoutlet.com is free to respond initially to a DNS lookup
from a user with a public IP such as 213.134.128.25 and then switch to
an address reserved for private networks, such as 10.0.0.1. Documents
loaded from both sources will be considered same-origin, giving the
attacker the ability to interact with the victim’s internal network.
The mitigating factor is that this interaction will not involve proper
ambient credentials that the victim normally has for the targeted site: As
far as the browser is concerned, it is still talking to bunnyoutlet.com and not
to, say, the aforementioned us-payroll site. Still, the prospect of the attacker
examining the internal network and perhaps trying to brute-force the
appropriate credentials or identify vulnerabilities is disconcerting.

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Simple Exploitation of XSS or XSRF Flaws
Even outside the realm of the same-origin policy, the mere possibility
of navigating to intranet URLs means that the attacker may attempt to
(blindly) target known or suspected vulnerabilities in locally running
software. Because internal applications are thought to be protected from
malicious users, they are often not engineered or maintained to the
same standards as externally facing code.
One striking example of this problem is the dozens of vulnerabilities
discovered over the years in internal-only web management interfaces
of home network routers manufactured by companies such as Linksys
(Cisco), Netgear, D-Link, Motorola, and Siemens. Cross-site request forgery vulnerabilities in these applications can, in extreme cases, permit
attackers to access the device and intercept or modify all network traffic
going to or through it.
So far, the disconnect between browser security mechanisms and network segmentation remains an unsolved problem in browser engineering.
Several browsers try to limit the impact of DNS rebinding by caching DNS
responses for a predefined time—a practice known as DNS pinning—but the
defense is imperfect, and the remaining attack vectors still remain.
NOTE

Unusually, Internet Explorer takes the lead on this front, offering an optional way to
mitigate the risk. Microsoft’s users are protected to some extent if they flip a cryptic zone
setting named “websites in less privileged web content zone can navigate into this zone”
to “disable” in the configuration options for local intranet. Unfortunately, the zone
model in Internet Explorer comes with some unexpected pitfalls, as we’ll discuss in
Chapter 15.

Prohibited Ports
Security researchers have cautioned that the ability of browsers to submit largely unconstrained cross-origin request bodies, for example with
<form method="POST" enctype="text/plain">, may interfere with certain other
fault-tolerant but non-HTTP network services. For example, consider SMTP,
the dominant mail transfer protocol: When interacting with an unsuspecting browser, most servers that speak SMTP will patiently ignore the first few
incomprehensible lines associated with HTTP headers and then honor any
SMTP commands that appear in the request body. In effect, the browser
could be used as a proxy for relaying spam.
A related but less well-explored concern, discussed in Chapter 3, is the
risk of an attacker talking to non-HTTP services running in the same domain
as the targeted web application and tricking the browser into misinterpreting
the returned, possibly partly attacker-controlled data as HTML delivered over
HTTP/0.9. This behavior could expose cookies or other credentials associated with the targeted site.
The design of HTTP makes it impossible to solve these problems in
a particularly robust way. Instead, browser vendors have responded in a
rather unconvincing manner: by shipping a list of prohibited TCP ports
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to which requests cannot be sent. For Internet Explorer versions 6 and 7,
the list consists of the following port numbers:
19
21
25
110
119
143

chargen
ftp
smtp
pop3
nntp
imap2

Versions 8 and 9 of Internet Explorer further prohibit ports 220 (imap3)
and 993 (ssl imap3).
All other browsers discussed in this book use a different, common list:
1
7
9
11
13
15
17
19
20
21
22
23
25
37
42
43
53
77
79
87
95
101
102
103
104
109
110
111
113

tcpmux
echo
discard
systat
daytime
netstat
qotd
chargen
ftp-data
ftp
ssh
telnet
smtp
time
name
nicname
domain
priv-rjs
finger
ttylink
supdup
hostriame
iso-tsap
gppitnp
acr-nema
pop2
pop3
sunrpc
auth

115
117
119
123
135
139
143
179
389
465
512
513
514
515
526
530
531
532
540
556
563
587
601
636
993
995
2049
4045
6000

sftp
uccp-path
nntp
ntp
loc-srv
netbios
imap2
bgp
ldap
ssl smtp
exec
login
shell
printer
tempo
courier
chat
netnews
uucp
remotefs
ssl nntp
smtp submission
syslog
ssl ldap
ssl imap
ssl pop3
nfs
lockd
X11
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There are, of course, various protocol-specific exceptions to these rules.
For example, ftp: URLs are obviously permitted to access port 21, normally
associated with that protocol.
The current solution is flawed in several ways, the most important of
which may be that both lists have numerous glaring omissions and, given the
number of network protocols devised to date, simply have no chance of ever
being exhaustive. For example, no rule would prevent the browser from talking to Internet Relay Chat (IRC) servers, which use a fault-tolerant, text-based
protocol not entirely unlike SMTP.
The lists are also not regularly updated to reflect the demise of nearly
extinct network protocols or the introduction of new ones. Lastly, they can
unfairly and unexpectedly penalize system administrators for picking nonstandard ports for certain services they want to hide from public view: Doing
so means opting out of this browser-level protection mechanism.

Limitations on Third-Party Cookies
Since their inception, HTTP cookies have been misunderstood as the tool
that enabled online advertisers to violate users’ privacy to an unprecedented
and previously unattainable extent. This sentiment has been echoed by the
mainstream press in the years since. For example, in 2001, the New York Times
published a lengthy exposé on the allegedly unique risks of HTTP cookies and
even quoted Lawrence Lessig, a noted legal expert and a political activist:1
Before cookies, the Web was essentially private. After cookies, the
Web becomes a space capable of extraordinary monitoring.

The high-profile assault on a single HTTP header continued over the
course of a decade, gradually shifting its focus toward third-party cookies in
particular. Third-party cookies are the cookies set by domains other than the
domain of the top-level document, and they are usually associated with the
process of loading images, frames, or applets from third-party sites. The reason they have attracted attention is that operators of advertising networks
have embraced such cookies as a convenient way to tag a user who sees their
ad embedded on fuzzybunnies.com and then recognize that user through a
similar embedded ad served on playboy.com.
Because the clearly undesirable possibility of performing this type of
cross-domain tracking has been erroneously conflated with the existence of
third-party cookies, the pressure on browser vendors has continued to mount.
In one instance, the Wall Street Journal flat out accused Microsoft of being in
bed with advertisers for not eliminating third-party cookies in the company’s
product.2
Naturally, the readers of this book will recognize that the fixation on
HTTP cookies is deeply misguided. There is no doubt that some parties use
the mechanism for vaguely sinister purposes, but nothing makes it uniquely
suited for this task; there are many other equivalent ways to store unique identifiers on visitors’ computers (such as cache-based tags, previously discussed
in Chapter 3). Besides, it is simply impossible to prevent cooperating sites
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from using existing unique fingerprints of every browser (exposed through
the JavaScript object model or plug-ins such as Flash) to correlate and mine
cross-domain browsing patterns at will. The sites that embed advertisements
for profit are quite willing to cooperate with the parties who pay their bills.
In fact, the common reliance on HTTP cookies offers a distinctive
advantage to users: Unlike many of the easily embraced alternatives, this
mechanism is purpose built and coupled with reasonably well-designed and
fine-grained privacy controls. Breaking cookies will not hinder tracking but
will remove any pretense of transparency from the end user. Another noted
privacy and security activist, Ed Felten, once said: “If you’re going to track
me, please use cookies.”3
Unscrupulous online tracking is a significant social issue, and new technical mechanisms may be needed so that users can communicate their privacy
preferences to well-behaved sites (such as the recently added DNT request
header4 rolled out in Firefox 4). In order to deal with the ill-behaved ones,
a regulatory framework may be required, too. In the absence of such a framework, in Internet Explorer 9, Microsoft is experimenting with a managed
blacklist of known bad sources of tracking cookies—but the odds that this
would discourage sleazy business practices are slim.
In any case, despite having little or no merit, the continued public outcry
against third-party cookies eventually resulted in several browser vendors
shipping half-baked and easily circumvented solutions that let them claim
they had done something.


In Internet Explorer, setting and reading third-party cookies is blocked
by default, except for session cookies accompanied by a satisfactory P3P
header. P3P (Platform for Privacy Preferences)5 is a method to construct
machine-readable, legally binding summaries of a site’s privacy policy, be
it as an XML file or as a compact policy in an HTTP header. For example,
the keyword TEL in an HTTP header means that the site uses the collected information for telemarketing purposes. (No technical measure
will prevent a site from lying in a P3P header, but the potential legal
consequences are meant to discourage that.)
NOTE



The incredibly ambitious, 111-page P3P specification caused the solution
to crumble under its own weight. Large businesses are usually very hesitant to embrace P3P as a solution to technical problems because of the
legal footprint of the spec, while small businesses and individual site
owners copy over P3P header recipes with little or no understanding of
what they are supposed to convey.

In Safari, the task of setting third-party cookies is blocked by default,
but previously issued cookies can be read freely. However, this behavior
can be overridden if the user interacts with the cookie-setting document
first. Such an interaction could be intentional but may very well not be:
The clickjacking-related tricks outlined in Chapter 11 apply to this scenario as well.

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

In other browsers, third-party cookies are permitted by default, but a
configuration option is provided to change the behavior. Enabling this
option limits the ability to set third-party cookies, but reading existing
ones is not limited in any way.

For the purpose of these checks, a cookie is considered to be coming from
a third party if it’s loaded from a completely unrelated domain. For example, a
frame pointing to bunnyoutlet.com loaded on fuzzybunnies.com meets this criterion, but www1.fuzzybunnies.com and www2.fuzzybunnies.com are considered to
be in a first-party relationship. The logic used to make this determination is
fragile, and it suffers from the same problems that cookie domain scoping
would. In Internet Explorer 6 and 7, for example, the comparisons in certain
country-level domains are performed incorrectly.
NOTE

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The crusade against third-party cookies could be seen as a harmless exercise, but it has
had negative consequences, too. Browsers that reject third-party cookies make it very difficult to build cookie-based authentication for embeddable gadgets and other types of
mashups, and they make it difficult to use “sandbox” domains to isolate untrusted but
private content from the main application to limit the impact of script-injection flaws.

Security Engineering Cheat Sheet
When Building Web Applications on Internal Networks
 Assume that determined attackers will be able to interact with those applications through
a victim’s browser, regardless of any network-level security controls. Ensure that proper
engineering standards are met and require HTTPS with secure cookies for all sensitive
applications in order to minimize the risk of origin infiltration attacks.

When Launching Non-HTTP Services, Particularly on Nonstandard Ports
 Evaluate the impact of browsers unintentionally issuing HTTP requests to the service
and the impact of having the response interpreted as HTTP/0.9. For vulnerable protocols, consider dropping the connection immediately if the received data begins with
“GET” or “POST” as one possible precaution.

When Using Third-Party Cookies for Gadgets or Sandboxed Content
 If you need to support Internet Explorer, be prepared to use P3P policies (and evaluate
their legal significance). If you need to support Safari, you may have to resort to an alternative credential storage mechanism (such as HTML5 localStorage).

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CONTENT RECOGNITION
MECHANISMS

So far, we have looked at a fair number of wellintentioned browser features that, as the technology
matured, proved to be short-sighted and outright dangerous. But now, brace for something special: In the
history of the Web, nothing has proven to be as misguided as content sniffing.
The original premise behind content sniffing was simple: Browser vendors
assumed that in some cases, it would be appropriate—even desirable—to
ignore the normally authoritative metadata received from the server, such as
the Content-Type header. Instead of honoring the developer’s declared intent,
implementations that support content sniffing may attempt to second-guess
the appropriate course of action by applying proprietary heuristics to the
returned payload in order to compensate for possible mistakes. (Recall from
Chapter 1 that during the First Browser Wars, vendors turned fault-tolerance
compatibility into an ill-conceived competitive advantage.)

It didn’t take long for content-sniffing features to emerge as a substantial
and detrimental aspect of the overall browser security landscape. To their
horror and disbelief, web developers soon noticed that they couldn’t safely
host certain nominally harmless document types like text/plain or text/csv on
behalf of their users; any attempt to do so would inevitably create a risk that
such content could be misinterpreted as HTML.
Perhaps partly in response to these concerns, in 1999 the practice of
unsolicited content sniffing was explicitly forbidden in HTTP/1.1:
If and only if the media type is not given by a Content-Type field, the
recipient may attempt to guess the media type via inspection of its
content and/or the name extension(s) of the URI used to identify
the resource.

Alas, this uncharacteristically clear requirement arrived a bit too late. Most
browsers were already violating this rule to some extent, and absent a convenient way to gauge the potential consequences, their authors hesitated to
simply ditch the offending code. Although several of the most egregious mistakes were cautiously reverted in the past decade, two companies—Microsoft
and Apple—largely resisted the effort. They decided that interoperability with
broken web applications should trump the obvious security problems. To
pacify any detractors, they implemented a couple of imperfect, secondary
security mechanisms intended to mitigate the risk.
Today, the patchwork of content-handling policies and the subsequently
deployed restrictions cast a long shadow on the online world, making it nearly
impossible to build certain types of web services without resorting to contrived
and sometimes expensive tricks. To understand these limitations, let’s begin
by outlining several scenarios where a nominally passive document may be
misidentified as HTML or something like it.

Document Type Detection Logic
The simplest and the least controversial type of document detection heuristics, and the one implemented by all modern browsers, is the logic implemented to handle the absence of the Content-Type header. This situation,
which is encountered very rarely, may be caused by the developer accidentally omitting or mistyping the header name or the document being loaded
over a non-HTTP transport mechanism such as ftp: or file:.
For HTTP specifically, the original RFCs explicitly permit the browser
to examine the payload for clues when the Content-Type value is not available.
For other protocols, the same approach is usually followed, often as a natural
consequence of the design of the underlying code.
The heuristics employed to determine the type of a document typically
amount to checking for static signatures associated with several dozen known
file formats (such as images and common plug-in-handled files). The response
will also be scanned for known substrings in order to detect signatureless formats such as HTML (in which case, the browser will look for familiar tags—
<body>, <font>, etc). In many browsers, noncontent signals, such as trailing .html
or .swf strings in the path segment of the URL, are taken into account as well.
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The specifics of content-sniffing logic vary wildly from one browser to
another and are not well documented or standardized. To illustrate, consider
the handling of Adobe Flash (SWF) files served without Content-Type: In Opera,
they are recognized unconditionally based on a content signature check; in
Firefox and Safari, an explicit .swf suffix in the URL is required; and Internet
Explorer and Chrome will not autorecognize SWF at all.
Rest assured, the SWF file format is not an exceptional case. For example,
when dealing with HTML files, Chrome and Firefox will autodetect the document only if one of several predefined HTML tags appears at the very beginning of the file; while Firefox will be eager to “detect” HTML based solely on
the presence of an .html extension in the URL, even if no recognizable markup
is seen. Internet Explorer, on the other hand, will simply always default to
HTML in the absence of Content-Type, and Opera will scan for known HTML
tags within the first 1000 bytes of the returned payload.
The assumption behind all this madness is that the absence of ContentType is an expression of an intentional wish by the publisher of the page—
but that assumption is not always accurate and has caused a fair number of
security bugs. That said, most web servers actively enforce the presence of a
Content-Type header and will insert a default value if one is not explicitly generated by the server-side scripts that handle user requests. So perhaps there is
no need to worry? Well, unfortunately, this is not where the story of content
sniffing ends.

Malformed MIME Types
The HTTP RFC permits content sniffing only in the absence of Content-Type
data; the browser is openly prohibited from second-guessing the intent of the
webmaster if the header is present in any shape or form. In practice, however,
this advice is not taken seriously. The next small step taken off the cliff was
the decision to engage heuristics if the server-returned MIME type was
deemed invalid in any way.
According to the RFC, the Content-Type header should consist of two
slash-delimited alphanumeric tokens (type/subtype), potentially followed by
other semicolon-delimited parameters. These tokens may contain any nonwhitespace, seven-bit ASCII characters other than a couple of special “separators” (a generic set that includes characters such as “@”, “?”, and the slash
itself). Most browsers attempt to enforce this syntax but do so inconsistently;
the absence of a slash is seen almost universally as an invitation to content
sniffing, and so is the inclusion of whitespaces and certain (but not all) control characters in the first portion of the identifier (the type token). On the
other hand, the technically illegal use of high-bit characters or separators
affects the validity of this field only in Opera.
The reasons for this design are difficult to understand, but to be fair, the
security impact is still fairly limited. As far as web application developers are concerned, care must be exercised not to make typos in Content-Type values and not
to allow users to specify arbitrary, user-controlled MIME types (merely validated
against a blacklist of known bad options). These requirements may be unexpected, but usually they do not matter a lot. So, what are we ultimately getting at?
Content Recognition Mechani sms

199

Special Content-Type Values
The first clear signal that content sniffing was becoming truly dangerous was
the handling of a seemingly unremarkable MIME type known as application/
octet-stream. This specific value is not mentioned at all in the HTTP specification but is given a special (if vague) role deep in the bowels of RFC 2046:1
The recommended action for an implementation that receives an
application/octet-stream entity is to simply offer to put the data in a
file, with any Content-Transfer-Encoding undone, or perhaps to use it
as input to a user-specified process.

The original intent of this MIME type may not be crystal clear from the
quoted passage alone, but it is commonly interpreted as a way for web servers
to indicate that the returned file has no special meaning to the server and
that it should not have one to the client. Consequently, most web servers
default to application/octet-stream on all types of opaque, nonweb files, such as
downloadable executables or archives, if no better Content-Type match can be
found. However, in rare cases of administrator errors (for example, due to
deletion of the essential AddType directives in Apache configuration files),
web servers may also fall back to this MIME type on documents meant for
in-browser consumption. This configuration error is, of course, very easy to
detect and fix, but Microsoft, Opera, and Apple nevertheless chose to compensate for it. The browsers from these vendors eagerly engage in content
sniffing whenever application/octet-stream is seen.*
This particular design decision has suddenly made it more difficult for
web applications to host binary files on behalf of the user. For example, any
code-hosting platform must exercise caution when returning executables or
source archives as application/octet-stream, because there is a risk they may be
misinterpreted as HTML and displayed inline. That’s a major issue for any
software hosting or webmail system and for many other types of web apps.
(It’s slightly safer for them to use any other generic-sounding MIME type,
such as application/binary, because there is no special case for it in the
browser code.)
In addition to the special treatment given to application/octet-stream, a
second, far more damaging exception exists for text/plain. This decision,
unique to Internet Explorer and Safari, traces back to RFC 2046. In that document, text/plain is given a dual function: first, to transmit plaintext documents (ones that “do not provide for or allow formatting commands, font attribute
specifications, processing instructions, interpretation directives, or content markup”)
and, second, to provide a fallback value for any text-based documents not
otherwise recognized by the sender.

*
In Internet Explorer, this implemented logic differs subtly from a scenario where no ContentType is present. Instead of always assuming HTML, the browser will scan the first 256 bytes for
popular HTML tags and other predefined content signatures. From the security standpoint,
however, it’s not a very significant difference.

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The distinction between application/octet-stream and text/plain fallback
made perfect sense for email messages, a topic that this RFC originally dealt
with, but proved to be much less relevant to the Web. Nevertheless, some
web servers adopted text/plain as the fallback value for certain types of
responses (most notably, the output of CGI scripts).
The text/plain logic subsequently implemented in Internet Explorer and
Safari in order to detect HTML in such a case is really bad news: It robs web
developers of the ability to safely use this MIME type to generate user-specific
plaintext documents and offers no alternatives. This has resulted in a substantial number of web application vulnerabilities, but to this day, Internet
Explorer developers seem to have no regrets and have not changed the
default behavior of their code.
Safari developers, on the other hand, recognized and tried to mitigate the
risk while keeping the functionality in place—but they failed to appreciate
the complexity of the Web. The solution implemented in their browser is to
rely on a secondary signal in addition to the presence of a plausible-looking
HTML markup in the document body. The presence of an extension such as
.html or .xml at the end of the URL path is interpreted by their implementation as a sign that content sniffing can be performed safely. After all, the
owner of the site wouldn’t name the file this way otherwise, right?
Alas, the signal they embraced is next to worthless. As it turns out, almost
all web frameworks support at least one of several methods for encoding parameters in the path segment of the URL instead of in the more traditionally
used query part. For example, in Apache, one such mechanism is known as
PATH_INFO, and it happens to be enabled by default. By leveraging such a
parameter-passing scheme, the attacker can usually append nonfunctional
garbage to the path, thereby confusing the browser without affecting how the
server will respond to the submitted request itself.
To illustrate, the following two URLs will likely have the same effect for
websites running on Apache or IIS:
http://www.fuzzybunnies.com/get_file.php?id=1234

and
http://www.fuzzybunnies.com/get_file.php/evil.html?id=1234

In some less-common web frameworks, the following approach may
also work:
http://www.fuzzybunnies.com/get_file.php;evil.html?id=1234

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Unrecognized Content Type
Despite the evident trouble with text/plain, the engineers working on Internet Explorer decided to take their browser’s heuristics even further. Internet
Explorer applies both content sniffing and extension matching* not only
to a handful of generic MIME types but also to any document type not
immediately recognized by the browser. This broad category may include
everything from JSON (application/json) to multimedia formats such as Ogg
Vorbis (audio/ogg).
Such a design is, naturally, problematic and causes serious problems when
hosting any user-controlled document formats other than a small list of universally supported MIME types registered internally in the browser or when
routed to a handful of commonly installed external applications.
Nor do the content-sniffing habits of Internet Explorer finally end there:
The browser will also resort to payload inspection when dealing with internally
recognized document formats that, for any reason, can’t be parsed cleanly.
In Internet Explorer versions prior to 8, serving a user-supplied but nonvalidated file claiming to be an JPEG image can lead to the response being
treated as HTML. And it gets even more hilarious: Even a subtle mistake,
such as serving a valid GIF file with Content-Type: image/jpeg, triggers the same
code path. Heck, several years ago, Internet Explorer even detected HTML
on any valid, properly served PNG file. Thankfully, this logic has since been
disabled—but the remaining quirks are still a minefield.
NOTE

In order to fully appreciate the risk of content sniffing on valid images, note that it is
not particularly difficult to construct images that validate correctly but that carry
attacker-selected ASCII strings—such as HTML markup—in the raw image data. In
fact, it is relatively easy to construct images that, when scrubbed, rescaled, and recompressed using a known, deterministic algorithm, will have a nearly arbitrary string
appear out of the blue in the resulting binary stream.
To its credit, in Internet Explorer 8 and beyond, Microsoft decided to
disallow most types of gratuitous content sniffing on known MIME types
in the image/* category. It also disallowed HTML detection (but not XML
detection) on image formats not recognized by the browser, such as image/
jp2 (JPEG2000).
This single tweak aside, Microsoft has proven rather unwilling to make
meaningful changes to its content-sniffing logic, and its engineers have publicly defended the need to maintain compatibility with broken websites.2
Microsoft probably wants to avoid the wrath of large institutional customers,
many of whom rely on ancient and poorly designed intranet apps and depend
on the quirks of the Internet Explorer–based monoculture on the client end.
In any case, due to the backlash that Internet Explorer faced over its text/
plain handling logic, newer versions offer a partial workaround: an optional
*
Naturally, path-based extension matching is essentially worthless for the reasons discussed
in the previous section; but in the case of Internet Explorer 6, it gets even worse. In this browser,
the extension can appear in the query portion of the URL. Nothing stops the attacker from simply
appending ?foo=bar.html to the requested URL, effectively ensuring that this check is always
satisfied.

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HTTP header, X-Content-Type-Options: nosniff, which allows website owners
to opt out of most of the controversial content heuristics. The use of this
header is highly recommended; unfortunately, the support for it has not
been backported to versions 6 and 7 of the browser and has only a limited
support in other browsers. In other words, it cannot be depended on as a
sole defense against content sniffing.
NOTE

Food for thought: According to the data collected in a 2011 survey by SHODAN and
Chris John Riley,3 only about 0.6 percent of the 10,000 most popular websites on the
Internet used this header on a site-wide level.

Defensive Uses of Content-Disposition
The Content-Disposition header, mentioned several times in Part I of this
book, may be considered a defense against content sniffing in some use cases.
The function of this header is not explained satisfactorily in the HTTP/1.1
specification. Instead, it is documented only in RFC 2183,4 where its role is
explained only as it relates to mail applications:
Bodyparts can be designated “attachment” to indicate that they are
separate from the main body of the mail message, and that their
display should not be automatic, but contingent upon some further action of the user. The MUA* might instead present the user
of a bitmap terminal with an iconic representation of the attachments, or, on character terminals, with a list of attachments from
which the user could select for viewing or storage.

The HTTP RFC acknowledges the use of Content-Disposition: attachment in
the web domain but does not elaborate on its intended function. In practice,
upon seeing this header during a normal document load, most browsers will
display a file download dialog, usually with three buttons: “open,” “save,” and
“cancel.” The browser will not attempt to interpret the document any further
unless the “open” option is selected or the document is saved to disk and
then opened manually. For the “save” option, an optional filename parameter
included in the header is used to suggest the name of the download, too. If
this field is absent, the filename will be derived from the notoriously unreliable URL path data.
Because the header prevents most browsers from immediately interpreting and displaying the returned payload, it is particularly well suited for
safely hosting opaque, downloadable files such as the aforementioned case of
archives or executables. Furthermore, because it is ignored on type-specific
subresource loads (such as <img> or <script>), it may also be employed to protect user-controlled JSON responses, images, and so on against content sniffing risks. (The reason why all implementations ignore Content-Disposition for
these types of navigation is not particularly clear, but given the benefits, it’s
best not to question the logic now.)

*
MUA stands for “mail user agent,” that is, a client application used to retrieve, display, and
compose mail messages.

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One example of a reasonably robust use of Content-Disposition and other
HTTP headers to discourage content sniffing on a JSON response may be
Content-Type: application/json; charset=utf-8
X-Content-Type-Options: nosniff
Content-Disposition: attachment; filename="json_response.txt"
{ "search_term": "<html><script>alert('Hi mom!')</script>", ... }

The defensive use of Content-Disposition is highly recommended where
possible, but it is important to recognize that the mechanism is neither mandated for all user agents nor well documented. In less popular browsers, such
as Safari Mobile, the header may have no effect; in mainstream browsers,
such as Internet Explorer 6, Opera, and Safari, a series of Content-Disposition
bugs have at one point or another rendered the header ineffective in
attacker-controlled cases.
Another problem with the reliance on Content-Disposition is that the user
may still be inclined to click “open.” Casual users can’t be expected to be wary
of viewing Flash applets or HTML documents just because a download prompt
gets in the way. In most browsers, selecting “open” puts the document in a
file: origin, which may be problematic on its own (the recent improvements
in Chrome certainly help), and in Opera, the document will be displayed in
the context of the originating domain. Arguably, Internet Explorer makes the
best choice: HTML documents are placed in a special sandbox using a markof-the-web mechanism (outlined in more detail in Chapter 15), but even in
that browser, Java or Flash applets will not benefit from this feature.

Content Directives on Subresources
Most content-related HTTP headers, such as Content-Type, Content-Disposition,
and X-Content-Type-Options, have largely no effect on type-specific subresource
loads, such as <img>, <script>, or <embed>. In these cases, the embedding party
has nearly complete control over how the response will be interpreted by the
browser.
Content-Type and Content-Disposition may also not be given much attention
when handling requests initiated from within plug-in-executed code. For
example, recall from Chapter 9 that any text/plain or text/csv documents may
be interpreted by Adobe Flash as security-sensitive crossdomain.xml policies
unless an appropriate site-wide metapolicy is present in the root directory on
the destination server. Whether you wish to call it “content sniffing” or just
“content-type blindness,” the problem is still very real.
Consequently, even when all previously discussed HTTP headers are
used religiously, it is important to always consider the possibility that a thirdparty page may trick the browser into interpreting that page as one of several
problematic document types; applets and applet-related content, PDFs, stylesheets, and scripts are usually of particular concern. To minimize the risk of
mishaps, you should carefully constrain the structure and character set of any
served payloads or use “sandbox” domains to isolate any document types that
can’t be constrained particularly well.
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Downloaded Files and Other Non-HTTP Content
The behavior of HTTP headers such as Content-Type, Content-Disposition, and
X-Content-Type-Options may be convoluted and exception ridden, but at the
very least, they add up to a reasonably consistent whole. Still, it is easy to forget that in many real-world cases, the metadata contained in these headers is
simply not available—and in that case, all bets are off. For example, the handling of documents retrieved over ftp:, or saved to disk and opened over the
file: protocol, is highly browser- and protocol-specific and often surprises
even the most seasoned security experts.
When opening local files, browsers usually give precedence to file extension
data, and if the extension is one of the hardcoded values known to the browser,
such as .txt or .html, most browsers will take this information at face value. Chrome
is the exception; it will attempt to autodetect certain “passive” document types,
such as JPEG, even inside .txt documents. (HTML, however, is strictly off-limits.)
When it comes to other extensions registered to external programs, the
behavior is a bit less predictable. Internet Explorer will usually invoke the
external application, but most other browsers will resort to content sniffing,
behaving as though they loaded the document over HTTP with no ContentType set. All browsers will also fall back to content sniffing if the extension is
not known (say, .foo).
The heavy reliance on file extension data and content sniffing for file:
documents creates an interesting contrast with the normal handling of
Internet-originating resources. On the Web, Content-Type is by and large the
authoritative descriptor of document type. File extension information is
ignored most of the time, and it is perfectly legal to host a functional JPEG
file at a location such as http://fuzzybunnies.com/gotcha.txt. But what happens
when this document is downloaded to disk? Well, in such case, the effective
meaning of the resource will unexpectedly change: When accessing it over
the file: protocol, the browser may insist on rendering it as a text file, based
strictly on the extension data.
The example above is
fairly harmless, but other content promotion vectors, such
as an image becoming an executable, may be more troubling. To that effect, Opera
and Internet Explorer will
attempt to modify the extension to match the MIME type Figure 13-1: Prompt displayed by Firefox when
saving a Content-Type: image/jpeg document
for a handful of known Content- served with Content-Disposition: attachment. The
Type values. Other browsers
“hello.exe” filename is derived by the browser from
do not offer this degree of
a nonfunctional PATH_INFO suffix appended by the
protection, however, and may attacker at the end of the URL. The prompt incorrectly
even be thoroughly confused claims that the .exe file is a “JPEG Image.” In fact,
when saved to disk, it will be an executable.
by the situation they find
themselves in. Figure 13-1
captures Firefox in one such
embarrassing moment.
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This problem underscores the importance of returning an explicit,
harmless filename value whenever using a Content-Disposition attachment, to
prevent the victim from being tricked into downloading a document format
that the site owner never intended to host.
Given the complex logic used for file: URLs, the simplicity of ftp: handling
may come as a shock. When accessing documents over FTP, most browsers
pay no special attention to file extensions and will simply indulge in rampant
content sniffing. One exception is Opera, where extension data still takes
precedence. From the engineering point of view, the prevalent approach to
FTP may seem logical: The protocol can be considered roughly equivalent to
HTTP/0.9. Nevertheless, the design also violates the principle of least astonishment. Server owners would not expect that by allowing users to upload .txt
documents to an FTP site, they are automatically consenting to host active
HTML content within their domain.

Character Set Handling
Document type detection is one of the more important pieces of the contentprocessing puzzle, but it is certainly not the only one. For all types of text-based
files rendered in the browser, one more determination needs to be made: The
appropriate character set transformation must be identified and applied to the
input stream. The output encoding sought by the browser is typically UTF-8
or UTF-16; the input, on the other hand, is up to the author of the page.
In the simplest scenario, the appropriate encoding method will be provided by the server in a charset parameter of the Content-Type header. In the case
of HTML documents, the same information may also be conveyed to some
extent through the <meta> directive. (The browser will attempt to speculatively
extract and interpret this directive before actually parsing the document.)
Unfortunately, the dangerous qualities of certain character encodings,
as well as the actions taken by the browser when the charset parameter is not
present or is not recognized, once again make life a lot more interesting
than the aforementioned simple rule would imply. To understand what can
go wrong, we first need to recognize three special classes of character sets
that may alter the semantics of HTML or XML documents:


Character sets that permit noncanonical representations of standard
7-bit ASCII codes. Such noncanonical sequences could be used to cleverly encode HTML syntax elements, such as angle brackets or quotes,
in a manner that survives a simple server-side check. For example, the
famously problematic UTF-7 encoding permits the “<” character to be
encoded as a five-character sequence of “+ADw-”, a string that most serverside filters will happily permit as is. In a similar vein, UTF-8 specification
formally prohibits, but technically permits, “<” to be represented by
unnecessarily verbose 2- to 5-byte sequences, from 0xC0 0xBC to 0xFC
0x80 0x80 0x80 0x80 0xBC.*

*
Today, this problem is mitigated by most browsers: Their parsers now have additional checks
to reject overlong UTF-8 encodings as a matter of principle. The same cannot be said of all
possible server-side UTF-8 libraries, however.

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

Variable length encodings that give special meaning to one or more bytes
that follow a special prefix. Such logic may result in legitimate HTML syntax elements being “consumed” as part of an unintentional multibyte literal. For example, the Shift JIS prefix code 0xE0 can cause the subsequent
angle bracket or a quote to be consumed in Internet Explorer, Firefox,
and Opera (but not in Chrome), possibly severely altering the meaning
of the inline markup.
The opposite problem may also occur: The server may be convinced
that it is outputting a multibyte literal, but this literal may be rejected by the
browser and interpreted as several individual characters. In EUC-KR, the
0x8E prefix is honored only if the subsequent character has an ASCII code
of 0x41 or higher. Any less and it will not have the expected effect, but
not all server-side implementations may notice.



Encodings that are completely incompatible with 8-bit ASCII. These
cases will simply lead to a very different view of document structure
between the client and the server. Common examples include UTF-16
or UTF-32.

The bottom line is that unless the server has a perfect command of the
character set it is generating and unless it is certain that the client will not
apply an unexpected transformation to the payload, serious complications may
arise. For example, consider a web application that removes angle brackets
from the highlighted user-controlled string in the following piece of HTML:
You are currently viewing:
<span class="blog_title">
+ADw-script+AD4-alert("Hi mom!")+ADw-/script+AD4</span>

If that document is interpreted as UTF-7 by the receiving party, the
actual parsed markup will look as follows:
You are currently viewing:
<span class="blog_title">
<script>alert("Hi mom!")</script>
</span>

A similar problem, this time related to byte consumption in Shift JIS encoding, is illustrated below. A multibyte prefix is permitted to consume a closing
quote, and as a result, the associated HTML tag is not terminated as expected,
enabling the attacker to inject an extra onerror handler into the markup:
<img src="http://fuzzybunnies.com/[0xE0]">
...this is still a part of the markup...
...but the server doesn't know...
" onerror="alert('This will execute!')"
<div>
...page content continues...
</div>

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It is simply imperative to prevent character set autodetection for all textbased documents that contain any type of user-controlled data. Most browsers
will engage in character set detection if the charset parameter is not found in
the Content-Type header or in the <meta> tag. Some marked differences exist
between the implementations (for example, only Internet Explorer is keen to
detect UTF-7), but you should never assume that the outcome of character
set sniffing will be safe.
Character set autodetection will also be attempted if the character set is
not recognized or is mistyped; this problem is compounded by the fact that
charset naming can be ambiguous and that web browsers are inconsistent in
how much tolerance they have for common name variations. As a single data
point, consider the fact that Internet Explorer recognizes both ISO-8859-2
and ISO8859-2 (with no dash after the ISO part) as valid character set identifiers in the Content-Type header but fails to recognize UTF8 as an alias for
UTF-8. The wrong choice can cause some serious pain.
NOTE

Fun fact: The X-Content-Type-Options header has no effect on character-sniffing
logic.

Byte Order Marks
We are not done with character set detection just yet! Internet Explorer needs
to be singled out for yet another dramatically misguided content-handling
practice: the tendency to give precedence to the so-called byte order mark
(BOM), a sequence of bytes that can be placed at the beginning of a file to
identify its encoding, over the explicitly provided charset data. When such a
marker is detected in the input file, the declared character set is ignored.
Table 13-1 shows several common markers. Of these, the printable
UTF-7 BOM is particularly sneaky.
Table 13-1: Common Byte Order Markers (BOMs)
Encoding name

NOTE

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Byte order mark sequence

UTF-7

“+/v” followed by “8”, “9”, “+”, or “/”

UTF-8

0xEF 0xBB 0xBF

UTF-16 little endian

0xFF 0xFE

UTF-16 big endian

0xFE 0xFF

UTF-32 little endian

0xFF 0xFE 0x00 0x00

UTF-32 big endian

0x00 0x00 0xFE 0xFF

GB-18030

0x84 0x31 0x95 0x33

Microsoft engineers acknowledge the problem with this design and, as of this writing,
say that the logic may be revised, depending on the outcome of compatibility tests. If
the problem is resolved by the time this book hits the shelves, kudos to them. Until then,
allowing the attacker to control the first few bytes of an HTTP response that is not otherwise protected by Content-Disposition may be a bad idea—and other than padding
the response, there is no way to work around this glitch.

Character Set Inheritance and Override
Two additional, little-known mechanisms should be taken into account when
evaluating the potential impact on character set handling strategies in contemporary web browsers. Both of these features may permit an attacker to
force undesirable character encoding upon another page, without relying
on character sniffing.
The first apparatus in question, supported by all but Internet Explorer, is
known as character set inheritance. Under this policy, any encoding defined for
the top-level frame may be automatically applied to any framed documents
that do not have their own, valid charset value set. Initially, such inheritance is
extended to all framing scenarios, even across completely unrelated websites.
However, when Stefan Esser, Abhishek Arya, and several other researchers
demonstrated a number of plausible attacks that leveraged this feature to
force UTF-7 parsing on unsuspecting targets, Firefox and WebKit developers
decided to limit the behavior to same-origin frames. (Opera still permits crossdomain inheritance. Although it does not support UTF-7, other problematic
encodings, such as Shift JIS, are fair game.)
The other mechanism that deserves mention is the ability to manually
override the currently used character set. This feature is available through
the View > Encoding menu or similar in most browsers. Using this menu to
change the character set causes the page and all its subframes (including
cross-domain ones!) to be reparsed using the selected encoding, regardless
of any charset directives encountered earlier for that content.
Because users may be easily duped into selecting an alternative encoding
for an attacker-controlled page (simply in order to view it correctly), this
design should make you somewhat uncomfortable. Casual users can’t be
expected to realize that their election will also apply to hidden <iframe> tags
and that such a seemingly innocuous action may enable cross-site scripting
attacks against unrelated web properties. In fact, let’s be real: Most of them
will not know—and should not have to know—what an <iframe> is.

Markup-Controlled Charset on Subresources
We are nearing the end of the epic journey through the web of contenthandling quirks, but we are not quite done yet. Astute readers may recall that
in “Type-Specific Content Inclusion” on page 82, I mentioned that on certain types of subresources (namely, stylesheets and scripts), the embedding
page can specify its own charset value in order to apply a specific transformation to the retrieved document, for example,
<script src="http://fuzzybunnies.com/get_js_data.php" charset="EUC-JP">

This parameter is honored by all browsers except for Opera. Where it is
supported, it typically does not take precedence over charset in Content-Type,
unless that second parameter is missing or unrecognized. But to every rule,
there is an exception, and all too often, the name of this exception is Internet Explorer 6. In that still-popular browser, the encoding specified by the
markup overrides HTTP data.
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Does this behavior matter in practice? To fully grasp the consequences,
let’s also quickly return to Chapter 6, where we debated the topic of securing
server-generated, user-specific, JSON-like code against cross-domain inclusion. One example of an application that needs such a defense is a searchable address book in a webmail application: The search term is provided in
the URL, and a JavaScript serialization of the matching contacts is returned
to the browser but must be shielded from inclusion on unrelated sites.
Now, let’s assume that the developer came up with a simple trick to
prevent third-party web pages from loading this data through <script src=...>:
A single “//” prefix is used to turn the entire response into a comment.
Same-origin callers that use the XMLHttpRequest API can simply examine the
response, strip the prefix, and pass the data to eval(...)—but remote callers,
trying to abuse the <script src=...> syntax, will be out of luck.
In this design, a request to /contact_search.php?q=smith may yield the following response:
// var result

= { "q": "smith", "r": [ "j.smith@example.com" ] };

As long as the search term is properly escaped or filtered, this scheme
appears safe. But when we realize that the attacker may force the response
to be interpreted as UTF-7, the picture changes dramatically. A seemingly
benign search term that, as far as the server is concerned, contains no illegal
characters could still unexpectedly decode to
// var result = { "q": "smith[CR][LF]
var gotcha = { "", "r": [ "j.smith@example.com" ] };

This response, when loaded via <script src=... charset=utf-7> inside
the victim’s browser, gives the attacker access to a portion of the user’s
address book.
This is not just a thought exercise: The “//” approach is fairly common
on the Web, and Masato Kinugawa, a noted researcher, found several popular web applications affected by this bug. And a more contrived variant of the
same attack is also possible against other execution-preventing prefixes, such
as while (1);. In the end, the problems with cross-domain charset override on
<script> tags is one of the reasons why in Chapter 6, we strongly recommend
using a robust parser-stopping prefix to prevent the interpreter from ever
looking at any attacker-controlled bits. Oh—and if you factor in the support
for E4X, the picture becomes even more interesting,5 but let’s leave it at that.

Detection for Non-HTTP Files
To wrap up this chapter, let’s look at the last missing detail: character set
encoding detection for documents delivered over non-HTTP protocols. As
can be expected, documents saved to disk and subsequently opened over
the file: protocol, or loaded by other means where the usual Content-Type
metadata is absent, will usually be subjected to character set detection logic.

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However, unlike with document determination heuristics, there is no substantial difference among all the possible delivery methods: In all cases, the
sniffing behavior is roughly the same.
There is no clean and portable way to address this problem for all textbased documents, but for HTML specifically, the impact of character set
sniffing can be mitigated by embedding a <meta> directive inside the document body:
<meta http-equiv="Content-Type" content="text/html;charset=...">

You should not ditch Content-Type in favor of this indicator. Unlike <meta>,
the header works for non-HTML content, and it is easier to enforce and audit
on a site-wide level. That said, documents that are likely to be saved to disk
and that contain attacker-controlled tidbits will benefit from a redundant
<meta> tag. (Just make sure that this value actually matches Content-Type.)

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Security Engineering Cheat Sheet
Good Security Practices for All Websites
 Instruct the web server to append the X-Content-Options: nosniff header to all HTTP
responses.
 Consult the cheat sheet in Chapter 9 to set up an appropriate /crossdomain.xml meta-policy.
 Configure the server to append default charset and Content-Type values on all responses
that would otherwise not have one.
 If you are not using path-based parameter passing (such as PATH_INFO), consider disabling this feature.

When Generating Documents with Partly Attacker-Controlled Contents
 Always return an explicit, valid, well-known Content-Type value. Do not use text/plain or
application/octet-stream.
 For any text-based documents, return a explicit, valid, well-known charset value in the
Content-Type header; UTF-8 is preferable to any other variable-width encodings. Do not
assume that application/xml+svg, text/csv, and other non-HTML documents do not need a
specified character set. For HTML, consider a redundant <meta> directive if it’s conceivable that the file may be downloaded by the user. Beware of typos—UTF8 is not a valid
alias for UTF-8.
 Use Content-Disposition: attachment and an appropriate, explicit filename value for responses
that do not need to be viewed directly—including JSON data.
 Do not allow the user to control the first few bytes of the file. Constrain the response as
much as possible. Do not pass through NULs, control characters, or high-bit values unless
absolutely necessary.
 When performing server-side encoding conversions, be sure that your converters reject
all unexpected or invalid inputs (e.g., overlong UTF-8).

When Hosting User-Generated Files
Consider using a sandbox domain if possible. If you intend to host unconstrained or unknown
file formats, a sandbox domain is a necessity. Otherwise, at the very minimum, do the following:
 Use Content-Disposition: attachment and an appropriate, explicit filename value that matches
the Content-Type parameter.
 Carefully validate the input data and always use the appropriate, commonly recognized
MIME type. Serving JPEG as image/gif may lead to trouble. Refrain from hosting MIME
types that are unlikely to be supported by popular browsers.
 Refrain from using Content-Type: application/octet-stream and use application/binary instead,
especially for unknown document types. Refrain from returning Content-Type: text/plain.
Do not permit user-specified Content-Type headers.

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DEALING WITH
ROGUE SCRIPTS

In the previous five chapters, we examined a fairly broad
range of browser security mechanisms—and looking
back at them, it is fair to say that almost all share a common goal: to stop rogue content from improperly interfering with any other, legitimate web pages displayed
in a browser. This is an important pursuit but also a
fairly narrow one; subverting the boundaries between unrelated websites
is a large part of every attacker’s repertoire but certainly not the only trick
in the book.
The other significant design-level security challenge that all browsers have
to face is that attackers may abuse well-intentioned scripting capabilities in
order to disrupt or impersonate third-party sites without actually interacting
with the targeted content. For example, if JavaScript code controlled by an
attacker is permitted to create arbitrary undecorated windows on a screen, the
attacker may find that, rather than look for a way to inject a malicious payload
into the content served at fuzzybunnies.com, it may be easier to just open a
window with a believable replica of the address bar, thus convincing the
user that the content displayed is from a trusted site.

Unfortunately for victims, in the early days of the Web, no real attention
was given to the susceptibility of JavaScript APIs to attacks meant to disrupt or
confuse users, and, unlike cross-domain content isolation issues, this class of
problems is still not taken very seriously. The situation is unlikely to change
anytime soon: Vendor resources are stretched thin between addressing comparatively more serious implementation-level flaws in the notoriously buggy
browser codebases and rolling out new, shiny security features that appease
web application developers, users, and the mainstream press alike.

Denial-of-Service Attacks
The possibility of an attacker crashing a browser or otherwise rendering it
inoperable is one of the most common, obvious, and least appreciated issues
affecting the modern Web. In the era of gadgets and mashups, it can have
unexpectedly unpleasant consequences, too.
The most prominent reason why most browsers are susceptible to
denial-of-service (DoS) attacks is due simply to a lack of planning: Neither the
underlying document formats nor the capabilities exposed through scripting
languages were designed to have a sensible, constrained worst-case CPU or
memory footprint. In other words, any sufficiently complex HTML file or an
endless JavaScript loop could bring the underlying operating system to its
knees. Worse, the attempts to mandate resource limits or to give users a way
to resume control of a runaway browser following a visit to a rogue page meet
with resistance. For example, the authors of many of the recently proposed
HTML5 APIs provide no advice on preventing resource exhaustion attacks,
nor do they even acknowledge this need, because they think that any limits
imposed today will likely hinder the growth of the Web 5 or 10 years from
now. Browser developers, in turn, refuse to take any action absent any
standards-level guidance.
A common utilitarian argument against any proposed DoS defenses
is that they are pointless—that the browser is hopelessly easy to crash in a
multitude of ways, so why take special measures to address a specific vector
today? It’s hard to argue with this view, but it’s also important to note that it
acts as a self-fulfilling prophecy: The steady increase in the number of DoS
vectors is making it more and more unlikely that the situation will be comprehensively addressed any time soon.
NOTE

To be fair, the computational complexity of certain operations is not the only reason why
browsers are easy to crash. Vendors are also constrained by the need to maintain a significant degree of synchronicity during page-rendering and script-execution steps (see
Chapter 6). This design eliminates the need for website developers to write reentrant
and thread-safe code and has substantial code complexity and security benefits. Unfortunately, it also makes it much easier for one document to lock up the entire browser, or
at least a good portion thereof.
Regardless of all these considerations, and even if browser vendors refuse
to acknowledge DoS risks as a specific flaw, the impact of such attacks is difficult to ignore. For one, whenever a browser is brought down, there is a

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substantial risk of data loss (in the browser itself or in any applications indirectly affected by the attack). Also, on some social-networking sites, an attacker
may be able to lock out the victim from the site simply by sharing a rogue gadget, or perhaps even a well-selected image, with the victim, preventing that
person from ever using that service again.
Some of the common tricks used to take a browser out of service include
loading complex XHTML or SVG documents, opening a very large number of
windows, running an endless JavaScript loop that allocates memory, queuing
a significant number of postMessage(...) calls, and so on. While these examples
are implementation-specific, every browser offers a fair number of ways to
achieve this goal. Even in Chrome, which uses separate renderer processes
to isolate unrelated pages, it’s not difficult to bring down the entire browser:
The top-level process mediates a variety of script-accessible and sometimes
memory- or CPU-intensive tasks.
Given the above, it’s no surprise that despite generally dismissive attitudes,
the major browsers nevertheless implement several DoS countermeasures.
They do not add up to a coherent strategy, and have they have been rolled
out only in response to the widespread abuse of specific APIs or to mitigate
nonmalicious but common programming errors. Nevertheless, let’s look at
them briefly.

Execution Time and Memory Use Restrictions
Because of the aforementioned need to enforce a degree of synchronicity
for many types of JavaScript operations, most browser vendors err on the side
of caution and execute scripts synchronously with most of the remaining
browser code. This design has an obvious downside: A good portion of the
browser may become completely unresponsive as the JavaScript engine is,
say, trying to evaluate a bogus while (1) loop. In Opera and Chrome, the toplevel user interface will still be largely responsive, if sluggish, but in most
other browsers, it won’t even be possible to close the browser window using
the normal UI.
Because endless loops are fairly easy to create by accident, in order to aid
developers, Internet Explorer, Firefox, Chrome, and Safari enforce a modest
time limit on any continuously or nearly continuously executing scripts. If
the script is making the browser unresponsive for longer than a couple of
seconds, the user will be shown a dialog and given the option to abort execution. Picking this option will have a result similar to encountering an unhandled exception, that is, of abandoning the current execution flow.
Regrettably, such a limit is not a particularly robust defense against malicious scripts. For example, regardless of the user’s choice, it is still possible to
resume execution through timers or event handlers, and it’s easy to avoid
triggering the prompt in the first place by periodically returning the CPU
briefly to an idle state in order to reset the counter. Too, as noted previously,
there are ways to hog CPU resources without resorting to busy loops: Rendering complex XHTML, SVG, or XSLT documents can be just as disruptive
and is not subject to any checks.

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Execution time aside, there have been attempts to control the memory
footprint of executed scripts. The size of the call stack is limited to a browserspecific value between 500 and 65535, and attempting a deeper recursion
will result in an unconditional stop. Script heap size, on the other hand, is
typically not restricted in a meaningful way; pages can allocate and use up
gigabytes of memory. In fact, most of the previously implemented restrictions (such as the 16MB cap in Internet Explorer 6) have been removed in
more recent releases.

Connection Limits
In many web applications, each web page consists not only of the proper
HTML document retrieved from the URL visible in the address bar but
also as many as several dozen other, separately loaded subresources, such
as images, stylesheets, and scripts. Because requesting all of these elements
through individually established HTTP connections can be slow, the reader
may recall from Chapter 3 that the protocol has been extended to offer keepalive sessions and request pipelining. But even with these improvements, one
stubborn problem remains. The inherent limitation of the protocol is that
the server must always send responses in the same order that it received the
requests, so if any of the subresources (no matter how inconsequential) takes
a bit longer to generate, the loading of all subsequent ones will be delayed.
To work around this problem, and to optimize performance when keepalive requests or pipelining can’t be used, all browsers permit the opening of
several simultaneous HTTP connections to the destination server. This way,
the browser can issue multiple requests in parallel.
Unfortunately, the parallel connection design can be expensive for the
destination website, especially if the server relies on the traditional fork()based connection-handling architecture.* Therefore, in order to limit the
risk of accidentally or intentionally launching a distributed DoS attack, the
number of parallel connections needs to be limited to a modest per-host
value, typically between 4 and 16. Furthermore, to prevent attackers from
overloading the browser itself (or affecting the performance of the nearby
networking equipment), the total number of simultaneous connections to
all destinations is also constrained to a low multiple of the per-host cap.
NOTE

In many implementations, the per-host connection limit is enforced by looking at DNS
labels, not at IP addresses. Therefore, an attacker may still be able to point several
bogus DNS entries in his own domains to any unrelated target IP and circumvent
the first restriction. The global connection limit will be still in effect, though.
Although the number of concurrent HTTP sessions is limited, there are
no practical restrictions on how long an active session may be kept alive (that
is, as long as no kernel-level TCP/IP timeouts are encountered). This design
*
The traditional design of most Unix services is to have a master “listener” process, and then
create a new process for handling every accepted connection. For the developer, this model is
remarkable in its simplicity; but it comes with many significant hidden costs for the operating
system, which sometimes finds handling more than several hundred simultaneous connections
at once challenging.

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may make it possible for attackers to simply exhaust the global connection
limit by talking to a couple of intentionally slow destinations, preventing the
user from doing anything useful in the meantime.

Pop-Up Filtering
The window.open(...) and window.showModalDialog(...)* APIs permit web pages
to create new browser windows, pointing them to any otherwise permitted
URLs. In both cases, the browser may be instructed not to show certain window decorations for the newly loaded document or to position the window
on the screen in a specific way. A simple use of window.open(...) might look
like this:
window.open("/hello.html", "_blank", "menubar=no,left=50,top=50");

In addition to these two JavaScript methods, new windows may also be
opened indirectly by programatically interacting with certain HTML elements. For example, it is possible to call the click() method on an HTML link
or to invoke the submit() method on a form. If the relevant markup includes
a target parameter, the resulting navigation will take place in a new window
of a specified name.
As could be expected, the ability for random web pages to open new
browser windows soon proved to be problematic. In the late 1990s, many
players in the then-young online advertising industry decided they needed
to attract attention to their ads at any cost, even at the expense of profoundly
annoying and alienating their audiences. Automatically spawning windows
solely to show a flashy advertisement seemed like a great way to do business
and make new friends.
Pop-up and pop-under† advertisements have quickly emerged as one of
the best-known and most reviled aspects of the Web. For good reason, too:
Especially with pop-unders, it would not be unusual to amass a dozen of
them after two to three hours of casual browsing.
Due to widespread complaints, browser vendors stepped in and implemented a simple restriction: Spurious attempts by non-whitelisted pages to
create new windows would be silently ignored.‡ Exceptions were made for
attempts made immediately after a mouse click or a similar user action. For
*
The little-known showModalDialog(...) method is a bit of a misnomer. It is essentially equivalent
to window.open(...), but it is supposed to vaguely emulate the behavior of a modal dialog by blocking the scripts in the calling context until such a “dialog” window is dismissed. The exact behavior of this API varies randomly from one browser to another. For example, it is sometimes possible
for other pages to navigate the underlying window or execute new scripts while the original JS
code that called showModalDialog(...) is in progress.
†
A “pop-under” is a pop-up window that, immediately after its creation, is moved to the back of
the window stack with the help of opener.window.focus() or window.blur(). Pop-unders are arguably
slightly less distracting than pop-ups, because the user does not have to take immediate action to
go back to the original document. They are no less despised, however.
‡

For example, a call to window.open(...) would not generate an exception. The return value in
such a case is not standardized, however, making it difficult to detect a blocked pop-up reliably.
In Internet Explorer and Firefox, the function will return null; in Safari, it will return another
special value, undefined; in Opera, a dummy window handle will be supplied; and in Chrome, the
returned window handle will even have a quasi-functional DOM.
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example, in the case of JavaScript, the ability to call window.open(...) would be
granted to code executed in response to an onclick event and revoked shortly
thereafter. (In Internet Explorer and WebKit, this permission expires the
moment the event handler is exited. Other browsers may recognize a short
grace period of one second or so.)
The pop-up blocking feature initially curtailed pop-up advertising but, ultimately, proved to be fairly ineffective: Many websites would simply wait for the
user to click anywhere on the page (in order to follow a link or even scroll the
document) and spawn new windows in response. Others simply moved on to
even more disruptive practices such as interstitials—full-page advertisements
you need to click through to get to the content you actually want to read.
The advertising arms race aside, the war on window.open(...) is also interesting from the DoS perspective. Creating hundreds of thousands of windows,
thereby exhausting OS-enforced limits on the number of UI handles, is a
sure way to crash the browser and to disrupt other applications as well. Any
mechanism that limits this capability would be, at least in theory, a valuable
defense. No such luck: Unbelievably, only Internet Explorer and Chrome
sensibly limit the actual number of times window.open(...) can be called in
response to a single click. In other browsers, once the temporary permission
to open windows is granted, the attacker can go completely nuts and open as
many windows as she desires.

Dialog Use Restrictions
Window-related woes aside, all web-originating scripts can open certain
browser- or OS-handled dialogs. The usefulness of these dialogs to modern
web applications is minimal, but they still constitute another interesting part
of the browser security landscape. Dialog-initiating APIs include window
.alert(...), used to display simple text messages; window.prompt(...) and window
.confirm(...), used to request basic user input; and window.print(...), which brings
up the OS-level printing dialog. A couple of obscure vendor extensions, such
as Mozilla’s window.sidebar.addPanel(...) and window.sidebar.addSearchEngine(...)
(to create bookmarks and register new search providers, respectively), are
also on this list.
The aforementioned JavaScript methods aside, several types of dialogs
can be spawned indirectly. For example, it is possible to invoke the click()
method on a file upload button or to navigate to a downloadable file, which
usually brings up the OS-supplied file selection dialog. Navigating to a URL
that requires HTTP authentication will also typically bring up a browser-level
prompt.
So, what makes dialogs so interesting? The challenge with these prompts
is quite different from that of programmatically created windows. Unlike
the largely asynchronous window.open(...) API, dialogs pause the execution of
JavaScript and defer many other actions (such as navigation or event delivery), effectively preventing dialogs from being created in large numbers to
exhaust resources and crash the application. But their modal behavior is
also their curse: They prevent any interaction with some portion of the
browser until the user dismisses the dialog itself.
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This creates an interesting loophole. If a new dialog is opened immediately after the old one is closed, the victim may be locked out of a vital portion
of the browser UI, often even losing the ability to close the window or navigate away from the offending page. Malware authors sometimes abuse that
quirk to force casual, panicked users to perform a dangerous action (such as
downloading and executing an untrusted executable) just to be permitted to
continue their work: Making any other choice in the script-initiated security
prompt will only make the same dialog reappear over and over again.
Probably because of this malware-related tangent, browser vendors have
begun experimenting with less disruptive prompting methods. In Chrome,
for example, some of the most common modal dialogs have a checkbox that
allows the user to suppress future attempts by the page to use the offending
API (until the next reload, that is). In Opera, it is possible to stop the execution of scripts on the page. And in both Opera and recent versions of Firefox,
many common dialogs are modal
only in relation to the documentcontrolled area of the window, still
allowing the tab to be closed or a
different URL to be entered in the
address bar. Nevertheless, the coverage of such improvements is
limited.
NOTE

*Many

browser-level dialogs do a poor
job of explaining where the prompt
originated and its intended purpose.
In some cases, such as the Firefox dialog shown in Figure 14-1, the result
can be comical—and there is a more
sinister side to such goofiness, too.
Spawning authoritative-sounding dialogs that claim to be coming from the
operating system itself is a common
trick used by malware authors to confuse less experienced users. It’s not
hard to imagine why that works.

Figure 14-1: Firefox generated a profoundly
confusing and vague prompt following the execution of an onbeforeunload handler on a web
page. The handler gives page authors a chance
to explain the consequences of navigating away
from their page (such as losing any unsaved
data) and requests a final decision from the
user.* In this screenshot, the first and the last
line come from the browser itself; the middle
two lines are an “explanation” supplied by an
(unnamed!) rogue website instead. The security
impact of this particular dialog is minimal, but
it is a remarkable example of poor UI design.
Sadly, a nearly identical dialog is also used by
Internet Explorer, and most other browser dialogs are not much better.

Window-Positioning and Appearance Problems
All right, all right—let’s move beyond the arguably uninspiring and unpopular topic of DoS flaws. There is a lot more to the various UI-related APIs—
and window.open(...) is a particularly curious case. Recall from the discussion
earlier in this chapter that this humble function permits web applications not
only to create new windows but also to position them in a specific spot on the
*

For usability reasons, random pages on the Internet are no longer permitted to abort pending
navigation by means other than this specific onbeforeunload dialog. (Surprisingly, the by-design
ability to trap the user on a rogue page forever and cancel any navigation attempts wasn’t
received well.)
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screen. Several other methods, such as window.moveTo(...), window.resizeTo(...),
window.focus(), or window.blur(), further permit such a window to be moved
around the screen, scaled, or stacked in a particular way. Finally, window.close()
allows it to be discreetly disposed of when the script no longer needs it.
As with most other UI-manipulation features, these APIs soon proved to
be a source of pain. Following a series of amusing hacks that involved creating “hidden” windows by placing them partly or completely off-screen or by
making them really tiny, these functions now require newly created windows
to have certain minimal dimensions and to stay entirely within the visible desktop area. (It is still possible to create a window that constantly hops around
the screen and evades all mouse-driven attempts to close it, but given what
you’ve read so far, this deserves nothing but a heavy sigh.)
The restrictions on window
size do not mean that the entire
contents of the address bar have
to be visible to the user, however. An undersized window
could be leveraged to mislead
the user as to the origin of a
document simply by carefully
truncating the hostname, as
shown in Figure 14-2. Browser
vendors have been aware of this
problem since at least my report
in 2010,1 but as of this writing,
only Internet Explorer uses a
somewhat convincing if subtle
mitigation: It appends “...” at
the end of any elided hostFigure 14-2: A window carefully sized by a script
names in the address bar.
so that the real origin of the displayed content is
Another interesting issue
elided in a confusing way. The actual URL of this
with script-controlled window
cat-themed page is http://www.example.com
.coredump.cx/, not http://www.example.com/.
positioning is the prospect of
creating several cleverly aligned,
overlapping windows to form what appears to be a single document window
with an address bar that doesn’t correspond to portions of the document displayed. This attack, which I like to call window splicing, is perhaps best illustrated in Figure 14-3.
Window positioning offers some interesting if far-fetched attack scenarios, but manipulating the contents of a programmatically created window is
also of some relevance to browser security. We have already mentioned that
one of the features of the window.open(...) API is its ability to hide certain elements of the browser chrome (scrollbars, menus, and so on) in the newly
opened window. An example of such a UI-restricting call is
window.open("http://example.com/", "_blank", "location=no,menubar=no");

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Figure 14-3: A window-splicing attack in Chrome. What may appear as a single document
is actually a composite of two overlapping, aligned windows. The user is led to believe that
the file upload button comes from the domain shown in the address bar of the top window,
but it does not. Certain visual cues indicate foul play (for example, part of the window border has a slightly different hue), but they are too subtle to be easily noticed by the user.

One of these settings, location=no, was meant to hide the address bar.
This is, of course, a horrible idea: It enables the attacker not only to hide the
actual address bar but also to load a page that simply provides a pixel-perfect
image of the address bar showing a completely unrelated URL. Heck, with
some minimal effort, that fake address bar may even be fully interactive.
Realizing the dangers of this design, most browsers eventually began
displaying a minimalistic, read-only address bar in any windows opened with
location=no; Apple, however, sees no harm in allowing this setting to work as
originally envisioned in the 1990s. Too bad: Figure 14-4 shows a simple attack
on its UI. (I contacted Apple about this attack sometime in 2010 but have yet
to hear back.)

Figure 14-4: Allowing websites to hide the address bar in Safari is a bad idea.
The displayed document is not retrieved from http://www.example.com/.
Instead, the page simply displays a screenshot of a real address bar in a window created by window.open("http://coredump.cx/...", "location=no").
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Microsoft has not fared much better: Although they patched up
window.open(...), they forgot about window.createPopup(...), an ancient
and obscure API still not subject to the necessary checks.

Timing Attacks on User Interfaces
The problems we’ve discussed so far in this chapter may be hard to fix, but at
least in principle, the solutions are not out of reach. Still, here’s a preposterous question: Could the current model of web scripting be fundamentally
incompatible with the way human beings work? By that, I do not mean merely
the dangers of web-delivered social engineering that targets the inattentive
and the easily confused; rather, I’m asking if it’s possible for scripts to consistently outsmart alert and knowledgeable victims simply due to the inherent
limitations of human cognition?
The question is outlandish enough not to be asked often, yet the answer
may be yes. Consider that in a typical, attentive human subject, the usual
latency between a visual stimulus and a voluntary motor response is between
100 and 300 milliseconds.2 Humans do not pause for that long to assess the
situation after every minute muscle movement; instead, we subconsciously
schedule a series of learned motor actions well in advance and process any
sensory feedback as it arrives later on. For a split second, we cannot abort a
premeditated action, even if something goes horribly wrong.
Alas, on today’s personal computers, a lot can happen in as little as onetenth of that interval. In particular, scripts can open new windows, move them
around, or close any existing ones; they can also initiate or abort system-level
prompts. In such a setting, designing security-sensitive UIs is not nearly as
simple as it seems, and some types of attacks may be simply impossible to
defend against without a major paradigm shift in how we design software.
To illustrate the issue, consider a page that attempts to start an unsolicited
download of a dangerous file type. The download will typically initiate a browserlevel dialog with three options: “open,” “save,” and “cancel.” Sane users will
make that last choice—but not if the attacker robs them of a chance to do so.
Let’s assume that just milliseconds after the dialog is opened, and perhaps before the user even registers its presence, a new window is created
on top that hides it from view. In that window, the attacker plants a carefully
positioned button or link that the user is likely to click, for example, a button
to dismiss an annoying interstitial advertisement. As the user attempts to perform this perfectly reasonable action, the rogue page may use onmousemove
events to monitor the position and velocity of the mouse pointer and fairly
accurately predict the timing of an upcoming click. Closing the overlay window several milliseconds before that click, only to reveal the “open” button
in the same position, will lead the user inevitably to make that choice in the
security prompt. There is simply nothing the user can do. (I demonstrated a
practical attack on Firefox along these lines in 2007.)3
In response to the attacks on security dialogs, a variety of security
delays have been implemented in the past few years, requiring anywhere
from 500 milliseconds to 5 seconds between the dialog coming into focus
and any dangerous buttons being enabled for user input. But such delays do
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not sit well with browser UI designers: They hate them, feeling that the product should be as responsive as possible and that annoying the user with nonclickable buttons or countdowns is a significant usability issue. Some have even
pushed to remove existing timeouts from legacy UIs.* HTML5 geolocationsharing prompts are impacted by this view. Many browsers are not protected
against the attack on this UI in any significant way.4
To further complicate the picture, browser-level user interfaces are not
the only concern for UI-timing attacks. The security- or privacy-sensitive functionality of many trusted websites can also be attacked, and fixing that problem is a lot harder than adding delay timers on a handful of known
dangerous system-level UIs.
NOTE

Millisecond-level click or keypress hijacking aside, it has been repeatedly demonstrated
that with minimal and seemingly innocuous conditioning, healthy and focused test
subjects can be reliably tricked into ignoring even very prominent and unusual visual
stimuli. The infamous Invisible Gorilla experiment,5 shown in Figure 14-5, is a particularly well-known example of this. Almost all viewers watching a clip prepared by the
researchers fail to notice a plainly visible gorilla in a crowd. The corollary is that even
savvy users can be conditioned to ignore cues such as changes to the address bar or to
SSL indicators in the browser—a very disconcerting thought. The only reason why we
are not trying to solve this problem today is that few exploit writers are behavioral scientists. But if you are a high-profile target, this seems like a risky bet.

Figure 14-5: A single frame from the Invisible Gorilla experiment, courtesy
of Daniel Simons6 (http://dansimons.com/). When asked to view this video
and count the number of times the players pass the basketball, most viewers fail to notice a person in a gorilla suit casually strolling across the room
halfway through the clip. Really! Go to http://theinvisiblegorilla.com/
videos.html and try it on a friend.
*
See, for example, Mozilla bug 561177, where one of the Firefox UI engineers proposed the
removal of a security delay from the plug-in installation prompt.

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Security Engineering Cheat Sheet
When Permitting User-Created <iframe> Gadgets on Your Site
 Don’t do so unless you are prepared to live with the consequences. You can’t reliably
prevent a malicious gadget from launching DoS attacks on your users. Any such gadget
will also be able to bring up various obscure dialogs that, as a rule, will not distinguish
between your top-level page and the domain the gadget is hosted in.

When Building Security-Sensitive UIs
 Because of the risk of UI race conditions, avoid situations where a vital setting can be
changed with a single keypress or a single click. Require at least two operations (such
as selecting a checkbox and then clicking Save). If single-click actions are unavoidable,
consider examining other signals. For example, was the mouse pointer in the current
window 500 milliseconds ago?

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EXTRINSIC SITE PRIVILEGES

To wrap up the discussion of all the noteworthy browser
security features, we’ll look at a handful of mechanisms
that grant special privileges to sites hand-picked by the
user or hardcoded by the authors of the browser itself.
The approach taken in these cases is in stark contrast to
the schemes we have discussed previously, all of which
rely on a fairly sensible examination of intrinsic properties of the displayed
content. Normally, the implementation would have us look at the source of
the document, the context it is displayed in, or the nature of the operation
that the document is attempting to perform, but barring the outcome of these
checks, the browser would never give preferential treatment to a single otherwise unremarkable origin.
Per-site privileges violate this principle of impartiality in a fairly brutal
way, for reasons ranging from questionable to—more commonly—just utilitarian. There are compelling usability reasons to bring certain inherently
dangerous features to the browser world, but there is no good way to

programmatically decide which web applications are trustworthy enough to
be given access to them. Delegating this task to a human being may be the
best thing we can do.*
Naturally, the creation of a caste of privileged applications can be very
problematic because the boundaries between any two web applications are
not particularly well defined to begin with, making it difficult to contain the
permissions precisely. And because the already imperfect boundaries apply
only to certain cross-site interactions, vulnerabilities such as XSS or XSRF may
further contribute to the misery. In the end, a significant disconnect may
develop between the intent of a per-site permission and the actual consequences of such a grant.

Browser- and Plug-in-Managed Site Permissions
When balancing security, privacy, and usability, browser vendors sometimes
find themselves between a rock and a hard place. Some proposed features
seem essential to the continued growth of the Web but are simply too dangerous to be made available to every website on the Internet. Examples of
such problematic mechanisms include giving access to video camera or microphone feeds,† allowing websites to query for user geolocation data,‡ installing
browser extensions or themes, or opening desktop notifications.
As a work-around for this problem, vendors require the user to approve
the application’s request in order for it to be allowed to access a privileged API.
On the first attempt to use restricted functionality, the user is typically provided with a visual cue (ranging from an icon to a modal prompt) and given
three choices: ignore the request, permit it once, or permanently authorize
the requesting site to access the API. Of these choices, the last one is the most
interesting: If selected, all future access from a matching host will be automatically approved, sometimes without any further visual indication.
NOTE

Most whitelists look only at the hostname, and not at the protocol or port. Any entry on
these lists will therefore match more than one SOP origin. In particular, authorizing
https://fuzzybunnies.com/ to access your camera may also authorize the nonencrypted site at http://fuzzybunnies.com/ to do the same.
Granting websites access to privacy- or security-sensitive features should
be done with care, because, as noted earlier, the implications of doing so
extend beyond merely trusting the authors of the whitelisted application.
*

It is fair to complain that browsers do not do much to equip users with affirmative signals about
the trustworthiness of a visited site, even though many robust indicators may plausibly be arrived
at in an automated way. Blacklist-driven attempts to block known malicious sites exist, but given
the negligible cost of registering a new domain (or compromising a random existing one), these
approaches are arguably of less value.

†
This functionality is currently supported only by plug-ins, such as Adobe Flash, but on track to
become a part of HTML5.
‡
This API derives user location from parameters such as the current IP address, the list of
nearby wireless networks or cell towers, or the data supplied by a hardware GPS receiver. With
the exception of GPS data, it may be necessary to consult an external service provider to map
these inputs to physical coordinates.

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Permission is granted to any content executed in the matching origin, regardless of how the payload got there, greatly amplifying the impact of simple (and,
in the long run, inevitable) implementation bugs. A script injection vulnerability in a privileged origin no longer merely exposes the data stored within
the application but may also leak client-originating sensitive data feeds.

Hardcoded Domains
In addition to the list of user-authorized privileged domains, some browsers
or browser plug-ins come with a list of vendor-selected sites or SOP origins
that are given substantial privileges to reconfigure or update portions of the
browser or the operating system. Some of the most prominent examples of
this trend include update.microsoft.com, which is recognized by ActiveX controls that ship with Microsoft Windows and is allowed to install software
updates; addons.mozilla.org and chrome.google.com, recognized by their corresponding browsers and given special privileges to install extensions or themes;
or www.macromedia.com, which is allowed to reconfigure Adobe Flash.
The designs of these mechanisms vary and, as a rule, are not documented
in a satisfactory way. Some features require second-level verification, such as
a cryptographic signature or user consent, but others do not. Broadly speaking, the proliferation of such privileged domains is troubling, because it is
clear that they will not be immune to the usual security problems that plague
the rest of the modern Web. Case in point: http://xssed.com/ lists six publicly
reported XSS vulnerabilities in addons.mozilla.org.1

Form-Based Password Managers
Surprised? Don’t be. Mentioning password managers may seem out of place,
but it is very useful to consider this technology as an indirect form of a sitebound privilege. Before we explain, let’s briefly review why password management is implemented in modern browsers to begin with and how it actually
operates.
The answer to the first question is fairly simple: Today, almost every
major website requires, or at least strongly encourages, all visitors to open an
account. Logging in is typically necessary in order to customize the appearance of the site and is a prerequisite for interacting with other registered
users. Unfortunately, these site-specific authentication systems are not synchronized (save for several limited-scale “federated login” experiments, such
as OpenID),2 and they effectively force the general population to create and
memorize several dozen robust passwords, one for every destination frequented. This approach is difficult to sustain and leads to rampant and dangerous password reuse; that’s where browser vendors decided to step in.
Form-based password managers are an inelegant but pragmatic solution
to the problem of coping with the proliferation of per-site credentials. They
apply simple heuristics to detect the submission of normal-looking login
forms (the browser looks for an <input type=password> field and then perhaps
examines the names of form fields for strings such as user and pass). When a
suitable form is detected, the browser will offer to save the associated login
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information in a persistent store on the hard drive,* and if the user consents,
it will then automatically retrieve and paste this data into matching forms
encountered later on. In Firefox, Chrome, and Safari, the process of retrieving a stored password is automatic; in Internet Explorer and Opera, an additional user gesture may be required to confirm the intent.
The design of password managers is fragile but has one clear benefit:
It works right away even without official support (or, for that matter, informed
consent) from any websites. Web applications that are unhappy about this
feature may opt out by appending a poorly named autocomplete=off parameter
to the offending password field,† but beyond that, the process is almost completely seamless.
The primary way that every in-browser password manager protects stored
data is by tying the credentials to the SOP origin where they were originally
entered—paying close attention to the hostname, protocol, and port. Some
browsers also consider secondary indicators, such as the ordering or naming
of form fields, the URL path to the form, or the address to which the credentials are sent. (As we know from Chapter 9, such scoping measures are not
particularly useful from the security standpoint due to the operation of the
same-origin policy.)
In browsers that autocomplete login forms without the need for human
interaction, it is sensible to look at the mechanism as a form of a privileged
API: Any content executing in the appropriate origin will be able to request
browser-stored credentials by constructing a believable-looking form and
then waiting for it to be automatically populated with login data. In order to
read back this information, the script merely needs to examine the value
property of the DOM element associated with the password field.
NOTE

Removing the ability to inspect values of password fields may seem like a simple way to
improve the scheme, but it is not a very good one. The data could still be stolen by, say,
waiting for password autocompletion, changing the data submission method from
POST to GET, and then calling submit() on the login form. These steps would result
in navigation to a page that has the password plainly visible in the location.search
string. (Plus, many web applications have legitimate uses for reading back these fields
on the client side, for example, to advise on password strength.)
As should be clear, the most serious risk associated with password managers
is the amplification of XSS bugs. In web applications that use httponly cookies,
a successful exploitation of an XSS flaw may give the attacker only transient
access to a user’s account, but if the same vulnerability can be leveraged to
steal a user’s password, the consequences are more dire and longer-lived.‡
*
This data may be stored on disk as a plaintext representation, a naïvely obfuscated string, or a
properly encrypted value protected with a “master” password that needs to be entered beforehand. All three methods are comparably vulnerable to determined attackers with access to the
local system, but the plaintext approach is sometimes frowned upon, as it is more exposed to
nosy but nontechnical users.
†
Despite the name, this stops the browser from recording the password and not just from
autocompleting it.
‡
Such consequences may extend beyond the affected application: Even with password managers
in place, password reuse is a common, unfortunate trend.

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More obscure side effects are possible, too. For example, any application that
allows users to construct custom form-based surveys must carefully restrict the
layout of the generated forms or risk doubling as a password-harvesting tool.

Internet Explorer’s Zone Model
Internet Explorer’s zone model3 is a proprietary attempt to reconcile the different security requirements that users (or system administrators) may have
for different types of web applications, for example, a banking page and an
online game. Microsoft’s approach is to establish several predefined classes
of websites—known as zones—each with its own set of configurable security
permissions. The five supported zones are these:


My computer (aka local machine) This hidden zone is used for all local
file: resources (with one exception—more about it soon). The user cannot add or remove any elements from this set and cannot change its
security settings through the normal user interface. Administrators and
developers can modify the registry or use urlmon.dll hooks to override
settings, however.



Local intranet This zone is meant to include trusted applications on a
user’s local network. By default, local intranet enjoys many problematic
privileges, such as unrestricted access to the system clipboard, the ability
to open windows without an address bar, or the ability to bypass the usual
frame navigation security checks (the descendant policy, outlined in
Chapter 11). Members of this set are detected automatically using several
configurable heuristics, and they may include destinations with non–fully
qualified hostnames, addresses on the HTTP proxy exemption list,* or
remote file: URLs accessed over SMB. Manual inclusion of sites in this
zone is also possible (in addition to or instead of the built-in heuristics).
NOTE

The local intranet zone makes an implicit connection between a local network and a trusted environment. This connection is often dubious in the
modern-day environment, especially given the prevalence of public Internet access over unencrypted Wi-Fi: Other uses of the network are not any
more trustworthy than a random website hosted across the globe.



Trusted sites These are nominally empty zones roughly equivalent to
local intranet in terms of their security settings but managed solely by the
user. Autodetection heuristics are unavailable, and all entries have to be
created by hand.



Restricted sites In these nominally empty zones, the user may add
“untrusted” destinations. The default settings for these zones remove
many rudimentary and generally harmless capabilities from the loaded
content (for example, Refresh headers will not work) while offering limited security benefits.

*
In configurations where a proxy is required to access protected internal systems but not
required to access the Internet, these may have the unintended and scary effect of classifying
the entire Web as a local network.

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The practicality of this zone seems unclear. Because of the need to
whitelist every untrusted site, the zone obviously can’t be relied upon as
an alternative to browsing the Internet with sensible default settings for
previously unseen destinations.


Internet This is a default zone for sites not included in any of the
remaining categories. Its default settings match the general browser
security model baseline discussed previously in this book.

The concept of zones, coupled with some of their security controls, seems to
be a step in the right direction. For example, it allows system administrators to
fine-tune the permissions for file: documents without affecting the security
or convenience of normal browsing—or to prohibit Internet sites from navigating to local, corporate systems (using the setting named “Websites in less
privileged web content zone can navigate into this zone”). Unfortunately, the
actual implementation of the zone model is muddied by a lack of focus, and
in practice, it is misused more often than it is genuinely benefited from.
The first problem evident to anyone trying to master the zone mechanism is its obtuse terminology and the almost-comical complexity of many
of the settings. Every zone comes with over 100 checkboxes; some of these
will alter the browser security model profoundly, while others have no security consequences whatsoever. (The aforementioned Refresh setting is one
example of a security no-op; the ability to disable form submission is another.)
These two classes of settings are not distinguished in any clear way, and many
are nearly impossible to comprehend at a glance. For example, the option
“Binary and script behaviors” can be set to “enable” or “disable,” but the help
subsystem offers no information about what either setting will actually do.
The only explanation is provided in the official developer documentation
posted on Microsoft’s site—but even this document can confuse.4 See for
yourself:
Internet Explorer contains dynamic binary behaviors: components
that encapsulate specific functionality for HTML elements to which
they were attached. These binary behaviors are not controlled by
any Internet Explorer security setting, allowing them to work on
Web pages in the Restricted Sites zone. In Windows Server 2003
Service Pack 1, there is a new Internet Explorer security setting for
binary behaviors. This new setting disables binary behaviors in the
Restricted Sites zone by default. In combination with the Local
Machine Lockdown security feature, it also requires administrative
approval for binary behaviors to run in the Local Machine zone by
default. This new binary behaviors security setting provides a general
mitigation to vulnerabilities in Internet Explorer binary behaviors.

There are many similar cases of settings that require a substantial effort
to understand. For example, it is unlikely that even the most seasoned administrators will understand the implications of tweaking settings named “Access
data sources across domains” or “Navigate windows and frames across different domains”. All this confusion has an interesting consequence: Trusted
parties unintentionally dispense dubious advice. For example, Charles Schwab,
a prominent investment bank, tells customers to disable the frame navigation
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descendant model,5 essentially making HTML frames unsafe to use not only
for Charles Schwab but also for any other website. One of the sites maintained by the Internal Revenue Service provides the same, extremely inconsiderate tip.6
The complexity and poor documentation of Internet Explorer’s zone
settings aside, the other problem with the zone model is the clustering of
unrelated permissions. The settings for local intranet and trusted sites containers
enable a random collection of features that may be required by some trusted
sites—but none of the trusted sites could possibly require all of the permissions
the zone entails. Because of this design, adding sites to privileged zones can
once more have unexpectedly far-ranging consequences in the case of, say,
a trivial XSS flaw.

Mark of the Web and Zone.Identifier
To maintain the integrity of the zone model on downloaded files, Internet
Explorer further utilizes two overlapping mechanisms to track the original
zone information for any externally retrieved document:


Mark of the Web (MotW) This simple pseudo-HTML tag is inserted at
the beginning of HTML documents downloaded via Internet Explorer
to indicate their initial source.7 One example of a MotW tag may be
<!-- saved from url=(0024)http://fuzzybunnies.com/ -->. The URL recorded in
this tag is mapped to an appropriate zone; the document is then opened
in a unique origin in that zone. The most important consesequence is
that the downloaded content is isolated from other file: URLs.
NOTE



The inline nature of MotW is one of its flaws. Faux tags can be preinserted by rogue parties into HTML documents downloaded through
non–Internet Explorer browsers, saved from email clients, or downloaded
by Internet Explorer with a non-HTML extension (and then subjected to
content sniffing). Though, to be fair, the privileges of file: documents
saved without any MotW tags are significant enough to keep attackers
relatively uninterested in hopping from the My Computer zone to, say,
Local Intranet.

Alternate Data Stream (ADS) Zone Identifier This is a piece of NTFS
metadata attached by Internet Explorer (and Chrome) to every downloaded file, indicating the numerical code of the zone the file was
retrieved from.8 The Zone.Identifier mechanism is less portable than
MotW, and the information is lost when files are saved to non-NTFS
filesystems. However, it is also more versatile, as it can be applied to
non-HTML documents.
Zone.Identifier metadata is recognized by Internet Explorer itself, by
the Windows GUI shell, and by some other Microsoft products, but thirdparty software almost universally ignores it. Where it is supported, it may
result in a more restrictive security policy being applied to the document; more commonly, it just pops up a security warning about the
unspecified risks of opening Internet-originating data.
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Security Engineering Cheat Sheet
When Requesting Elevated Permissions from Within a Web Application
Keep in mind that requesting access to geolocation data, video or microphone feeds, and other
privileged APIs comes with responsibility. If your site is prone to XSS vulnerabilities, you are
gambling not only with the data stored in the application but with the privacy of your users.
Plan accordingly and compartmentalize the privileged functionality well. Never ask your users
to lower their Internet Explorer security settings to accommodate your application, and do
not blindly follow this advice when given by others—no matter who they are.

When Writing Plug-ins or Extensions That Recognize Privileged Origins
You are putting your users at elevated risk due to inevitable web application security bugs.
Design APIs robustly and try to use secondary security measures, such as cryptography, to further secure communications with your server. Do not whitelist nonencrypted origins, as they
are prone to spoofing on open wireless networks.

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PART III
A GLIMPSE OF THINGS
TO COME

Following nearly a decade of stagnation, the world of
browsers is once more a raging battlefield. In a manner all too reminiscent of the First Browser Wars in the
late 1990s, vendors compete by bringing new features
to market monthly. The main difference is that security is now seen as a clear selling point.
Of course, objectively measuring the robustness of any sufficiently
complex piece of software is an unsolved problem in computing, doubly so
if your codebase happens to carry almost two decades worth of bloat. Therefore, much of the competitive effort goes into inventing and then rapidly
deploying new security-themed additions, often with little consideration for
how well they actually solve the problem they were supposed to address.
In the meantime, standards bodies, mindful of their earlier misadventures,
have ditched much of their academic rigor in favor of just letting a dedicated
group of contributors tweak the specifications as they see fit. There is talk of
making HTML5 the last numbered version of the standard and transitioning
to a living document that changes every day—often radically. The relaxation

of the requirement has helped keep ongoing much of the work around W3C
and WHATWG, but it has also undermined some of the benefits of having a
central organization to begin with. Many recent proposals gravitate toward
quick, narrowly scoped hacks that do not even try to form a consistent and
well-integrated framework. When this happens, no robust feedback mechanism is in place to allow external experts to review reasonably stable specifications and voice concerns before any implementation work takes place. The
only way to stay on top of the changes is to immerse oneself in the day-to-day
dynamics of the working group.
It is difficult to say if this new approach to standardization is a bad thing.
In fact, its benefits may easily outweigh any of the speculative risks; for one,
we now have a chance at a standard that is reasonably close to what browsers
actually do. Nevertheless, the results of this frantic and largely unsupervised
process can be unpredictable, and they require the security community to be
very alert.
In this spirit, the last part of the book will explore some of the more plausible and advanced proposals that may shape the future of the Web . . . or that
may just as likely end up in the dustbin of history a few years from now.

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NEW AND UPCOMING
SECURITY FEATURES

You will soon find out that there is little rhyme and reason to how all the new browser features mesh, but we
still need to organize the discussion in some way. Perhaps the best approach is to look at their intended
purposes and begin with all the mechanisms created
specifically to tweak the Web’s security model for a
well-defined gain.
The dream of inventing a brand-new browser security model is strong
within the community, but it is always followed by the realization that it would
require rebuilding the entire Web. Therefore, much of the practical work
focuses on more humble extensions to the existing approach, necessarily
increasing the complexity of the security-critical sections of the browser
codebase. This complexity is unwelcome, but its proponents invariably see it
as justified, whether because they aim to mitigate a class of vulnerabilities,

build a stopgap for some other hard problem that nobody wants to tackle
right now,* or simply enable new types of applications to be built in the
future. All these benefits usually trump the vague risk.

Security Model Extension Frameworks
Some of the most successful security enhancements proposed in the past few
years boil down to adding flexibility to the original constraints imposed by the
same-origin policy and its friends. For example, one formerly experimental
proposal that has now crossed into the mainstream is the postMessage(...) API
for communicating across origins, discussed in Chapter 9. Surprisingly, the
act of relaxing SOP checks in certain carefully chosen scenarios is more intuitive and less likely to cause problems than locking the policy down. So, to
begin on a lighter note, we’ll focus on this class of frameworks first.

Cross-Domain Requests
Under the original constraints of the same-origin policy, scripts associated
with one origin have no clean and secure way to communicate with clientside scripts executing in any other origin and no safe way to retrieve potentially useful data from a willing third-party server.
Web developers have long complained about these constraints, and
in recent years, browser vendors have begun to listen to their demands. As
you recall, the more pressing task of arranging client-side communications
between scripts was solved with postMessage(...). The client-to-server scenario
was found to be less urgent and still awaits a canonical solution, but there
has been some progress to report.
The most successful attempt to create a method for retrieving documents from non-same-origin servers began in 2005. Under the auspices of
W3C, several developers working on VoiceXML, an obscure document format for building Interactive Voice Response (IVR) systems, drafted a proposal for Cross-Origin Resource Sharing (CORS).1 Between 2007 and 2009, their
awkward, XML-based design gradually morphed into a much simpler and
more widely useful scheme, which relied on HTTP header–level signaling
to communicate consent to cross-origin content retrieval using a natural
extension of the XMLHttpRequest API.
CORS Request Types
As specified today, CORS relies on differentiating between two types of calls
to the XMLHttpRequest API. When the site attempts to load a cross-origin document through the API, the browser first needs to distinguish between simple
requests, where the resulting HTTP traffic is deemed close enough to what
*

Malicious URL blacklists, a feature supported by (and usually enabled in) all modern browsers,
are a prime example of this trend. The blacklist is a lightweight, crude substitute for an antivirus,
which is, in turn, a poor substitute for up-to-date and well-designed software. Antimalware features do not make individual attacks any more difficult; they are simply meant to stop the largescale distribution of unsophisticated malware, based on the assumption that most users are not
interesting enough to be specifically targeted or attacked with something clever.

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can be generated through other, existing methods of navigation, and nonsimple requests, which encompass everything else. The operation of these two
classes of requests vary significantly, as we’ll see.
The current specification says that simple requests must have a method
of GET, POST, or HEAD. Additionally, if any custom headers are specified
by the caller, they must belong to the following set:


Cache-Control



Content-Language



Content-Type



Expires



Last-Modified



Pragma

Today, browsers that support CORS simply do not allow methods other
than GET, POST, and HEAD. At the same time, they ignore the recommended whitelist of headers, unconditionally demoting any requests with
custom header values to non-simple status. The implementation in WebKit
also considers any payload-bearing requests to be non-simple. (It is not clear
whether this is an intentional design decision or a bug.)
Security Checks for Simple Requests
The CORS specification allows simple requests to be submitted to the destination server immediately, without attempting to confirm whether the destination is willing to engage in cross-domain communications to begin with.
This decision is based on the fact that the attacker may initiate fairly similar
cookie-authenticated traffic by other means (for example, by automatically
submitting a form) and, therefore, that there is no point in introducing an
additional handshake specifically for CORS.*
The crucial security check is carried out only after the response is
retrieved from the server: The data is revealed to the caller through the
XMLHttpRequest API only if the response includes a suitable, well-formed
Access-Control-Allow-Origin header. To assist the server, the original request
will include a mandatory Origin header, specifying the origin associated
with the calling script.
To illustrate this behavior, consider the following cross-domain
XMLHttpRequest call performed from http://www.bunnyoutlet.com/:
var x = XMLHttpRequest();
x.open('GET', 'http://fuzzybunnies.com/get_message.php?id=42', false);
x.send(null);

*
That assumption is not completely correct. For example, prior to the introduction of this
scheme, attackers would not have been able to initiate a cross-domain request completely
indistinguishable from the submission of a file upload form, but under CORS, such forgery
is possible.

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The result will be an HTTP request that looks roughly like this:
GET /get_message.php?id=42 HTTP/1.0
Host: fuzzybunnies.com
Cookie: FUZZYBUNNIES_SESSION_ID=EA7E8167CE8B6AD93D43AC5AA869A920
Origin: http://www.bunnyoutlet.com

To indicate that the response should be readable across domains, the
server needs to respond with
HTTP/1.0 200 OK
Access-Control-Allow-Origin: http://www.bunnyoutlet.com
The secret message is: "It's a cold day for pontooning."

NOTE

It is possible to use a wildcard (“*”) in Access-Control-Allow-Origin, but do so with
care. It is certainly unwise to indiscriminately set Access-Control-Allow-Origin: *
on all HTTP responses, because this step largely eliminates any assurances of the sameorigin policy in CORS-compliant browsers.
Non-simple Requests and Preflight
In the early drafts of the CORS protocol, almost all requests were meant to
be submitted without first checking to see if the server was actually willing to
accept them. Unfortunately, this design undermined an interesting property
leveraged by some web applications to prevent cross-site request forgery:
Prior to CORS, attackers could not inject arbitrary HTTP headers into crossdomain requests, so the presence of a custom header often served as a proof
that the request came from the same origin as the destination and was issued
through XMLHttpRequest.
Later CORS revisions corrected this problem by requiring a more complicated two-step handshake for requests that did not meet the strict “simple
request” criteria outlined in “CORS Request Types” on page 236. The handshake for non-simple requests aims to confirm that the destination server is
CORS compliant and that it wants to receive nonstandard traffic from that particular caller. The handshake is implemented by sending a vanilla OPTIONS
request (“preflight”) to the target URL containing an outline of the parameters of the underlying XMLHttpRequest call. The most important information
is conveyed to the server in three self-explanatory headers: Origin, AccessControl-Request-Method, and Access-Control-Request-Headers.
This handshake is considered successful only if these parameters are
properly acknowledged in the response through the use of Access-ControlAllow-Origin, Access-Control-Allow-Method, and Access-Control-Allow-Headers. Following a correct handshake, the actual request is made. For performance
reasons, the result of the preflight check for a particular URL may be cached
by the client for a set period of time.

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Current Status of CORS
As of this writing, CORS is available only in Firefox and WebKit-based browsers and is notably absent in Opera or Internet Explorer. The most important
factor hindering its adoption may be simply that the API is not as critical as
postMessage(...), its client-side counterpart, because it can be often replaced
by a content-fetching proxy on the server side. But the scheme is also facing
three principal, if weak, criticisms, some of which come directly from one of
the vendors. Obviously, these criticisms don’t help matters.
The first complaint, voiced chiefly by Microsoft developers and echoed
by some academics, is that the scheme needlessly abuses ambient authority.
They argue that there are very few cases where data shared across domains
would need to be tailored based on the credentials available for the destination site. The critics believe that the risks of accidentally leaking sensitive
information far outweigh any benefits and that a scheme permitting only
nonauthenticated requests to be made would be preferable. In their view,
any sites that need a form of authentication should instead rely on explicitly
exchanged authentication tokens.*
The other, more pragmatic criticism of CORS is that the scheme is needlessly complicated: It extends an already problematic and error-prone API
without clearly explaining the benefits of some of the tweaks. In particular, it
is not clear if the added complexity of preflight requests is worth the peripheral benefit of being able to issue cross-domain requests with unorthodox
methods or random headers.
The last of the weak complaints hinges on the fact that CORS is susceptible to header injection. Unlike some other recently proposed browser features, such as WebSockets (Chapter 17), CORS does not require the server to
echo back an unpredictable challenge string to complete the handshake. Particularly in conjunction with preflight caching, this may worsen the impact of
certain header-splitting vulnerabilities in the server-side code.

XDomainRequest
Microsoft’s objection to CORS appears to stem from the aforementioned
concerns over the use of ambient authority, but it also bears subtle overtones
of their dissatisfaction with interactions with W3C. In 2008, Sunava Dutta, a
program manager at Microsoft, offered this somewhat cryptic insight:2
During the [Internet Explorer 8] Beta 1 timeframe there were
many security based concerns raised for cross domain access of
third party data using cross site XMLHttpRequest and the Access
Control framework. Since Beta 1, we had the chance to work with
other browsers and attendees at a W3C face-to-face meeting to
improve the server-side experience and security of the W3C’s
Access Control framework.

*
The same claim can be made about the use of HTTP cookies in any other setting and seems
equally futile. It is true that ambient credentials cause problems more frequently than some
other forms of explicit authentication would, but they are also a lot more convenient to use
and are simply not going away.

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Instead of embracing the CORS extensions to XMLHttpRequest, Microsoft decided to implement a counterproposal, dubbed XDomainRequest.3 This
remarkably simple, new API differs from the variant available in other browsers in that the resulting requests are always anonymous (that is, devoid of any
browser-managed credentials) and that it does not allow for any custom HTTP
headers or methods to be used.
The use of Microsoft’s API is otherwise very similar to XMLHttpRequest:
var x = new XDomainRequest();
x.open("GET", "http://www.fuzzybunnies.com/get_data.php?id=1234");
x.send();

Borrowing from W3C’s proposal, the resulting request will bear an Origin
header, and the response data will be revealed to the caller only if a matching Access-Control-Allow-Origin header is present in the response.* Preflight
requests and permission caching are not a part of the design.
For all intents and purposes, Microsoft’s solution is far more reasonable
than CORS: It is simpler, safer, and probably just as functional in all the plausible uses. That said, it is also unpopular. It is supported only in Internet
Explorer 8 and up, and owing to W3C backing CORS, others have no reason
to embrace XDomainRequest anytime soon.
In the meantime, a separate group of researchers have proposed a third
solution, again acting under the auspices of W3C. Their design, known as Uniform Messaging Policy (complete with a corresponding UniformRequest API),4
embraces an approach nearly identical to Microsoft’s. It is not supported in
any existing browser, but there is some talk of unifying it with CORS.

Other Uses of the Origin Header
The Origin header is an essential part of CORS, XDomainRequest, and UMP,
but it actually evolved somewhat independently with other uses in mind. In
their 2008 paper, Adam Barth, Collin Jackson, and John C. Mitchell5 advocated the introduction of a new HTTP header that would offer a more reliable and privacy-conscious alternative to Referer. It would also serve as a way
to prevent cross-site request vulnerabilities by providing the server with the
information needed to identify the SOP-level origin of a request, without
disclosing the potentially more sensitive path or query data.
Of course, it was unclear whether the subtle improvement between
Referer and its proposed successor would actually make a difference for the
small but nonnegligible population of users who block that first header on
privacy grounds. The proposal consequently ended up in a virtual limbo,
not being deployed in any existing browsers but also discouraging others
from pursuing other solutions such as XSRF or XSSI.6 (To be fair, the concept was very recently revived under the new name of From-Origin and may
not be completely dead yet.)7

*

The reason for this check, even if the response is not authenticated, is to prevent the use of the
browser as a proxy (for example, to crawl internal networks or send out spam).

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The fate of the original idea aside, the utility of the Origin header in specialized cases such as CORS was pretty clear. Around 2009, this led to Barth
submitting an IETF draft specifying the syntax of the header,8 while shying
away from making any statements about when the header should be sent, or
what specific security problems it might solve:
The user agent MAY include an Origin header in any HTTP
request.
[…]
Whenever a user agent issues an HTTP request from a “privacysensitive” context, the user agent MUST send the value “null” in
the Origin header.
NOTE: This document does not define the notion of a privacysensitive context. Applications that generate HTTP requests can
designate contexts as privacy-sensitive to impose restrictions on
how user agents generate Origin headers.

The bottom line of this specification is that whatever the decision process is, once the client chooses to provide the header, the value is required to
accurately represent the SOP origin from which the request is being made.
For example, when a particular operation takes place from http://www
.bunnyoutlet.com:1234/bunny_reports.php, the transmitted value should be
Origin: http://www.bunnyoutlet.com:1234

For origins that do not meaningfully map to a protocol-host-port tuple,
the browser must send the value of null instead.
Despite all of these plans, as of this writing only one browser includes the
Origin header on non-CORS navigation: WebKit-based implementations send
it when submitting HTML forms. Firefox seems to be considering a different
approach, but nothing specific seems to have been implemented yet.

Security Model Restriction Frameworks
Designs that extend the bounds of the same-origin policy are fairly simple to
understand and typically fail securely. If the proposed change is not accounted
for in one of the possible code paths, or is simply not supported in a particular browser, the previously implemented, more restrictive logic will kick in.
Compared with this, it is far more dangerous to try to erect new boundaries
on top of the existing browser security model. That’s because every securitysensitive code path must be tweaked to recognize the new scheme and every
browser must comply right away, or unexpected problems will arise.
In this section, we will take a quick look at some of the more accomplished
attempts to take this dangerous but potentially rewarding path—and explore
where they come apart.

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Content Security Policy
Content Security Policy (CSP) is an unusually comprehensive security framework first proposed by Brandon Sterne of Mozilla in 2008.9 The framework
was originally envisioned as an all-encompassing way to mitigate the impact
of common web vulnerabilities, from XSRF to XSS, and as a tool for website
owners to perform a variety of non-security content-policing tasks.
In the years that followed, CSP evolved rapidly, and on several occasions,
its scope changed in major ways. (For example, the author quickly abandoned
the plan to address XSRF vulnerabilities, delegating the job to the yet unrealized extensions of the Origin header.) In fact, as of this writing, the canonical
Mozilla specification is being rewritten as a W3C draft,10 resulting in substantial
differences in the implementation shipped in Firefox and the partial support
implemented in WebKit by Adam Barth. (Internet Explorer and Opera do not
support CSP and have not announced any specific plans to embrace it.)
Primary CSP Directives
At its core, Sterne’s design permits site owners to specify per-document policies that constrain the ability of the subject document to perform actions that
would normally be permitted under the same-origin policy. For example, CSP
may prevent a page from loading any external subresources except for images
and restrict image sources to only a set of trusted origins, like so:
X-Content-Security-Policy: default-src 'none'; img-src http://*.example.com

As should be evident from this example, the policies may be encoded in
an HTTP header. Under the W3C draft, it is also possible to embed them in
the document itself (using <meta> tags) or host the policy at an external URL
and point to it with policy-uri.
For every content source directive, the author of the policy may specify
any number of fully qualified origins or wildcard expressions that match multiple hosts, protocols, or ports. Three special keywords (none, self, and data:)
correspond to an empty set, the origin associated with the policy-bearing
page, or all inline data: URLs, in corresponding order.
As of today, the following behaviors can be controlled with CSP directives:


Script execution A script-src directive can be used to specify the protocol, host, and port for permissible <script src=...> URLs. Normally, the CSP
disables the ability to embed scripts inline in the document (whether
through standalone <script> blocks or via event handlers) and of existing
scripts to carelessly pass strings to functions such as eval(...), setTimeout(...),
setInterval(...), and so on. Because of this, the script-src directive is useful
for limiting the impact of XSS vulnerabilities: Any markup injected by
the attacker will be limited to loading scripts legitimately hosted in one
of the approved origins.*

*
CSP offers several ways to shoot yourself in the foot here. For one, it is possible to re-enable script
execution with settings such as inline-script (Mozilla’s naming, changed to disable-xss-protection in
W3C draft) or eval-script. Perhaps less obviously, it is also possible to make the mistake of permitting data: or * as a permissible origin or allowing an HTTP origin on an HTTPS site.

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NOTE



Plug-in content This is controlled through object-src. Because plug-ins
such as Java or Flash may have unconstrained access to the embedding
page, the directive should be considered largely analogous to script-src,
and the two directives must be restricted in a comparable way to achieve
any security benefits.



Stylesheets and fonts This is controlled by style-src and font-src. Unlike its
handling of scripts, CSP originally did not prevent inline <style> blocks
or style= parameters from appearing on the page. Therefore, any attacker
exploiting an XSS flaw could dramatically alter the appearance and function of the vulnerable page (or worse),* and these two directives only served
nonsecurity goals, with the possible exception of limiting mixed-content
bugs. Only moments before the publication of the book, the specifications have been amended to include a more robust approach to CSS.



Passive multimedia Directives such as img-src or media-src control the
ability to embed multimedia content from specific origins. As with the
original design of CSS controls, this could not have been considered a
security feature. For example, in the case of an XSS bug, CSP would not
have prevented the attacker from leveraging stylesheets to draw arbitrary
shapes on the vulnerable page or even animating them to some extent.



Subframes The frame-src directive specifies the acceptable destinations
for any <iframe> tags encountered on the page; the policy of the parent
page is not inherited by the framed document. To preserve the value of
other XSS mitigations, steps must be taken not to allow data: URLs here
(see Chapter 10).



Default policy Known as default-src in the W3C draft, and under a more
cryptic name (allow) in Mozilla documentation, the directive specifies fallback behavior for any content not covered by a more specific directive.
The directive is required, even in cases where it is technically unnecessary.

It may be unfortunate that CSP directives are selected to map very closely to individual
HTML tags, instead of grouping functionally similar behaviors. Because of this, it is difficult to appreciate the tricky interactions among settings such as script-src, frame-src,
and object-src. Also, the approach is simply not very future-safe: There already are
some peripheral classes of subresources (such as “favicons”) that are excluded from
CSP altogether, and that list will probably unintentionally grow.
In an unusual departure from the subresource-driven model outlined thus
far, CSP also features an oddball directive called frame-ancestors. This parameter
is meant to mitigate the impact of clickjacking by specifying the allowed ancestors for the current document in a manner similar to the better-established
X-Frame-Options header (outlined in Chapter 11). The frame-ancestors logic is
completely independent of default-src or any other parts of CSP; its default
value is “*”.

*
Remember advanced selectors in CSS3? By cleverly leveraging them in injected stylesheets,
some information about the strings appearing on the page may be conveniently relayed to a
third-party server without the use of JavaScript.

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Many other possible extensions of the policy are being discussed as of
this writing. These include a script-nonce directive that could be used to more
securely embed inline scripts (every script block must begin with a policyspecified, unpredictable token, often making XSS exploitation harder) and
a sandbox directive, which offers an alternative interface to another security
mechanism, discussed in “Sandboxed Frames” on page 245.
Policy Violations
The policy specified according to these rules constrains the behavior of the
underlying document. Violations normally result in a failed subresource load,
the failure to execute an inline script, or the inhibition of page rendering
(in the special case of frame-ancestors).
Because CSP controls a wide range of content behaviors, and because the
default failure mode is fairly brutal, the authors perceived a need to ease the
worries of webmasters. To make CSP more user-friendly, and perhaps also in
a naïve attempt to offer exploit detection, an optional feature of CSP allows
the browser to report all policy violations immediately back to the owner of the
site. This feature can be enabled through the report-uri keyword in the policy.
To further simplify deployment, it is also possible to roll out any policy—or
part thereof—in a “soft” mode, where violations result only in an HTTP notification but do not actually break the page. This is achieved by specifying the
policy inside a header named X-Content-Security-Policy-Report-Only.*
Criticisms of CSP
CSP is a remarkably sensible and consistent design compared to most of the
one-off security features proposed or deployed in the browser world. Nevertheless, from its inception, the proposal has been haunted by recurring
design and implementation concerns.
Perhaps the most prosaic complaint about CSP is a nonsecurity one: In
order to benefit from the XSS defenses offered by the framework, webmasters
have to move all inline scripts on the page (often hundreds of individual snippets of code) to a separately requested document; in the new drafts of CSP, the
same will be required for all stylesheets. The complexity of retrofitting existing pages to work with CSP and the performance penalty of an additional
HTTP request are often prohibitive. (It may be possible to resolve this problem with the script-nonce extension proposed in the most recent drafts.)
A more fundamental concern with the design of CSP is that the currently
envisioned origin-level granularity of the rulesets may not offer a sufficiently
robust defense against XSS. Consider the fact that any complex, real-life
domain may well host a dozen largely separate web applications, each consisting of hundreds of possibly unrelated static scripts and JavaScript APIs. Attackers exploiting an XSS vulnerability in a CSP-protected site are prevented from
directly executing a malicious script, but they may be able to put the application into an inconsistent and possibly dangerous state by loading the existing
*
As a side note, this feature is useful not only for short-term experiments but also for detecting
noncritical issues on an ongoing basis. For example, the owner of a site may leverage it to detect
mixed-content issues by creating a report-only policy for HTTPS pages that will be violated by
any HTTP scripts.

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scripts in the wrong context or in an incorrect sequence. The history of vulnerabilities in nonweb software suggests that such state corruption conditions are exploitable more often than we may think.
An even more troubling prospect is that an attacker can load a subresource that is not truly a script but that might be mistaken for one. An
extreme example of this may be a browser supporting E4X (see Chapter 6):
Any valid XHTML document in which the attacker can place a nominally
harmless string—say, {alert("Hi mom!")}—may result in code execution when
loaded via <script src=...>. Recognizing this problem, the developers decided
to require whitelisted Content-Type values for any scripts loaded under CSP,
but even this approach is often insufficient.
To understand what may go wrong, consider the exceedingly common
practice of hosting public JSONP APIs in which the client can specify the
name of the callback function:
GET /store_locator_api.cgi?zip=90210&callback=myResultParser HTTP/1.0
...
HTTP/1.0 200 OK
Content-Type: application/x-javascript
...
myResultParser({ "store_name": "Spacely Space Sprockets",
"street": … });

Such an API anywhere within a CSP-permitted origin may be leveraged
by an attacker to call arbitrary existing functions in the client-side code, perhaps together with attacker-controlled parameters. And if the callback string
is not constrained to alphanumerics (and why should it be?), specifying
callback=alert(1);// will lead to straightforward code injection.
Issues with granularity aside, CSP deserves some gentle criticism for its
sometimes puzzling and detrimental lack of focus. On one hand, through
the inclusion of directives such as frame-descendants or sandbox, it seems to be
flirting with the idea of building a single, unifying browser security framework—only to unexpectedly exclude XSRF flaws from its scope without offering a viable alternative beyond a vague mention of Origin. On the other hand,
the proposal often aspires to be just a “Content Policy,” with no special attention paid to offering sufficiently robust and intuitive security properties. The
ease of creating dangerous script policies, coupled with the originally ineffective policing of stylesheets and images, is a testament to this trend.

Sandboxed Frames
Sandboxed frames11 are an extension of the normal <iframe> behavior.
They allow the owner of the top-level page to place certain additional restrictions on the embedded document along with any of that document’s subframes. The goal is to make it safer for web applications to embed potentially

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untrusted advertisements, gadgets, or preformatted HTML documents on an
otherwise sensitive site. The refinement of the design and the initial implementation of this feature in WebKit (which is currently the only engine supporting it) was driven by Adam Barth.
NOTE

Curiously, sandboxed frames are not exactly a novel idea: Microsoft came up with a
similar proposal almost a decade earlier. Since version 6, Internet Explorer has supported a proprietary security=restricted parameter, which forces the target frame to be
rendered in the Restricted Zone, effectively removing its ability to execute scripts, navigate to other locations, and so on. However, no one seemed interested in using this feature for anything other than bypassing certain client-side JavaScript security mechanisms
(most notably, anticlickjacking checks). We will soon know whether the HTML5 successor fares any better.
The design of sandboxed frames is fairly simple: Any frame embedded in
a document may be constrained by specifying the sandbox parameter on the
appropriate <iframe> tag. By default, the document subject to this restriction
is prevented from executing scripts and performing certain types of navigations. The permissions may be fine-tuned with one or more whitespacedelimited keywords, specified as a value for the sandbox parameter itself:

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

Allow-scripts In the absence of this keyword, the document displayed
inside the frame will be unable to execute JavaScript code. The primary
function of this feature is to prevent the embedded document from performing DoS attacks, opening browser dialogs, or employing any other
complex automation of the page.



Allow-forms When this keyword is absent, any HTML forms encountered
in the embedded document will not work. This mechanism is designed
to prevent the framed content from exploiting its placement on a trusted
website to phish for sensitive information. (Note that with allow-scripts
enabled, there is little or no point in allow-forms. Scripts may easily construct form-like controls and automatically relay the collected information to another site without the need for a functioning <form> tag.)



Allow-top-navigation This keyword re-enables the ability of the embedded page to navigate the top-level window. This type of navigation is normally permitted as one of the exceptions to the same-origin policy (see
Chapter 11), and it may be abused simply to prevent the user from interacting with the embedding site or to carry out phishing attacks.



Allow-same-origin Without this flag, the content inside a sandboxed
frame is assigned a unique, randomly selected, synthetic origin. This
prevents the page from accessing any origin-bound content that would
normally be available to scripts executing in the domain it is nominally
hosted in. The inclusion of allow-same-origin removes the synthetic origin
and permits same-origin data access.

Scripting, Forms, and Navigation
The first three restrictions available to sandboxed frames—scripting, forms,
and navigation—are fairly intuitive and safe to use. Their value is diminished
only by the need to also disable all plug-ins whenever the sandbox attribute is
used, because frameworks such as Flash or Java do not honor the extension
and would allow any embedded applets to bypass the newly added browser
checks. Unfortunately, the three most obvious use cases for sandboxed
frames—embedded advertisements, videos, and games—rely heavily on
Flash, thus rendering this security mechanism much less useful than it
might otherwise be.
Synthetic Origins
The last mechanism on the list, synthetic origins, is far more problematic and
is likely misguided. It is envisioned primarily as a way to make it possible for
untrusted documents (such as incoming HTML-based emails in a webmail
interface) to be served as is, along with the rest of the application, while preventing these untrusted documents from accessing sensitive data.
Unfortunately, the concept of synthetic origins creates more problems
than it solves. For one, unless the URL of the embedded document is unpredictable, the attacker may simply navigate to it directly in a new browser window, in which case the browser will not see the sandbox attribute at all.
As an attempt to work around this problem, the authors of the specification
eventually proposed the use of a specialized MIME type (text/html-sandboxed)
for content meant to be shown only in a sandboxed frame. Their reasoning
is that browsers will normally not recognize this MIME type and will not display it inline and that a special case may be created in the <iframe> handling
code. Of course, as should be clear from Chapter 13, such a defense is inadequate, because some browsers and plug-ins will render text/html-sandboxed
responses inline or interpret the returned data in other troubling ways (say,
as crossdomain.xml).
The concept of synthetic origins is also highly problematic given the
fragmentation of origin- or domain-level security mechanisms in a typical
browser. For example, dangerous interactions are possible with password
managers, which must be explicitly prevented from autocompleting login
forms in the sandboxed documents. Also, special logic must be added to
security prompts, such as the one associated with the geolocation API.
After some trial and error, the implementation currently available in
WebKit resolved many of these issues on a case-by-case basis. That said, future
implementations are likely to fall for this trap repeatedly, especially since the
HTML5 specification considers the behavior of these features to be out of
scope and does not specify the required behavior in any way.
NOTE

Removing synthetic origins leads to trouble, too: If the user clicks on a same-site link in
a sandboxed advertisement and that link opens in a new window, the browser probably
should prevent the unrestricted scripts in the new window from traversing the opener
object to perform actions that its parent is prohibited from performing on its own.

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Strict Transport Security
One of the most significant weaknesses in the design of HTTPS is that users
often begin navigation by typing in a protocol-less URL in the address bar
(such as bankofamerica.com rather than https://www.bankofamerica.com), in
which case the browser will presume HTTP and send the initial request in
plaintext. Even if the site immediately redirects this traffic to HTTPS, any
active attacker on the victim’s network may intercept and modify that initial
response, preventing the user from ever upgrading to a secure protocol. In
such case, the absence of a tiny lock icon in the browser UI will be very easy
to miss.
This problem, as well as several peripheral issues related to mixed content and cookie scoping, prompted Jeff Hodges and several other researchers to draft a proposal for HTTP Strict Transport Security (HSTS, or STS for
short).12 Their approach (currently supported in WebKit and Firefox) allows
any site on the Internet to instruct the browser that all future requests made
to a particular hostname or domain should always use HTTPS and that any
HTTP traffic should be automatically upgraded and submitted only over
HTTPS.
The reasoning behind the design of HSTS is that the user’s first interaction with a particular domain is unlikely to occur over a connection that is
being actively tampered with—but that, over time, as the user roams on open
wireless networks, the chances of encountering an attacker increase rapidly.
HSTS is, therefore, an imperfect defense, but in practice it is usually good
enough.
The HSTS opt-in header may appear in HTTPS responses, looking something like this:
Strict-Transport-Security: max-age=3000000; includeSubDomains

NOTE

For HSTS to offer reasonable protection, max-age (the number of seconds that the STS
record may be stored in the browser) must be set to a value substantially higher than the
usual worst-case time between visits to the site. Because there is no easy way to disable or
override HSTS when something goes wrong with the HTTPS site, website owners will be
tempted to choose a value small enough to minimize disruption when they mess something up and have to revert. It is not clear whether this conflict of interests will lead web
programmers to make optimal choices.
The negative security consequences of this design are fairly unremarkable:
There is a slightly elevated risk of DoS attacks, because an attacker could inject
this response header into a domain that is not fully HTTPS enabled. There is
also the possibility of using a unique combination of HSTS settings for several decoy hostnames to tag a particular instance of a browser, offering yet
another alternative to cookie-based user tracking. Neither of these concerns
is particularly pronounced, however.

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Unfortunately, as with other restriction-adding frameworks discussed in
this section of the book, the mechanism sounds great in principle, but it’s
difficult to fully account for how it may interact with other legacy code. In
particular, unless the includeSubDomains flag is used, HSTS offers unexpectedly little protection for HTTP cookies: Cookies not marked as secure may still
be intercepted simply by inventing a nonexistent subdomain and intercepting the HTTP request made to that destination.* (Even secure cookies could
be clobbered in a similar fashion, just not read back.)
In a similar vein, the enforcement of HSTS on requests originating from
plug-in-based content is unlikely to work well.

Private Browsing Modes
Private browsing, colloquially known as the “porn mode,” is a nonstandardized feature available in most up-to-date browsers. It is meant to create a nonpersistent browsing sandbox, isolated from the main browser session, which
is completely discarded as soon as the last private browsing window is closed.
In a sense, this mechanism can be considered a form of content isolation
added on top of the existing browser security paradigms, so it seems fitting to
briefly mention it now.
With the exception of Chrome, most browser vendors do not accurately
explain the security assurances associated with private browsing. Unfortunately, the intuitive understanding of the term is quite different from what
browsers can actually deliver.
Arguably, the most straightforward interpretation of the feature is that a
private browsing session should be perfectly anonymous and that no data about
the user’s activity will persist on the system. These two assumptions are already
partly undermined by the constraints imposed by the networking stacks and
the memory management practices of modern operating systems. But even
within the browser itself, the goal of reasonable anonymity is nearly impossible to achieve. Almost every stateful browser mechanism, from geolocation or
pop-up permissions to Strict Transport Security to form autocompletion to
plug-in-based persistent data storage, must be modified in order to properly
account for the distinction between the two browsing modes, and for each
vendor, achieving that goal is an uphill battle. Perhaps more frustratingly,
anonymity is also undermined by the ability of scripts to uniquely fingerprint
any given system simply by examining its characteristics—such as the set of
installed plug-ins, fonts, screen resolutions, window sizes, clock drift, or even
the behavior of noncryptographically secure PRNGs.13
In the end, despite appearances to the contrary, private browsing mode
is suitable only for preventing casual data disclosure to other nontechnical
users of the same machine, and even that goal is sometimes difficult to
achieve.

*

Recall from Chapter 9 that host-scoped cookies are fairly tricky to create in some browsers and
outright impossible to have in Internet Explorer.
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Other Developments
The security features discussed previously in this chapter aim to shift the
boundaries between web applications and change the way sites interact with
each other. Another group of proposed mechanisms escapes this simple classification yet is important or mature enough to briefly mention here. We’ll
review some of them now.

In-Browser HTML Sanitizers
XSS vulnerabilities are by far the most common security issue encountered
in modern web applications. It must be surprising, then, that so few of the
proposed security frameworks aim to address the problem in a comprehensive way. True, CSP is a strong contender, but it requires a radical change in
how web applications are written, and it can’t be deployed particularly gradually or selectively. Sandboxed frames, on the other hand, are probably too
resource-intensive and too awkward to use for the most common task of displaying hundreds of individual, short snippets of user-supplied data.
Perhaps the best solution to many XSS woes would be a method for web
frameworks to provide the browser with a parsed, unambiguous, binary DOM
tree. Such a solution would eliminate many of the issues associated with template escaping and HTML sanitization. A more down-to-earth alternative
might be to equip web developers with a robust tool to mark the boundaries
of an attacker-supplied string and restrict the behavior or appearance of the
embedded payload without having to escape or sanitize it. One might think
of syntax such as this:
<sandbox token="random_value12345" settings="allow_static_html">
...any unsanitized text or HTML...
</sandbox token="random_value12345">

Were such a tool to be used, the attacker would be unable to escape such
a sandbox and remove the restriction on scripting without guessing the correct value of the randomly generated token boundary.
Sadly, such a proposal is unlikely to become a part of HTML5 or to ship
in any browser, because this serialization is fundamentally incompatible with
XML, and revising XML itself to allow an obscure use case in HTML is a difficult act to pull off. Depressingly, XML already offers a similar method of
encapsulating arbitrary data inside a <![CDATA[...]]> block, but absent a tokenbased guard, this sandbox can be escaped easily when exploiting XSS.
On the flip side, it is considerably easier to restrict the privileges of
any HTML generated by scripts on the client side. Beginning with Internet
Explorer 8, Microsoft offers a simple and somewhat inflexible toStaticHTML(...)
API,14 which promises to remove JavaScript from any fully qualified bit of
HTML passed to it as a parameter. The output of this method is designed to
be safe to assign to the innerHTML property somewhere in the existing DOM.*
*
Amusingly, the HTML parser in Internet Explorer is apparently so obtuse that even the authors
of toStaticHTML(...) had some trouble following it. Since its introduction, the API has suffered
from a fair number of bypass vulnerabilities, most frequently related to the handling of CSS data.

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Microsoft’s proposal is fine, but it dances around the most common and
problematic task of safely displaying server-supplied documents. And its API has
a minor but entirely unnecessary weakness: It makes it unexpectedly dangerous to trim or concatenate the sanitized toStaticHTML(...) output after the call
but before the innerHTML assignment, a practice that many web developers
will probably attempt. A more sensible approach would be to allow content
sanitization only upon assignment to innerHTML. In fact, WebKit engineers
briefly discussed a proposal for such an API (alternately named innerStaticHTML
or safeInnerHTML), but the effort seems to have fizzled out long ago.

XSS Filtering
Reducing the incidence of cross-site vulnerabilities is difficult, and so is
limiting their impact. Because of this, some researchers have concluded that
detecting and stopping the exploitation of such flaws may be a better choice.
And so, around 2008, David Ross of Microsoft announced the inclusion of
XSS-detection logic in the upcoming release of Internet Explorer 8;15 several
months later, Adam Barth implemented a similar feature in WebKit. The
implementations compare portions of the current URL with any strings
appearing on the retrieved page or passed to APIs such as document.write(...)
and innerHTML. If that comparison reveals that a portion of JavaScript present
on the page may have originated with an improperly escaped URL parameter,
the relevant portion of the page may be substituted with a harmless string.
Sadly, this seemingly elegant idea is known to cause serious problems. Accidental false positives aside (users of Internet Explorer 8 will have unexpected
trouble visiting http://www.google.com/search?q=<script>), the filter may also be
tripped for ill purposes by appending a legitimate portion of the page as a nonfunctional parameter in the URL. In one extreme and now resolved case, this
behavior was leveraged to create XSS vectors where none had existed before,
simply by tricking the browser into haphazardly rearranging the markup.16
But more fundamentally, it’s risky for any complex web application to selectively disable attacker-selected script blocks, even if the structure of the page
is otherwise correctly preserved, and such a tweak may easily put the clientside code in an inconsistent or dangerous state. For example, consider an
online document editor that implements each of the following in a separate
<script> block:
1.

Initializes the internal state of the editor and creates the UI with an
empty starting document.

2.

Loads the current version of the document requested by the user in a URL
parameter with error checking to catch any potential network problems.

3.

If no errors are detected, enters an interactive editing mode and automatically saves the current state of the document every 30 seconds under
the URL-derived ID.

In this not entirely unreasonable design, the ability to remove step two
can be disastrous because the next step could overwrite the existing, serverstored document with a blank copy. D’oh.
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This problem could have been avoided by using much simpler design
whereby any suspected XSS attacks would result in the browser simply refusing to render the document. Alas, the relatively high incidence of accidental
false positives prevented the authors from taking this route. Only after some
debate did Microsoft decide to offer a “strict” blocking mode on an opt-in
basis, toggled by a response header such as this:
XSS-Protection: 1; mode=block

NOTE

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In addition to the risk of false positives, XSS filters are also prone to false negatives,
a situation that probably can’t be improved by much. By design, these filters will never
be able to detect the arguably more dangerous stored XSS vulnerabilities, where incorrectly escaped data comes from a source other than the followed link. But even beyond
that, the multitude of (often implicit) input escaping schemes and the growing use of
location.hash or pushState (Chapter 17) as a method to store application state make
it difficult to formulate an accurate connection between what the browser sees in the
address bar and what the application makes of the received URL.

Security Engineering Cheat Sheet
Approach experimental browser security features with care, particularly when dealing with
mechanisms that create finer-grained security boundaries. Ensure that any application leveraging these mechanisms will degrade safely in a noncompliant browser.
 Cross-domain XMLHttpRequest (CORS): Fairly safe, but easy to misuse. Avoid non-simple
requests and do not permit arbitrary headers or methods. If you have control over the
server-side application framework, consider automatically stripping Cookie headers on
incoming CORS requests with nonwhitelisted Origin values to minimize the risk of accidentally sharing user-specific data. To minimize the incidence of mixed-content bugs,
consider rejecting HTTPS Origin values on any requests received over plain HTTP.
Be wary of Access-Control-Allow-Origin: *, and if you need to use it, make sure it is only
returned for the location you intend to share.
 XDomainRequest: This is safe to use. As with XMLHttpRequest, restricting access to HTTP
APIs from HTTPS origins may be a good way to stamp out mixed-content bugs.
 Content Security Policy: This is safe to use as defense in depth. Review the caveats related
to the interactions among script-src, object-src, and so on, and the dangers of permitting
data: origins. Do not accidentally allow mixed content: Always specify protocols in the
rulesets and make sure they match the protocol the requesting page is served over.
 Sandboxed frames: This is safe to use as a way to embed gadgets from other origins, but
the mechanism will fail dramatically in noncompliant browsers. You should not sandbox
same-origin documents.
 Strict Transport Security: This is safe to use as defense in depth. Be sure to mark all relevant cookies as secure and be prepared for the possibility of cookie injection via spoofed,
non-STS locations in your domain. Use includeSubDomains where feasible to mitigate
this risk.
 toStaticHTML(...): This is safe to use where available, but it is difficult to substitute on
the client side in noncompliant browsers. Bypass vulnerabilities have an above-average
chance of recurring in the API due to the design of the filter.
 Private browsing: Do not rely on this mechanism for security purposes.
 XSS filtering: Do not rely on this mechanism for security purposes. Always explicitly specify XSS-Protection: 1; mode=block or XSS-Protection: 0 in HTTP responses. The default is fairly
unsafe.

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OTHER BROWSER
MECHANISMS OF NOTE

To conclude the third part of the book, we briefly enumerate some of the recently implemented or simply
planned APIs that, although not designed for security
purposes, may substantially change the security landscape in the coming years. For example, some change
the types of data that web applications have access to
or alter the way the browser communicates with the
outside world.
The following list is necessarily incomplete: New, reasonably plausible
designs are drafted every week, and old approaches are scrapped at a moment’s
notice, often long before shipping in an actual browser. Still, this chapter
should serve as an interesting snapshot of what the future may bring.

URL- and Protocol-Level Proposals
These features seek to change the processes surrounding the behavior of
links, the address bar, and the exchange of data over the wire.
Protocol registration
Web applications commonly assume the handling of URL schemes previously reserved for “real” desktop software. One prime example of this
may be the mailto: protocol, which was originally meant to instantiate a
stand-alone mail application but which is often more sensibly routed to
webmail interfaces today. To this end, Mozilla proposed and WebKit
embraced a simple navigator.registerProtocolHandler(...) API.1 When this
API is invoked, the user is presented with a simple security prompt, and
if the action is approved, a URL-based handler is associated with a particular scheme. As of today, the associated prompts are vulnerable to
the race conditions outlined in Chapter 14, and they seem to be lacking
in other ways, as shown in Figure 17-1.

l

Figure 17-1: A seriously confusing prompt in Firefox. The prompt shown in the upper
area of the browser window was generated by the browser in response to a call to
the registerProtocolHandler(...) API, with the protocol name set to “doing really awesome stuff” and application name set to “Firefox (mozilla.org)”. This particular example
is harmless, but more sinister abuse is within reach.

Address bar manipulation
The newly introduced HTML5 history.pushState(...) API,2 supported by
Firefox, WebKit, and Opera, permits the currently displayed document
to change the contents of the address bar to any other same-origin URL,
without actually triggering a page transition normally associated with this
step. The API offers a superior alternative to the widespread abuse of
location.hash to store application state. Interestingly, despite its simplicity,
it has already led to a fair number of interesting security bugs. For example,
some implementations briefly allowed not only the top-level document
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but also any dodgy third-party frames to change the top-level URL shown
in the address bar, and they permitted origins such as about:blank to put
largely unconstrained gibberish in the URL field.
Binary HTTP
SPDY3 (“Speedy”) is a simple, encrypted drop-in replacement for HTTP
that preserves the protocol’s key design principles (including the layout
and function of most headers). At the same time, it mini- mizes the overhead associated with delivering concurrent requests or with the parsing
of text-based requests and response data. The protocol is currently supported only in Chrome, and other than select Google services, it is not
commonly encountered on the Web. It may be coming to Firefox soon,
too, however.
HTTP-less networking
WebSocket4 is a still-evolving API designed for negotiating largely
unconstrained, bidirectional TCP streams for when the transactional
nature of TCP gets in the way (e.g., in the case of a low-latency chat application). The protocol is bootstrapped using a keyed challenge-response
handshake, which looks sort of like HTTP and which is (quite remarkably)
impossible to spoof by merely exploiting a header-splitting flaw in the destination site. Following a successful handshake, raw data may be exchanged
bidirectionally within the resulting long-lived TCP connection, with each
message enveloped inside a simple protocol frame. The mechanism is
supported in WebKit and is probably coming soon to Firefox.
P2P networking
WebRTC5 is a proposed set of APIs and network protocols designed to facilitate the discovery of and communication with other browsers without the
need for a centralized server infrastructure. The primary use case for such
a protocol is the implementation of IP telephony and video-conferencing
features within web apps. No stable browser support is available yet.
Offline applications
Cache manifests6 are a relatively simple way for a web server to instruct
the browser that copies of certain documents should be stored indefinitely and reused whenever the client appears to have no network connectivity. In conjunction with client-side storage mechanisms such as
localStorage (Chapter 9), this allows certain self-sufficient JavaScript applications to be used in offline mode. Offline operation is supported in Firefox, the WebKit browser, and Opera. As with localStorage, the persistent
nature of this mechanism could exacerbate the long-term consequences
of visiting an untrusted network.
Better cookies
Cake 7 is a now-expired proposal drafted by Adam Barth that aims to create a more lightweight and secure alternative to HTTP cookies: one origin-bound, browser-generated nonce for every destination site. A more
current but incomplete proposal appears to flirt with normal but originbased cookies as an alternative. Neither approach is available in any
browser today.
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257

Content-Level Features
The proposals outlined in this section aim to enable new classes of web applications to be built on top of HTML and JavaScript.
Client-side databases
Several APIs for creating and manipulating locally stored databases have
been proposed over the years, including the notorious WebSQL API,8
which would have brought the famously dangerous SQL syntax to clientside JavaScript. The WebSQL proposal was ditched in favor of a more sensible IndexedDB design,9 which offers a clean API without serialized queries
and has a security model comparable to that of localStorage—but not until
WebSQL support had shipped in a couple of browsers. Meanwhile, the
new API has shipped in Chrome and is expected to appear in Firefox.
Background processes
The Worker API,10 available in Firefox, WebKit, and Opera, permits the
creation of background JavaScript processes to perform computationally
expensive tasks without having to worry about blocking the browser UI.
Each worker runs in an isolated environment that lacks the usual window
or document DOM and may communicate with its creator asynchronously
through the postMessage(...) API. Dedicated workers are directly reachable
only by their creator, while shared workers may be “attached” to several different sites at any given time. (Persistent workers, which would run independently of any sustained demand for their services, were proposed early on
but then dropped.) The concept of worker threads raises some peripheral DoS concerns but otherwise poses no apparent security risks.
Geolocation discovery
The navigator.geolocation.getCurrentPosition(...) API11 permits any website
to request information about the physical location of the client device,
subject to a user’s (largely hijackable) consent. The computed geolocation data may be derived from GPS information on a system with a suitable hardware module, or it may be looked up based on the names of
nearby wireless access points, cell towers, and so forth. The API is supported in all major browsers except for Internet Explorer.
Device orientation
A nonrestricted event-driven DeviceOrientation API12 allows websites to
read back the orientation of the device, based on accelerometer data.
This API, which is probably geared toward mobile gaming, is available in
Firefox, WebKit, and Opera on systems equipped with the appropriate
hardware. Two researchers at the University of California, Davis have
recently demonstrated a fatal flaw: On smartphones, minute movements
of the device may be used to reliably reconstruct on-screen keyboard
input, including passwords entered on unrelated websites.13
Page prerendering
This experimental feature in Chrome allows pages to be prefetched in
anticipation of the user following a particular link, and it permits the entire
HTML document to be prerendered in a hidden tab14 and momentarily
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Chapter 17

revealed once the predicted navigation action takes place. The mechanism has some interesting browser security consequences if the prerendered page turns out to be malicious. The implementation in Chrome
is careful to defer any disruptive actions until the tab is revealed, but mistakes will be very easy to make across all browser codebases.
Navigation timing
Several complementary APIs, currently available only in Chrome, permit
certain types of navigation, including cross-domain page loads, to be very
accurately benchmarked from client-side JavaScript.15 This interface is
designed to allow site owners to identify obvious performance bottlenecks, as experienced by a typical visitor. The API allows some privacyrelated information to be collected by profiling the time needed to load
certain third-party content, but because the same attack is possible in
many other ways (for example with onload handlers on subresources),
that probably does not matter much.

I/O Interfaces
The features listed below offer new input and output capabilities to webbased scripts.
UI notifications
Notification and window.notifications16 APIs allow the creation of text-only or
HTML-based, always-on-top pop-ups in the corner of the screen, allowing
select web applications to gently notify users of important developments
(such as a new mail message). User consent to receiving notifications is
required on a per-site basis, limiting the risk of abuse. Nevertheless, care
must be taken to properly communicate the origin of the tiny notification
window and any dialogs or prompts it subsequently creates, an aspect that
took some time to refine. The API is available only in WebKit today.
Full-screen mode
Several proposals have been circulated to allow JavaScript to maximize
the current browser window and hide all the browser chrome. This functionality is essential to tasks such as viewing presentations or watching
movies, but it is obviously very dangerous from the security standpoint:
Once in control of the entire screen, any malicious page may draw a fake
browser window with a fake address bar. So far, no specific implementation seems to be available for review. An early-stage proposal for mouse
cursor locking is being discussed, too.
Media capture
A proposed suite of navigator.device.capture APIs17 has been postulated for
giving websites access to webcam and microphone data. Obvious security
and privacy concerns arise around this mechanism, especially around
the resilience of any associated security prompts with respect to race condition attacks. The API has no stable browser support today.

O t h e r B r ows e r Me c h a n i s m s of No t e

259

COMMON WEB
VULNERABILITIES

Up until this point, we have paid little attention to
the taxonomy of common web vulnerabilities. Gaining
insight into the underlying mechanics of web applications is far more important than memorizing several
thousand random and often unnecessary terms; nomenclature such as improper restriction of operations within the
bounds of a memory buffer (Common Weakness Enumeration) or insecure direct object references (Open Web Application Security Project) finds no place in a reasonable
conversation—and rightly so.
Nevertheless, the industry has come up with a handful of reasonably
precise phrases that security researchers use every day. Having thoroughly
discussed the inner workings of the browser, it seems useful to recap and
highlight the terminology the average reader is likely to see.

Vulnerabilities Specific to Web Applications
The terms outlined in this section are unique to the technologies used on
the Web and often have no immediate counterparts in the world of “traditional” application security.
Cross-site request forgery (XSRF, CSRF)
A vulnerability caused by the failure to verify that a particular statechanging HTTP request received by the server-side portion of the web
application was initiated from the expected client-side origin. This flaw
permits any third-party website loaded in the browser to perform actions
on behalf of the victim.


See Chapter 4 for a more detailed discussion of XSRF.

Cross-site script inclusion (XSSI)
A flaw caused by the failure to secure sensitive JSON-like responses
against being loaded on third-party sites via <script src=...>. User-specific
information in the response may be leaked to attackers.


See Chapter 6 for an overview of the problem (and potential fixes).

Cross-site scripting (XSS)
Insufficient input validation or output escaping can allow an attacker to
plant his own HTML markup or scripts on a vulnerable site. The injected
scripts will have access to the entirety of the targeted web application
and, in many cases, to HTTP cookies stored by the client.
The qualifier reflected refers to cases where the injected string is
simply a result of incorrectly echoing back a portion of the request,
whereas stored or persistent refers to a situation where the payload takes a
more complex route. DOM-based may be used to denote that the vulnerability is triggered by the behavior of the client-side portion of the web
app (i.e., JavaScript).


See Chapter 4 for common XSS vectors in HTML documents.



See Chapter 6 for an overview of DOM-based XSS risks.



See Chapter 13 for XSS vectors associated with content sniffing.



See Chapter 9 for a discussion of the normal security model for JS code.

Header injection (response splitting)
Insufficient escaping of newlines (or equivalent characters) in HTTP
responses generated by the server-side portion of a web application.
This behavior will typically lead to XSS, browser, or proxy cache poisoning and more.


See Chapter 3 for a detailed discussion of the flaw.

Mixed content
A catch-all name for loading non-HTTPS subresources on HTTPS pages.
In the case of scripts and applets, this behavior makes the application
trivially vulnerable to active attackers, particularly on open wireless
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Chapter 18

networks (at cafés, airports, and so on), and undoes almost all benefits of
HTTPS. The consequences of mixed content bugs with stylesheets, fonts,
images, or frames are usually also fairly serious but more constrained.


See Chapters 4 and 8 for content-specific precautions on HTTPS sites.



See Chapter 11 for an overview of mixed-content handling rules.

Open redirection
A term used to refer to applications that perform HTTP- or script-based
requests to user-supplied URLs without constraining the possible destinations in any meaningful way. Open redirection is not advisable and
may be exploitable in some scenarios, but it is typically not particularly
dangerous by itself.


See Chapter 10 for cases where unconstrained redirection may lead to XSS.

Referer leakage
Accidental disclosure of a sensitive URL by embedding an off-site subresource or providing an off-site link. Any security- or privacy-relevant
data encoded in the URL of the parent document will be leaked in the
Referer header, with the exception of the fragment identifier.


See Chapter 3 for an overview of the Referer logic.

Problems to Keep in Mind in Web Application Design
The problems outlined in this section are an unavoidable circumstance of
doing business on the Internet and must be properly accounted for when
designing or implementing new web apps.
Cache poisoning
The possibility of long-term pollution of the browser cache (or any
interim proxies) with a fabricated, malicious version of the targeted
web application. Encrypted web applications may be targeted due to
response-splitting vulnerabilities. For nonencrypted traffic, active network attackers may be able to modify the responses received by the
requestor, too.


See Chapter 3 for an overview of HTTP-caching behaviors.

Clickjacking
The possibility of framing or otherwise decorating or obscuring a portion of another web application so that the victim, when interacting with
the attacker’s site, is not aware that individual clicks or keystrokes are
delivered to the other site, resulting in undesirable actions being taken
on behalf of the user.


See Chapter 11 for a discussion of clickjacking and related UI issues.

C o m m on W eb V u l n e r a b i l i t i es

263

Content and character set sniffing
Describes the possibility that the browser will ignore any authoritative
content type or character set information provided by the server and
interpret the returned document incorrectly.


See Chapter 13 for a discussion of content-sniffing logic.



See Chapters 4 and 8 for scenarios where Content-Type data is ignored.

Cookie forcing (or cookie injection)
The possibility of blindly injecting HTTP cookies into the context of an
otherwise impenetrable web application due to issues in how the mechanism is designed and implemented in modern browsers. Cookie injection is of particular concern to HTTPS applications. (Cookie stuffing is a
less common term referring specifically to maliciously deleting cookies
belonging to another application by overflowing the cookie jar.)


See Chapter 9 for more information on cookie scoping.



See Chapter 3 for a general discussion of the operation of HTTP cookies.

Denial-of-service (DoS) attacks
A broad term denoting any opportunities for the attacker to bring down
a browser or server or otherwise make the use of a particular targeted
application significantly more difficult.


See Chapter 14 for an overview of DoS considerations with JavaScript.

Framebusting
The possibility of a framed page navigating the top-level document to a
new URL without having to satisfy same-origin checks. The behavior may
be exploited for phishing attacks or simply for petty mischief.


See Chapter 11 for this and other frame navigation quirks.

HTTP downgrade
The ability for active attackers to prevent the user from reaching an
HTTPS version of a particular site or to downgrade an existing HTTPS
session to HTTP.


See Chapter 3 for an overview of HTTPS.



See Chapter 16 for Strict Transport Security, a proposed solution to the
problem.

Network fenceposts
The prospect of websites on the Internet leveraging the browser to interact with destinations not directly accessible to the attacker, for example,
with the systems on a victim’s internal network. Such attacks can be performed blindly, or (with the help of attacks such as DNS rebinding) the
attacker may be able to see responses to all requests.

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Chapter 18



See Chapter 12 for an overview of non-SOP boundaries in a browser.



See Chapter 15 for Internet Explorer zone model, a potential approach to
this risk.

NOTE

Beware non-buzzword bugs! Not all vulnerabilities have catchy names. Web developers
should be wary of many other implementation and design issues that are outside the
scope of this book but that can nevertheless bite hard. Examples include weak pseudorandom number generators (especially for session management purposes); insufficient
authentication and authorization checks (in particular, overly trusting the browseroriginating data); incorrect uses of cryptography (inventing one’s own algorithms is
usually a no-no); and so on. For a remarkably detailed discussion of these and many
other failure patterns, see The Art of Software Security Assessment by Dowd,
McDonald, and Schuh (Addison-Wesley, 2006).

Common Problems Unique to Server-Side Code
The following issues are commonly encountered in the server-hosted portion
of any web application and, by virtue of being tied to specific programming
languages or software components, are unlikely to occur on the client side.
Buffer overflow
A condition where a program allows more information to be stored in
a particular memory region than there is space to accommodate the
incoming data, leading to the unexpected overwrite of other vital data
structures. Buffer overflows happen chiefly in low-level programming
languages, such as C or C++, and in these languages, they can be frequently leveraged to execute attacker-supplied code.
Command injection (SQL, shell, PHP, and so on)
A problem where, due to insufficient input filtering or output escaping,
attacker-controlled strings may be unintentionally processed as statements in an interpreted language used by the application. (In a distant
sense, this is similar to XSS.) The consequences depend on the capabilities of the language, but in most cases, code execution is the eventual
outcome.
Directory traversal
A problem where, due to insufficient input filtering (most commonly,
the failure to properly recognize and handle “../” segments in filenames),
an application can be tricked into reading or writing files at arbitrary locations on the disk. Any consequences depend on additional constraints,
but unconstrained file-writing bugs are usually easily exploitable to run
attacker-supplied code.
File inclusion
If used without a qualifier or prefixed with local (LFI), the term is largely
synonymous with read-related directory traversal. Remote file inclusion
(RFI), on the other hand, is an alternative way to exploit file-inclusion
vulnerabilities by specifying a URL rather than a valid file path. In some
scripting languages, a single, common API opens local files and fetches
remote URLs. In these cases, the ability to retrieve the file from an
attacker-controlled server may offer substantial benefits, depending
on how the data is subsequently processed.
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265

Format-string vulnerability
A handful of commonly used library functions accept templates (“format
strings”), followed by a set of parameters that the function is expected
to insert into the template at predefined locations. Such an approach is
particularly common in C (printf(...), syslog(...), and so on), but it is not
limited to that language. Format-string vulnerabilities are caused by
unintentionally permitting attackers to supply the template to one of
these functions. Depending on the capabilities of the template system
and the specifics of the language, this error may lead to anything from
minor data leaks to code execution.
Integer overflow
A vulnerability specific to languages with limited or no range checking. The
flaw is caused by the developer failing to detect that an integer exceeded
the maximum possible value and rolled back to zero, to a very large negative integer, or to some other hardware-specific and unexpected result.
Depending on how the value is used, this may put the program in an
inconsistent state or, worse, lead to the reading or writing of data at an
incorrect memory location (which, in turn, may lend itself to code execution). Integer underflow is the opposite effect: crossing the minimum
permissible value and rolling over to a very large positive integer.
Pointer management vulnerabilities
In languages that encourage or require the use of raw memory pointers
(chiefly C and C++), it is possible to use pointers that are either uninitialized or no longer valid (“dangling”), leading to vulnerabilities such as use
after free, double free, and many more. These vulnerabilities will corrupt the
internal state of the program and usually allow an attacker to execute
attacker-supplied code.

266

Chapter 18

EPILOGUE

Well, who would have thought. This concludes The
Tangled Web! I hope you’ve enjoyed reading this book
as much as I’ve enjoyed exploring the world of browser
security over the last decade or so. I also hope that what
you’ve discovered on these pages will guide you in your
future journeys, wherever they may be.
As for what to make of it all: To me, the stark contrast between the amazing robustness of the modern Web and the inexplicable unsoundness of its
foundations was difficult to reconcile at first. In retrospect, I think it offers
an important insight into our own, unreasonable attitude about securing the
online world.
I am haunted by the uncomfortable observation that in real life,
modern societies are built on remarkably shaky ground. Every day, each of
us depends on the sanity, moral standards, and restraint of thousands of random strangers—from cab drivers, to food vendors, to elevator repair techs.
The rules of the game are weakly enforced through a series of deterrence

mechanisms, but if crime statistics are to be believed, their efficacy is remarkably low. The problem isn’t just that most petty criminals think they can get
away with their misdeeds but that they are usually right.
In this sense, our world is little more than an incredibly elaborate honor
system that most of us voluntarily agree to participate in. And that’s probably
okay: Compliance with self-imposed norms has proven to be a smart evolutionary move, and it is a part of who we are today. A degree of trust is simply
essential to advancing our civilization at a reasonable pace. Too, paradoxically, despite short-term weaknesses, accelerated progress makes us all a lot
stronger and more adaptable in the long run.
It is difficult to understand, then, why we treat our online existence in
such a dramatically different way. For example, why is it that we get upset at
developers who use cryptography incorrectly, but we don’t mind that the
locks on our doors can be opened with a safety pin? Why do we scorn web
developers who can’t get input validation right, but we don’t test our breakfast for laxatives or LSD?
The only explanation I can see is that humankind has had thousands of
years to work out the rules of social engagement in the physical realm. During that time, entire societies have collapsed, new ones have emerged, and
an increasingly complex system of behavioral norms, optimized for the preservation of communities, has emerged in the process. Unfortunately for us,
we have difficulty transposing these rules into the online ecosystem, and this
world is so young, it hasn’t had the chance to develop its own, separate code
of conduct yet.
The phenomenon is easy to see: While your neighbor will not try to sneak
into your house, he may have no qualms about using your wireless network
because doing so feels much less like a crime. He may oppose theft, but he
may be ambivalent about unlawfully duplicating digital content. Or he may
frown upon crude graffiti in the neighborhood but chuckle at the sight of a
defaced website. The parallels are there but just aren’t good enough.
What if our pursuit of perfection in the world of information security
stems from nothing but a fundamental misunderstanding of how human communities can emerge and flourish? The experts of my kind preach a model of
networked existence based on complete distrust, but perhaps wrongly so: As
the complexity of our online interactions approaches that of real life, the odds
of designing perfectly secure software are rapidly diminishing. Meanwhile, the
extreme paranoia begins to take a heavy toll on how quickly we can progress.
Perhaps we are peddling a recipe for a failure. What if our insistence on
absolute security only takes us closer to the fate of so many other early civilizations, which collapsed under the weight of their flaws and ultimately vanished? I find this perverse thought difficult to dismiss. Fortunately, we know
that from the rubble, new, more enlightened societies will certainly emerge
one day. Their ultimate nature is anyone’s guess.

268

Epil og ue

NOTES

Chapter 1
1.

D.E. Bell and L.J. La Padula, Secure Computer System: Unified Exposition
and Multics Interpretation (ESD-TR-75-306), Bedford, MA: MITRE Corporation for US Air Force (1976), http://csrc.nist.gov/publications/history/
bell76.pdf.

2.

C.E. Landwehr, C.L. Heitmeyer, and J.D. McLean, “A Security Model for
Military Message Systems: Retrospective,” paper presented at the 17th
Annual Computer Security Applications Conference, New Orleans, LA
(2001), http://www.acsa-admin.org/2001/papers/141.pdf.

3.

V. Bush, “As We May Think,” Atlantic Monthly (July 1945), http://
www.theatlantic.com/doc/194507/bush/.

4.

R. Dhamija, J.D. Tygar, and M. Hearst, “Why Phishing Works,” paper
presented at the Conference on Human Factors in Computing Systems,
Montreal, Canada (2006), http://people.seas.harvard.edu/~rachna/papers/
why_phishing_works.pdf.

5.

C. Jackson, D.R. Simon, D.S. Tan, and A. Barth, “An Evaluation of
Extended Validation and Picture-in-Picture Phishing Attacks,” paper
presented at Usable Security, Lowlands, Trinidad and Tobago (2007),
http://usablesecurity.org/papers/jackson.pdf.

6.

C. Jackson and A. Barth, “Beware of Finer-Grained Origins,” paper presented at Web 2.0 Security and Privacy, Oakland, CA (2008), http://seclab
.stanford.edu/websec/origins/fgo.pdf ; C. Jackson, and A. Barth, “Beware of
Coarser-Grained Origins,” paper presented at Web 2.0 Security and Privacy, Oakland, CA (2008), http://seclab.stanford.edu/websec/origins/scheme/.

7.

“Security Exploit Uses Internet Explorer to Attack Mozilla Firefox,”
MozillaZine ( July 11, 2007), http://www.mozillazine.org/talkback
.html?article=22198.

Page 19
1.

Net Applications website, http://marketshare.hitslink.com/browser-market-share
.aspx?qprid=0, http://marketshare.hitslink.com/browser-market-share.aspx?qprid=2
(accessed June 13, 2011).

Chapter 2
1.

T. Berners-Lee, R. Fielding, and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” IETF Request for Comments 3986 (2005),
http://www.ietf.org/rfc/rfc3986.txt.

2.

T. Berners-Lee, L. Masinter, and M. McCahill, “Uniform Resource Locators (URL),” IETF Request for Comments 1738 (1994), http://www.ietf
.org/rfc/rfc1738.txt.

3.

R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, and T.
Berners-Lee, “Hypertext Transfer Protocol—HTTP/1.1,” IETF Request
for Comments 2616 (1999), http://www.ietf.org/rfc/rfc2616.txt.

4.

“Uniform Resource Identifer (URI) Schemes per RFC4395,” Internet
Assigned Numbers Authority ( June 6, 2011), http://www.iana.org/
assignments/uri-schemes.html.

5.

P. Mockapetris, “Domain Names—Implementation and Specification,”
IETF Request for Comments 1035 (1987), http://www.ietf.org/rfc/
rfc1035.txt.

6.

T. Berners-Lee, “Universal Resource Identifiers in WWW,” IETF Request
for Comments 1630 (1994), http://www.w3.org/Addressing/rfc1630.txt.

7.

P. Hoffman, L. Masinter, and J. Zawinski, “The mailto URL Scheme,”
IETF Request for Comments 2368 (1998), http://www.ietf.org/rfc/
rfc2368.txt.

8.

“HTML 4.01 Specification: Forms,” World Wide Web Consortium (1999),
http://www.w3.org/TR/html401/interact/forms.html#h-17.13.4.1.

9.

P. Faltstrom, P. Hoffman, and A. Costello, “Internationalizing Domain
Names in Applications (IDNA),” IETF Request for Comments 3490
(2003), http://www.ietf.org/rfc/rfc3490.txt.

10. A. Costello, “Punycode: A Bootstring Encoding of Unicode for Internationalized Domain Names in Applications (IDNA),” IETF Request for
Comments 3492 (2003), http://www.ietf.org/rfc/rfc3492.txt.
11. E. Gabrilovich and A. Gontmakher, “The Homograph Attack,” Communications of the ACM (2002), http://www.cs.technion.ac.il/~gabr/papers/
homograph_full.pdf.
270

N o t e s f o r P a ge s 17 – 3 5

12. E. Rescorla, “HTTP Over TLS,” IETF Request for Comments 2818 (2000),
http://www.ietf.org/rfc/rfc2818.txt.
13. J. Postel and J. Reynolds, “File Transfer Protocol (FTP),” IETF Request
for Comments 959 (1985), http://www.ietf.org/rfc/rfc959.txt.
14. F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey, and B.
Alberti, “The Internet Gopher Protocol,” IETF Request for Comments
1436 (1993), http://www.ietf.org/rfc/rfc1436.txt.
15. E. Rescorla and A. Schiffman, “The Secure HyperText Transfer Protocol,” IETF Request for Comments 2660 (1999), http://www.ietf.org/rfc/
rfc2660.txt.
16. L. Masinter, “The ‘data’ URL Scheme,” IETF Request for Comments
2397 (1998), http://www.ietf.org/rfc/rfc2397.txt.
17. “What Are rss: and feed: Links?” http://www.brindys.com/winrss/
feedformat.html.
18. M. Zalewski, “A Note on an MHTML Vulnerability,” Lcamtuf’s blog
(March 11, 2011), http://lcamtuf.blogspot.com/2011/03/note-on-mhtmlvulnerability.html.
Chapter 3
1.

T. Berners-Lee, “The Original HTTP as defined in 1991.” World Wide
Web Consortium archives (1991), http://www.w3.org/Protocols/HTTP/
AsImplemented.html.

2.

T. Berners-Lee, R. Fielding, and H. Frystyk, “Hypertext Transfer
Protocol—HTTP/1.0,” IETF Request for Comments 1945 (1996),
http://www.ietf.org/rfc/rfc1945.txt.

3.

R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, and
T. Berners-Lee, “Hypertext Transfer Protocol—HTTP/1.1,” IETF Request
for Comments 2616 (1999), http://www.ietf.org/rfc/rfc2616.txt.

4.

HTTPbis Working Group, “Httpbis Status Pages,” http://tools.ietf.org/wg/
httpbis/.

5.

A. Luotonen, “Tunneling TCP-Based Protocols Through Web Proxy
Servers,” IETF draft (1998), http://tools.ietf.org/id/draft-luotonen-web-proxytunneling-01.txt.

6.

S. Chen, Z. Mao, Y.M. Wang, and M. Zhang, “Pretty-Bad-Proxy: An
Overlooked Adversary in Browsers’ HTTPS Deployments,” Microsoft
Research (2009), http://research.microsoft.com/pubs/79323/pbp-final-withupdate.pdf.

7.

“Mozilla Cross-Reference mozilla1.8.0,” Mozilla code repository, http://
mxr.mozilla.org/mozilla1.8.0/source/nsprpub/pr/src/misc/prtime.c#1045.

8.

K. Moore, “MIME (Multipurpose Internet Mail Extensions) Part Three:
Message Header Extensions for Non-ASCII Text,” IETF Request For
Comments 2047 (1996), http://www.ietf.org/rfc/rfc2047.txt.
N ot e s f or P a g e s 3 6 – 5 0

271

9.

N. Freed and K. Moore, “MIME Parameter Value and Encoded Word
Extensions: Character Sets, Languages, and Continuations,” IETF
Request for Comments 2231 (1997), http://www.ietf.org/rfc/rfc2231.txt.

10. Mozilla Bug Tracking System, Mozilla bug #418394, https://bugzilla
.mozilla.org/show_bug.cgi?id=418394.
11. T. Berners-Lee, “Basic HTTP as defined in 1992: Methods,” World Wide
Web Consortium archives (1992), http://www.w3.org/Protocols/HTTP/
Methods.html.
12. L. Dusseault, “HTTP Extensions for Web Distributed Authoring and
Versioning (WebDAV),” IETF Request for Comments 4918 (2007),
http://www.ietf.org/rfc/rfc4918.txt.
13. See note 12 above.
14. M. Pool, “Meantime: Non-Consensual HTTP User Tracking Using
Caches” (2000), http://sourcefrog.net/projects/meantime/.
15. L. Montulli, “Persistent Client State HTTP Cookies” (1994), http://curl
.haxx.se/rfc/cookie_spec.html.
16. D. Kristol and L. Montulli, “HTTP State Management Mechanism,” IETF
Request for Comments 2109 (1997), http://www.ietf.org/rfc/rfc2109.txt.
17. D. Kristol and L. Montulli, “HTTP State Management Mechanism,” IETF
Request for Comments 2965 (2000), http://tools.ietf.org/rfc/rfc2965.txt.
18. A. Barth, “HTTP State Management Mechanism,” IETF Request for
Comments 6265 (2011), http://www.ietf.org/rfc/rfc6265.txt.
19. J. Franks, P. Hallam-Baker, J. Hostetler, S. Lawrence, P. Leach, A.
Luotonen, and L. Stewart, “HTTP Authentication: Basic and Digest
Access Authentication,” IETF Request for Comments 2617 (1999),
http://www.ietf.org/rfc/rfc2617.txt.
20. R. Tschalär, “NTLM Authentication Scheme for HTTP” (2003), http://
www.innovation.ch/personal/ronald/ntlm.html.
21. E. Rescorla, “HTTP Over TLS,” IETF Request for Comments 2818 (2000),
http://www.ietf.org/rfc/rfc2818.txt.
22. P. Hallam-Baker, “The Recent RA Compromise,” Comodo IT Security
(blog) (March 23, 2011), http://blogs.comodo.com/it-security/data-security/
the-recent-ra-compromise/.
23. S. Chen, R. Wang, X. F. Wang, and K. Zhang, “Side-Channel Leaks
in Web Applications: A Reality Today, a Challenge Tomorrow,”
Microsoft Research (2010), http://research.microsoft.com/pubs/119060/
WebAppSideChannel-final.pdf.
24. C. Evans, “Open Redirectors: Some Sanity,” Security: Hacking Everything
(blog) (June 25, 2010), http://scarybeastsecurity.blogspot.com/2010_06_01
_archive.html.

272

N o t e s f o r P a ge s 50 – 6 6

Chapter 4
1.

T. Berners-Lee, “HTML Tags,” World Wide Web Consortium archives
(1991), http://www.w3.org/History/19921103-hypertext/hypertext/WWW/
MarkUp/Tags.html.

2.

T. Berners-Lee and D. Connolly, “Hypertext Markup Language—2.0,”
IETF Request for Comments 1866 (1995), http://www.ietf.org/rfc/
rfc1866.txt.

3.

D. Raggett, “HTML 3.2 Reference Specification,” World Wide Web
Consortium (1997), http://www.w3.org/TR/REC-html32.

4.

D. Raggett, A. Le Hors, and I. Jacobs, “HTML 4.01 Specification,” World
Wide Web Consortium (1999), http://www.w3.org/TR/html401/.

5.

I. Hickson, “HTML5,” World Wide Web Consortium draft, revision
1.5019 (2011), http://dev.w3.org/html5/spec/Overview.html.

6.

G. Coldwind, “Too general charset = detection in meta,” Mozilla
bug 640529 (2011), https://bugzilla.mozilla.org/show_bug.cgi?id=640529.

Chapter 5
1.

H. Wium Lie and B. Bos, “Cascading Style Sheets, Level 1,” World
WideWeb Consortium, (1996), http://www.w3.org/TR/CSS1/.

2.

T. Çelik, E.J. Etemad, D. Glazman, I. Hickson, P. Linss, and J. Williams,
“Selectors Level 3: Selectors,” World Wide Web Consortium (2009), http://
www.w3.org/TR/css3-selectors/#selectors.

3.

I. Hickson, “XML Binding Language (XBL) 2.0,” World Wide Web
Consortium (2007), http://www.w3.org/TR/xbl/.

4.

G. Heyes, D. Lindsay, and E.V. Nava, “The Sexy Assassin: Tactical
Exploitation Using CSS” (2009), http://www.scribd.com/doc/54664700/
Tactical-Xploit-Css.

Chapter 6
1.

Netscape Communications Corporation, “Netscape and Sun Announce
JavaScript, the Open, Cross-Platform Object Scripting Language for Enterprise Networks and the Internet” (press release) (December 4, 1995),
http://web.archive.org/web/20070916144913/http://wp.netscape.com/newsref/
pr/newsrelease67.html.

2.

ECMA International, “ECMA-262: ECMAScript Language Specification,” 3rd ed. (1999), http://www.ecma-international.org/publications/files/
ECMA-ST-ARCH/ECMA-262,%203rd%20edition,%20December%201999.pdf.

3.

ECMA International, “ECMA-262: ECMAScript Language Specification,”
5th ed. (2009), http://www.ecma-international.org/publications/files/ECMA-ST/
ECMA-262.pdf.
N ot e s f or P a g e s 6 9 – 9 6

273

4.

D. Crockford, “The Application/JSON Media Type for JavaScript Object
Notation (JSON),” IETF Request for Comments 4627 (2006), http://
www.ietf.org/rfc/rfc4627.txt.

5.

J. Schneider, R. Yu, and J. Dyer, eds., “Standard ECMA-357: ECMAScript
for XML (E4X) Specification,” 2nd ed., ECMA International (2005), http://
www.ecma-international.org/publications/standards/Ecma-357.htm.

6.

P. Le Hégaret, R. Whitmer, and L. Wood, “Document Object Model
(DOM),” World Wide Web Consortium (2005), http://www.w3.org/DOM/.

7.

E. Vela Nava, “Bug 38922—innerHTML decompilation issues in textarea” (WebKit bug-tracking system post) (2010), https://bugs.webkit.org/
show_bug.cgi?id=38922.

8.

“Windows Scripting 5.8: MsgBox Function,” Microsoft Developer
Network Platforms (2009), http://msdn.microsoft.com/en-us/library/
sfw6660x%28v=vs.85%29.aspx.

9.

D. Crockford, “JSON in JavaScript,” GitHub Social Coding (blog) (March 5,
2011), https://github.com/douglascrockford/JSON-js/blob/master/json2.js.

Chapter 7
1.

J. Ferraiolo, F. Jun, and D. Jackson, “Scalable Vector Graphics (SVG) 1.1
Specification,” World Wide Web Consortium (2003), http://www.w3.org/
TR/2003/REC-SVG11-20030114/.

2.

D. Carlisle, P. Ion, and R. Miner, “Mathematical Markup Language
(MathML) Version 3.0,” World Wide Web Consortium WC3 Recommendation 21 (2010), http://www.w3.org/TR/MathML3/.

3.

A. Mechelynck, “XUL,” Mozilla Developer Network (2011), https://
developer.mozilla.org/en/xul.

4.

Wireless Application Protocol Forum, “Wireless Application Protocol:
Wireless Markup Language Specification version 30” (1998), http://www
.wapforum.org/what/technical/wml-30-apr-98.pdf.

5.

RSS Advisory Board, “RSS 2.0 Specification version 2.0.11” (2009), http://
www.rssboard.org/rss-specification.

6.

M. Nottingham and R. Sayre, eds., “The Atom Syndication Format,” IETF
Request for Comments 4287 (2005), http://www.ietf.org/rfc/rfc4287.txt.

Chapter 8

274

1.

E. Mills, “Security Labs Report: January–June 2010 Recap,” M86 Security
(2010), http://www.m86security.com/documents/pdfs/security_labs/m86_security
_labs_report_1H2010.pdf.

2.

B. Rios, “Sun Fixes GIFARs” (December 17, 2008), http://xs-sniper.com/
blog/2008/12/17/sun-fixes-gifars/.

N o t e s f o r P a ge s 10 4 – 1 2 9

3.

A.K. Sood, “PDF Silent HTTP Form Repurposing Attacks,” SecNiche
Security Labs (2009), http://secniche.org/papers/SNS_09_03_PDF_Silent
_Form_Re_Purp_Attack.pdf.

4.

P.D. Petkov, “Universal PDF XSS Afterparty” (January 4, 2007), http://
www.gnucitizen.org/blog/universal-pdf-xss-after-party/.

5.

S. Jobs, “Thoughts on Flash” (2010), http://www.apple.com/hotnews/
thoughts-on-flash/.

6.

“Adobe Shockwave Player,” Adobe Systems Incorporated, http://www
.adobe.com/products/shockwaveplayer/.

7.

“ActionScript 3.0 Reference for the Adobe Flash Platform,” Adobe Systems Incorporated, http://help.adobe.com/en_US/FlashPlatform/reference/
actionscript/3/index.html.

8.

“Content Played Back in Flash Player Reaches 99% of Internet Viewers,”
Adobe Systems Incorporated (March 2011), http://www.adobe.com/products/
player_census/flashplayer/.

9.

“Web Browser Plugin Market Share,” StatOwl (May 2011), http://www
.statowl.com/plugin_overview.php.

10. “ActionScript 3.0 Reference for the Adobe Flash Platform: ExternalInterface,” Adobe Systems Incorporated, http://livedocs.adobe.com/flex/
3/langref/flash/external/ExternalInterface.html#includeExamplesSummary.
11. “XAML Overview (WPF),” Microsoft Corporation, http://msdn.microsoft
.com/en-us/library/ms752059.aspx.
12. “Rich Internet Application Statistics” (July 2011), http://www.riastats.com/
. See also StatOwl (Chapter 8, note 9).
13. “Secunia Half Year Report 2010,” Secunia (2010), http://secunia.com/gfx/
pdf/Secunia_Half_Year_Report_2010.pdf.
14. “WPF XAML Browser Applications Overview,” Microsoft Corporation,
http://msdn.microsoft.com/en-us/library/aa970060.aspx.
15. “Akamai Download Manager Help,” Microsoft Corporation, https://
msdn.microsoft.com/en-us/subscriptions/manage/bb153537.aspx.
Chapter 9
1.

A. Klein, “IE + Some Popular Forward Proxy Servers = XSS, Defacement
(Browser Cache Poisoning)” (May 22, 2006), http://seclists.org/webappsec/
2006/q2/352; M. Zalewski, “Web 2.0 Backdoors Made Easy with MSIE &
XMLHttpRequest” (February 3, 2007), http://seclists.org/fulldisclosure/
2007/Feb/81.

2.

A. van Kesteren, ed., “Cross-Origin Resource Sharing,” working draft,
World Wide Web Consortium (July 27, 2010), http://www.w3.org/TR/cors/.

3.

I. Hickson, “Web Storage,” editor’s draft, World Wide Web Consortium
(July 28, 2011), http://dev.w3.org/html5/webstorage/.

N ote s f or P ag e s 130– 148

275

4.

J. Stenback, “Make sessionStorage Use Principals Instead of String
Domains,” Mozilla bug #495337 (May 28, 2009), https://bugzilla.mozilla
.org/show_bug.cgi?id=495337.

5.

T. Ormandy, “Common DNS Misconfiguration Can Lead to ‘Same Site’
Scripting” (January 18, 2008), http://seclists.org/bugtraq/2008/Jan/270.

6.

R. Singel, “ISPs’ Error Page Ads Let Hackers Hijack Entire Web,
Researcher Discloses,” Wired (April 19, 2008), http://www.wired.com/
threatlevel/2008/04/isps-error-page/.

7.

“APSB10-14 Security Update Available for Adobe Flash Player,” Adobe
Systems Incorporated (June 10, 2010), http://www.adobe.com/support/
security/bulletins/apsb10-14.html.
“Understanding Flash Player 9 April 2008: Security Update Compatibility,” Adobe Systems Incorporated (April 8, 2008), http://www.adobe.com/
devnet/flashplayer/articles/flash_player9_security_update.html.
“ActionScript® 3.0 Reference for the Adobe® Flash® Platform: URLRequestHeader,” Adobe Systems Incorporated, http://help.adobe.com/
en_US/FlashPlatform/reference/actionscript/3/flash/net/URLRequestHeader.html.
“ActionScript® 3.0 Reference for the Adobe® Flash® Platform: Security,”
Adobe Systems Incorporated, http://livedocs.adobe.com/flash/9.0/
ActionScriptLangRefV3/flash/system/Security.html.
“Adobe Cross Domain Policy File Specification,” version 2.0, Adobe Systems Incorporated (August 2, 2010), http://learn.adobe.com/wiki/download/
attachments/64389123/CrossDomain_PolicyFile_Specification.pdf?version=1.
M. Zalewski, “[RAZOR] Linux Kernel IP Masquerading Vulnerability”
(July 30, 2001), http://seclists.org/bugtraq/2001/Jul/733.
“Silverlight: WebHeaderCollection Class,” Microsoft, http://msdn.microsoft
.com/en-us/library/system.net.webheadercollection%28v=VS.95%29.aspx.
“Class HttpURLConnection,” Sun Microsystems/Oracle, http://download
.oracle.com/javase/1.4.2/docs/api/java/net/HttpURLConnection.html.
“Class URLConnection,” Sun Microsystems/Oracle, http://download
.oracle.com/javase/1.4.2/docs/api/java/net/URLConnection.html.
“Class Socket,” Sun Microsystems/Oracle, http://download.oracle.com/
javase/1.4.2/docs/api/java/net/Socket.html.
“Java-to-Javascript Communication,” Sun Microsystems/Oracle, http://
download.oracle.com/javase/1.4.2/docs/guide/plugin/developer_guide/
java_js.html.
“Java-to-Javascript Communication: Common DOM API,” Sun Microsystems/Oracle, http://download.oracle.com/javase/1.4.2/docs/guide/plugin/
developer_guide/java_js.html#common_dom.
B. “Snowhare” Franz, “Triple Dot Cookies” (1998), http://snowhare
.com/utilities/triple_dot/.
“Adobe ActionScript 3.0: Security Sandboxes,” Adobe Systems Incorporated, http://help.adobe.com/en_US/ActionScript/3.0_ProgrammingAS3/
WS5b3ccc516d4fbf351e63e3d118a9b90204-7e3f.html.

8.

9.

10.

11.

12.
13.
14.
15.
16.
17.

18.

19.
20.

276

N o t e s f o r P a ge s 14 8 – 1 6 0

Chapter 10
1.

L. Masinter, “The ‘data’ URL scheme,” IETF Request for Comments
2397 (1998), http://www.ietf.org/rfc/rfc2397.txt.

2.

M. Zalewski, “about:neterror, certerror permit URL spoofing by being
same-origin with about:blank,” Mozilla bug #602780 (CVE-2010-3774)
(2010), https://bugzilla.mozilla.org/show_bug.cgi?id=602780.

Chapter 11
1.

G. Guninski, “Frame spoofing using loading two frames,” Mozilla bug
#13871 (1999), https://bugzilla.mozilla.org/show_bug.cgi?id=13871.

2.

R. Zilberman, “Frame spoofing is possible within a short time frame
while the window is loading,” Mozilla bug #381300 (CVE-2007-3089)
(2008), https://bugzilla.mozilla.org/show_bug.cgi?id=381300.

3.

A. Barth, C. Jackson, and J.C. Mitchell, “Securing Frame Communication
in Browsers,” Communications of the ACM 52, no. 6 (2009): 83-91, http://
www.adambarth.com/papers/2009/barth-jackson-mitchell-cacm.pdf.

4.

R. Hansen and J. Grossman, “Clickjacking” (2008), http://www.sectheory
.com/clickjacking.htm.

5.

M. Zalewski, “Dealing with UI redress vulnerabilities inherent to the current web” (post to whatwg.org list) (September 25, 2008), http://lists.whatwg
.org/htdig.cgi/whatwg-whatwg.org/2008-September/thread.html#16292.

6.

E. Lawrence, “IE8 Security Part VII: ClickJacking Defenses,” IEBlog
( January 27, 2009), http://blogs.msdn.com/b/ie/archive/2009/01/27/
ie8-security-part-vii-clickjacking-defenses.aspx.
SHODAN, “HTTP Header Survey” (March 14, 2011), http://www.shodanhq
.com/research/infodisc/report.
P. Stone, “Next Generation Clickjacking,” Blackhat Europe (2010), http://
blog.c22.cc/2010/04/14/blackhat-europe-next-generation-clickjacking-3/.
M. Zalewski, “The curse of inverse strokejacking,” Icamtuf’s blog (June 8,
2010), http://lcamtuf.blogspot.com/2010/06/curse-of-inverse-strokejacking.html.
C. Evans, “Generic cross-browser cross-domain theft,” Security (blog)
(December 28, 2009) http://scarybeastsecurity.blogspot.com/2009/12/
generic-cross-browser-cross-domain.html.
C. Evans, “IE8 CSS-based forced tweeting,” Security (blog) (September 29,
2010), http://scarybeastsecurity.blogspot.com/2010/09/ie8-css-based-forcedtweeting.html.
I. Hickson, “HTML: 4.8.11 The canvas element,” WHATWG (2011),
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvaselement.html.
E.W. Felten and M.A. Schneider, “Timing Attacks on Web Privacy,”
Proceedings of the 7th ACM Conference on Computer and Communications
Security (2000), http://sip.cs.princeton.edu/pub/webtiming.pdf.

7.
8.
9.
10.

11.

12.

13.

N ote s f or P ag e s 167– 184

277

14. C. Evans, “Cross-domain search timing,” Security (blog) (December 11,
2009), http://scarybeastsecurity.blogspot.com/2009/12/cross-domain-searchtiming.html.
15. C. Wilson, P. Le Hégaret, and V. Apparao, “Document Object Model
CSS: 2.2.1 Override and computed style sheet,” World Wide Web Consortium (2000), http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSSOverrideAndComputed.
16. “currentStyle Object,” Microsoft Corporation MSDN Library, http://
msdn.microsoft.com/en-us/library/ms535231%28v=vs.85%29.aspx.
17. A. Clover, “CSS visited pages disclosure” (February 20, 2002), http://
seclists.org/bugtraq/2002/Feb/271.
18. Z. Weinberg, E.Y. Chen, P.R. Jayaraman, and C. Jackson, “I Still Know
What You Visited Last Summer” (2011), http://websec.sv.cmu.edu/visited/
visited.pdf.
Chapter 12
1.

J. Schwartz, “Giving Web a Memory Cost Its Users Privacy,” New York
Times (September 4, 2001), http://www.nytimes.com/2001/09/04/technology/04COOK.html.

2.

N. Wingfield, “Microsoft Quashed Effort to Boost Online Privacy,”
Wall Street Journal (August 2, 2010), http://online.wsj.com/article/
SB10001424052748703467304575383530439838568.html.

3.

E. Felten, “If You’re Going to Track Me, Please Use Cookies,” Freedom
to Tinker (blog) (July 7, 2009), http://www.freedom-to-tinker.com/blog/felten/
if-youre-going-track-me-please-use-cookies.

4.

J. Mayer, A. Narayanan, and S. Stamm, “Do Not Track: A Universal ThirdParty Web Tracking Opt Out,” IETF Request for Comments (2011), http://
datatracker.ietf.org/doc/draft-mayer-do-not-track/?include_text=1.

5.

L. Cranor, M. Langheinrich, M. Marchiori, M. Presler-Marshall, and
J. Reagle, “The Platform for Privacy Preferences 1.0 (P3P1.0) Specification,” World Wide Web Consortium (2002), http://www.w3.org/TR/P3P/.

Chapter 13

278

1.

N. Freed and N. Borenstein, “Multipurpose Internet Mail Extensions
(MIME) Part Two: Media Types,” IETF Request for Comments 2046
(1996), http://www.ietf.org/rfc/rfc2046.txt.

2.

V. Gupta, “IE Content-Type Logic,” IEBlog (February 1, 2005), http://
blogs.msdn.com/b/ie/archive/2005/02/01/364581.aspx.

3.

SHODAN, “HTTP Header Survey” (2011), http://www.shodanhq.com/
research/infodisc/download_latest.

N o t e s f o r P a ge s 18 4 – 2 0 3

4.

R. Troost, S. Dorner, and K. Moore, “Communicating Presentation
Information in Internet Messages: The Content-Disposition Header
Field,” IETF Request for Comments 2183 (1997), http://www.ietf.org/
rfc/rfc2183.txt.

5.

G. Heyes, “Inline UTF-7 E4X Javascript Hijacking,” The Spanner (blog)
(February 24, 2009), http://www.thespanner.co.uk/2009/02/24/inline-utf7-e4x-javascript-hijacking/.

Chapter 14
1.

M. Zalewski, “URL Spoofing Is Likely Possible Through Address Bar Eliding” (2010), https://bugzilla.mozilla.org/show_bug.cgi?id=581313.

2.

R. J. Kosinski, “A Literature Review on Reaction Time,” Clemson University (2010), http://biae.clemson.edu/bpc/bp/Lab/110/reaction.htm
#Type%20of%20Stimulus.

3.

M. Zalewski, “Bug 376473: File Action Dialog Controls Vulnerable to
Refocus Race” (2007), https://bugzilla.mozilla.org/show_bug.cgi?id=376473.

4.

M. Zalewski, “Geolocation Spoofing and Other UI Woes,” Bugtraq (mailing list) (August 17, 2010), http://seclists.org/bugtraq/2010/Aug/201.

5.

D. Simons and C. Chabris, “Selective Attention Test” (1999), http://www
.youtube.com/watch?v=vJG698U2Mvo&feature=player_embedded.

6.

D.J. Simmons and C.F. Chabris, “Gorillas in our midst: Sustained inattentional blindness for dynamic events,” Perception, 28, 1059–1074 (1999),
http://www.cnbc.cmu.edu/~behrmann/dlpapers/Simons_Chabris.pdf.

Chapter 15
1.

http://www.xssed.com/search?key=addons.mozilla.org

2.

http://openid.net/

3.

“Internet Explorer: Security Zones,” Microsoft, http://technet.microsoft
.com/en-us/library/dd361896.aspx.

4.

“Internet Explorer Binary Behaviors Security Setting,” Microsoft, http://
technet.microsoft.com/en-us/library/cc776248(WS.10).aspx.

5.

Charles Schwab, “Technical Support,” http://www.visualwebcaster.com/
charles_schwab/support/ (accessed September 9, 2011).

6.

Internal Revenue Service, “Streaming Media System Requirements &
Troubleshooting Assistance,” http://www.irsvideos.gov/sbv_1099webinar/
player/IRS_Webinar_Technical_Support.pdf (accessed September 9, 2011).

7.

“.NET Framework 3.0: Mark of the Web,” Microsoft, http://msdn.microsoft
.com/en-us/library/ms537628%28VS.85%29.aspx.

8.

“Persistent Zone Identifier Object,” Microsoft, http://msdn.microsoft.com/
en-us/library/ms537029%28VS.85%29.aspx.

N ote s f or P ag e s 203– 231

279

Chapter 16
1.

A. van Kesteren, “Cross-Origin Resource Sharing,” (working draft) World
Wide Web Consortium (July 27, 2010), http://www.w3.org/TR/cors/.

2.

S. Dutta, “Updates for AJAX in IE8 Beta 2,” IEBlog (2008), http://blogs
.msdn.com/b/ie/archive/2008/10/06/updates-for-ajax-in-ie8-beta-2.aspx.

3.

“.NET Framework 3.0: XDomainRequest Object,” Microsoft Developer
Network, http://msdn.microsoft.com/en-us/library/cc288060%28v=vs
.85%29.aspx.

4.

T. Close and M. Miller, “Uniform Messaging Policy, Level One,” (working draft) World Wide Web Consortium (January 26, 2010), http://www
.w3.org/TR/UMP/.

5.

A. Barth, C. Jackson, and J.C. Mitchell, “Robust Defenses for Cross-Site
Request Forgery,” ACM Conference on Computer and Communications
Security (2008), http://seclab.stanford.edu/websec/csrf/csrf.pdf.

6.

B. Sterne, “Origin Header Proposal,” http://people.mozilla.com/~bsterne/
content-security-policy/origin-header-proposal.html.

7.

A. van Kesteren, “The From-Origin Header,” (working draft) World
Wide Web Consortium (July 21, 2011), http://www.w3.org/TR/2011/
WD-from-origin-20110721/.

8.

A. Barth, “The Web Origin Concept (v. 9),” IETF Draft (November 26,
2010), http://tools.ietf.org/html/draft-abarth-origin-09.

9.

B. Sterne, “Content Security Policy” (2008), http://people.mozilla.com/
~bsterne/content-security-policy/.

10. B. Sterne, “Content Security Policy,” (draft) World Wide Web Consortium (March 15, 2011), https://dvcs.w3.org/hg/content-security-policy/raw-file/
tip/csp-unofficial-draft-20110315.html.
11. I. Hickson, “HTML Living Standard,” WHATWG (2011), http://www
.whatwg.org/specs/web-apps/current-work/multipage/the-iframe-element.html
#attr-iframe-sandbox.
12. J. Hodges, C. Jackson, and A. Barth, “HTTP Strict Transport Security
(HSTS),” (draft) IETF Request for Comments (August 5, 2011), http://
tools.ietf.org/html/draft-ietf-websec-strict-transport-sec-02.
13. A. Klein, “Google Chrome 6.0 and Above: Math.random Vulnerability”
(2010), http://www.trusteer.com/sites/default/files/Google_Chrome_6.0_and
_7.0_Math.random_vulnerability.pdf.
14. “.NET Framework 3.0: toStaticHTML Method,” Microsoft, http://msdn
.microsoft.com/en-us/library/cc848922%28v=vs.85%29.aspx.
15. D. Ross, “IE8 Security Part IV: The XSS Filter,” IEBlog (2008), http://blogs
.msdn.com/b/ie/archive/2008/07/02/ie8-security-part-iv-the-xss-filter.aspx.
16. E. Vela Nava and D. Lindsay, “Abusing Internet Explorer 8’s XSS Filters”
(2009), http://p42.us/ie8xss/Abusing_IE8s_XSS_Filters.pdf.
280

N o t e s f o r P a ge s 23 6 – 2 5 1

Chapter 17
1.

“navigator.registerProtocolHandler,” Mozilla Developer Network, https://
developer.mozilla.org/en/DOM/window.navigator.registerProtocolHandler.

2.

“Manipulating the Browser History,” Mozilla Developer Network, https://
developer.mozilla.org/en/DOM/Manipulating_the_browser_history/.

3.

A. Langley and M. Belsche, “SPDY: An Experimental Protocol for a
Faster Web,” The Chromium Projects, http://www.chromium.org/spdy/
spdy-whitepaper/.

4.

I. Fette and A. Melnikov, “The WebSocket Protocol,” IETF Request
for Comments draft (2011), http://tools.ietf.org/html/draft-ietf-hybithewebsocketprotocol-10/.

5.

J. Rosenberg, M. Kaufman, M. Hiie, and F. Audet, “An Architectural
Framework for Browser Based Real-Time Communications,” IETF
Request for Comments draft (2011), http://tools.ietf.org/html/draftrosenberg-rtcweb-framework-00/.

6.

I. Hickson, “HTML5: 5.6—Offline Web Applications,” World Wide Web
Consortium (2011), http://www.w3.org/TR/html5/offline.html.

7.

A. Barth, “Simple HTTP State Management Mechanism,” IETF Request
for Comments draft (2010), http://tools.ietf.org/html/draft-abarth-cake-00/.

8.

I. Hickson, “Web SQL Database: W3C Working Group Note 18,” World
Wide Web Consortium (2010), http://www.w3.org/TR/webdatabase/.

9.

N. Mehta, J. Sicking, E. Graff, A. Popescu, and J. Orlow, “Indexed Database API: W3C Working Draft 19,” World Wide Web Consortium (2011),
http://www.w3.org/TR/IndexedDB/.

10. I. Hickson, “Web Applications 1.0: Web Workers,” WHATWG (2011),
http://www.whatwg.org/specs/web-apps/current-work/complete/workers.html.
11. A. Popescu, “Geolocation API Specification: Editor’s Draft,” World
Wide Web Consortium (February 10, 2010), http://dev.w3.org/geo/api/
spec-source.html.
12. “Detecting Device Orientation,” Mozilla Developer Network, https://
developer.mozilla.org/en/detecting_device_orientation/.
13. L. Cai and H. Chen, “TouchLogger: Inferring Keystrokes on Touch
Screen from Smartphone Motion,” Usenix HOTSEC (2011), http://
www.usenix.org/event/hotsec11/tech/final_files/Cai.pdf.
14. “Web Developer’s Guide to Prerendering in Chrome,” Google code labs,
http://code.google.com/chrome/whitepapers/prerender.html.
15. Z. Wang, “Navigation Timing: Editor’s Draft,” World Wide Web Consortium (July 27, 2011), https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/
NavigationTiming/Overview.html.

N ote s f or P ag e s 256– 259

281

16. J. Gregg, “Web Notifications Overview: W3C Editor’s Draft,” World Wide
Web Consortium (October 12, 2010), http://dev.w3.org/2006/webapi/
WebNotifications/publish/.
17. D.D. Tran, I. Oksanen, and I. Kliche, “The Media Capture API: W3C
Working Draft,” World Wide Web Consortium (September 28, 2010),
http://www.w3.org/TR/media-capture-api/.

282

Notes for P age 259

INDEX
Symbols & Numbers
& (ampersand), in HTML, 71
< > (angle brackets)
browser interpretation, 74–75
in HTML, 71
<![CDATA[...]]> blocks, 72, 78, 250
<!DOCTYPE> directive, 71
<!ENTITY> directive, 76
<!-- and -->, for HTML comments, 72
<% ... %> blocks, Internet Explorer
and, 75
@ directives, in CSS, 89–90
\ (backslashes) in URLs, browser acceptance of, 29
` (backticks), as quote characters, 74, 111
!- directives, 76
// fixed string, in URLs, 25
% (percent sign), for character
encoding, 31
. (period), hostnames with, and cookiesetting algorithms, 159
?-directives, 76
<?xml-stylesheet href=... ?> directive, 88
; (semicolon), as delimiter
in HTTP headers, 48–49
in URLs, 29
200–299 status codes, 54
300–399 status codes, 55
400–499 status codes, 55–56
500–599 status codes, 56

A
<a href=...> tag (HTML), 79
target parameter, 174–175
about:blank document, origin inheritance,
165, 166–167
about:config (Firefox), navigation risks, 188
absolute URLs, vs. relative, 25
Accept-Language request header, 43

Accept request header, 43
Access-Control-Allow-Origin header,
237–238, 240
acrobat: scheme, 36
action parameter, for <form> tag, 80
ActionScript, 132–134
Active Server Pages, 75
ActiveX, 129, 136–137
address bars, 220
and EV SSL, 65
hiding, 221
manipulation, 256–257
Adobe Flash, 119, 130, 132–134
and cross-domain HTTP headers, 147n
file handling without Content-Type, 199
HTML parser offered by plug-in, 133
policy file spoofing risks, 156–157
security rules, 154–157
Adobe Reader, 130
Adobe Shockwave Player, 132
ADS (Alternate Data Stream) Zone
Identifier, 231
advertisements, new window for, 217
Akamai Download Manager, 137
Allow-forms keyword, for sandbox
parameter, 246
AllowFullScreen parameter, for Flash, 155
AllowNetworking parameter, for Flash, 155
Allow-same-origin keyword, for sandbox
parameter, 246
AllowScriptAccess parameter, for Flash, 154
Allow-scripts keyword, for sandbox
parameter, 246
Allow-top-navigation keyword, for sandbox
parameter, 246
Alternate Data Stream (ADS) Zone
Identifier, 231
ambient authority, 60, 60n
ampersand (&), in HTML, 71
anchor element (HTML), specifying
name of, 28

angle brackets (< >)
browser interpretation, 74–75
in HTML, 71
anonymity, scripts and, 249
anonymous requests, in CORS, 239
anonymous windows, 175
antimalware, 236n
Apache
and Host headers, 47
PATH_INFO, 201
APNG file format, 83
Apple QuickTime, 119, 130, 132
Apple Safari. See Safari (Apple)
<applet> tag (HTML), 83, 128, 135, 183
application/binary, 212
application/javascript document type, 118
application/json document type, 118, 202
application/mathml+xml document type, 119
application/octet-stream document type,
200–201, 212
application/x-www-for-urlencoded, 81
Arce, Ivan, 2n
Arya, Abhishek, 209
asynchronous XMLHttpRequest, 146
Atom, 123
<audio> tag (HTML), 84, 119
authentication, in HTTP, 62–63
authorization, vs. authentication, 62n
Authorization header (HTTP), 63

B
background parameter for HTML tags, 83
background processes, in JavaScript, 258
backslashes (\) in URLs, browser acceptance of, 29
backticks (`), as quote characters, 74, 111
Bad Request status error (400), 55
bandwidth, and XML, 123n
Barth, Adam, 16, 177, 240, 241, 246, 257
Base64 encoding, 50n
basic credential-passing method, 63
Bell-La Padula security model, 2, 4
Berners-Lee, Tim, 9, 41, 69
and semantic web, 72–73
World Wide Web browser, 9
World Wide Web Consortium, 11
<bgsound> tag (HTML), 84, 119
binary HTTP, 257
bitmap images, browser recognition of, 118
blacklists
of HTTP headers in XMLHttpRequest, 147
malicious URLs, 236n
_blank, as link target, 80

284

INDEX

BMP file format, 83
<body> tag (HTML), 83
BOM (byte order marks), 208
Breckman, John, 52n
browser cache
information in, 59
poisoning, 60
browser extensions and UI, 161
browser-managed site permissions, 226–227
browser market share, May 2011, 19
browser-side scripts, 95–116
browser wars, 10–11, 233
buffer overflow, 265
bugs, preventing classes of, 7
Bush, Vannevar, 8
byte order marks (BOM), 208

C
cache. See browser cache
Cache-Control directive, 48, 59
cache manifests, 257
cache poisoning, 189, 263
caching behavior, in HTTP, 58–60
caching HTTP proxy, keepalive
sessions and, 57
Caja, 116
Cake (proposal), 257
call stack, limiting size, 216
callto: scheme, 36
<canvas> tag (HTML5), 183
CAPTCHA, 184–185, 185n
Cascading Style Sheets (CSS), 11, 12, 73,
83, 87–93
basic syntax, 88–90
character encoding, 91–92
interaction with HTML, 90
opacity property, 179
parser resynchronization risks, 90–91
property definitions, 89
case of tags, HTML vs. XML, 72
<![CDATA[...]]> blocks, 72, 78, 250
certificate authorities, 64
certificates
extended validation, 65
warning dialog example, 66
cf: scheme, 36
characters
delimiting, in URLs, 29
encoding in CSS, 91–92
encoding in filenames, 49–51
encoding in HTML, 76–78
encoding in JavaScript, 112–113
encoding in URLs, 31–35
printable, browser treatment of, 32

reserved, 31–35
unreserved, 32
character sets
byte order marks and detection, 208
detection for non-HTTP files, 210–211
handling, 206–211
for headers, 49–51
inheritance and override, 209
markup-controlled, on subresources,
209–210
sniffing, 264
in URLs, 33
@charset (CSS), 89
children objects in JavaScript, 108
Chrome
autodetection of passive document
types, 205
cached pages in, 37
characters in URL scheme name
ignored by, 25
deleting JavaScript function, 103
and file extensions in URLs, 130
local file access, 160
modal dialogs for prompts, 219
navigation timing, 259
prerendering page, 258–259
printable characters in, 32
privileged JavaScript in, 161
and realm string, 63
and RFC 2047 encoding, 50
stored password retrieval, 228
SWF file handling without
Content-Type, 199
time limits on continuously executing
scripts, 215
WebKit parsing engine, 70n
window.open() function and, 218
Windows Presentation Foundation
plug-ins, 136
chunked data transfers, 57–58
clickjacking, 179, 180–181, 263
click() method, 218
client certificates, 64–66
client-server architecture, 17–18
client-side data, 165
client-side databases, 258
client-side errors (400–499), 55–56
client-side scripts, restricting privileges of
HTML generated by, 250–251
cloud, 15
Clover, Andrew, 184
command injection, 265
comments
in CSS syntax, 89
in XHTML and HTML, 72

Common UNIX Printing System (CUPS),
152–153
Common Vulnerability Scoring System
(CVSS), 6–7
Common Weakness Enumeration (CWE), 6
complex selectors, in CSS, 88
computer proficiency of user, 14
conditionals, explicit and implicit, in
HTML, 75–76
conflicting headers, resolution of, 47–48
CONNECT requests, 46, 54
Connolly, Dan, 9
content directives, on subresources, 204
Content-Disposition directive, 48, 84, 122
defensive uses, 203–204
NUL character and, 51
plug-in-executed code and, 204
user-controlled filenames in, 67
content inclusion in HTML
hyperlinking and, 79–84
type-specific, 82–84
Content-Length header, 43, 52, 147
in keepalive sessions, 56–58
content recognition, 197–211
content rendering, plug-ins for, 127–138
Content Security Policy (CSP), 242–245,
250, 253
criticisms of, 244–245
violations, 244
content sniffing, 197–198, 205, 264
Content-Type directive, 49, 71, 84
application/binary, 212
application/JavaScript, 118
application/json, 118, 202
application/mathml+xml, 119
application/octet-stream, 200–201, 212
charset parameter, 206, 208
image/jpeg, 118, 202, 205
image/svg+xml, 124
logic to handle absence, 198–199
plug-ins and, 128, 204
slash-delmited alphanumeric
tokens in, 199
special values, 200–201
text/css, 118
text/html, 124
text/plain, 118, 156, 200–201, 204, 212
unrecognized, 202–203
and XML document parsing, 120
control characters, JavaScript shorthand
notation, 112
cookie-authenticated text, reading, 181
Cookie header. See cookies
cookie injection, 264

INDEX

285

cookies, 11, 257
deleting, 62
and DNS hijacking, 153
forcing, 264
limitations on third-party, 192–194
and same-origin policy, 150–151
security policy for, 149–153
semantics, 60–62
user data in, 67
CORS. See Cross-Origin Resource
Sharing (CORS)
CR characters, stripping from HTTP
headers, 45
credential-passing methods, 63
credentials, in URLs, 26
CRLF (newline), 45
cross-browser interactions, 16–17
cross-document links, 8, 9
cross-domain communications, and frame
descendant policy, 176–178
cross-domain content inclusion, 181–183
cross-domain policy files, 155–156
cross-domain requests, 236–239
Cross-Origin Resource Sharing (CORS),
148, 236
current status, 239
non-simple requests and preflight, 238
request types, 236–237
security checks, 237–238
cross-origin subresources, 183
cross-site request forgery (XSRF, CSRF),
84, 190, 262
exploitation of flaws, 190
login forms and, 145–146
cross-site script inclusion (XSSI), 104n, 262
cross-site scripting (XSS), 71, 262
bugs, and password managers, 228
exploitation of flaws, 190
filtering, 251–252, 253
crossdomain.xml file, 155, 162
CSP (Content Security Policy), 242–245,
250, 253
CSRF (cross-site request forgery), 84,
190, 262
exploitation of flaws, 190
login forms and, 145–146
CSS. See Cascading Style Sheets (CSS)
CUPS (Common UNIX Printing System),
152–153
currentStyle API, 184
CVSS (Common Vulnerability Scoring
System), 6–7
CWE (Common Weakness Enumeration), 6
Cyrillic alphabet, homoglyphs in, 35

286

INDEX

D
daap: scheme, 36
data: scheme, 37, 167–168
data transfers, chunked, 57–58
Date/If-Modified-Since header pair, 59
deceptive framing, 180
dedicated workers, for background
processes, 258
default policy, CSP directive for, 243
default ports, for protocols, overriding, 27
DELETE method (HTTP), 53
deleting
cookies, 62
JavaScript functions, 102–103
delimiting characters, in URLs, 29
denial-of-service (DoS) attacks, 214–219,
248, 264
DeviceOrientation API, 258
dialog use restrictions, 218–219
digest credential-passing method, 63
Digital Rights Management (DRM), 131
directory traversal, 265
disable-xss-protection, 242n
<div> tag (HTML), 73
DNS hijacking, and cookies, 153
DNS labels, security mechanisms
based on, 142n
DNS names, in URLs, browser
acceptance, 27
DNS pinning, 142n, 190
DNS rebinding, 142n, 189
DNT request header, 193
<!DOCTYPE> directive, 71
document.cookie API (JavaScript), 61
document.domain property (JavaScript),
143–144
document-level scrollbar, 180
document namespace, mapping HTML
elements to, 110
document object (JavaScript), 108
Document Object Model, 12, 108,
109–111, 142–146
document rendering helpers, 130–131
documents
changing location of existing, 174–178
script access to other, 111–112
document type detection logic, 198–206
Domain parameter, for cookie, 61
domains
hardcoded, 227
problems with restrictions, 151–152
DOMService mechanism, 158
DoS (denial-of-service) attacks, 214–219,
248, 264

downloaded files, 205–206
drag-and-drop, 180
DRM (Digital Rights Management), 131
duplicate headers, resolution of, 47–48
Dutta, Sunava, 239

E
E4X. See ECMAScript for XML (E4X)
Earthlink, 153
ECMA (European Computer Manufacturers Association), 11, 96
ECMAScript, 96
escape codes, 112
strict mode, 104
ECMAScript for XML (E4X), 106–107
Eich, Brendan, 95
Electronic Frontier Foundation, 109
Eloquent JavaScript (Haverbeke), 97
<embed> tag (HTML), 83
mixed content, 183
src=..., 128
EMF file format, 83
encapsulating pseudo-protocols, 37–38
encoding schemes, for headers, 49–51
encryption, protocol-level, 64–66
enctype="text/plain", for <form> tag, 81
endless loop, 101, 215
ENQUIRE, 9, 10
<!ENTITY> directive, 76
entity encoding, in HTML, 76–78
error-handling rules, for certificates, 65–66
escaping reserved characters, in HTML, 71
escaping scheme, 91
Esser, Stefan, 209
ETag/If-None-Match header pair, 59
European Computer Manufacturers Association (ECMA), 11, 96
eval() function, 102
eval-script, 242n
Evans, Chris, 181, 182
EV SSL (Extended Validation SSL), 65
exception
for eval() function, 102
recovery in JavaScript, 100
execution time for scripts, 215–216
Expires directive, 48, 59
Expires parameter, for cookie, 61
explicit conditionals, in HTML, 75–76
expression(...) function (CSS), 89
Extended Validation SSL (EV SSL), 65
Extensible Application Markup Language
(XAML), 134
extension matching, 202n

ExternalInterface.call() API, 133
External XML Entity (XXE) attack, 76

F
false positives, risk in XSS filtering, 251–252
fault tolerance, 11
feeds, 123–124
feed: scheme, 37
Felten, Ed, 193
file extensions, browser response to, 205
file formats. See also plug-ins
audio and video, 119
bitmap images, 118
HTML. See HTML
non-renderable, 124
plaintext, 64, 85, 117–118
XML. See XML
file inclusion, 265
file path, hierarchical, in URLs, 27–28
file: protocol, 159–160, 188
files, downloaded, 205–206
File Transfer Protocol (FTP), 26n, 205–206
filtering
pop-up, 217–218
reserved characters, in HTML, 71
Firefox (Mozilla), 13, 17
and ActiveX, 137
cached pages in, 37
character set inheritance, 209
CORS in, 239
and credential portion of URLs, 26
data: URLs in, 168
DNT request header, 193
entity names, 77
external content directives, 90
Gecko parsing engine, 70n
history.pushState() API, 256
javascript: URLs in, 169
local file access, 160
modal dialogs for prompts, 219
multiple cookies for, 62
printable characters in, 32
privileged JavaScript in, 161
prompt displayed when saving ContentType: image/jpeg document, 205
redirects to about:blank, 166
and RFC 2047 encoding, 50
RSS and Atom renderers for, 124
same-origin policy loopholes, 185
stored password retrieval, 228
Strict Transport Security support, 248
SWF file handling without
Content-Type, 199

INDEX

287

Firefox (continued)
time limits on continuously executing
scripts, 215
UTF-8 text in, 50
Windows Presentation Foundation
plug-ins, 136
Worker API, 258
firefoxurl: protocol, 17, 36
Flash applets, 11
fonts
CSP directive for, 243
Flash programs enumeration of, 132
Forbidden status code (403), 56
forecasting, statistical, 6
format-string vulnerability, 266
form-based password managers, 227–229
form feed character, in HTML tag, 74
forms, 80–82
Found status code (302), 55
fragment ID, in URLs, 28–29
frame-ancestors directive, 243
framebusting, 264
frame descendant policy, and cross-domain
communications, 176–178
frames, 82
disabling navigation descendant model,
230–231
hijacking risks, 175–176
name attribute of, 175
sandboxed, 245–247
unsolicited, 178–181
and window interactions, 174–181
frame-src directive, 243
From-Origin header, 240
FTP (File Transfer Protocol), 26n, 205–206
ftp: scheme, 36
full-screen mode, proposals for, 259
fully qualified absolute URLs, 24
fully restricted URL scheme, 188
functional notation, in CSS, 89
functions
JavaScript, overriding, 102–103
resolution for JavaScript, 98–99

G
Gabrilovich, Evgeniy, 35
Gecko parsing engine, 70n
Generalized Markup Language
(GML), 8–9
geolocation data, 226
geolocation discovery, 258
geolocation-sharing prompts, 223
getComputedStyle API, 184
getElementById() function, 109

288

INDEX

getElementsByTagName() function, 109
GET method (HTTP), 42, 52, 58, 80–81
GetRight download utility, 137
getters, in JavaScript, 103
getURL() function, 133
GIFAR vulnerability, 129
GIF file format, 83, 129
GML (Generalized Markup Language),
8–9
Gontmakher, Alex, 35
Gonzalez, Albert, 5n
gopher: scheme, 36
Gosling, James, 134
GPS data, 226n
Grossman, Jeremiah, 179
Guninski, Georgi, 176

H
Hansen, Robert, 179
hardcoded domains, 227
Haverbeke, Marijn, Eloquent JavaScript, 97
HDP file format, 83
header injection, 45, 239, 262
headers
character set and encoding schemes,
49–51
Content Security Policy encoded in, 242
in HTTP requests, 43
resolution of duplicate or conflicting,
47–48
semicolon-delimited values, 48–49
HEAD request (HTTP), 53
hexadecimal notation, 77, 112
hierarchical file path, in URLs, 27–28
history object (JavaScript), 108
history.pushState() API, 256
Hodges, Jeff, 248
homoglyphs, in Cyrillic alphabet, 35
Host request header, 43
hostnames
extra periods, and cookie-setting
algorithms, 159
non-fully qualified, 159
HTML (Hypertext Markup Language), 9,
69–86
basic concepts, 70–73
case of tags, 72
converting to plaintext, 85
CSS interaction with, 90
document misidentified as, 198
document parsing modes, 71–72
embedded in feed formats, 124
entity encoding, 76–78
explicit and implicit conditionals, 75–76

HTTP integration semantics, 78–79
hyperlinking and content inclusion,
79–84
in-browser sanitizers, 250–251
mapping elements to document
namespace, 110
parser behavior, 73–76
tag interactions, 74–75
type-specific content inclusion, 82–84
version 4, 12
version 5, 70, 119, 131
HTTP (HyperText Transfer Protocol), 9,
41–67
authentication, 62–63
basic syntax, 42–51
binary, 257
caching behavior, 58–60
cookie semantics, 60–62
downgrade, 264
history, 41–42
HTML integration semantics, 78–79
newline handling, 45
proxy requests, 46–47
request types, 52–54
semantics battle, 72–73
simultaneous connections, 216
version 0.9, 42–43, 44
version 1.0, 42, 43, 44, 48, 59
version 1.1, 42–43, 45, 48, 57, 198
httponly flag, for cookie, 61, 150
http: scheme, 36
HTTPS, 65
documents, 138n, 183
downgrade risks, 248
https: scheme, 36
hyperlinking, and content inclusion, 79–84
Hypertext Markup Language (HTML).
See HTML (Hypertext Markup
Language)
HyperText Transfer Protocol (HTTP). See
HTTP (HyperText Transfer Protocol)

I
IANA (Internet Assigned Numbers
Authority), 24, 152
ICO file format, 83
IDNA (Internationalized Domain Names
in Applications), 34–35
IETF (Internet Engineering Task Force), 11
If-Modified-Since header, 59
If-None-Match header, 59
<iframe> tag (HTML), 82, 176, 209, 245–247
image/jpeg document type, 118, 202, 205

images
bitmap, 118
in HTML, 83
risk of content sniffing on, 202
Scalable Vector Graphics (SVG), 83,
121–122
image/svg+xml document type, 124
<img> tag (HTML), 83
src parameter, 181
for SVG images, 122
implicit caching, 59
implicit conditionals, in HTML, 75–76
@import, in CSS, 89–90
IndexedDB design, 258
indicator of hierarchical URLs, 25–26
information security, 1–8
inheritance, for vbscript: scheme, 169–170
inline-script setting, 242n
innerHTML property, 110–111
innerStaticHTML API, 251
integer overflow, 266
Interactive Voice Response (IVR)
systems, 236
interconnected systems, losses in, 5
internal networks, access to, 189–190
Internal Revenue Service, 231
Internal Server Error (500), 56
International Organization for Standardization (ISO), 11
Internationalized Domain Names in
Applications (IDNA), 34–35
Internet Assigned Numbers Authority
(IANA), 24, 152
Internet Engineering Task Force (IETF), 11
Internet Explorer, 10, 11–12
ActiveX and, 137
and <% ... %> blocks, 75
\ (backslash) in URLs, 29
acceptance of backtick as quote, 74
characters in URL scheme name
ignored by, 25
clickjacking, 182
content sniffing, 202
cookies, 149
data: URLs in, 168
delete attempt of JavaScript function, 103
extension matching, 202
fallback display, 118
and file extensions in URLs, 130
frames, 177
JavaScript in, 96
JSON.parse() function alternative, 104
local file access, 160
markup controlled charset on, 209

INDEX

289

Internet Explorer (continued)
and multiline headers, 45
multiline string literals support, 91
non-recognition of vertical tab, 112
NUL character and, 73, 74
origin check and port number, 142
printable characters in, 32
proprietary security-restricted
parameter, 246
redirects to about:blank, 166–167
and RFC 2047 encoding, 50
same-origin policy and, 143n, 185
Silverlight and, 134
stored password retrieval, 228
SWF file handling without
Content-Type, 199
text/plain document type, 200–201
third-party cookies blocking, 193
time limits on continuously executing
scripts, 215
Trident parsing engine, 70n
VBScript, 96, 114
window.open() function and, 218
Windows Presentation Foundation
plug-ins, 136
XDomainRequest approach to, 148
XSS-detection logic, 251
Zone.Identifier metadata, 231
zone model, 229–231
Internet Information Server, and Host
headers, 47
Internet service providers, 153
Internet zone, for Internet Explorer, 230
interstitials, 218
intrusions
escalation of, 5
nonmonetary costs, 5
Invisible Gorilla experiment, 223
IP addresses, and cookies, 158
ISO (International Organization for Standardization), 11
ISO-8859-1 (Western European
code page), 50
itms: scheme, 36
itpc: scheme, 36
IVR (Interactive Voice Response)
systems, 236

J
Jackson, Collin, 16, 177, 184, 240
jar: scheme, 37
Java, 134–135, 157–158
Java Runtime Environment (JRE), 135

290

INDEX

JavaScript, 10, 11n, 83, 95–107
character encoding in, 112–113
code and object inspection capabilities,
101–102
code execution, 100
code inclusion modes and nesting risks,
113–114
document.domain property, 143–144
Document Object Model, 12, 108,
109–111
embedded in PDF documents, 130
execution order control, 100–101
labeled statements support, 105n
MIME type, 118n
Netscape and, 95–96
runtime environment for, 102–104
script processing model, 97–100
setters and getters, 103
standard object hierarchy, 107–112
variable declaration, 99
and WML Script (WMLS), 123
JavaScript Object Notation (JSON),
104–106, 112
javascript: scheme, 37, 169–170
Jobs, Steve, 131
JPEG file format, 83
JScript, 11n
JScript.Encode, 113n
JSObject mechanism, 158
JSON (JavaScript Object Notation),
104–106, 112
JSONP (JSON with padding), 106n, 245
JSON.parse() function, alternatives, 104

K
Kaminsky, Dan, 153
katakana, 33
keepalive sessions, 56–57, 216
keystroke redirection, 180
Kinugawa, Masato, 210

L
language parameter, for <script> tag, 113n
Lessig, Lawrence, 192
LF (newline), HTTP quirks in
handling, 45
LFI (local file inclusion), 265
Lie, Wium, Håkon, 87
<link rel=stylesheet> directive, 88
<link rel=stylesheet href=...> tag, 181
LiveScript, 95
livescript: scheme, 37

loadPolicyFile() method, 155–156
local file inclusion (LFI), 265
local files, access issues, 159–160
local intranet zone, for Internet
Explorer, 229
local machine zone, for Internet
Explorer, 229
localhost, danger of, 152–153
localStorage object ( JavaScript), 148
location.hash, 256
location headers, sending usercontrolled, 67
location.host property, 173
location object (JavaScript), 108, 153–154
location of documents, changing, 174–178
login forms, autocompletion by
browsers, 228
lookup functions, in Document Object
Model, 109
loopback interfaces, 152n
Lynx, 10

M
Macromedia Flash, 132
mailto: protocol, 25, 36, 256
mail user agent (MUA), 203n
malicious sites, blacklist-driven attempts to
block, 226
managed code, 134n
Mark of the Web (MotW), 204, 231
markup filter for user content, 86
mashups, 176
MathML (Mathematical Markup Language), 72, 122
Math.random() function, 109
max-age parameter
for cookie, 61
for STS record, 248
media capture, 259
Memex, 8
memory pointers, 266
memory use restrictions for scripts,
215–216
<meta> directive, 206, 208
<meta http-equiv=> directive, 78–79
meta-policies, for Flash, 156–157
mhtml protocol, 38
Microsoft. See also Internet Explorer
descendant policy development, 177
.NET Framework with XPAB
plug-ins, 136
objections to CORS, 239
Sun suit over Java virtual machine, 135n

Threats Against and Protection of Microsoft’s
Internal Network, 5n
Windows operating system, 10
Microsoft Office, 130
Microsoft Silverlight, 119, 134, 157
MIME (Multipurpose Internet Mail Extensions), 43n, 81n
malformed types, 199
mapping types to plaintext, 118
for plug-ins, 128
specialized for content in sandboxed
frame, 247
Mitchell, John C., 177, 240
mixed content, 183, 262–263
mmst: scheme, 36
mmsu: scheme, 36
Mocha language, 95
mocha: scheme, 37
modal behavior of dialogs, 218–219
Montulli, Lou, 60
Mosaic, 10. See also Netscape
MotW (Mark of the Web), 204, 231
mouse cursors, redefining, 89n
Moved Permanently status code (301), 55
Mozilla Firefox. See Firefox (Mozilla)
Mozilla specification, 242
msbd: scheme, 36
MsgBox (VBScript), 114
MUA (mail user agent), 203n
multiline headers, support for, 45
multiline string literals
Internet Explorer support, 91
in JavaScript, 113
multimedia playback, 130
Multipurpose Internet Mail Extensions
(MIME). See MIME (Multipurpose
Internet Mail Extensions)
My computer zone, for Internet
Explorer, 229

N
name attribute, of frames, 175
named entities, 76
namespace in JavaScript, 107
name: value pairs, in HTTP requests, 43
name=value pairs
cookies for storing, 60
for forms, 81
National Science Foundation, backbone
network, 10
Naval Research Laboratory, 3–4
navigateToURL() function, 133

INDEX

291

navigation
to sensitive schemes, 188
timing, 259
navigator.device.capture API, 259
navigator.geolocation.getCurrentPosition()
API, 258
navigator object (JavaScript), 108
navigator:registerProtocolHandler() API, 256
Negotiate authentication method, 63
.NET runtime, 135
Netflix, 134
Netscape
cookie specification, 151–152
and JavaScript, 95–96
and same-origin policy, 142
Netscape Navigator, 11
network fenceposts, 264
networking, HTTP-less, 257
New York Times, 192
newline, HTTP quirks in handling, 45
news: scheme, 36
NLS, 9
nntp: scheme, 36
no-cache value, for Cache-Control header, 59
No Content status code (204), 54
noncanonical encodings, 32n
nonencapsulating pseudo-protocols, 37
non-HTTP resources
character set detection for, 210–211
proxies allowing requests for, 46
non-renderable file types, 124
non-US-ASCII text, in URLs, 32–35
no-store value, for Cache-Control header, 59
Not Found status code (404), 56
Not Modified status code (304), 55, 59
Notification API, 259
NTLM authentication method, 63
NUL character, and HTTP headers, 51
NUL-containing strings,
JavaScript and, 109

O
Object Linking and Embedding
(OLE), 136
<object> tag, 83, 84
data=..., 128
mixed content, 183
octal character codes, JavaScript
support, 112
Ogg Theora, 119
OK status code (200), 54
OLE (Object Linking and
Embedding), 136
onbeforeunload dialog, 219n

292

INDEX

onerror handler, on <img> tag, 184
onerror parameter, 74
onkeydown event (JavaScript), 180
onload handler, to measure load time for
document, 184
onmousemove events, 222
opacity property (CSS2), and
JavaScript code, 179
opener.window.focus() function, 217n
OpenGL-based 3D graphics, 131n
open redirection, 263
Opera, 10
data: URLs in, 168
deleting JavaScript function, 103
and file extensions in URLs, 130
history.pushState() API, 256
local file access, 160
modal dialogs for prompts, 219
and multiline headers, 45
period-counting problem in, 159
Presto parsing engine, 70n
printable characters in, 32
redirects to about:blank, 167
Refresh redirection to javascript:, 170
and RFC 2047 encoding, 50
RSS and Atom renderers for, 124
stored password retrieval, 228
SWF file handling without
Content-Type, 199
Worker API, 258
OPTIONS method (HTTP), 53
Origin header, 240–241
origin inheritance, 165–171
about:blank document, 166–167
for javascript: scheme, 169–170
origins
ambiguous or unexpected, 158–161
attempts to broaden, 143
Ormandy, Tavis, 152
outerHTML property, 110–111
overwriting cookie, 62

P
P2P networking, 257
P3P (Platform for Privacy Preference), 193
Panopticlick, 109
parallel HTTP connection design, 216
<param> tag (HTML), for plug-ins, 128
_parent, as link target, 80
parsing
behavior fundamentals, 73–76
JavaScript, 97–98
modes for HTML documents, 71–72
resynchronization risks, 90–91

parsing engines, for browsers, 70n
Partial Content status code (206), 54
partly restricted URL scheme, 188
passive multimedia, CSP directive for, 243
password
in credentials portion of URLs, 26
form-based managers, 227–229
methods for passing, 63
Path parameter, for cookie, 61
path value, for cookie, 149–150
payload inspection, by Internet
Explorer, 202
PDF documents130–131
percent encoding, 31
percent sign (%), for character
encoding, 31
per-host connection limit, 216
period (.), hostnames with, and cookiesetting algorithms, 159
permissions, browser- and plug-inmanaged, 226–227
permitted-cross-domain-policies parameter,
for crossdomain.xml file, 162
persistent workers, for background
processes, 258
Petkov, Petko D., 131
phishing, 176n
plaintext
converting HTML to, 85
as file format, 117–118
for HTTP session information, 64
<plaintext> tag (HTML), 72
Platform for Privacy Preference (P3P), 193
plug-ins, 10–11
ActiveX, 129, 136–137
Adobe Flash. See Adobe Flash
application frameworks as basis, 131–136
content, 83
for content rendering, 127–138
CSP directive for, 243
document rendering helpers, 130–131
invoking, 128–130
Microsoft Silverlight, 119, 134, 157
for PDF documents, 130–131
perils of content-type handling, 129–130
protocols claimed by, 36–37
security rules, 153–158
site permissions management, 226–227
Sun Java, 134–135, 157–158
XML browser applications (XBAP),
135–136
PNG file format, 83
pointers, management vulnerabilities, 266
poisoned browser cache, on trusted
network, 60

pop-under, 217
pop-up filtering, 217–218
ports
default, for protocols, overriding, 27
prohibited, 190–192
positioning windows, 219–222
postMessage(...) API, 144–145, 258
POST method (HTTP), 52, 81
postponing JavaScript execution, 101
Pragma: no-cache request header, 59
prerendering web page, 258–259
presentation, HTML tags for, 73
PresentationHost.exe, 135
pressed key, examining code of, 180
Presto parsing engine, 70n
printable characters, browser
treatment of, 32
privacy-related side channels, 184–185
private browsing modes, 249, 253
private value, for Cache-Control header, 59
privileges, site, 225–234
prohibited ports, 190–192
properties, definitions in CSS, 89
proposals
content-level, 258–259
I/O interfaces, 259
URL- and protocol-level, 256–257
protocol-host-port tuple, 142, 241
protocol-level information
encryption, 64–66
preserving, 78
protocol-level proposals, 256–257
protocols
claimed by third-party applications, 36–37
default ports for, overriding, 27
registration, 256
in URL scheme name, 24
proxy-originating error responses, browser
processing, 47
proxy requests, 46–47
pseudo-functions (CSS), 89
pseudo-protocols
encapsulating, 37–38
nonencapsulating, 37
pseudo-URLs, 23, 24, 165
restricted, 170–171
and same-origin policy, 161
public key cryptography, 64, 64n
Public Suffix List, 159
public value, for Cache-Control header, 59
public Wi-Fi networks, and HTTP
caching risk, 60
Punycode, 34
purging browser cache, 60
PUT request (HTTP), 53
INDEX

293

Q
query string in URLs, 28
QuickTime (Apple), 119, 130, 132
quote characters, in HTML, 71, 74
quoted-printable encoding scheme, 50n
quoted-string syntax, 48–49
and cookies, 62
for CSS property values, 89

R
race conditions, in JavaScript, 101
raw text, for CSS property values, 89
Really Simple Syndication (RSS), 123
realm string, 62
RealNetworks RealPlayer, 130, 132
redirect headers, sending usercontrolled, 67
Redirection status codes (300–399), 55
Referer header, 43, 51
alternative to, 240
leakage, 263
relative URLs, 24
vs. absolute, 25
input filters, 40
resolution of, 38–39
remote file inclusion (RFI), 265
Request for Comments (RFC). See RFC
(Request for Comments)
request headers, in HTTP, 43
request types
form-triggered, 80–82
HTTP, 52–54
reserved characters, in HTML, 31–35, 71
resource exhaustion attacks, 214
response codes, server, 54–56
response splitting, 45
Restricted sites zone, for Internet
Explorer, 229–230
revalidation, 59
RFC (Request for Comments)
1630
on query string format, 28
on reference parser, 25–26
1738, on URLs, 24, 25
1866, on HTML 2.0, 69
1945
on HTTP, 42
and TEXT token, 50
2046, on application/octet-stream, 200
2047, for non-ISO-8859-1 string
format, 50
2109, on cookies, 60, 61, 62
2183, on Content-Disposition header, 203
2368, on query string format, 28

294

INDEX

2616, 44
on GET requests, 58
on HTTP, 42
on resolving ambiguities, 47
status codes for server response, 54
on URLs, 24
2617, on authentication, 62
2818, on encapsulation, 64
2965, on Cookie2, 60
3490, 34
3492, 34
3986, 24, 25, 33
4627, on JSON, 104
4918, on WebDAV, 54
6265, on cookies, 61
browser permissions to examine
payload, 198
on HTTP, 48
RFI (remote file inclusion), 265
rgb(...) pseudo functions (CSS), 89
Riley, Chris John, 203
Rios, Billy, 129
risk management, 4–6
root object in JavaScript, 107
Ross, David, 251
rotate(...) pseudo functions (CSS), 89
RSS (Really Simple Syndication), 123
rtsp: scheme, 36
runtime environment, for JavaScript,
102–104

S
Safari (Apple), 13
and credential portion of URLs, 26
deleting JavaScript function, 103
hiding address bar, 221
and multiline headers, 45
and realm string, 63
RSS and Atom renderers for, 124
SOP bypass flaws, 142n
stored password retrieval, 228
SWF file handling without
Content-Type, 199
text/plain document type, 200–201
third-party cookies, 193
time limits on continuously executing
scripts, 215
WebKit parsing engine, 70n
safeInnerHTML API, 251
same-origin policy mechanism, 16
cookies impact on, 150–151
for Document Object Model, 142–146
limitations, 173–186
loopholes, 185

and pseudo-URLs, 161
for web storage, 148
for XMLHttpRequest API, 146–148
sandbox directive, 244
sandboxed frames, 245–247, 250, 253
scripting, forms and navigation
restrictions, 247
synthetic origins, 247
sanitization
in-browser HTML, 250–251
of tags, 76
Scalable Vector Graphics (SVG), 83,
121–122
scale(...) pseudo functions (CSS), 89
schemes
current list of valid names, 24
input filters, 40
name in URLs, 24–25
navigation to sensitive, 188
Schwab, Charles, 230–231
screen object (JavaScript), 108
script-nonce directive, 244
scripts, 83
access to other documents, 111–112
browser-side, 95–116
connection limits, 216–217
dialog use restrictions, 218–219
execution time and memory use restrictions, 215–216
pop-up filtering, 217–218
rogue, 213–224
specifying charset, 209
script-src directive (CSP), 242
<script> tag (HTML), 72
JSON and, 104–105
language parameter, 113n
parsing and, 98
src parameter, 181
<script> tag (XHTML), 78
scrollbar, document-level, 180
Secure attribute, for cookie, 61
secure cookies, 150, 162
security
actions subject to checks, 141
definition, 2–4
new and upcoming features, 235–253
practical approaches, 7–8
quality assurance, 7
Security.allowDomain(...) method, for
Flash, 155
security dialogs, attacks on, 222–223
security engineering cheat sheet
building web applications on internal
networks, 195
Content Security Policy (CSP), 253

converting HTML to plaintext, 85
cross-domain communications in
JavaScript, 162, 186
cross-domain resources, 186
cross-domain XMLHttpRequest
(CORS), 253
data: and javascript: URLs, 172
decoding parameters received
through URLs, 40
embedding plug-in-handled active content from third parties, 162
enabling plug-in-handled files, 138
filtering user-supplies CSS, 93
generating documents with partly
attacker-controlled contents, 212
generating HTML documents with
attacker-controlled bits, 85
good practices for all websites, 212
hosting user-generated files, 212
hosting XML-based document
formats, 125
hosting your own plug-in-executed
content, 163
hygiene for all HTML documents, 85
interacting with browser objects on
client side, 115
launching non-HTTP services, 195
loading remote scripts, 115
loading remote stylesheets, 93
markup filter for user content, 86
non-HTML document types, 125
parsing JSON from server, 115
permitting user-created <iframe>
gadgets on site, 224
private browsing modes, 253
putting attacker-controlled values
into CSS, 93
relying on HTTP cookies for
authentication, 162
requesting elevated permissions within
web application, 232
sandboxed frames, 253
security hygiene for all websites, 186
security policy hygiene for all
websites, 162
security-sensitive UIs, 224
sending user-controlled location
headers, 67
sending user-controlled redirect
headers, 67
serving plug-in-handled files, 138
Strict Transport Security, 253
third-party cookies for gadgets or sandboxed content, 195
toStaticHTML() API, 253
INDEX

295

security engineering cheat sheet
(continued)
URL input filters, 40
URLs constructed based on user
input, 40
user-controlled filenames in
Content-Disposition headers, 67
user-controlled scripts, 116
user data in HTTP cookies, 67
user-specified class values on HTML
markup, 93
user-supplied data inside JavaScript
blocks, 115
writing browser extensions, 163
writing plug-ins or extensions recognizing privileged origins, 232
XDomainRequest, 253
XSS filtering, 253
security model extension frameworks,
236–241
cross-domain requests, 236–239
XDomainRequest, 239–240
security model restriction frameworks,
241–249
See Other status code (303), 55
selector suffixes, in CSS, 88
_self, as link target, 80
self-closing tag syntax, 72
semantic web, 72–73
semicolon (;), as delimiter
in HTTP headers, 48–49
in URLs, 29
server address, in URLs, 26–27
server port, in URLs, 27
server response codes, 54–56
server-side code, common problems
unique to, 265–266
server-side errors (500–599), 56
Service Unavailable error (503), 56
sessionStorage object (JavaScript), 148
Set-Cookie headers, 61
setters, in JavaScript, 103
SGML (Standard Generalized Markup
Langauge), 9
shared workers, for background
processes, 258
Shockwave Flash, 132
SHODAN, 203
showModalDialog() method, 217
shttp: scheme, 36
Simple Mail Transfer Protocol (SMTP),
27, 44, 190
sip: scheme, 36
<site-control permitted-cross-domainpolicies=.".."> parameter, 157

296

INDEX

site privileges, 225–234
browser- and plug-in-managed permissions, 226–227
skew(...) pseudo functions (CSS), 89
SMTP (Simple Mail Transfer Protocol),
27, 44, 190
social engineering attacks, 32n
software, difficulty analyzing behavior of, 3
<span> tag (HTML), 73
SPDY (Speedy), 257
Spyglass Mosaic, 10
SSL, warnings appearance, 66
Standard Generalized Markup Langauge
(SGML), 9
statistical forecasting, 6
Sterne, Brandon, 242
Stone, Paul, 180
Strict Transport Security (STS),
248–249, 253
strict XML mode, 72
stylesheets
CSP directive for, 243
specifying charset, 209
<style> tag (HTML), 72
<style> tag (XHTML), 78
subframes, CSP directive for, 243
subresources
cross-origin, 183
markup-controlled charset on, 209–210
Sun Java, 134–135, 157–158
Sun Microsystems, 129
SVG (Scalable Vector Graphics), 83,
121–122
<svg> tag (HTML5), 122
synchronous XMLHttpRequest, 146
syntax-delimiting characters, in URLs, 31
“syntax error” message, retrieved file
snippet in, 181

T
<table> tag (HTML), 83
tags, in HTML, 70
handling those not closed before end
of file, 75
interactions, 74–75
sanitization, 76
target parameter, for <a href=...> tag
(HTML), 79, 174–175
taxonomy, 6–7
TCP/IP, HTTP and, 42
TCP (Transmission Control Protocol), 42n
connections via XMLSocket, 156
list of prohibited ports, 190–192
Temporary Redirect status code (307), 55

testing, for Internet Explorer use, 112
text/css document type, 118
text/csv document type, 198
text/html document type, 124
text message, sending to window with
valid JavaScript handle, 144
text/plain document type, 118, 156,
200–201, 204, 212
TEXT token, 50
<textarea> tag (HTML), 72, 111
third-party applications, protocols claimed
by, 36–37
third-party cookies, limitations, 192–194
threat evolution, 14–18
cloud, 15
nonconvergence of visions, 15–16
user as security flaw, 14–15
Threats Against and Protection of Microsoft’s
Internal Network (Microsoft), 5n
three-step TCP handshake, 56
TIFF file format, 83
timer, in JavaScript, 101
timing attacks, on user interfaces, 222–223
TLS (Transport Layer Security), 64
_top, as link target, 80
top-level domains, 152
toSource() method (JavaScript), 101
toStaticHTML() API, 250–251, 253
toString() method (JavaScript), 101
TRACE method (HTTP), 53
tracking, unscrupulous online, 193
tragedy of the commons dilemma, 3
Transfer-Encoding: chunked scheme, 58
Transmission Control Protocol (TCP). See
TCP (Transmission Control Protocol)
Transport Layer Security (TLS), 64
Trident parsing engine, 70n
Trusted sites zone, for Internet
Explorer, 229
Turing, Alan, 3n
type parameter, for plug-in tag, 128

U
UI spoofing attacks, and Flash, 132
unauthenticated requests, by browser, 62
Unauthorized status error (401), 55, 62
unhandled exception, in JavaScript, 100
Unicode, 33
decimal &#number; notation for, 77
escaping method based on, 113
JavaScript support, 112
whitespace, 74n

Uniform Messaging Policy, 240
Uniform Resource Locators (URLs), 23–40
browser processing, 29–31
common schemes, 36–38
constructing based on user input, 40
encoding, 31
encoding data in fragment
identifiers, 144n
fully qualified absolute, 24
hiding with encapsulating protocols, 38
navigation based on tiers of schemes, 188
resolution of relative, 38–39
structure, 24–31
credentials, 26
fragment ID, 28–29
hierarchical file path, 27–28
indicator of hierarchical URLs, 25–26
query string, 28
scheme name, 24–25
server address, 26–27
server port, 27
UniformRequest API, 240
University of Illinois, 10
Unix services, listener process, 216n
unreserved characters, in HTML, 32
unrestricted URL scheme, 188
URLs (Uniform Resource Locators). See
Uniform Resource Locators (URLs)
URL-handling APIs, 133
URL-level proposals, 256–257
url(...) pseudo-functions (CSS), 89
user
browsing habits, Referer header and, 51
collecting information about
interaction, 184
as security flaw, 14–15
URL construction based on input, 40
User-Agent request header, 43
user content, markup filter for, 86
user-controlled filenames in ContentDisposition headers, 67
user data in HTTP cookies, 67
user interfaces
browser extensions and, 161
notifications, 259
timing attacks on, 222–223
username, in credentials portion
of URLs, 26
UTF-7 charset, 78
UTF-8 charset, 33, 206
in HTTP headers, 50
UTF-16 charset, 78, 206
UTF-32 charset, 78

INDEX

297

V
valid scheme names, current list, 24
variables, declaration in JavaScript, 99
VBScript, 96
vbscript: scheme, 37, 169–170
vertical tab, in HTML tag, 74
<video> tag (HTML5), 84, 119, 131
view-cache: scheme, 37
View > Encoding menu, 209
view-source: scheme, 37
Visual Basic, 10, 114, 130
VoiceXML, 236

W
W3C (World Wide Web Consortium),
12, 70
w3m, 10
WAP (Wireless Application Protocol
suite), 123
WBXML, 123n
WDP file format, 83
Web, the. See World Wide Web
web 2.0, 12–13
web applications
design issues, 263–265
vulnerabilities specific to, 262–263
WebDAV, 54
WebGL, 131, 131n
Web Hypertext Application Technology
Working Group (WHATWG), 13
WebKit parsing engine, 70n, 242
character set inheritance, 209
CORS in, 237, 239
data: URLs in, 168
history.pushState() API, 256
Refresh redirection to javascript:, 170
Strict Transport Security support, 248
Worker API, 258
XSS-detection logic, 251
web page, prerendering, 258–259
web storage, same-origin policy mechanism for, 148
WebRTC, 257
WebSocket API, 257
WebSQL API, 258
Western European code page
(ISO-8859-1), 50
WHATWG (Web Hypertext Application
Technology Working Group), 13
whitelists, 226
whitespace, 74, 92
window.alert() API, 218

298

INDEX

window.blur() function, 217n, 220
window.confirm() API, 218
window.createPopup() API, 222
window.focus() method, 220
window handles, 175
window.moveTo() method, 220
window.name property, of frames, 175
window.notifications API, 259
window.open() function, 111, 174–175, 217,
217n, 219, 222
window.print() API, 218
window.prompt() API, 218
window.resizeTo() method, 220
windows
anonymous, 175
creating new in browser, 217
and frame interactions, 174–181
positioning, 219–222
window.showModalDialog() API, 217
Windows Media Player, 119, 130, 132
Windows operating system, 10, 13
window splicing, 220–221
Windows Presentation Foundation,
134, 136
Wireless Application Protocol
suite (WAP), 123
Wireless Markup Language (WML), 123
WMF file format, 83
WML Script (WMLS), and JavaScript, 123
WML (Wireless Markup Language), 123
Worker API, 258
World Wide Web
browser wars, 10–11, 233
history, 8–13
threat of hostile takeover, 131
World Wide Web Consortium (W3C),
12, 70
creation of, 11
Microsoft and, 239
worms, 12
WWW-Authenticate header, 62, 63
wyciwyg: scheme, 37

X
XAML (Extensible Application Markup
Language), 134
Xanadu, 9
XBAP (XML browser applications), 135–136
XBL bindings, 89–90
X-Content-Type-Options header, 208
X-Content-Type-Options: nosniff header, 203
XDomainRequest API, 239–240, 253
X-Frame-Options header, 179–180, 243

XHTML, 12
and HTML entities, 78
minimal fault-tolerance of parser, 73
named entities, 76
syntax, 70
XML (Extensible Markup Language)
and bandwidth, 123n
binary-only serialization, 123n
case of tags, 72
<![CDATA[...]]> blocks, 72, 78, 250
XML Binding Language files, 90
XML browser applications (XBAP),
135–136
XML documents
browser support, 119–124
generic view, 120–121
XMLHttpRequest API, 12, 54, 210, 236,
237–238
httponly cookies and, 150
same-origin policy mechanism for,
146–148
xmlns namespace, 72, 119
<?xml-stylesheet href=... ?> directive, 88
XML User Interface Language (XUL),
122–123
XMLSocket, TCP connections via, 156
<xmp> tag (HTML), 72
XSRF (cross-site request forgery), 84,
190, 262
exploitation of flaws, 190
login forms and, 145–146
XSS (cross-site scripting), 71, 262
bugs, and password managers, 228
exploitation of flaws, 190
filtering, 251–252, 253
XUL (XML User Interface Language),
122–123
XXE (External XML Entity) attack, 76

Z
ZIP files, extracting content from, 37
Zone.Identifier metadata, Internet Explorer
and, 231
zone model, for Internet Explorer,
229–231

INDEX

299

UPDATES
Visit http://nostarch.com/tangledweb.htm for updates, errata, and other
information.

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