OWASP Code Review Guide V2

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CODE
REVIEW
GUIDE
RELEASE
Creative Commons (CC) Attribution
Free Version at: https://www.owasp.org
Project leaders: Larry Conklin and Gary Robinson
2.0
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Foreword Acknowledgements
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3 6 85
Introduction How To Use The Code Review Guide
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Code Review Do’s And Dont’s
Code Review Checklist
Threat Modeling Example
Code Crawling
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A1 Injection
A2 Broken Authentication And Session Management
A3 Cross-Site Scripting (XSS)
A4 Insecure Direct Object Reference
A5 Security Misconguration
A6 Sensitive Data Exposure
A7 Missing Function Level Access Control
A8 Cross-Site Request Forgery (CSRF)
A9 Using Components With Know Vulnerabilities
A10 Unvalidated Redirects And Forwards
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Technical Reference For Secure Code Review Appendix
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HTML5
Same Origin Policy
Reviewing Logging Code
Error Handling
Reviewing Security Alerts
Review For Active Defence
Race Conditions
Buer Overruns
Client Side JavaScript
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Secure Code Review
Methodology
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Introduction
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Code Review Guide Foreword - By Eoin Keary
By Eoin Keary,
Long Serving OWASP Global Board Member
The OWASP Code Review guide was originally born from the
OWASP Testing Guide. Initially code review was covered in the
Testing G uide, as i t seem ed like a good i dea at the t ime. Howe v-
er, the topic of security code review is too big and evolved into
its own stand-alone guide.
I started the Code Review Project in 2006. This current edition
was started in April 2013 via the OWASP Project Reboot initia-
tive and a grant from the United States Department of Home-
land Security.
The OWASP Code Review team consists of a small, but talented,
group of volunteers who should really get out more often. The
volunteers have experience and a drive for the best practices
in secure code review in a variety of organizations, from small
start-ups to some of the largest software development organi-
zations in the world.
It is common knowledge that more secure software can be pro-
duced and developed in a more cost eective way when bugs
are detected early on in the systems development lifecycle. Or-
ganizations with a proper code review function integrated into
the software development lifecycle (SDLC) produced remark-
ably better code from a security standpoint. To put it simply “We
can’t hack ourselves secure”. Attackers have more time to nd
vulnerabilities on a system than the time allocated to a defend-
er. Hacking our way secure amounts to an uneven battleeld,
asymmetric warfare, and a losing battle.
By necessity, this guide does not cover all programming lan-
guages. It mainly focuses on C#/.NET and Java, but includes C/
C++, PHP and other languages where possible. However, the
techniques advocated in the book can be easily adapted to al-
most any code environment. Fortunately (or unfortunately), the
security aws in web applications are remarkably consistent
across programming languages.
Eoin Keary, June 2017
FOREWORD
1
4Acknowledgements
APPRECIATION TO UNITED STATES DEPARTMENT OF
HOMELAND SECURITY
FEEDBACK
OWASP community and Code Review Guide project leaders wish to expresses its deep ap-
preciation to United States Department of Homeland Security for helping make this book
possible by funds provided to OWASP thru a grant. OWASP continues be to the preeminent
organization for free unbiased/unfretted application security.
We have seen a disturbing rise in threats and attacks on community institutions thru appli-
cation vulnerabilities, only by joining forces, and with unfretted information can we help
turn back the tide these threats. The world now runs on software and that software needs
to be trust worthy. Our deepest appreciation and thanks to DHS for helping and in sharing
in this goal.
If you have any feedback for the OWASP Code Review team, and/or nd any mistakes or
improvements in this Code Review Guide please contact us at:
owasp-codereview-project@owasp.org
5
Acknowledgements
ACKNOWLEDGEMENTS
Content Contributors
Larry Conklin
Gary Robinson
Johanna Curiel
Eoin Keary
Islam Azeddine Mennouchi
Abbas Naderi
Carlos Pantelides
Michael Hidalgo
Reviewers
Alison Shubert
Fernando Galves
Sytze van Koningsveld
Carolyn Cohen
Helen Gao
Jan Masztal
David Li
Lawrence J Timmins
Kwok Cheng
Ken Prole
David D’Amico
Robert Ferris
Lenny Halseth
Kenneth F. Belva
VERSION 2.0, 2017
Project Leaders
Larry Conklin Gary Robinson
Project Leader
Eoin Keary
Content Contributors
Jenelle Chapman
Andrew van der Stock
Paolo Perego
David Lowry
David Rook
Dinis Cruz
Je Williams
Reviewers
Je Williams
Rahin Jina
VERSION 1.0, 2007
7
How to use
The contents and the structure of the book have been carefully designed. Further, all the contributed chapters have been judi-
ciously edited and integrated into a unifying framework that provides uniformity in structure and style.
This book is written to satisfy three dierent perspectives.
1. Management teams who wish to understand the reasons of why code reviews are needed and why they are included in best
practices in developing secure enterprise software for todays organizations. Senior management should thoroughly read sec-
tions one and two of this book. Management needs to consider the following items if doing secure coding is going to be part of
the organizations software development lifecycle:
• Does organization project estimation allot time for code reviews?
• Does management have the capability to track the relevant metrics of code review and static analysis for each project and
programmer?
• Management needs to decide when in the project life cycle will that code reviews should be done in the project lifecycle and
what changes to existing projects require review of previously completed code reviews.
2. Software leads who want to give manfully feedback to peers in code review with ample empirical artifacts as what to look for
in helping create secure enterprise software for their organizations. They should consider:
•As a peer code reviewer, to use this book you rst decided on the type of code review do you want to accomplish. Lets spend a
few minutes going over each type of code review to help in deciding how this book can be assistance to you.
• API/design code reviews. Use this book to understand how architecture designs can lead to security vulnerabilities. Also if the
API is a third party API what security controls are in place in the code to prevent security vulnerabilities.
• Maintainability code reviews. These types of code reviews are more towards the organizations internal best coding practices.
This book does cover code metrics, which can help the code reviewer, better understand what code to look at for security vul-
nerabilities if a section of code is overly complex.
• Integration code reviews. Again these types of code reviews are more towards the organizations internal coding policies. Is
the code being integrated into the project fully vetted by IT management and approved? Many security vulnerabilities are now
being implemented by using open source libraries whichh may bring in dependencies that are not secure.
• Testing code reviews. Agile and Test Driven design where programmer creates unit tests to prove code methods works as the
programmer intended. This code is not a guide for testing software. The code reviewer may want to pay attention to unit test
cases to make sure all methods have appropriate exceptions; code fails in a safe way. If possible each security control in code has
the appropriate unit test cases.
3. Secure code reviewer who wants an updated guide on how secure code reviews are integrated in to the organizations secure
software development lifecycle. This book will also work as a reference guide for the code review as code is in the review process.
This book provides a complete source of information needed by the code reviewer. It should be read rst as a story about code
reviews and seconds as a desktop reference guide.
HOW TO USE THE CODE REVIEW GUIDE
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2
9
SECURE CODE REVIEW
Technical Reference For Secure Code Review
Here the guide drills down into common vulnerabilities and technical controls, including XSS, SQL injection,
session tracking, authentication, authorization, logging, and information leakage, giving code examples in
various languages to guide the reviewer.
This section can be used to learn the important aspects of the various controls, and as an on-the-job reference
when conducting secure code reviews.
We start with the OWASP Top 10 issues, describing technical aspects to consider for each of these issues. We
then move onto other common application security issues not specic to the OWASP Top 10
Secure code review is probably the single-most eective technique for identifying security bugs early in the
system development lifecycle. When used together with automated and manual penetration testing, code
review can signicantly increase the cost eectiveness of an application security verication eort.
This guide does not prescribe a process for performing a security code review. Rather, it provides guidance on
how the eort should be structured and executed. The guide also focuses on the mechanics of reviewing code
for certain vulnerabilities.
Manual secure code review provides insight into the “real risk” associated with insecure code. This contextual,
white-box approach is the single most important value. A human reviewer can understand the relevance of
a bug or vulnerability in code. Context requires human understanding of what is being assessed. With ap-
propriate context we can make a serious risk estimate that accounts for both the likelihood of attack and the
business impact of a breach. Correct categorization of vulnerabilities helps with priority of remediation and
xing the right things as opposed to wasting time xing everything.
5.1 Why Does Code Have Vulnerabilities?
MITRE has catalogued circa 1000 dierent kinds of software weaknesses in the CWE project. These are all
dierent ways that software developers can make mistakes that lead to insecurity. Every one of these weak-
nesses is subtle and many are seriously tricky. Software developers are not taught about these weaknesses in
school and most do not receive any on the job training about these problems.
These problems have become so important in recent years because we continue to increase connectivity
and add technologies and protocols at an extremely fast rate. The ability to invent technology has seriously
outstripped the ability to secure it. Many of the technologies in use today simply have not received enough
(or any) security scrutiny.
There are many reasons why businesses are not spending the appropriate amount of time on security. Ulti-
mately, these reasons stem from an underlying problem in the software market. Because software is essen-
tially a black box, it is extremely dicult for a customer to tell the dierence between good code and insecure
code. Without this visibility vendors are not encouraged to spend extra eort to produce secure code. Nev-
ertheless, information security experts frequently get pushback when they advocate for security code review,
with the following (unjustied) excuses for not putting more eort into security:
“We never get hacked (that I know of), we don’t need security”
Secure Code Review
10
“We have a rewall that protects our applications”
“We trust our employees not to attack our applications”
Over the last 10 years, the team involved with the OWASP Code Review Project has performed thousands of
application reviews, and found that every non-trivial application has had security vulnerabilities. If code has
not been reviewed for security holes, the likelihood that the application has problems is virtually 100%.
Still, there are many organizations that choose not to know about the security of their code. To them, consider
Rumsfeld’s cryptic explanation of what we actually know:
“...we know, there are known knowns; there are things we know we know. We also know there are known un-
knowns; that is to say we know there are some things we do not know. But there are also unknown unknowns
-- the ones we don’t know we don’t know.”- Donald Rumsfeld
If informed decisions are being made based on a measurement of risk in the enterprise, which will be fully
supported. However, if risks are not being understood, the company is not being duly diligent, and is being
irresponsible both to shareholders and customers.
5.2 What is Secure Code Review?
Code review aims to identify security aws in the application related to its features and design, along with the
exact root causes. With the increasing complexity of applications and the advent of new technologies, the
traditional way of testing may fail to detect all the security aws present in the applications. One must under-
stand the code of the application, external components, and congurations to have a better chance of nding
the aws. Such a deep dive into the application code also helps in determining exact mitigation techniques
that can be used to avert the security aws.
It is the process of auditing the source code of an application to verify that the proper security and logical
controls are present, that they work as intended, and that they have been invoked in the right places. Code
review is a way of helping ensure that the application has been developed so as to be “self-defending” in its
given environment.
Secure code review allows a company to assure application developers are following secure development
techniques. A general rule of thumb is that a penetration test should not discover any additional application
vulnerabilities relating to the developed code after the application has undergone a proper security code
review. At the least very few issues should be discovered.
All security code reviews are a combination of human eort and technology support. At one end of the spec-
trum is an inexperienced person with a text editor. At the other end of the scale is an expert security team with
advanced static analysis (SAST) tools. Unfortunately, it takes a fairly serious level of expertise to use the current
application security tools eectively. They also don’t understand dynamic data ow or business logic. SAST
tools are great for coverage and setting a minimum baseline.
Tools can be used to perform this task but they always need human verication. They do not understand
context, which is the keystone of security code review. Tools are good at assessing large amounts of code and
pointing out possible issues, but a person needs to verify every result to determine if it is a real issue, if it is
actually exploitable, and calculate the risk to the enterprise. Human reviewers are also necessary to ll in for
the signicant blind spots, which automated tools, simply cannot check.
Secure Code Review
11
Secure Code Review
5.3 What is the dierence between Code Review and Secure Code Review?
The Capability Maturity Model (CMM) is a widely recognized process model for measuring the development
processes of a software development organization. It ranges from ‘level 1’ where development processes are
ad hoc, unstable and not repeatable, to ‘level 5’ where the development processes are well organized, docu-
mented and continuously improving. It is assumed that a company’s development processes would start out
at level 1 when starting out (a.k.a start-up mode) and will become more dened, repeatable and generally
professional as the organization matures and improves. Introducing the ability to perform code reviews (note
this is not dealing with secure code review yet) comes in when an organization has reached level 2 (Repeat-
able) or level 3 (Dened).
Secure Code Review is an enhancement to the standard code review practice where the structure of the re-
view process places security considerations, such as company security standards, at the forefront of the de-
cision-making. Many of these decisions will be explained in this document and attempt to ensure that the
review process can adequately cover security risks in the code base, for example ensuring high risk code is
reviewed in more depth, ensuring reviewers have the correct security context when reviewing the code, en-
suring reviewers have the necessary skills and secure coding knowledge to eectively evaluate the code.
5.4 Determining the Scale of a Secure Source Code Review?
The level of secure source code review will vary depending on the business or regulatory needs of the software, the
size of the software development organization writing the applications and the skills of the personnel. Similar to
other aspects of software development such as performance, scalability and maintainability, security is a measure of
maturity in an application. Security is one of the non-functional requirements that should be built into every serious
application or tool that is used for commercial or governmental purposes.
If the development environment consists of one person programming as a hobby and writing a program to track
their weekly shopping in visual basic (CMM level 1), it is unlikely that that programmer will use all of the advice
within this document to perform extensive levels of secure code review. On the other extreme, a large organization
with thousands of developers writing hundreds of applications will (if they wish to be successful) take security very
seriously, just like they would take performance and scalability seriously.
Not every development organization has the necessity, or resources, to follow and implement all of the topics in
this document, but all organizations should be able to begin to write their development processes in a way that
can accommodate the processes and technical advice most important to them. Those processes should then
be extensible to accommodate more of the secure code review considerations as the organization develops and
matures.
In a start-up consisting of 3 people in a darkened room, there will not be a ‘code review team’ to send the code to,
instead it’ll be the bloke in the corner who read a secure coding book once and now uses it to prop up his monitor.
In a medium sized company there might be 400 developers, some with security as an interest or specialty, however
the organizations processes might give the same amount of time to review a 3 line CSS change as it gives to a
redesign of the agship products authentication code. Here the challenge is to increase the workforce’s secure
coding knowledge (in general) and improve the processes through things like threat modelling and secure code
review.
For some larger companies with many thousands of developers, the need for security in the S-SDLC is at its greatest,
but process eciency has an impact on the bottom line. Take an example of a large company with 5,000 developers.
If a change is introduced to the process that results in each developer taking an extra 15 minutes a week to perform
a task, suddenly that’s 1,250 hours extra each week for the company as a whole This results in a need for an extra 30
full time developers each year just to stay on track (assuming a 40 hour week). The challenge here is to ensure the
security changes to the lifecycle are ecient and do not impede the developers from performing their task.
12
It must be remembered though, no matter what size the organization, the reason to perform secure code re-
view is to catch more bugs and catch them earlier in the S-SDLC. It is quicker to conduct a secure code review
and nd bugs that way, compared to nding the bugs in testing or in production. For the 5,000-person orga-
nization, how long will it take to nd a bug in testing, investigate, re-code, re-review, re-release and re-test?
What if the code goes to production where project management and support will get involved in tracking the
issue and communicating with customers? Maybe 15 minutes a week will seem like a bargain.
5.5 We Can’t Hack Ourselves Secure
Penetration testing is generally a black-box point in time test and should be repeated on each release (or
build) of the source code to nd any regressions. Many continuous integration tools (e.g. Jenkins/Hudson)
allow repeatable tests, including automated penetration tests, to be run against a built and installed version
of a product.
As source code changes, the value of the ndings of an unmaintained penetration tests degrade with time.
There are also privacy, compliance and stability and availability concerns, which may not covered by penetra-
tion testing, but can be covered in code reviews. Data information leakage in a cloud environment for example
may not be discovered, or allowed, via a penetration test. Therefore penetration testing should be seen as an
important tool in the arsenal, but alone it will not ensure product software is secure.
The common methods of identifying vulnerabilities in a software project are:
• Source Code Scanning using automated tools that run against a source code repository or module, nding
string patterns deemed to potentially cause security vulnerabilities.
Secure Code Review
There seems to be a catch-22 with the following sentiment; as many code developers are not aware or skilled in
security, a company should implement peer secure code reviews amongst developers.
How does a workforce introduce the security skills to implement a secure code review methodology? Many
security maturity models (e.g. BSIMM or OpenSAMM) discuss the concept of a core security team, who are skilled
developers and skill security subject matter experts (SMEs). In the early days of a company rolling out a se-
cure code review process, the security SMEs will be central in the higher risk reviews, using their experience and
knowledge to point out aspects of the code that could introduce risk.
As well as the core security team a further group of developers with an interest in security can act as team local
security SMEs, taking part in many secure code reviews. These satellites (as BSIMM calls them) will be guided by
the core security team on technical issues, and will help encourage secure coding.
Over time, an organization builds security knowledge within its core, and satellite teams, which in turns spreads
the security knowledge across all developers since most code reviews will have a security SME taking part.
The ‘on-the-job’ training this gives to all developers is very important. Whereas an organization can send their de-
velopers on training courses (classroom or CBT) which will introduce them to common security topics and create
awareness, no training course can be 100% relevant to a developer’s job. In the secure code review process, each
developer who submits their code will receive security related feedback that is entirely relevant to them, since
the review is of the code they produced.
Skilling a Workforce for Secure Code Review
13
Secure Code Review
• Automated Penetration Testing (black/grey box) through penetrating testing tools automatic scans, where
the tool is installed on the network with the web site being tested, and runs a set of pre-dened tests against
the web site URLs.
• Manual Penetration Testing, again using tools, but with the expertise of a penetration tester performing more
complicated tests.
• Secure Code Review with a security subject matter expert.
It should be noted that no one method will be able to identify all vulnerabilities that a software project might
encounter, however a defense-in-depth approach will reduce the risk of unknown issues being including in
production software.
During a survey at AppSec USA 2015 the respondents rated which security method was the most eective in
nding:
1) General security vulnerabilities
2) Privacy issues
3) Business logic bugs
4) Compliance issues (such as HIPPA, PCI, etc.)
5) Availability issues
The results are shown in gure 1.
Vulnerabilities
Source Code Scanning Tools
Automated Scan
Manual Pen Test
Manual Code Review
0
5
10
15
20
25
30
35
Privacy Business Logic Compliance
(HIPPA)
Availability
Figure 1: Survey relating detection methods to general vulnerability types
14 Secure Code Review
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10
Source Code Scanning Tool
Automated Scan
Manual Pen Test
Manual Code Review
0
5
10
15
20
25
30
35
Figure 2: Survey relating detection methods to OWASP Top 10 vulnerability types
Figure 3: Code Review and Penetration Testing Interactions
These surveys show that manual code review should be a component of a company’s secure lifecycle, as in
many cases it is as good, or better, than other methods of detecting security issues.
5.6 Coupling Source Code Review and Penetration Testing
The term “360 review” refers to an approach in which the results of a source code review are used to plan and
execute a penetration test, and the results of the penetration test are, in turn, used to inform additional source
code review.
CODE
REVIEW
SUSPECTED KNOWN
VULNERABILITIES
EXPLOITED
VULNERABILITIES
PENETRATION
TEST
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Secure Code Review
Knowing the internal code structure from the code review, and using that knowledge to form test cases and
abuse cases is known as white box testing (also called clear box and glass box testing). This approach can lead to
a more productive penetration test, since testing can be focused on suspected or even known vulnerabilities. Us-
ing knowledge of the specic frameworks, libraries and languages used in the web application, the penetration
test can concentrate on weaknesses known to exist in those frameworks, libraries and languages.
A white box penetration test can also be used to establish the actual risk posed by a vulnerability discovered
through code review. A vulnerability found during code review may turn out not to be exploitable during pen-
etration test due to the code reviewer(s) not considering a protective measure (input validation, for instance).
While the vulnerability in this case is real, the actual risk may be lower due to the lack of exposure. However there
is still an advantage to adding the penetration test encase the protective measure is changed in the future and
therefore exposes the vulnerability.
While vulnerabilities exploited during a white box penetration test (based on secure code review) are certainly
real, the actual risk of these vulnerabilities should be carefully analyzed. It is unrealistic that an attacker would
be given access to the target web application’s source code and advice from its developers. Thus, the risk that
an outside attacker could exploit the vulnerabilities found by the white box penetration tester is probably low-
er. However, if the web application organization is concerned with the risk of attackers with inside knowledge
(former employees or collusion with current employees or contractors), the real-world risk may be just as high.
The results of the penetration test can then be used to target additional areas for code review. Besides address-
ing the par-ticular vulnerability exploited in the test, it is a good practice to look for additional places where that
same class of vulnerability is present, even if not explicitly exploited in test. For instance, if output encoding is
not used in one area of the application and the penetration test exploited that, it is quite possible that output
encoding is also not used elsewhere in the application.
5.7 Implicit Advantages of Code Review to Development Practices
Integrating code review into a company’s development processes can have many benets which will depend
upon the processes and tools used to perform code reviews, how well that data is backed up, and how those
tools are used. The days of bringing developers into a room and displaying code on a projector, whilst recording
the review results on a printed copy are long gone, today many tools exist to make code review more ecient
and to track the review records and decisions. When the code review process is structured correctly, the act of
reviewing code can be ecient and provide educational, auditable and historical benets to any organization.
This section provides a list of benets that a code review procedure can add to development organization.
Provides an historical record
If any developer has joined a company, or moved teams within a company, and had to maintain or enhance a
piece of code written years ago, one of the biggest frustrations can be the lack of context the new developer has
on the old code. Various schools of opinion exist on code documentation, both within the code (comments) and
external to the code (design and functional documents, wikis, etc.). Opinions range from zero-documentation
tolerance through to near-NASA level documentation, where the size of the documentation far exceeds the size
of the code module.
Many of the discussions that occur during a code review, if recorded, would provide valuable information (con-
text) to module maintainers and new programmers. From the writer describing the module along with some of
their design decisions, to each reviewers comments, stating why they think one SQL query should be restruc-
tured, or an algorithm changed, there is a development story unfolding in front of the reviewers eyes which can
be used by future coders on the module, who are not involved in the review meetings.
16 Secure Code Review
Capturing those review discussions in a review tool automatically and storing them for future reference will pro-
vide the development organization with a history of the changes on the module which can be queried at a lat-
er time by new developers. These discussions can also contain links to any architectural/functional/design/test
specications, bug or enhancement numbers.
Verication that the change has been tested
When a developer is about to submit code into the repository, how does the company know they have sucient-
ly tested it? Adding a description of the tests they have run (manually or automated) against the changed code
can give reviewers (and management) condence that the change will work and not cause any regressions. Also
by declaring the tests the writer has ran against their change, the author is allowing reviewers to review the tests
and suggest further testing that may have been missed by the author.
In a development scenario where automated unit or component testing exists, the coding guidelines can require
that the developer include those unit/component tests in the code review. This again allows reviewers within this
environment to ensure the correct unit/component tests are going to be included in the environment, keeping
the quality of the continuous integration cycles.
Coding education for junior developers
After an employee learns the basics of a language and read a few of the best practices book, how can they get
good on-the-job skills to learn more? Besides buddy coding (which rarely happens and is never cost eective)
and training sessions (brown bag sessions on coding, tech talks, etc.) the design and code decisions discussed
during a code review can be a learning experience for junior developers. Many experienced developers admit to
this being a two way street, where new developers can come in with new ideas or tricks that the older developers
can learn from. Altogether this cross pollination of experience and ideas can only be benecial to a development
organization.
Familiarization with code base
When a new feature is developed, it is often integrated with the main code base, and here code review can be a
conduit for the wider team to learn about the new feature and how its code will impact the product. This helps
prevent functional duplication where separate teams end up coding the same small piece of functionality.
This also applies for development environments with siloed teams. Here the code review author can reach out to
other teams to gain their insight, and allow those other teams to review their modules, and everyone then learns
a bit more about the company’s code base.
Pre-warning of integration clashes
In a busy code base there will be times (especially on core code modules) where multiple developers can write
code aecting the same module. Many people have had the experience of cutting the code and running the
tests, only to discover upon submission that some other change has modied the functionality, requiring the
author to recode and retest some aspects of their change. Spreading the word on upcoming changes via code
reviews gives a greater chance of a developer learning that a change is about to impact their upcoming commit,
and development timelines, etc., can be updated accordingly.
Many development environments have coding guidelines which new code must adhere to. Coding guidelines
can take many forms. It’s worth pointing out that security guidelines can be a particularly relevant touch point
Secure Coding Guidelines Touch Point
17
Secure Code Review
5.8 Technical Aspects of Secure Code Review
Security code reviews are very specic to the application being reviewed. They may highlight some aws that
are new or specic to the code implementation of the application, like insecure termination of execution ow,
synchronization errors, etc. These aws can only be uncovered when we understand the application code ow
and its logic. Thus, security code review is not just about scanning the code for set of unknown insecure code
patterns but it also involves understanding the code implementation of the application and enumerating the
aws specic to it.
The application being reviewed might have been designed with some security controls in place, for example a
centralized blacklist, input validation, etc. These security controls must be studied carefully to identify if they
are fool-proof. According to the implementation of the control, the nature of attack or any specic attack vec-
tor that can be used to bypass it, must be analyzed. Enumerating the weakness in the existing security control
is another important aspect of the security code reviews.
There are various reasons why security aws manifest in the application, like a lack of input validation or
parameter mishandling. In the process of a code review the exact root cause of aws are exposed and the
complete data ow is traced. The term ‘source to sink analysis’ means to determine all possible inputs to the
application (source) and how they are being processed by it (sink). A sink could be an insecure code pattern
like a dynamic SQL query, a log writer, or a response to a client device.
Consider a scenario where the source is a user input. It ows through the dierent classes/components of the
application and nally falls into a concatenated SQL query (a sink) and there is no proper validation being
applied to it in the path. In this case the application will be vulnerable to SQL injection attack, as identied
by the source to sink analysis. Such an analysis helps in understanding, which vulnerable inputs can lead to a
possibility of an exploit in the application.
Once a aw is identied, the reviewer must enumerate all the possible instances present in the application.
This would not be a code review initiated by a code change, this would be a code scan initiated by manage-
ment based on a aw being discovered and resources being committed to nd if that aw exists in other
parts of the product. For example, an application can be vulnerable to XSS vulnerability because of use of
un-validated inputs in insecure display methods like scriptlets ‘response.write’ method, etc. in several places.
5.9 Code Reviews and Regulatory Compliance
Many organizations with responsibility for safeguarding the integrity, condentiality and availability of their
software and data need to meet regulatory compliance. This compliance is usually mandatory rather than a
voluntary step taken by the organization.
Compliance regulations include:
• PCI (Payment Card Industry) standards
• Central bank regulations
within a code review, as unfortunately the secure coding issues are understood only by a subset of the develop-
ment team. Therefore it can be possible to include teams with various technical expertise into the code reviews,
i.e. someone from the security team (or that person in the corner who knows all the security stu) can be invited
as a technical subject expert to the review to check the code from their particular angle. This is where the OWASP
top 10 guidelines could be enforced.
18 Secure Code Review
• Auditing objectives
• HIPPA
Compliance is an integral part of software security development life-cycle and code review is an important
part of compliance as many rules insist on the execution of code reviews in order to comply with certain reg-
ulations.
To execute proper code reviews that meet compliance rules it is imperative to use an approved methodolo-
gy. Compliance requirements such as PCI, specically requirement 6: “Develop and maintain secure systems”,
while PCI-DSS 3.0, which has been available since November 2013, exposes a series of requirements which
apply to development of software and identifying vulnerabilities in code. The Payment Card Industry Data
Security Standard (PCI-DSS) became a mandatory compliance step for companies processing credit card pay-
ments in June 2005. Performing code reviews on custom code has been a requirement since the rst version
of the standard.
The PCI standard contains several points relating to secure application development, but this guide will focus
solely on the points, which mandate code reviews. All of the points relating to code reviews can be found in
requirement 6 “Develop and maintain secure systems and applications”.
5.10 PCI-DSS Requirements Related to Code Review
Specically, requirement 6.3.2 mandates a code review of custom code. Reviewing custom code prior to re-
lease to production or customers in order to identify any potential coding vulnerability (using either manual
or automated processes) to include at least the following:
• Code changes are reviewed by individuals other than the originating code author, and by individuals knowl-
edgeable about code review techniques and secure coding practices.
• Code reviews ensure code is developed according to secure coding guidelines
• Appropriate corrections are implemented prior to release.
• Code review results are reviewed and approved by management prior to release.
Requirement 6.5 address common coding vulnerabilities in software-development processes as follows:
• Train developers in secure coding techniques, including how to avoid common coding vulnerabilities, and
understanding how sensitive data is handled in memory.
• Develop applications based on secure coding guidelines.
The PCI Council expanded option one to include internal resources performing code reviews. This added
weight to an internal code review and should provide an additional reason to ensure this process is performed
correctly.
The Payment Application Data Security Standard (PA-DSS) is a set of rules and requirements similar to PCI-DSS.
However, PA-DSS applies especially to software vendors and others who develop payment applications that
store, process, or transmit cardholder data as part of authorization or settlement, where these payment appli-
cations are sold, distributed, or licensed to third parties.
19
Secure Code Review
PA-DSS Requirements Related to Code Review
Requirements regarding code review are also applied since these are derived from PA-DSS in requirement 5
(PCI, 2010):
5.2 Develop all payment applications (internal and external, and including web administrative access to prod-
uct) based on secure coding guidelines.
5.1.4 Review of payment application code prior to release to customers after any signicant change, to identi-
fy any potential coding vulnerability.
Note: This requirement for code reviews applies to all payment application components (both internal and
public-facing web applications), as part of the system development life cycle. Code reviews can be conducted
by knowledgeable internal personnel or third parties.
20
METHODOLOGY
Code review is systematic examination of computer source code and reviews are done in various forms and
can be accomplished in various stages of each organization S-SDLC. This book does not attempt to tell each
organization how to implement code reviews in their organization but this section does go over in generic
terms and methodology of doing code reviews from informal walkthroughs, formal inspections, or Tool-assist-
ed code reviews.
6.1 Factors to Consider when Developing a Code Review Process
When planning to execute a security code review, there are multiple factors to consider since every code
review is unique to its context. In addition to the elements discussed in this section, one must consider any
technical or business related factors (business decisions such as deadlines and resources) that impact the
analysis as these factors and may ultimately decide the course of the code review and the most eective way
to execute it.
Risks
It is impossible to secure everything at 100%, therefore it is essential to prioritize what features and components
must be securely reviewed with a risk based approach. While this project highlights some of the vital areas of design
security peer programmers should review all code being submitted to a repository, not all code will receive the atten-
tion and scrutiny of a secure code review.
Purpose & Context
Computer programs have dierent purposes and consequently the grade of security will vary depending on the
functionality being implemented. A payment web application will have higher security standards than a promotion-
al website. Stay reminded of what the business wants to protect. In the case of a payment application, data such as
credit cards will have the highest priority however in the case of a promotional website, one of the most important
things to protect would be the connection credentials to the web servers. This is another way to place context into a
risk-based approach. Persons conducting the security review should be aware of these priorities.
Lines of Code
An indicator of the amount of work is the number of lines of code that must be reviewed. IDEs (Integrated De-
velopment Environments) such as Visual Studio or Eclipse contain features, which allows the amount of lines
of code to be calculated, or in Unix/Linux there are simple tools like ‘wc’ that can count the lines. Programs
written in object-oriented languages are divided into classes and each class is equivalent to a page of code.
Generally line numbers help pinpoint the exact location of the code that must be corrected and is very useful
when reviewing corrections done by a developer (such as the history in a code repository). The more lines of
code a program contains, the greater the chances that errors are present in the code.
Programming language
Programs written in typed safe languages (such as C# or Java) are less vulnerable to certain security bugs
such as buf- fer overows than others like C and C++. When executing code review, the kind of language will
determine the types of expected bugs. Typically software houses tend towards a few languages that their
programmers are experienced in, however when a decision is made to create new code in a language new to
the developer management must be aware of the increased risk of securely reviewing that code due to the
lack of in-house experience. Throughout this guide, sections explain the most common issues surrounding
the specic programming language code to be reviewed, use this as a reference to spot specic security issues
in the code.
Methodology
21
Resources, Time & Deadlines
As ever, this is a fundamental factor. A proper code review for a complex program will take longer and it will
need higher analysis skills than a simple one. The risks involved if resources are not properly provided are
higher. Make sure that this is clearly assessed when executing a review.
6.2 Integrating Code Reviews in the S-SDLC
Code reviews exist in every formal Secure Software Development Lifecycle (S-SDLC), but code reviews also
vary widely in their level of formality. To confuse the subject more, code reviews vary in purpose and in rela-
tion to what the code reviewer is looking for, be it security, compliance, programming style, etc. Throughout
the S-SDLC (XP, Agile, RAD, BSIMM, CMMI, Microsoft ALM) there are points where an application security SME
should to be involved. The idea of integrating secure code reviews into an S-SLDC may sound daunting as
there is another layer of complexity or additional cost and time to an already over budget and time con-
strained project. However it is proven to be cost eective and provides an additional level of security that
static analyzers cannot provide.
In some industries the drive for secure enhancements to a company’s S-SDLC may not be driven purely by
the desire to produce better code, these industries have regulations and laws that demand a level of due care
when writing software (e.g. the governmental and nancial industries) and the nes levelled at a company
who has not attempted to secure their S-SDLC will be far greater than the costs of adding security into the
development lifecycle.
When integrating secure code reviews into the S-SDLC the organization should create standards and policies
that the secure code reviewer should adhere to. This will create the right importance of the task so it is not just
looked at as a project task that just needs to be checked o. Project time also needs to be assigned to the task
so there is enough time to complete the tasks (and for any remedial tasks that come out of the secure code
review). Standards also allow management and security experts (e.g. CISOs, security architects) to direct em-
ployees on what secure coding is to be adhered to, and allows the employees to refer to the (standard) when
review arbitration is necessary.
Methodology
A standard report template will provide enough information to enable the code reviewer to clas-
sify and prioritize the software vulnerabilities based on the applications threat model. This report
does not need to be pages in length, it can be document based or incorporated into many auto-
mated code review tools. A report should provide the following information:
• Date of review.
• Application name, code modules reviewed.
• Developers and code reviewer names.
• Task or feature name, (TFS, GIT, Subversion, trouble ticket, etc.).
Code Review Reports
22
Today most org anizatio ns have modi ed thei r S -S DLC pro ces s to add agi le into thei r S-S DLC pro ces s. Bec ause of this
the organization is going to need to look at their own internal development practices to best determine where and
how often secure code reviews need to happen. If the project is late and over budget then this increases the chance
that a software x could cause a secure vulnerability since now the emphasis is on getting the project to deployment
quicker. Code reviews for code in production may nd software vulnerabilities but understand that there is a race
with hackers to nd the bug and the vulnerable software will remain in production while the remedial x is being
worked on.
6.3 When to Code Review
Once an organization decides to include code reviews part of their internal code process. The next big question to
ask is to determine what stages of the SDLC will the code be reviewed. This section talks about three possible ways to
include code reviews. There are three stages be in the SDLC when code can be reviewed:
When code is about to be checked in (pre-commit)
The development organization can state in their process that all code has to be reviewed before the code can be
submitted to the source code repository. This has the disadvantage of slowing the check-in process down, as the
review can take time, however it has many advantages in that below standard code is never placed in the code line,
and management can be condent that (if processes are being followed) the submitted code is at the quality that
has been stipulated.
For example, processes may state that code to be submitted must include links to requirements and design docu-
mentation and necessary unit and automated tests. This way the reviewers will have context on the exact code mod-
ication being done (due to the documentation) and they will know how the developer has tested the code (due to
the tests). If the peer reviewers do not think the documentation is complete, or the tests are extensive enough, they
can reject the review, not because of the code itself, but because the necessary docs or tests are not complete. In an
environment using CI with automated tests running nightly, the development team as a whole will know the next
day (following check-in) if the submitted code was of enough quality. Also management know that once a bug or
feature is checked in that the developer has nished their task, there’s no “I’ll nish those tests up next week” scenar-
ios which adds risk to the development task.
When code has just been checked into a code base (post-commit)
Here the developer submits their code change, and then uses the code repository change-lists to send the code
di for review. This has the advantage of being faster for the developer as there’s no review gate to pass before they
check-in their code. The disadvantage is that, in practice, this method can lead to a lesser quality of code. A develop-
Methodology
• A brief sentence(s) to classify and prioritize software vulnerability if any and what if any remedial
tasks need to be accomplished or follow up is needed.
• Link to documents related to task/feature, including requirements, design, testing and threat
modeling documents.
• Code Review checklist if used, or link to organization Code Review Checklist. (see Appendix A)
• Testing the developer has carried out on the code. Preferably the unit or automated tests them-
selves can be part of the review submission.
• If any tools such as FxCop, BinScope Binary Analyzer, etc. were used prior to code review.
23
er will be less inclined to x smaller issues once the code has been checked in, usually with a mantra of “Well the code
is in now, it’ll do.”. There also a risk of timing, as other developers could write other code xes into the same module
before the review is done or changes and tests have been written, meaning the developer not only has to implement
the code changes from the peer or security review, but they also have to do so in a way that does not break other
subsequent changes. Suddenly the developer has to re-test the subsequent xes to ensure no regressions.
Some development organizations using the Agile methodology add a ‘security sprint’ into their processes. During the
security sprint the code can be security reviewed, and have security specic test cases (written or automated) added.
When code audits are done
Some organizations have processes to review code at certain intervals (i.e. yearly) or when a vulnerable piece of
code is suspected of being repeated throughout the code base. Here static code analyzers, or simple string searches
through the code (for specic vulnerability patterns) can speed up the process. This review is not connected to the
submission of a feature or bug x, they are triggered by process considerations and are likely to involve the review of
an entire application or code base rather than a review of a single submission.
6.4 Security Code Review for Agile and Waterfall Development
Today agile development is an umbrella term for a lot of practices that include programming, continuous inte -
gration, testing, project management, etc. There are many avors of agile development, perhaps as many avors
as there are practitioners. Agile development is a heterogeneous reference framework where the development
team can pick what practices they want to use.
Agile has some practices that could aect how and when code is reviewed, for example agile tries to keep code
review and testing as near as possible to the development phase. It is a common practice to dene short devel-
opment cycles (a.k.a. Iterations or Sprints). At the end of each cycle, all the code should be production quality
code. It can be incomplete, but it must add some value. That aects the review process, as reviewing should be
continuous. From the point of view of secure coding review, it shouldn’t make a dierence if the development
Methodology
Some organizations assume secure code review can be a job for a security or risk-analysis team member. How-
ever all developers need to understand the exposure points of their applications and what threats exist for their
applications.
Many companies have security teams that do not have members with coding backgrounds, which can make
interactions with development teams challenging. Because of this development teams are usually skeptical of
security input and guidance. Security teams are usually willing to slow things down to ensure condentiality
and integrity controls are in place while developers are face with pressure from business units they support to
create and update code as quickly as possible. Unfortunately the more critical the application to operational or
business needs, the more pressure to deploy the code to production.
It is best to weave secure code reviews into the SDLC processes so that development organizations do not see
security as a hindrance, but as an assistance. As mentioned previously, spreading secure coding SMEs through-
out an organization (satellites in BSIMM terminology) allows the secure code review tasks to scale and reach
more development teams. As the process grows, more of the developers gain awareness of secure coding issues
(as they have reviews rejected on secure coding grounds) and the frequency of secure coding issues in code
reviews should drop.
Who Should Perform Secure Code Reviews
24
organization uses agile or waterfall development practices. Code review is aligned to the code submitted, not the
order of feature development vs testing, or the time patterns assigned to the coding task. In many organizations
the line between waterfall and agile is becoming blurred, with traditional waterfall departments introducing the
continuous integration (CI) aspects from agile, including nightly builds, automated testing, test driven develop-
ment, etc.
6.5 A Risk Based Approach to Code Review
A development house will have various degrees of code changes being reviewed, from simple one line bug xes
in backend scripts that run once a year, to large feature submissions in critical business logic. Typically the inten-
sity of the code review varies based on the perceived risk that the change presents.
In the end, the scale of the code review comes down to the management of resources (skilled persons, company
time, machines, etc.). It would not be scalable to bring in multiple security experts for every code change occur-
ring on a product, the resources of those persons or those teams would not be large enough to handle every
change. Therefore companies can make a call on which changes are important and need to be closely scruti-
nized, and which ones can be allowed through with minimal inspection. This will allow management to better
size the development cycle, if a change is going to be done in an area which is high risk, management can know
to set aside sucient time for code review and ensure persons with relevant skills will be available. The process
of deciding which changes need which level of code review is based on the risk level of the module the change
is within.
If the review intensity of code changes is based on the risk level of the module being changed, who should
decide the level of risk? Ultimately management is responsible for the output of a company, and thus they are
responsible for the risk associated with products sold by the company. Therefore it is up to management (or per-
sons delegated by management) to create a reproducible measure or framework for deciding the risk associated
with a code change.
Decisions on the risk of a module or piece of code should be based on solid cost benet analysis and it would be
irresponsible to decide all modules are high risk. Therefore management should meet with persons who have
an understanding of the code base and security issues faced by the products, and create a measure of risk for
various ele-ments of code. Code could be split up into modules, directories, products, etc., each with a risk level
associated with it.
Various methods exist in the realm of risk analysis to assign risk to entities, and many books have been dedicated
to this type of discussion. The three main techniques for establishing risk are outlined in table 1 below.
Methodology
Technique Method
Quantitative Bring people together and establish a monetary value on the potential loss associated with the code. Gauge the likeli-
hood that the code could be compromised. Use dollar values produced from these calculations to determine the level
of risk.
Qualitative
Delphi
Bring people together and discuss opinions on what level of loss is associated with the modules, and opinions on likeli-
hood of compromise. Qualitative does not attempt to nail down monetary associations with the loss, but tends towards
the perception or opinion of associated losses.
Independently interview or question people on the losses and compromises of the modules, whilst letting them know
the feedback will be anonymous. The impression here is that the people will give more honest answers to the questions
and will not be swayed by other people’s arguments and answers.
Table 1: Options For Establishing Risk
25
Risk is chance of something bad happening and the damage that can be caused if it occurs. The criteria for decid-
ing the risk prole of dierent code modules will be up to the management team responsible for delivering the
changes, examples are provided in table 2.
When levels of risk have been associated with products and modules, then the policies can be created deter-
mining what level of code review must be conducted. It could be that code changes in a level one risk module
must be reviewed by 3 persons including a Security Architect, whereas changes in a level 4 risk module only
need a quick one person peer review.
Other options (or criteria) for riskier modules can include demands on automated testing or static analysis, e.g.
code changes in high risk code must include 80% code coverage on static analysis tools, and sucient auto-
mated tests to ensure no regressions occur. These criteria can be demanded and checked as part of the code
review to ensure they are capable of testing the changed code.
Some companies logically split their code into diering repositories, with more sensitive code appearing in a
repository with a limited subset of developers having access. If the code is split in this fashion, then it must be
remembered that only developers with access to the riskier code should be able to conduct reviews of that
code.
Risk analysis could also be used during the code review to decide how to react to a code change that introduc-
es risk into the product, as in table 3. In a typical risk analysis process, the team needs to decide whether to
accept, transfer, avoid or reduce the risks. When it comes to code reviews it is not possible to transfer the risk
as transferring risk normally means taking out insurance to cover the cost of exposure.
Methodology
Criteria Explanation
Ease of exposure Is the code change in a piece of code directly exposed to the internet? Does an insider use the interface directly?
Value of loss
Regulatory controls
How much could be lost if the module has a vulnerability introduced? Does the module contain some critical password
hashing mechanism, or a simple change to HTML border on some internal test tool?
If a piece of code implements business logic associated with a standard that must be complied with, then these mod-
ules can be considered high risk as the penalties for non-conformity can be high.
Table 2: Common Criteria For Establishing The Risk Prole Of A Code Module
Risk Resolution Explanation
Reduce This is the typical resolution path. When a code reviewer nds that the code change introduces risk into an element of
business logic (or simply a bug) the code will be changed to x the bug or code in a way that reduces the risk.
Accept When the code change introduces a risk in the code but there is no other way to implement the business logic, the code
change can pass code review if the risk is considered acceptable. The risk and any workarounds or mitigating factors
should be documented correctly so that it is not ignored.
Table 3: Options For Handling Risks Identied In A Code Review
Avoid When the code change introduces a risk that is too great to be accepted, and it is not possible to reduce the risk by imple-
menting a code change, then the team need to consider not performing the change. Ideally this decision should be reached
before the code review stage, but it is entirely possible that factors can arise during code implementation that changes the
understood risk prole of a code module and prompts management to reconsider if a change should go ahead.
26
6.6 Code Review Preparation
A security review of the application should uncover common security bugs as well as the issues specic to business
logic of the application. In order to eectively review a body of code it is important that the reviewers understand
the business purpose of the application and the critical business impacts. The reviewers should understand the attack
surface, identify the dierent threat agents and their motivations, and how they could potentially attack the applica-
tion.
For the software developer whose code is being reviewed, performing code review can feel like an audit and devel-
opers may nd it challenging to not take the feedback personally. A way to approach this is to create an atmosphere
of collaboration be-tween the reviewer, the development team, the business representatives, and any other vested
interests. Por-traying the image of an advisor and not a policeman is important to get co-operation from the devel-
opment team.
The extent to which information gathering occurs will depend on the size of the organization, the skill set of the re-
viewers, and the criticality/risk of the code being reviewed. A small change to the CSS le in a 20- person start-up will
not result in a full threat model and a separate secure review team. At the same time a new single sign-on authentica-
tion module in a multi-billion dollar company will not be secure code reviewed by a person who once read an article
on secure coding. Even within the same organization, high-risk modules or applications may get threat modeled,
where the lower risk modules can be reviewed with a lesser emphasis on the reviewer understanding the security
model of the module.
This section will present the basic items the reviewer (or review team) should attempt to understand about the ap-
plication subjected to a secure code review. This can be used in smaller companies that don’t have the resources to
create a full security baseline, or on low risk code within larger companies. A later section goes into detail on threat
modeling, which would be used by larger companies on their highest risk code bases.
In an ideal world the reviewer would be involved in the design phase of the application, but this is rarely the case.
However regardless of the size of the code change, the engineer initiating the code review should direct reviewers
to any relevant architecture or design documents. The easiest way to do this is to include a link to the documents (as-
suming they’re stored in an online document repository) in the initial e-mail, or in the code review tool. The reviewer
can then verify that the key risks have been properly addressed by security controls and that those controls are used
in the right places.
To e ec ti vel y conduc t the review the re viewer shou ld de velop familiar it y with the fo llowin g asp ec ts :
Application features and Business Rules
The reviewer should understand all the features currently provided by the application and capture all the business
restrictions/rules related to them. There is also a case for being mindful of potential future functionality that might be
on the roadmap for an application, thereby future-proong the security decisions made during current code reviews.
What are the consequences of this system failing? Shall the enterprise be aected in any great way if the application
cannot perform its functions as intended?
Context
All security is in context of what we are trying to secure. Recommending military standard security mechanisms on
an application that vends apples would be overkill and out of context. What type of data is being manipulated or
processed, and what would the damage to the company be if this data was compromised? Context is the “Holy Grail”
of secure code inspection and risk assessment.
Methodology
27
Sensitive Data
The reviewer should also make a note of the data entities like account numbers and passwords that are sensitive to
the application. The categorizing the data entities based on their sensitivity will help the reviewer to determine the
im-pact of any kind of data loss in the application.
User roles and access rights
It is important to understand the type of users allowed to access the application. Is it externally facing or internal to
“trusted” users? Generally an application that is accessible only for the internal users of an organization might be
exposed to threats that are dierent than the one that is available for anyone on the Internet. Hence, knowing the
users of the application and its deployed environment would allow the reviewer to realize the threat agents correctly.
In addition to this, the dierent privileges levels present in the application must also be understood. It would help
the reviewer to enumerate dierent security violations/privilege escalation attacks that can be applicable to the ap-
plication.
Application type
This refers to understanding whether the application is browser based application, a desktop based standalone ap-
plication, a web-service, a mobile applications or a hybrid application. Dierent type of application faces dierent
kinds of security threats and understanding the type of the application would help the reviewer to look for specic
security aws, determine correct threats agents and highlight necessary controls suitable to the application.
Code
The language(s) used, the features and issues of that language from a security perspective. The issues a programmer
needs to look out for and language best practices from a security and performance perspective.
Design
Generally web applications have a well-dened code layout if they are developed using MVC design principle. Appli-
cations can have their own custom design or they may use some well-known design frameworks like Struts/Spring
etc. Where are the application properties/conguration parameters stored? How is the business class identied for
any feature/URL? What types of classes get executed for any processing any request? (e.g. centralized controller,
command classes, view pages etc.) How is the view rendered to the users for any request?
Company Standards and Guidelines
Many companies will have standards and guidelines dictated by management. This is how the management (ulti-
mately responsible for the organizations information security) control what levels of security are applied to various
functions, and how they should be applied. For example, if the company has a Secure Coding Guidelines document,
reviewers should know and understand the guidelines and apply them during the code review.
6.7 Code Review Discovery and Gathering Information
The reviewers will need certain information about the application in order to be eective. Frequently, this informa-
tion can be obtained by studying design documents, business requirements, functional specications, test results,
and the like. However, in most real-world projects, the documentation is signicantly out of date and almost never
has appropriate security information. If the development organization has procedures and templates for architecture
and design documents, the reviewer can suggest updates to ensure security is considered (and documented) at
these phases.
If the reviewers are initially unfamiliar with the application, one of the most eective ways to get started is to talk
with the developers and the lead architect for the application. This does not have to be a long meeting, it could be a
whiteboard session for the development team to share some basic information about the key security considerations
Methodology
28
and controls. A walkthrough of the actual running application is very helpful to give the reviewers a good idea about
how the application is intended to work. Also a brief overview of the structure of the code base and any libraries used
can help the reviewers get started.
If the information about the application cannot be gained in any other way, then the reviewers will have to spend
some time doing reconnaissance and sharing information about how the application appears to work by examining
the code. Preferably this information can then be documented to aid future reviews.
Security code review is not simply about the code structure. It is important to remember the data; the reason that
we review code is to ensure that it adequately protects the information and assets it has been entrusted with, such
as money, intellectual property, trade secrets, or lives. The context of the data with which the application is intended
to process is very important in establishing potential risk. If the application is developed using an inbuilt/well-known
design framework the answers to the most of these questions would be pre-dened. But, in case it is custom then this
information will surely aid the review process, mainly in capturing the data ow and internal validations. Knowing
the architecture of the application goes a long way in understanding the security threats that can be applicable to
the application.
A design is a blueprint of an application; it lays a foundation for its development. It illustrates the layout of the appli-
cation and identies dierent application components needed for it. It is a structure that determines execution ow
of the application. Most of the application designs are based on a concept of MVC. In such designs dierent compo-
nents interact with each other in an ordered sequence to serve any user request. Design review should be an integral
part of secure software development process. Design reviews also help to implementing the security requirements
in a better way.
Collecting all the required information of the proposed design including ow charts, sequence diagrams, class dia-
grams and requirements documents to understand the objective of the proposed design. The design is thoroughly
studied mainly with respect to the data ow, dierent application component interactions and data handling. This is
achieved through manual analysis and discussions with the design or technical architect’s team. The design and the
architecture of the application must be understood thoroughly to analyze vulnerable areas that can lead to security
breaches in the application.
After understanding the design, the next phase is to analyze the threats to the design. This involves observing the
design from an attacker’s perspective and uncovering the backdoors and insecure areas present in it. Tab le 4 below
Methodology
Existing security controls • Are there any known weaknesses in third-part security controls
• Is the placements of security controls correct?
Architecture • Are connections to external servers secure?
• Are inputs from external sources validated?
Conguration les and
data stores
• Is there any sensitive data in conguration les?
• Who has access to conguration or data les?
Authentication and
access control
• Does the design implement access control for all resources?
• Are sessions handled correctly?
• What functionality can be accessed without authentication?
Design Area Questions to consider
Data Flow • Are user inputs used to directly reference business logic?
• Is there potential for data binding aws?
• Is the execution ow correct in failure cases?
Table 4: Example Design Questions During Secure Code Review
29
highlights some questions that can be asked of the architecture and design to aid secure code reviews.
Every security requirement should be associated with a security control best suited for the design. Here, we would
identify exact changes or additions to be incorporated in the design that are needed to meet any requirement or
mitigate a threat. The list of security requirements and proposed controls can be then discussed with the develop-
ment teams. The queries of the teams should be addressed and feasibility of incorporating the controls must be
determined. Exceptions, if any must be taken into account and alternate recommendations should be proposed. In
this phase a nal agreement on the security controls is achieved. The nal design incorporated by the development
teams can be reviewed again and nalized for further development process.
6.8 Static Code Analysis
Static Code Analysis is carried out during the implementation phase of S-SDLC. Static code analysis commonly
refers to running static code analysis tools that attempt to highlight possible vulnerabilities within the ‘static’
(non-running) source code.
Ideally static code analysis tools would automatically nd security aws with few false positives. That means it should
have a high degree of condence that the bugs that it nds are real aws. However, this ideal is beyond the state of
the art for many types of application security aws. Thus, such tools frequently serve as aids for an analyst to help
them zero in on security relevant portions of code so they can nd aws more eciently, rather than a tool that nds
all aws automatically.
Bugs may exist in the application due to insecure code, design or conguration. Automated analysis can be carried on
the application code to identify bugs through either of the following two options:
Methodology
Dening a generic checklist, which the development team can ll out can give reviewers the desired context. The
checklist is a good barometer for the level of security the developers have attempted or thought of. If security
code review becomes a common requirement, then this checklist can be incorporated into a development pro-
cedure (e.g. document templates) so that the information is always available to code reviewers. See Appendix A
for a sample code review checklist.
The checklist should cover the most critical security controls and vulnerability areas such as:
• Data Validation
• Authentication
• Session Management
• Authorization
• Cryptography
• Error Handling
• Logging
• Security Conguration
• Network Architecture
Code Review Checklist
30
1. Static code scanner scripts based on a pattern search (in-house and open source).
2. Static code analyzers (commercial and open source).
Advantages and disadvantages of source code scanners are shown in tables 5 and 6.
Though code scanning scripts and open source tools can be ecient at nding insecure code patterns, they often
lack the capability of tracing the data ow. This gap is lled by static code analyzers, which identify the insecure code
patterns by partially (or fully) compiling the code and investigating the execution branches, allowing for source to
sink analysis. Static code analyzers and scanners are comprehensive options to complement the process of code
review.
Choosing a static analysis tool
Choosing a static analysis tool is a dicult task since there are a lot of choices. The comparison charts below could
help organization decide which tool is right for them, although this list is not exhaustive.
Methodology
Source to sink analysis Some analyzers can trace the code and identify the vulnerabilities through source to sink analysis. They identify possible
inputs to the application and trace them thoroughly throughout the code until they nd them to be associated with any
insecure code pattern. Such a source to sink analysis helps the developers in understanding the aws better as they get
a complete root cause analysis of the aw
Elaborate reporting
format
Scanners provide a detailed report on the observed vulnerabilities with exact code snippets, risk rating and complete
description of the vulnerabilities. This helps the development teams to easily understand the aws and implement
necessary controls
Advantage Explanation
Reduction in manual
eorts
The type of patterns to be scanned for remains common across applications, computers are better at such scans than
humans. In this scenario, scanners play a big role is automating the process of searching the vulnerabilities through
large codebases.
Find all the instances of
the vulnerabilities
Scanners are very eective in identifying all the instances of a particular vulnerability with their exact location. This is
helpful for larger code base where tracing for aws in all the les is dicult.
Table 5: Advantages To Using Source Code Scanners
Limitation Explanation
Business logic aws
remain untouched
The aws that are related to application’s business logic, transactions, and sensitive data remain untouched by the scan-
ners. The security controls that need to be implemented in the application specic to its features and design are often
not pointed by the scanners. This is considered as the biggest limitation of static code analyzers.
Limited scope Static code analyzers are often designed for specic frameworks or languages and within that scope they can search
for a certain set of vulnerable patterns. Outside of this scope they fail to address the issues not covered in their search
pattern repository.
Design aws Design aws are not specic to the code structure and static code analyzers focus on the code. A scanner/analyzer will
not spot a design issue when looking at the code, whilst a human can often identify design issues when looking at their
implementation.
False positives Not all of the issues agged by static code analyzers are truly issues, and thus the results from these tools need to be
understood and triaged by an experienced programmer who understands secure coding. Therefore anyone hoping
that secure code checking can be automated and run at the end of the build will be disappointed, and there is still a
deal of manual intervention required with analyzers.
Table 6: Disadvantages To Using Source Code Scanners
31
Some of the criteria for choosing a tool are:
• Does the tool support the programming language used?
• Is there a preference between commercial or free tools? Usually the commercial tools have more features and are
more reliable than the free ones, whilst their usability might dier.
• What type of analysis is being carried out? Is it security, quality, static or dynamic analysis?
The next step requires that some work is done since it is quite subjective. The best thing to do is to test a few tools to
see if the team is satised with dierent aspects such as the user experience, the reporting of vulnerabilities, the level
of false positives and the customization and the customer support. The choice should not be based on the number of
features, but on the features needed and how they could be integrated in the S-SDLC. Also, before choosing the tool,
the expertise of the targeted users should be clearly evaluated in order to choose an appropriate tool.
6.9 Application Threat Modeling
Threat modeling is an in-depth approach for analyzing the security of an application. It is a structured approach that
enables employees to identify, quantify, and address the security risks associated with an application. Threat model-
ing is not an approach to reviewing code, but it complements the secure code review process by providing context
and risk analysis of the application.
The inclusion of threat modeling in the S-SDLC can help to ensure that applications are being developed with securi-
ty built-in from the very beginning. This, combined with the documentation produced as part of the threat modeling
process, can give the reviewer a greater understanding of the system, allows the reviewer to see where the entry
points to the application are (i.e. the attack surface) and the associated threats with each entry point (i.e. attack vec-
tors).
The concept of threat modeling is not new but there has been a clear mind-set change in recent years. Modern threat
modeling looks at a system from a potential attacker’s perspective, as opposed to a defender’s viewpoint. Many com-
panies have been strong advocates of the process over the past number of years, including Microsoft who has made
threat modeling a core component of their S-SDLC, which they claim to be one of the reasons for the increased se-
curity of their products in recent years.
When source code analysis is performed outside the S-SDLC, such as on existing applications, the results of the threat
modeling help in reducing the complexity of the source code analysis by promoting a risk based approach. Instead of
reviewing all source code with equal focus, a reviewer can prioritize the security code review of components whose
threat modeling has ranked with high risk threats.
The threat modeling process can be decomposed into 3 high level steps:
6.9.1. Step 1: Decompose the Application.
The rst step in the threat modelling process is concerned with gaining an understanding of the application and how
it interacts with external entities. This involves creating use-cases to understand how the application is used, identi-
fying entry points to see where a potential attacker could interact with the application, identifying assets i.e. items/
areas that the attacker would be interested in, and identifying trust levels which represent the access rights that the
application will grant to external entities. This information is documented in the threat model document and it is also
used to produce data ow diagrams (DFDs) for the application. The DFDs show the dierent data paths through the
system, highlighting the privilege (trust) boundaries.
Methodology
32
Items to consider when decomposing the application include
External Dependencies
External dependencies are items external to the code of the application that may pose a threat to the application.
These items are typically still within the control of the organization, but possibly not within the control of the de-
velopment team. The rst area to look at when investigating external dependencies is how the application will be
deployed in a production environment.
This involves looking at how the application is or is not intended to be run. For example if the application is expected
to be run on a server that has been hardened to the organization’s hardening standard and it is expected to sit behind
a rewall, then this information should be documented.
Entry Points
Entry points (aka attack vectors) dene the interfaces through which potential attackers can interact with the appli-
cation or supply it with data. In order for a potential attacker to attack an application, entry points must exist. Entry
points in an application can be layered, for example each web page in a web application may contain multiple entry
points.
Assets
The system must have something that the attacker is interested in; these items/areas of interest are dened as assets.
Assets are essentially threat targets, i.e. they are the reason threats will exist. Assets can be both physical assets and
abstract assets. For example, an asset of an application might be a list of clients and their personal information; this is
a physical asset. An abstract asset might be the reputation of an organization.
Determining the Attack Surface
The attack surface is determined by analyzing the inputs, data ows and transactions. A major part of actually per-
forming a security code review is performing an analysis of the attack surface. An application takes inputs and pro-
duces output of some kind. The rst step is to identify all input to the code.
Inputs to the application may include the bullet points below and gure 4 describes an example process for identi-
fying an applications input paths:
• Browser input
• Cookies
• Property les
• External processes
• Data feeds
• Service responses
• Flat les
• Command line parameters
• Environment variables
Methodology
33
Methodology
TRANSITIONAL
ANALYSIS
INITIATION
IDENTIFY INPUT PATHS
IDENTIFY AREAS
OF LATE & DY-
NAMIC BINDING
FOLLOW PATH
EACH PARAME-
TER THROUGH
CODE
IDENTIFY AREAS
OF CONFIG FILE
REFERENCE
IDENTIFY
ATTACK
SURFACE
INPUT
PARAMETERS
(USER)
INPUT
PARAMETERS
(CONFIG)
IDENTIFY
ATTACK
SURFACE
IDENTIFY
ATTACK
SURFACE
IDENTIFY
ATTACK
SURFACE
INPUT
PARAMETERS
(CONTROL)
INPUT
PARAMETERS
(BLACKEND)
Figure 4: Example process diagram for identifying input paths
34 Methodology
Trust Levels
Trust levels represent the access rights that the application will grant to external entities. The trust levels are cross-ref-
erenced with the entry points and assets. This allows a team to dene the access rights or privileges required at each
entry point, and those required to interact with each asset.
Data ow analysis
Exploring the attack surface includes dynamic and static data ow analysis. Where and when variables are set and
how the variables are used throughout the workow, how attributes of objects and parameters might aect other
data within the program. It determines if the parameters, method calls, and data exchange mechanisms implement
the required security.
Transaction analysis
Transaction analysis is needed to identify and analyze all transactions within the application, along with the relevant
security functions invoked.
The areas that are covered during transaction analysis are:
• Data/Input Validation of data from all untrusted sources
• Authentication
• Session Management
• Authorization
• Cryptography (data at rest and in transit)
• Error Handling /Information Leakage
• Logging /Auditing
Data Flow Diagrams
All of the information collected allows an accurately model the application through the use of Data Flow Diagrams
(DFDs). The DFDs will allow the employee to gain a better understanding of the application by providing a visual
representation of how the application processes data. The focus of the DFDs is on how data moves through the
application and what happens to the data as it moves. DFDs are hierarchical in structure, so they can be used to
decompose the application into subsystems. The high level DFD will allow the employee to clarify the scope of the
application being modelled. The lower level iterations will allow more focus on the specic processes involved when
processing specic data.
There are a number of symbols that are used in DFDs for threat modelling, as show in the following table 7 below:
The process shape represents a task that handles data within
the application. The task may process the data or perform an
action based on the data.
PROCESS
Methodology
ELEMENT IMAGE DESCRIPTION
The external entity shape is used to represent any entity
outside the application that interacts with the application via
an entry point.
EXTERNAL ENTITY
Table 7: Threat Modeling Symbols
35
DFDs show how data moves logically through the system and allows the identication data entering or leav-
ing the system along with the storage of data and the ow of control through these components. Trust bound-
aries show any location where the level of trust changes. Process components show where data is processed,
such as web servers, application servers, and database servers. Entry points show where data enters the sys-
tem (i.e. input elds, methods) and exit points are where it leaves the system (i.e. dynamic output, methods),
respectively. Entry and exit points dene a trust boundary.
6.9.2 Step 2: Determine and rank threats
Critical to the identication of threats is using a threat categorization methodology. A threat categorization
such as STRIDE can be used, or the Application Security Frame (ASF) that denes threat categories such as Au-
diting & Logging, Authentication, Authorization, Conguration Management, Data Protection in Storage and
Transit, Data Validation and Exception Management.
The goal of the threat categorization is to help identify threats both from the attacker (STRIDE) and the defen-
sive perspective (ASF). DFDs produced in step 1 help to identify the potential threat targets from the attacker’s
perspective, such as data sources, processes, data ows, and interactions with users. These threats can be
identied further as the roots for threat trees; there is one tree for each threat goal.
From the defensive perspective, ASF categorization helps to identify the threats as weaknesses of security
controls for such threats. Common threat-lists with examples can help in the identication of such threats. Use
and abuse cases can illustrate how existing protective measures could be bypassed, or where a lack of such
protection exists.
The determination of the security risk for each threat can be determined using a value-based risk model such
as DREAD or a less subjective qualitative risk model based upon general risk factors (e.g. likelihood and im-
pact).
The rst step in the determination of threats is adopting a threat categorization. A threat categorization pro-
Methodology
The multiple process shape is used to present a collection of
subprocesses. The multiple process can be broken down into
its subprocesses in another DFD.
MULTIPLE PROCESS
The data ow shape represents data movement within the
application. The direction of the data movement is represent-
ed by the arrow.
DATA FLOW
The privilege boundary shape is used to represent the
change of privilege levels as the data ows through the
application.
PRIVILEGE BOUNDARY
The data store shape is used to represent locations where
data is stored. Data stores do not modify the data, they only
store data.
DATA STORE
36
vides a set of threat categories with corresponding examples so that threats can be systematically identied
in the application in a structured and repeatable manner.
STRIDE
Threat lists based on the STRIDE model are useful in the identication of threats with regards to the attacker
goals. For example, if the threat scenario is attacking the login, would the attacker brute force the password to
break the authentication? If the threat scenario is to try to elevate privileges to gain another user’s privileges,
would the attacker try to perform forceful browsing?
A threat categorization such as STRIDE is useful in the identication of threats by classifying attacker goals
such as shown in table 8.
It is vital that all possible attack vectors should be evaluated from the attacker’s point of view. For example, the login
page allows sending authentication credentials, and the input data accepted by an entry point has to validate for
potential malicious input to exploit vulnerabilities such as SQL injection, cross site scripting, and buer overows.
Additionally, the data ow passing through that point has to be used to determine the threats to the entry points
to the next components along the ow. If the following components can be regarded critical (e.g. the hold sensitive
data), that entry point can be regarded more critical as well. In an end to end data ow the input data (i.e. username
and password) from a login page, passed on without validation, could be exploited for a SQL injection attack to ma-
nipulate a query for breaking the authentication or to modify a table in the database.
Exit points might serve as attack points to the client (e.g. XSS vulnerabilities) as well for the realization of information
disclosure vulnerabilities. In the case of exit points from components handling condential data (e.g. data access
components), any exit points lacking security controls to protect the condentiality and integrity can lead to disclo-
sure of such condential information to an unauthorized user.
In many cases threats enabled by exit points are related to the threats of the corresponding entry point. In the login
example, error messages returned to the user via the exit point might allow for entry point attacks, such as account
Methodology
STRIDE Explanation
Spoong “Identity spoong” is a key risk for applications that have many users but provide a single execution context at the ap-
plication and database level. In particular, users should not be able to become any other user or assume the attributes
of another user.
Tampering Users can potentially change data delivered to them, return it, and thereby potentially manipulate client-side valida-
tion, GET and POST results, cookies, HTTP headers, and so forth. The application should also carefully check data re-
ceived from the user and validate that it is sane and applicable before storing or using it.
Repudiation Users may dispute transactions if there is insucient auditing or recordkeeping of their activity. For example, if a user
says they did not make a nancial transfer, and the functionality cannot track his/her activities through the application,
then it is extremely likely that the transaction will have to be written o as a loss.
Information Disclosure
Denial of Service
Elevation of Privilege
Users are rightfully wary of submitting private details to a system. Is possible for an attacker to publicly reveal user data
at large, whether anonymously or as an authorized user?
Application designers should be aware that their applications may be subject to a denial of service attack. The use of
expensive resources such as large les, complex calculations, heavy-duty searches, or long queries should be reserved
for authenticated and authorized users, and not available to anonymous users.
If an application provides distinct user and administrative roles, then it is vital to ensure that the user cannot elevate
his/her role to a higher privilege one.
Table 8: Explanation Of The Stride Attributes
37
Methodology
harvesting (e.g. username not found), or SQL injection (e.g. SQL exception errors). From the defensive perspective,
the identication of threats driven by security control categorization such as ASF allows a threat analyst to focus on
specic issues related to weaknesses (e.g. vulnerabilities) in security controls. Typically the process of threat identi-
cation involves going through iterative cycles where initially all the possible threats in the threat list that apply to
each component are evaluated. At the next iteration, threats are further analyzed by exploring the attack paths, the
root causes (e.g. vulnerabilities) for the threat to be exploited, and the necessary mitigation controls (e.g. counter-
measures).
Once common threats, vulnerabilities, and attacks are assessed, a more focused threat analysis should take in con-
sideration use and abuse cases. By thoroughly analyzing the use scenarios, weaknesses can be identied that could
lead to the realization of a threat. Abuse cases should be identied as part of the security requirement engineering
activity. These abuse cases can illustrate how existing protective measures could be bypassed, or where a lack of such
protection exists. Finally, it is possible to bring all of this together by determining the types of threat to each compo-
nent of the decomposed system. This can be done by repeating the techniques already discussed on a lower level
threat model, again using a threat categorization such as STRIDE or ASF, the use of threat trees to determine how the
threat can be exposed by vulnerability, and use and misuse cases to further validate the lack of a countermeasure to
mitigate the threat.
Microsoft DREAD threat-risk ranking model
In the Microsoft DREAD threat-risk ranking model, the technical risk factors for impact are Damage and Aected Us-
ers, while the ease of exploitation factors are Reproducibility, Exploitability and Discoverability. This risk factorization
allows the assignment of values to the dierent inuencing factors of a threat.
To de ter mine the rank ing of a t hreat, the threat analyst has to answer basic questi ons for each factor of ri sk , for ex-
ample:
The impact mainly depends on the damage potential and the extent of the impact, such as the number of
components that are aected by a threat.
Damage How big would the damage be if the attack succeeded?
Can an attacker completely take over and manipulate the system?
Can an attacker crash the system?
Reproducibility How easy is it to reproduce an attack to work?
Can the exploit be automated?
Exploitability How much time, eort, and expertise is needed to exploit the threat?
Does the attacker need to be authenticated?
Aected Users
Discoverability
If a threat were exploited, what percentage of users would be aected?
Can an attacker gain administrative access to the system?
How easy is it for an attacker to discover this threat?
DREAD Questions
Table 9: Explanation Of The Dread Attributes
38
These questions help in the calculation of the overall risk values by assigning qualitative values such as High,
Medium and Low to Likelihood and Impact factors. In this case, using qualitative values, rather than numeric
ones like in the case of the DREAD model, help avoid the ranking becoming overly subjective.
6.9.3 Step 3: Determine countermeasures and mitigation.
A lack of protection against a threat might indicate a vulnerability whose risk exposure could be mitigated
with the implementation of a countermeasure. Such countermeasures can be identied using threat-counter-
measure mapping lists. Once a risk ranking is assigned to the threats, it is possible to sort threats from the high-
est to the lowest risk, and prioritize the mitigation eort, such as by responding to such threats by applying the
identied countermeasures.
The risk mitigation strategy might involve evaluating these threats from the business impact that they pose
and establishing countermeasures (or design changes) to reduce the risk.
Other options might include accepting the risk, assuming the business impact is acceptable because of com-
pensating controls, informing the user of the threat, removing the risk posed by the threat completely, or the
least preferable option, that is, to do nothing. If the risk identied is extreme, the functionality or product
could be discontinued, as the risk of something going wrong is greater than the benet.
The purpose of the countermeasure identication is to determine if there is some kind of protective measure
(e.g. security control, policy measures) in place that can prevent each threat previously identied via threat
analysis from being realized. Vulnerabilities are then those threats that have no countermeasures.
Since each of these threats has been categorized either with STRIDE or ASF, it can be possible to nd appropri-
ate countermeasures in the application within the given category. Each of the above steps is documented as
they are carried out. The resulting set of documents is the threat model for the application. Detailed examples
of how to carry out threat modeling is given in Appendix B.
Threat Prole
Once threats and corresponding countermeasures are identied it is possible to derive a threat prole with the
following criteria:
A more generic risk model takes into consideration the Likelihood (e.g. probability of an attack) and the Impact
(e.g. damage potential):
Risk = Likelihood x Impact
Note that this is a conceptual formula and is not expected to use actual values for likelihood and impact. The
likelihood or probability is dened by the ease of exploitation, which mainly depends on the type of threat and
the system characteristics, and by the possibility to realize a threat, which is determined by the existence of an
appropriate countermeasure.
Likelihood
Methodology
39
6.10 Metrics and Code Review
Metrics measure the size and complexity of a piece of code. There is a long list of quality and security char-
acteristics that can be considered when reviewing code (such as, but not limited to, correctness, eciency,
portability, maintainability, reliability and securability). No two-code review sessions will be the same so some
judgment will be needed to decide the best path. Metrics can help decide the scale of a code review.
Metrics can also be recorded relating to the performance of the code reviewers and the accuracy of the review
process, the performance of the code review function, and the eciency and eectiveness of the code review
function.
The gure 5 describes the use of metrics throughout the code review process.
Some of the options for calculating the size of a review task include:
Lines of Code (LOC):
A count of the executable lines of code (commented-out code or blank lines are not counted). This gives a
rough estimate but is not particularly scientic.
Function Point:
The estimation of software size by measuring functionality. The combination of a number of statements which
perform a specic task, independent of programming language used or development methodology. In an
object orientated language a class could be a functional point.
Defect Density:
The average occurrence of programming faults per Lines of Code (LOC). This gives a high level view of the code
quality but not much more. Fault density on its own does not give rise to a pragmatic metric. Defect density
would cover minor issues as well as major security aws in the code; all are treated the same way. Security of
code cannot be judged accurately using defect density alone.
Risk Density:
Similar to defect density, but discovered issues are rated by risk (high, medium & low). In doing this we can
give insight into the quality of the code being developed via a [X Risk / LoC] or [Y Risk / Function Point] value
(X&Y being high, medium or low risks) as dened by internal application development policies and standards.
For example:
4 High Risk Defects per 1000 (Lines of Code)
2 Medium Risk Defects per 3 Function Points
Threat Type Description
Non-mitigated threats Threats which have no countermeasures and represent vulnerabilities that can be fully exploited and cause an impact.
Partially mitigated
threats
Threats partially mitigated by one or more countermeasures, which represent vulnerabilities that can only partially be
exploited and cause a limited impact.
Fully mitigated threats These threats have appropriate countermeasures in place and do not expose vulnerability and cause impact.
Table 10: Types Of Mitigated Threats
Methodology
40 Methodology
PERSIST METRICS
PERFORM
REVIEW
CODE
SUBMITED
FOR SCR
CODE
RESUBMITED
FOR
RE-REVIEW
HAS CONTEXT
OF CODE BEEN
DEFINED
RECOMENDED
CODE TRIAGE
MEETING
DEFINE CRITERIA
PROJECT OR
VULNERABILITY
BASED
CODE REVIEW
DATABASE
TREND
ANALYSIS
RECORD
FINDINGS
DEVELOP
METRICS
PREVIOUS
FINDINGS
STANDARDS
GUIDELINES
POLICIES
NO
YES
COMMUNICATE
RESULTS TO TEAM
The Use of Metrics
Throughout The
Code Review
Process
Figure 5: Example process diagram for identifying input paths
41
Cyclomatic complexity (CC):
A static analysis metric used to assist in the establishment of risk and stability estimations on an item of code,
such as a class, method, or even a complete system. It was dened by Thomas McCabe in the 70’s and it is easy
to calculate and apply, hence its usefulness.
The McCabe cyclomatic complexity metric is designed to indicate a program’s testability, understandability
and maintainability. This is accomplished by measuring the control ow structure, in order to predict the di-
culty of understanding, testing, maintaining, etc. Once the control ow structure is understood one can gain a
realization of the extent to which the program is likely to contain defects. The cyclomatic complexity metric is
intended to be independent of language and language format that measures the number of linearly indepen-
dent paths through a program module. It is also the minimum number of paths that should be tested.
By knowing the cyclomatic complexity of the product, one can focus on the module with the highest com-
plexity. This will most likely be one of the paths data will take, thus able to guide one to a potentially high risk
location for vulnerabilities. The higher the complexity the greater potential for more bugs. The more bugs the
higher the probability for more security aws.
Does cyclomatic complexity reveal security risk? One will not know until after a review of the security posture
of the module. The cyclomatic complexity metric provides a risk-based approach on where to begin to review
and analyze the code. Securing an application is a complex task and in many ways complexity an enemy of
security as software complexity can make software bugs hard to detect. Complexity of software increases over
time as the product is updated or maintained.
Cyclomatic complexity can be calculated as:
CC = Number of decisions +1
… where a decision would be considered as commands where execution is branched with if/else, switch, case,
catch, while, do, templated class calls, etc.,
As the decision count increases, so do the complexity and the number of paths. Complex code leads to less
stability and maintainability.
The more complex the code, the higher risk of defects. A company can establish thresholds for cyclomatic
complexity for a module:
0-10: Stable code, acceptable complexity
11-15: Medium Risk, more complex
16-20: High Risk code, too many decisions for a unit of code.
Modules with a very high cyclomatic complexity are extremely complex and could be refactored into smaller
methods.
Bad Fix Probability:
This is the probability of an error accidentally inserted into a program while trying to x a previous error,
known in some companies as a regression.
Cyclomatic Complexity: 1 – 10 == Bad Fix Probability: 5%
Cyclomatic Complexity: 20 –30 == Bad Fix Probability: 20%
Cyclomatic Complexity: > 50 == Bad Fix Probability: 40%
Cyclomatic Complexity: Approaching 100 == Bad Fix Probability: 60%
As the complexity of software increase so does the probability to introduce new errors.
Methodology
42
Inspection Rate:
This metric can be used to get a rough idea of the required duration to perform a code review. The inspection
rate is the rate of coverage a code reviewer can cover per unit of time. For example, a rate of 250 lines per hour
could be a baseline. This rate should not be used as part of a measure of review quality, but simply to deter-
mine duration of the task.
Defect Detection Rate:
This metric measure the defects found per unit of time. Again, can be used to measure performance of the
code review team, but not to be used as a quality measure. Defect detection rate would normally increase as
the inspection rate (above) decreases.
Re-inspection Defect Rate:
The rate at which upon re-inspection of the code more defects exist, some defects still exist, or other defects
manifest through an attempt to address previously discovered defects (regressions).
6.11 Crawling Code
Crawling code is the practice of scanning a code base of the review target and interface entry points, looking
for key code pointers wherein possible security vulnerability might reside. Certain APIs are related to inter-
facing to the external world or le IO or user management, which are key areas for an attacker to focus on. In
crawling code we look for APIs relating to these areas. We also need to look for business logic areas which may
cause security issues, but generally these are bespoke methods which have bespoke names and cannot be de-
tected directly, even though we may touch on certain methods due to their relationship with a certain key API.
We also need to look for common issues relating to a specic language; issues that may not be security related
but which may aect the stability/availability of the application in the case of extraordinary circumstances.
Other issues when performing a code review are areas such a simple copyright notice in order to protect one’s
intellectual property. Generally these issues should be part of a companies Coding Guidelines (or Standard),
and should be enforceable during a code review. For example a reviewer can reject a code review because
the code violates something in the Coding Guidelines, regardless of whether or not the code would work in
its current state.
Crawling code can be done manually or in an automated fashion using automated tools. However working
manually is probably not eective, as (as can be seen below) there are plenty of indicators, which can apply to
a language. Tools as simple as grep or wingrep can be used. Other tools are available which would search for
keywords relating to a specic programming language. If a team is using a particular review tool that allows
it to specify strings to be highlighted in a review (e.g. Python based review tools using pygments syntax high-
lighter, or an in-house tool for which the team can change the source code) then they could add the relevant
string indicators from the lists below and have them highlighted to reviewers automatically.
The basis of the code review is to locate and analyze areas of code, which may have application security impli-
cations. Assuming the code reviewer has a thorough understanding of the code, what it is intended to do, and
the context in which it is to be used, rstly one needs to sweep the code base for areas of interest.
Appendix C gives practical examples of how to carry out code crawling in the following programming lan-
guages:
• .Net
• Java
• ASP
• C++/Apache
Methodology
43
A1
44 A1 - Injection
7.1 Overview
What is Injection?
Injection attacks allow a malicious user to add or inject content and commands into an application in order to
modify its behaviour. These types of attacks are common, widespread, an easy for a hacker to test if a web site
is vulnerable and easy and quick for the attacker to take advantage of. Today they are very common in legacy
applications that haven’t been updated.
7.2 SQL Injection
The most common injection vulnerability is SQL injection. Injection vulnerability is also easy to remediate and
protect against. This vulnerability covers SQL, LDAP, Xpath, OS commands, XML parsers.
Injection vulnerability can lead to…
1. Disclosure/leaking of sensitive information.
2. Data integrity issues. SQL injection may modify data, add new data, or delete data.
3. Elevation of privileges.
4. Gaining access to back-end network.
SQL commands are not protected from the untrusted input. SQL parser is not able to distinguish between
code and data.
Using string concatenation to generate a SQL statement is very common in legacy applications where de-
velopers were not considering security. The issue is this coding technique does not tell the parser which part
of the statement is code and which part is data. In situations where user input is concatenated into the SQL
statement, an attacker can modify the SQL statement by adding SQL code to the input data.
1. Untrusted input is acceptable by the application. There are several ways to mitigate injection vulnerability,
whitelisting, regex, etc. The ve best ways are. All ve should be used together for a defense in depth approach.
1. HtmlEncode all user input.
2. Using static analysis tools. Most static analysis for languages like .Net, Java, python are accurate. However
static analysis can become an issue when injection comes from JavaScript and CSS.
3. Parameterize SQL queries. Use SQL methods provided by the programming language or framework that
parameterize the statements, so that the SQL parser can distinguish between code and data.
INJECTION
A1
SELECT custName, address1 FROM cust_table WHERE custID= ‘“ + request.GetParameter(“id”) + ““
String custQuery =
Code Data
45
4. Use Stored Procedures. Stored procedures will generally help the SQL parser dierentiate code and data.
However Stored Procedures can be used to build dynamic SQL statements allowing the code and data to be-
come blended together causing the it to become vulnerable to injection.
5. Provide developer training for best practices for secure coding.
Blind SQL Injection
Typically SQL queries return search results that are presented to a user. However, there are cases where SQL
queries are happening behind the scenes that inuence how the page is rendered, and unfortunately attack-
ers can still glean information based on the error responses from various UI elements. Blind SQL injection is a
type of attack that asks the database true or false questions and determines the answer based on the applica-
tions response.
Eectively the attacker uses SQL queries to determine what error responses are returned for valid SQL, and
which responses are returned for invalid SQL. Then the attacker can probe; for example check if a table called
“user_password_table” exists. Once they have that information, they could use an attack like the one described
above to maliciously delete the table, or attempt to return information from the table (does the username
“john” exist?). Blind SQL injections can also use timings instead of error messages, e.g. if invalid SQL takes 2
seconds to respond, but valid SQL returns in 0.5 seconds, the attacker can use this information.
Parameterized SQL Queries
Parameterized SQL queries (sometimes called prepared statements) allow the SQL query string to be dened
in such a way that the client input can’t be treated as part of the SQL syntax.
Take the example in sample 7.1:
In this example the string ‘query’ is constructed in a way that it does not rely on any client input, and the
‘PreparedStatement’ is constructed from that string. When the client input is to be entered into the SQl, the
‘setString’ function is used and the rst question mark “?” is replaced by the string value of ‘rstname’, the sec-
ond question mark is replaced by the value of ‘lastname’. When the ‘setString’ function is called, this function
automatically checks that no SQL syntax is contained within the string value. Most prepared statement APIs
allow you to specify the type that should be entered, e.g. ‘setInt’, or ‘setBinary’, etc.
Safe String Concatenation?
So does this mean you can’t use string concatenation at all in your DB handling code? It is possible to use string
concatenation safely, but it does increase the risk of an error, even without an attacker attempting to inject
SQL syntax into your application.
You should never use string concatenation in combination with the client input value. Take an example where
the existence (not the value) of a client input variable “surname” is used to construct the SQL query of the
prepared statement;
Sample 7.1
1 String query = “SELECT id, rstname, lastname FROM authors WHERE forename = ? and surname = ?”;
2 PreparedStatement pstmt = connection.prepareStatement( query );
3 pstmt.setString( 1, rstname );
4 pstmt.setString( 2, lastname );
A1 - Injection
46
Here the value of ‘lastname’ is not being used, but the existance of it is being evaluated. However there is still a risk
when the SQL statement is larger and has more complex business logic involved in creating it. Take the following
example where the function will search based on rstname or lastname:
This logic will be ne when either rstname, or lastname is given, however if neither were given then the SQL state-
ment would not have any WHERE clause, and the entire table would be returned. This is not an SQL injection (the
attacker has done nothing to cause this situation, except not passing two values) however the end result is the same,
information has been leaked from the database, despite the fact that a parameterized query was used.
For this reason, the advice is to avoid using string concatenation to create SQL query strings, even when using param-
eterized queries, especially if the concatenation involves building any items in the where clause.
Using Flexible Parameterized Statements
Functional requirements often need the SQL query being executed to be exible based on the user input, e.g. if the
end user species a time span for their transaction search then this should be used, or they might wish to query based
on either surname or forename, or both. In this case the safe string concatenation above could be used, however from
1 String query = “Select id, rstname, lastname FROM authors WHERE forename = ?”;
2 if (lastname!= NULL && lastname.length != 0) {
3 query += “ and surname = ?”;
4 }
5 query += “;”;
6
7 PreparedStatement pstmt = connection.prepareStatement( query );
8 pstmt.setString( 1, rstname);
9
10 if (lastname!= NULL && lastname.length != 0) { pstmt.setString( 2, lastname ); }
Sample 7.2
1 String query = “select id, rstname, lastname FROM authors”;
2
3 if (( rstname != NULL && rstname.length != 0 ) && ( lastname != NULL && lastname.length != 0 )) {
4 query += “WHERE forename = ? AND surname = ?”;
5 }
6 else if ( rstname != NULL && rstname.length != 0 ) {
7 query += “WHERE forename = ?”;
8 }
9 else if ( lastname!= NULL && lastname.length != 0 ) {
10 query += “WHERE surname = ?”;
11 }
12
13 query += “;”;
14
15 PreparedStatement pstmt = connection.prepareStatement( query )
Sample 7.3
A1 - Injection
47
a maintenance point of view this could invite future programmers to misunderstand the dierence between safe
concatenation and the unsafe version (using input string values directly).
One option for exible parameterized statements is to use ‘if’ statements to select the correct query based on the
input values provided, for example:
PHP SQL Injection
An SQL injection attack consists of injecting SQL query portions in the back-end database system via the client
interface in the web application. The consequence of a successful exploitation of an SQL injection varies from
just reading data to modifying data or executing system commands. SQL Injection in PHP remains the number
one attack vector, and also the number one reason for data compromises as shown in sample 7.5.
Example 1 :
1 String query;
2 PreparedStatement pstmt;
3
4 if ( (rstname!= NULL && rstname.length != 0) &&
5 lastname!= NULL && lastname.length != 0) ) {
6 query = “Select id, rstname, lastname FROM authors WHERE forename = ? and surname = ?”
7 pstmt = connection.prepareStatement( query );
8 pstmt.setString( 1, rstname );
9 pstmt.setString( 2, lastname );
10 }
11 else if (rstname != NULL && rstname.length != 0) {
12 query = “Select id, rstname, lastname FROM authors WHERE forename = ?”;
13 pstmt = connection.prepareStatement( query );
14 pstmt.setString( 1, rstname );
15 }
16 else if (lastname != NULL && lastname.length != 0){
17 query = “Select id, rstname, lastname FROM authors WHERE surname= ?”;
18 pstmt = connection.prepareStatement( query );
19 pstmt.setString( 1, lastname);
20 }
21 else{
22 throw NameNotSpeciedException(); }
Sample 7.4
1 <?php
1 $pass=$_GET[“pass”];
2 $con = mysql_connect(‘localhost’, ‘owasp’, ‘abc123’);
3 mysql_select_db(“owasp_php”, $con);
4 $sql=”SELECT card FROM users WHERE password = ‘”.$pass.”’”;
5 $result = mysql_query($sql);
6 ?>
Sample 7.5
A1 - Injection
48
The most common ways to prevent SQL Injection in PHP are using functions such as addslashes() and mysql_
real_escape_string() but those function can always cause SQL Injections in some cases.
Addslashes :
You will avoid Sql injection using addslashes() only in the case when you wrap the query string with quotes.
The following example would still be vulnerable
mysql_real_escape_string():
mysql_real_escape_string() is a little bit more powerful than addslashes() as it calls MySQL’s library function
mysql_real_escape_string, which prepends backslashes to the following characters: \x00, \n, \r, \, ‘, “ and \x1a.
As with addslashes(), mysql_real_escape_string() will only work if the query string is wrapped in quotes. A
string such as the following would still be vulnerable to an SQL injection:
SQL injections occur when input to a web application is not controlled or sanitized before executing to the
back-end database.
The attacker tries to exploit this vulnerability by passing SQL commands in her/his input and therefore will
create a undesired response from the database such as providing information that bypasses the authorization
and authentication programmed in the web application. An example of vulnerable java code is shown in
sample 7.7
An example of a vulnerable java code
The input parameter “name” is passed to the String query without any proper validation or verication. The
query ‘SELECT* FROM users where name” is equal to the string ‘username’ can be easily misused to bypass
something dierent that just the ‘name’. For example, the attacker can attempt to pass instead in this way ac-
cessing all user records and not only the one entitled to the specic user
“ OR 1=1.
1 $id = addslashes( $_GET[‘id’] );
2 $query = ‘SELECT title FROM books WHERE id = ‘ . $id;
Sample 7.6
1 HttpServletRequest request = ...;
2 String userName = request.getParameter(“name”);
3 Connection con = ...
4 String query = “SELECT * FROM Users WHERE name = ‘” + userName + “’”;
5 con.execute(query);
Sample 7.7
A1 - Injection
49
.NET Sql Injection
Framework 1.0 & 2.0 might be more vulnerable to SQL injections than the later versions of .NET. Thanks to the
proper implementation and use of design patters already embedded in ASP.NET such as MVC(also depending
on the version), it is possible to create applications free from SQL injections, however, there might be times
where a developer might prefer to use SQL code directly in the code.
Example.
A developer creates a webpage with 3 elds and submit button, to search for employees on elds ‘name’,
‘lastname’ and ‘id’
The developer implements a string concatenated SQL statement or stored procedure in the code such as in
sample 7.8.
This code is equivalent to the executed SQL statement in sample 7.9.
A hacker can then insert the following employee ID via the web interface “123’;DROP TABLE pubs --” and exe-
cute the following code:
SELECT name, lastname FROM authors WHERE ei_id = ‘123’; DROP TABLE pubs --’
The semicolon “;” provides SQL with a signal that it has reached the end of the sql statement, however, the
hacker uses this to continue the statement with the malicious SQL code
; DROP TABLE pubs;
Parameter collections
Parameter collections such as SqlParameterCollection provide type checking and length validation. If you use
a parameters collection, input is treated as a literal value, and SQL Server does not treat it as executable code,
and therefore the payload can not be injected.
Using a parameters collection lets you enforce type and length checks. Values outside of the range trigger an
exception. Make sure you handle the exception correctly. Example of the SqlParameterCollection:
Hibernate Query Language (HQL)
1 SqlDataAdapter thisCommand = new SqlDataAdapter(
2 “SELECT name, lastname FROM employees WHERE ei_id = ‘” +
idNumber.Text + “’”, thisConnection);
Sample 7.8
1 SqlDataAdapter thisCommand = new SqlDataAdapter(
2 “SearchEmployeeSP ‘” + idNumber.Text + “’”, thisConnection);
Sample 7.9
A1 - Injection
50
Hibernate facilitates the storage and retrieval of Java domain objects via Object/Relational Mapping (ORM).
It is a very common misconception that ORM solutions, like hibernate, are SQL Injection proof. Hibernate allows the
use of “native SQL” and denes a proprietary query language, called HQL (Hibernate Query Language); the former is
prone to SQL Injection and the later is prone to HQL (or ORM) injection.
What to Review
• Always validate user input by testing type, length, format, and range.
• Test the size and data type of i nput and enforce a ppropr iate lim its.
• Test the content of string variables and accept only expected values. Reject entries that contain binary data, escape
sequences, and comment characters.
• When you are working with XML documents, validate all data against its schema as it is entered.
• Never build SQL statements directly from user input.
• Use stored procedures to validate user input, when not using stored procedures use SQL API provided by platform.
i.e. Parameterized Statements.
• Implement multiple layers of validation.
• Never concatenate user input that is not validated. String concatenation is the primary point of entry for script
injection.
You sh oul d re vie w a ll co de tha t c all s E XEC UTE , EXE C, any S QL ca lls th at ca n c all ou tsi de re sou rc es or co mm and li ne.
OWASP References
• https://www.owasp.org/index.php/SQL_Injection_Prevention_Cheat_Sheet OWASP SQL Injection Prevention
Cheat Sheet
• https://www.owasp.org/index.php/Query_Parameterization_Cheat_Sheet OWASP Query Parameterization
Cheat Sheet
• https://www.owasp.org/index.php/Command_Injection OWASP Command Injection Article
• https://www.owasp.org/index.php/XXE OWASP XML eXternal Entity (XXE) Reference Article
1 using (SqlConnection conn = new SqlConnection(connectionString)) {
2 DataSet dataObj = new DataSet();
3 SqlDataAdapter sqlAdapter = new SqlDataAdapter( “StoredProc”, conn); sqlAdapter.SelectCommand.
CommandType =
4 CommandType.StoredProcedure;
5 sqlAdapter.SelectCommand.Parameters.Add(“@usrId”, SqlDbType.VarChar, 15);
6 sqlAdapter.SelectCommand.Parameters[“@usrId “].Value = UID.Text;
Sample 7.10
A1 - Injection
51
• https://www.owasp.org/index.php/ASVS ASVS: Output Encoding/Escaping Requirements (V6)
• https://www.owasp.org/index.php/Testing_for_SQL_Injection_(OWASP-DV-005) OWASP Testing Guide: Chap-
ter on SQL Injection Testing
External References
• http://cwe.mitre.org/data/denitions/77.html CWE Entry 77 on Command Injection
• http://cwe.mitre.org/data/denitions/89.html CWE Entry 89 on SQL Injection
• http://cwe.mitre.org/data/denitions/564.html CWE Entry 564 on Hibernate Injection
• Livshits and Lam, 2005 “Finding Security Vulnerabilities in Java Applications with Static Analysis” available at
https://www.usenix.org/legacy/event/sec05/tech/full_papers/livshits/livshits_html/#sec:sqlinjexample
• http://www.php.net/manual/en/book.pdo.php PDO
• https://technet.microsoft.com/en-us/library/ms161953(v=sql.105).aspx
7.3 JSON (JavaScript Object Notation)
JSON (JavaScript Object Notation) is an open standard format that uses easy to read text to transmit data be-
tween a server and web applications. JSON data can be used by a large number of programming Languages
and is becoming the de-facto standard in replacing XML.
JSON main security concern is JSON text dynamically embedded in JavaScript, because of this injection is a very real
vulnerability. The vulnerability in the program that may inadvertently to run a malicious script or store the malicious
script to a database. This is a very real possibility when dealing with data retrieved from the Internet.
The code reviewer needs to make sure the JSON is not used with Javascript eval. Make sure JSON.parse(…) is used.
Var parsed_object = eval(“(“ + Jason_text + “)”); // Red ag for the code reviewer.
JSON.parse(text[, reviver]); .. // Much better then using javascript eval function.
Code reviewer should check to make sure the developer is not attempting to reject known bad patterns in text/
string data, Using regex or other devices is fraught with error and makes testing for correctness very hard. Allow only
whitelisted alphanumeric keywords and carefully validated numbers.
Do not allow JSON data to construct dynamic HTML. Always us safe DOM features like innerText or CreateText-
Node(…)
Object/Relational Mapping (ORM)
Object/Relation Mapping (ORM) facilitates the storage and retrieval of domain objects via HQL (Hibernate Query
Language) or .NET Entity framework.
It is a very common misconception that ORM solutions, like hibernate are SQL Injection proof. They are not. ORM’s
allow the use of “native SQL”. Thru proprietary query language, called HQL is prone to SQL Injection and the later is
prone to HQL (or ORM) injection. Linq is not SQL and because of that is not prone to SQL injection. However using
excutequery or excutecommand via linq causes the program not to use linq protection mechanism and is vulnera-
bility to SQL injection.
A1 - Injection
52
Bad Java Code Examples
List results = session.createQuery(“from Items as item where item.id = “ + currentItem.getId()).list();
NHibernate is the same as Hibernate except it is an ORM solution for Microsoft .NET platform. NHibernate is also vul-
nerable to SQL injection if used my dynamic queries.
Bad .Net Code Example
Code Reviewer Action
Code reviewer needs to make sure any data used in an HQL query uses HQL parameterized queries so that it
would be used as data and not as code. They can also use the Criteria API at https://docs.jboss.org/hibernate/
orm/3.3/reference/en/html/querycriteria.html
7.5 Content Security Policy (CSP)
Is a W3C specication oering the possibility to instruct the client browser from which location and/or which
type of resources are allowed to be loaded. To dene a loading behavior, the CSP specication use “directive”
where a directive denes a loading behavior for a target resource type. CSP helps to detect and mitigate cer-
tain types of attacks, including Cross Site Scripting (XSS) and data injection attacks. These attacks are used for
everything from data theft to site defacement or distribution of malware
Directives can be specied using HTTP response header (a server may send more than one CSP HTTP header
eld with a given resource representation and a server may send dierent CSP header eld values with dif-
ferent representations of the same resource or with dierent resources) or HTML Meta tag, the HTTP headers
below are dened by the specs:
• Content-Security-Policy : Dened by W3C Specs as standard header, used by Chrome version 25 and later,
Firefox version 23 and later, Opera version 19 and later.
• X-Content-Security-Policy : Used by Firefox until version 23, and Internet Explorer version 10 (which partially
implements Content Security Policy).
• X-WebKit-CSP : Used by Chrome until version 25
Risk
The risk with CSP can have 2 main sources:
• Policies misconguration,
• Too permissive policies.
What to Review
Code reviewer needs to understand what content security policies were required by application design and
how these policies are tested to ensure they are in use by the application.
string userName = ctx.GetAuthenticatedUserName();
String query = “SELECT * FROM Items WHERE owner = ‘”
+ userName + “’ AND itemname = ‘”
+ ItemName.Text + “’”;
List items = sess.CreateSQLQuery(query).List()
A1 - Injection
53
Useful security-related HTTP headers
In most architectures these headers can be set in web servers conguration without changing actual applica-
tion’s code. This oers signicantly faster and cheaper method for at least partial mitigation of existing issues,
and an additional layer of defense for new applications.
Note the Spring Security library can assist with these headers, see http://docs.spring.io/spring-security/site/
docs/current/reference/html/headers.html
Header name Description Example
Strict-Transport-Security
https://tools.ietf.org/
html/rfc6797
X-Frame-Options
https://tools.ietf.org/
html/draft-ietf-websec-x-
frame-options-01
Frame-Options
https://tools.ietf.org/
html/draft-ietf-websec-
frame-options-00
HTTP Strict-Transport-Security (HSTS) enforces secure (HTTP over SSL/TLS) connections
to the server. This reduces impact of bugs in web applications leaking session data
through cookies and external links and defends against Man-in-the-middle attacks.
HSTS also disables the ability for user’s to ignore SSL negotiation warnings.
Provides Click jacking protection. Values: deny - no rendering within a frame, sameorigin
- no rendering if origin mismatch, allow-from: DOMAIN - allow rendering if framed by
frame loaded from DOMAIN
Strict-Transport-Security:
max-age=16070400;
includeSubDomains
X-Frame-Options: deny
Table 11: Security Related HTTP Headers
X-Content-Type-Options
https://blogs.msdn.
microsoft.com/
ie/2008/09/02/ie8-securi-
ty-part-vi-beta-2-update/
Content-Security-Policy,
X-Content-Security-poli-
cy,X-WebKit-CSP
https://www.w3.org/TR/
CSP/
Content-Security-Poli-
cy-Report_Only
https://www.w3.org/TR/
CSP/
The only dened value, “nosni”, prevents Internet Explorer and Google Chrome from
MIME-sning a response away from the declared content-type. This also applies to Goo-
gle Chrome, when downloading extensions. This reduces exposure to drive-by down-
load attacks and sites serving user uploaded content that, by clever naming, could be
treated by MSIE as executable or dynamic HTML les.
Content Security Policy requires careful tuning and precise denition of the policy. If
enabled, CSP has signicant impact on the way browser renders pages (e.g., inline JavaS-
cript disabled by default and must be explicitly allowed in policy). CSP prevents a wide
range of attacks, including Cross-site scripting and other cross-site injections.
Like Content-Security-Policy, but only reports. Useful during implementation, tuning
and testing eorts.
X-Content-Type-Options:
nosni
Content-Security-Policy:
default-src ‘self’
Content-Security-Pol-
icy-Report-Only: de-
fault-src ‘self’; report-uri
http://loghost.example.
com/reports.jsp
X-XSS-Protection
[http://blogs.
msdn.com/b/ie/ar-
chive/2008/07/02/
ie8-security-part-iv-the-
xss-lter.aspx X-XSS-Pro-
tection]
This header enables the Cross-site scripting (XSS) lter built into most recent web brows-
ers. It’s usually enabled by default anyway, so the role of this header is to re-enable the
lter for this particular website if it was disabled by the user. This header is supported in
IE 8+, and in Chrome (not sure which versions). The anti-XSS lter was added in Chrome
4. Its unknown if that version honored this header.
X-XSS-Protection: 1;
mode=block
A1 - Injection
54
References
Apache: http://httpd.apache.org/docs/2.0/mod/mod_headers.html
IIS: http://technet.microsoft.com/pl-pl/library/cc753133(v=ws.10).aspx
7.6 Input Validation
Input validation is one of the most eective technical controls for application security. It can mitigate numer-
ous vulnerabilities including cross-site scripting, various forms of injection, and some buer overows. Input
validation is more than checking form eld values.
All data from users needs to be considered untrusted. Remember one of the top rules of secure coding is
“Don’t trust user input”. Always validate user data with the full knowledge of what your application is trying
to accomplish.
Regular expressions can be used to validate user input, but the more complicated the regular express are the
more chance it is not full proof and has errors for corner cases. Regular expressions are also very hard fro QA
to test. Regular expressions may also make it hard for the code reviewer to do a good review of the regular
expressions.
Data Validation
All external input to the system (and between systems/applications) should undergo input validation. The
validation rules are dened by the business requirements for the application. If possible, an exact match vali-
dator should be implemented. Exact match only permits data that conforms to an expected value. A “Known
good” approach (white-list), which is a little weaker, but more exible, is common. Known good only permits
characters/ASCII ranges dened within a white-list.
Such a range is dened by the business requirements of the input eld. The other approaches to data valida-
tion are “known bad,” which is a black list of “bad characters”. This approach is not future proof and would need
maintenance. “Encode bad” would be very weak, as it would simply encode characters considered “bad” to a
format, which should not aect the functionality of the application.
Business Validation
Business validation is concerned with business logic. An understanding of the business logic is required prior
to reviewing the code, which performs such logic. Business validation could be used to limit the value range
or a transaction inputted by a user or reject input, which does not make too much business sense. Reviewing
code for business validation can also include rounding errors or oating point issues which may give rise to
issues such as integer overows, which can dramatically damage the bottom line.
Canonicalization
Canonicalization is the process by which various equivalent forms of a name can be resolved to a single stan-
dard name, or the “canonical” name.
The most popular encodings are UTF-8, UTF-16, and so on (which are described in detail in RFC 2279). A single
character, such as a period/full-stop (.), may be represented in many dierent ways: ASCII 2E, Unicode C0 AE,
and many others.
With the myriad ways of encoding user input, a web application’s lters can be easily circumvented if they’re
not carefully built.
A1 - Injection
55
Bad Example
Good Example
.NET Request Validation
One solution is to use .Net “Request Validation”. Using request validation is a good start on validating user
data and is useful. The downside is too generic and not specic enough to meet all of our requirements to
provide full trust of user data.
You can never use request validation for securing your application against cross-site scripting attacks.
The following example shows how to use a static method in the Uri class to determine whether the Uri provid-
ed by a user is valid.
var isValidUri = Uri.IsWellFormedUriString(passedUri, UriKind.Absolute);
However, to suciently verify the Uri, you should also check to make sure it species http or https. The follow-
ing example uses instance methods to verify that the Uri is valid.
var uriToVerify = new Uri(passedUri);
var isValidUri = uriToVerify.IsWellFormedOriginalString();
var isValidScheme = uriToVerify.Scheme == “http” || uriToVerify.Scheme == “https”;
Before rendering user input as HTML or including user input in a SQL query, encode the values to ensure ma-
licious code is not included.
You can HTML encode the value in markup with the <%: %> syntax, as shown below.
<span><%: userInput %></span>
Or, in Razor syntax, you can HTML encode with @, as shown below.
public static void main(String[] args) {
File x = new File(“/cmd/” + args[1]);
String absPath = x.getAbsolutePath();
}
Sample 7.11
public static void main(String[] args) throws IOException {
File x = new File(“/cmd/” + args[1]);
String canonicalPath = x.getCanonicalPath();
Sample 7.12
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56
<span>@userInput</span>
The next example shows how to HTML encode a value in code-behind.
var encodedInput = Server.HtmlEncode(userInput);
Managed Code and Non-Managed Code
Both Java and .Net have the concept of managed and non-managed code. To oer some of these protections
during the invocation of native code, do not declare a native method public. Instead, declare it private and
expose the functionality through a public wrapper method. A wrapper can safely perform any necessary input
validation prior to the invocation of the native method:
Java Sample code to call a Native Method with Data Validation in place
Data validations checklist for the Code Reviewer.
• Ensure that a Data Validation mechanism is present.
public nal class NativeMethodWrapper {
private native void nativeOperation(byte[] data, int oset, int len);
public void doOperation(byte[] data, int oset, int len) {
// copy mutable input
data = data.clone();
// validate input
// Note oset+len would be subject to integer overow.
// For instance if oset = 1 and len = Integer.MAX_VALUE,
// then oset+len == Integer.MIN_VALUE which is lower
// than data.length.
// Further,
// loops of the form
// for (int i=oset; i<oset+len; ++i) { ... }
// would not throw an exception or cause native code to
// crash.
if (oset < 0 || len < 0 || oset > data.length - len) {
throw new IllegalArgumentException();
}
nativeOperation(data, oset, len);
}
}
Sample 7.13
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57
• Ensure all input that can (and will) be modied by a malicious user such as HTTP headers, input elds, hidden
elds, drop down lists, and other web components are properly validated.
• Ensure that the proper length checks on all input exist.
• Ensure that all elds, cookies, http headers/bodies, and form elds are validated.
• Ensure that the data is well formed and contains only known good chars if possible.
• Ensure that the data validation occurs on the server side.
• Examine where data validation occurs and if a centralized model or decentralized model is used.
• Ensure there are no backdoors in the data validation model.
• “Golden Rule: All external input, no matter what it is, will be examined and validated.”
Resources:
http://msdn.microsoft.com/en-us/library/vstudio/system.uri
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A2
59
8.1 Overview
Web applications and Web services both use authentication as the primary means of access control from log-
ins via user id and passwords. This control is essential to the prevention of condential les, data, or web pages
from being access by hackers or users who do not have the necessary access control level.
8.2 Description
Authentication is important, as it is the gateway to the functionality you are wishing to protect. Once a user
is authenticated their requests will be authorized to perform some level of interaction with your application
that non-authenticated users will be barred from. You cannot control how users manage their authentication
information or tokens, but you can ensure there is now way to perform application functions without proper
authentication occurring.
There are many forms of authentication with passwords being the most common. Other forms include client
certicates, biometrics, one time passwords over SMS or special devices, or authentication frameworks such as
Open Authorization (OAUTH) or Single Sign On (SSO).
Typically authentication is done once, when the user logs into a website, and successful authentication results
in a web session being setup for the user (see Session Management). Further (and stronger) authentication
can be subsequently requested if the user attempts to perform a high risk function, for example a bank user
could be asked to conrm an 6 digit number that was sent to their registered phone number before allowing
money to be transferred.
Authentication is just as important within a companies rewall as outside it. Attackers should not be able to
run free on a companies internal applications simply because they found a way in through a rewall. Also
separation of privilege (or duties) means someone working in accounts should not be able to modify code in
a repository, or application managers should not be able to edit the payroll spreadsheets.
8.3 What to Review
When reviewing code modules which perform authentication functions, some common issues to look out for
include:
• Ensure the login page is only available over TLS. Some sites leave the login page has HTTP, but make the form
submission URL HTTPS so that the users username and password are encrypted when sent to the server. How-
ever if the login page is not secured, a risk exists for a man-in-the-middle to modify the form submission URL
to an HTTP URL, and when the user enters their username & password they are sent in the clear.
• Make sure your usernames/user-ids are case insensitive. Many sites use email addresses for usernames and
email addresses are already case insensitive. Regardless, it would be very strange for user ‘smith’ and user
‘Smith’ to be dierent users. Could result in serious confusion.
• Ensure failure messages for invalid usernames or passwords do not leak information. If the error message
indicates the username was valid, but the password was wrong, then attackers will know that username exists.
If the password was wrong, do not indicate how it was wrong.
• Make sure that every character the user types in is actually included in the password.
BROKEN AUTHENTICATION AND
SESSION MANAGEMENT
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60 A2 - Broken Authentication and Session Management
• Do not log invalid passwords. Many times an e-mail address is used as the username, and those users will
have a few passwords memorized but may forget which one they used on your web site. The rst time they
may use a password that in invalid for your site, but valid for many other sites that this user (identied by the
username). If you log that username and password combination, and that log leaks out, this low level compro-
mise on your site could negatively aect many other sites.
• Longer passwords provide a greater combination of characters and consequently make it more dicult for
an attacker to guess. Minimum length of the passwords should be enforced by the application. Passwords
shorter than 10 characters are considered to be weak. Passphrases should be encouraged. For more on pass-
word lengths see the OWASP Authentication Cheat Sheet.
• To prevent brute force attacks, implement temporary account lockouts or rate limit login responses. If a user
fails to provide the correct username and password 5 times, then lock the account for X minutes, or implement
logic where login responses take an extra 10 seconds. Be careful though, this could leak the fact that the user-
name is valid to attackers continually trying random usernames, so as an extra measure, consider implement-
ing the same logic for invalid usernames.
• For internal systems, consider forcing the users to change passwords after a set period of time, and store a
reference (e.g. hash) of the last 5 or more passwords to ensure the user is not simply re-using their old pass-
word.
• Password complexity should be enforced by making users choose password strings that include various type of
characters (e.g. upper- and lower-case letters, numbers, punctuation, etc.). Ideally, the application would indicate to
the user as they type in their new password how much of the complexity policy their new password meets. For more
on password complexity see the OWASP Authentication Cheat Sheet.
• It is common for an application to have a mechanism that provides a means for a user to gain access to their
account in the event they forget their password. This is an example of web site functionality this is invoked by unau-
thenticated users (as they have not provided their password). Ensure interfaces like this are protected from misuse, if
asking for password reminder results in an e-mail or SMS being sent to the registered user, do not allow the password
reset feature to be used to spam the user by attackers constantly entering the username into this form. Please see
Forgot Password Cheat Sheet for details on this feature.
• It is critical for an application to store a password using the right cryptographic technique. Please see
Password Storage Cheat Sheet for details on this feature.
When reviewing MVC .NET is is important to make sure the application transmits and received over a secure link. It is
recommended to have all web pages not just login pages use SSL.
We need to protect session cookies, which are useful as users credentials.
In the global.asax le we can review the RegisterGlobalFilters method.
public static void RegisterGlobalFilters(GlobalFilterCollection lters) {
“ ters.Add(new RequireHttpsAttribute()); “
}
Sample 8.1
61
A2 - Broken Authentication and Session Management
The attribute RequireHttpsAttribute() can be used to make sure the application runs over SSL/TLS
It is recommended that this be enabled for SSL/TLS sites.
• For high risk functions, e.g. banking transactions, user prole updates, etc., utilize multi-factor authentication
(MFA). This also mitigates against CSRF and session hijacking attacks. MFA is using more than one authentication
factor to logon or process a transaction:
• Something you know (account details or passwords)
• Something you have (tokens or mobile phones)
• Something you are (biometrics)
• If the client machine is in a controlled environment utilize SSL Client Authentication, also known as two-way
SSL authentication, which consists of both browser and server sending their respective SSL certicates during the TLS
handshake process. This provides stronger authentication as the server administrator can create and issue client cer-
ticates, allowing the server to only trust login requests from machines that have this client SSL certicate installed.
Secure transmission of the client certicate is important.
References
• https://www.owasp.org/index.php/Authentication_Cheat_Sheet
• http://csrc.nist.gov/publications/nistpubs/800-132/nist-sp800-132.pdf
• http://www.codeproject.com/Articles/326574/An-Introduction-to-Mutual-SSL-Authentication
• https://cwe.mitre.org/data/denitions/287.html Improper Authentication
• OWASP ASVS requirements areas for Authentication (V2)
8.4 Forgot Password
Overview
If your web site needs to have user authentication then most likely it will require user name and password to
authenticate user accesses. However as computer system have increased in complexity, so has authenticating
users has also increased. As a result the code reviewer needs to be aware of the benets and drawbacks of user
authentication referred to as “Direct Authentication” pattern in this section. This section is going to emphasise
design patterns for when users forget user id and or password and what the code reviewer needs to consider
when reviewing how user id and passwords can be retrieved when forgotten by the user and how to do this
in a secure manner.
General considerations
Notify user by (phone sms, email) such that the user has to click a link in the email that takes them to your site
and ask the user to enter a new password.
Ask user to enter login credentials they already have (Facebook, Twitter, Google, Microsoft Live, OpenID etc) to
validate user before allowing user to change password.
Send notication to user to confrm registration or forgot password usage.
Send notications that account information has been changed for registered email. Set appropriate time out
value. I.e. If user does not respond to email within 48 hours then user will be frozen out of system until user
re-arms password change.
• The identity and shared secret/password must be transferred using encryption to provide data condenti-
ality.
62
• A shared secret can never be stored in clear text format, even if only for a short time in a message queue.
• A shared secret must always be stored in hashed or encrypted format in a database.
• The organization storing the encrypted shared secret does not need the ability to view or decrypt users
passwords. User password must never be sent back to a user.
• If the client must cache the username and password for presentation for subsequent calls to a Web service
then a secure cache mechanism needs to be in place to protect user name and password.
• When reporting an invalid entry back to a user, the username and or password should not be identied as be-
ing invalid. User feed back/error message must consider both user name and password as one item “user credential”.
I.e. “The username or password you entered is incorrect.”
8.5 CAPTCHA
Overview
CAPTCHA (an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart”.) is
an access control technique.
CAPTCHA is used to prevent automated software from gaining access to webmail services like Gmail, Hotmail
and Yahoo to create e-mail spam, automated postings to blogs, forums and wikis for the purpose of promotion
(commercial, and or political) or harassment and vandalism and automated account creation.
CAPTCHA’s have proved useful and their use has been upheld in court. Circumventing CAPTCHA has been
upheld in US Courts as a violation Digital Millennium Copyright Act anti-circumvention section 1201(a)(3) and
European Directive 2001/29/EC.
General considerations
Code review of CAPTCHA’s the reviewer needs to pay attention to the following rules to make sure the CAPT-
CHA is built with strong security principals.
• Do not allow the user to enter multiple guesses after an incorrect attempt.
• The software designer and code review need to understand the statics of guessing. I.e. One CAPTCHA
design shows four (3 cats and 1 boat) pictures, User is requested to pick the picture where it is not in the same cat-
egory of the other pictures. Automated software will have a success rate of 25% by always picking the rst picture.
Second depending on the xed pool of CAPTCHA images over time an attacker can create a database of correct
answers then gain 100% access.
• Consider using a key being passed to the server that uses a HMAC (Hash-based message authentication
code) the answer.
Text base CAPTCHA’s should adhere to the following security design principals...
1. Randomize the CAPTCHA length: Don’t use a xed length; it gives too much information to the attacker.
2. Randomize the character size: Make sure the attacker can’t make educated guesses by using several font
sizes / several fonts.
3. Wave the CAPTCHA: Waving the CAPTCHA increases the diculty for the attacker.
4. Don’t use a complex charset: Using a large charset does not improve signicantly the CAPTCHA scheme’s
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63
security and really hurts human accuracy.
5. Use anti-recognition techniques as a means of strengthening CAPTCHA security: Rotation, scaling and
rotating some characters and using various font sizes will reduce the recognition eciency and increase secu-
rity by making character width less predictable.
6. Keep the line within the CAPTCHAs: Lines must cross only some of the CAPTCHA letters, so that it is
impossible to tell whether it is a line or a character segment.
7. Use large lines: Using lines that are not as wide as the character segments gives an attacker a robust
discriminator and makes the line anti-segmentation technique vulnerable to many attack techniques.
8. CAPTCHA does create issues for web sites that must be ADA (Americans with Disabilities Act of 1990)
compliant. Code reviewer may need to be aware of web accessibilities and security to review the CAPTCHA
implementation where web site is required to be ADA complaint by law.
References
• UNITED STATES of AMERICA vs KENNETH LOWSON, KRISTOFER KIRSCH, LOEL STEVENSON Federal Indictment.
February 23, 2010. Retrieved 2012-01-02.
• http://www.google.com/recaptcha/captcha
• http://www.ada.gov/anprm2010/web%20anprm_2010.htm
• Inaccessibility of CAPTCHA - Alternatives to Visual Turing Tests on the Web http://www.w3.org/TR/turingtest/
8.6 Out-of-Band Communication
Overview
The term ‘out-of-band’ is commonly used when an web application communicates with an end user over a
channel separate to the HTTP request/responses conducted through the users’ web browser. Common exam-
ples include text/SMS, phone calls, e-mail and regular mail.
Description
The main reason an application would wish to communicate with the end user via these separate channels
is for security. A username and password combination could be sucient authentication to allow a user to
browse and use non-sensitive parts of a website, however more sensitive (or risky) functions could require
a stronger form of authentication. A username and password could have been stolen through an infected
computer, through social engineering, database leak or other attacks, meaning the web application cannot
put too much in trust a web session providing the valid username and password combination is actually the
intended user.
Examples of sensitive operations could include:
• Changing password
• Changing account details, such as e-mail address, home address, etc
• Transferring funds in a banking application
• Submitting, modifying or cancelling orders
In these cases many applications will communicate with you via a channel other than a browsing session.
Many large on-line stores will send you conrmation e-mails when you change account details, or purchase
items. This protects in the case that an attacker has the username and password, if they buy something, the
legitimate user will get an e-mail and have a chance to cancel the order or alert the website that they did not
modify the account.
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64
When out-of-band techniques are performed for authentication it is termed two-factor authentication. There
are three ways to authenticate:
1. Something you know (e.g. password, passphrase, memorized PIN)
2. Something you have (e.g. mobile phone, cash card, RSA token)
3. Something you are (e.g. iris scan, ngerprint)
If a banking website allows users to initiate transactions online, it could use two-factor authentication by tak-
ing 1) the password used to log in and 2) sending an PIN number over SMS to the users registered phone, and
then requiring the user enter the PIN before completing the transaction. This requires something the user
knows (password) and has (phone to receive the PIN).
A ‘chip-and-pin’ banking card will use two-factor authentication, by requiring users to have the card with them
(something they have) and also enter a PIN when performing a purchase (something they know). A ‘chip-and-
pin’ card is not use within the PIN number, likewise knowing the PIN number is useless if you do not have the
card.
What to Review
When reviewing code modules which perform out-of-band functions, some common issues to look out for
include:
1. Recognize the risk of the system being abused. Attackers would like to ood someone with SMS messages
from your site, or e-mails to random people. Ensure:
2. When possible, only authenticated users can access links that cause an out-of-band feature to be invoked
(forgot password being an exception).
3. Rate limit the interface, thus users with infected machines, or hacked accounts, can’t use it to ood
out-of-band messages to a user.
4. Do not allow the feature to accept the destination from the user, only use registered phone numbers,
e-mails, addresses.
5. For high risk sites (e.g. banking) the users phone number can be registered in person rather than via the
web site.
6. Do not send any personal or authentication information in the out-of-band communication.
7. Ensure any PINs or passwords send over out-of-band channels have a short life-span and are random.
8. A consideration can be to prevent SMS messages being sent to the device currently conducting the
browsing session (i.e. user browsing on their iPhone, were the SMS is sent to). However this can be hard to enforce.
If possible give users the choice of band they wish to use. For banking sites Zitmo malware on mobile
devices (see references) can intercept the SMS messages, however iOS devices have not been aected by this mal-
ware yet, so users could choose to have SMS PINs sent to their Apple devices, and when on Android they could use
recorded voice messages to receive the PIN.
9. In a typical deployments specialized hardware/software separate from the web application will handle the
out-of-band communication, including the creation of temporary PINs and possibly passwords. In this scenario there
is no need to expose the PIN/password to your web application (increasing risk of exposure), instead the web appli-
cation should query the specialized hardware/software with the PIN/password supplied by the end user, and receive
A2 - Broken Authentication and Session Management
65
a positive or negative response.
Many sectors including the banking sector have regulations requiring the use of two-factor authentication for certain
types of transactions. In other cases two-factor authentication can reduce costs due to fraud and re-assure customers
of the security of a website.
References
• https://www.owasp.org/index.php/Forgot_Password_Cheat_Sheet
• http://securelist.com/blog/virus-watch/57860/new-zitmo-for-android-and-blackberry/
8.7 Session Management
Overview
A web session is a sequence of network HTTP request and response transactions associated to the same user. Session
management or state is needed by web applications that require the retaining of information or status about each
user for the duration of multiple requests. Therefore, sessions provide the ability to establish variables – such as access
rights and localization settings – which will apply to each and every interaction a user has with the web application
for the duration of the session.
Description
Code reviewer needs to understand what session techniques the developers used, and how to spot vulnerabilities
that may create potential security risks. Web applications can create sessions to keep track of anonymous users after
the very rst user request. An example would be maintaining the user language preference. Additionally, web ap-
plications will make use of sessions once the user has authenticated. This ensures the ability to identify the user on
any subsequent requests as well as being able to apply security access controls, authorized access to the user private
data, and to increase the usability of the application. Therefore, current web applications can provide session capabil-
ities both pre and post authentication.
The session ID or token binds the user authentication credentials (in the form of a user session) to the user
HTTP trac and the appropriate access controls enforced by the web application. The complexity of these
three components (authentication, session management, and access control) in modern web applications,
plus the fact that its implementation and binding resides on the web developer’s hands (as web development
framework do not provide strict relationships between these modules), makes the implementation of a secure
session management module very challenging.
The disclosure, capture, prediction, brute force, or xation of the session ID will lead to session hijacking (or
sidejacking) attacks, where an attacker is able to fully impersonate a victim user in the web application. Attack-
ers can perform two types of session hijacking attacks, targeted or generic. In a targeted attack, the attacker’s
goal is to impersonate a specic (or privileged) web application victim user. For generic attacks, the attacker’s
goal is to impersonate (or get access as) any valid or legitimate user in the web application.
With the goal of implementing secure session IDs, the generation of identiers (IDs or tokens) must meet the
following properties:
• The name used by the session ID should not be extremely descriptive nor oer unnecessary details about the
purpose and meaning of the ID.
• It is recommended to change the default session ID name of the web development framework to a generic
name, such as “id”.
• The session ID length must be at least 128 bits (16 bytes) (The session ID value must provide at least 64 bits
of entropy).
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66
• The session ID content (or value) must be meaningless to prevent information disclosure attacks, where an
attacker is able to decode the contents of the ID and extract details of the user, the session, or the inner work-
ings of the web application.
It is recommended to create cryptographically strong session IDs through the usage of cryptographic hash
functions such as SHA2 (256 bits).
What to Review
Require cookies when your application includes authentication. Code reviewer needs to understand what in-
formation is stored in the application cookies. Risk management is needed to address if sensitive information
is stored in the cookie requiring SSL for the cookie
.Net ASPX web.cong
Java web.xml
PHP.INI
Session Expiration
In reviewing session handling code the reviewer needs to understand what expiration timeouts are needed
by the web application or if default session timeout are being used. Insucient session expiration by the web
<authentication mode=”Forms”>
<forms loginUrl=”member_login.aspx”
cookieless=”UseCookies”
requireSSL=”true”
path=”/MyApplication” />
</authentication>
Sample 8.2
<session-cong>
<cookie-cong>
<secure>true</secure>
</cookie-cong>
</session-cong>
Sample 8.3
void session_set_cookie_params ( int $lifetime [, string $path [, string $domain [, bool $secure = true [, bool
$httponly = true ]]]] )
Sample 8.4
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67
application increases the exposure of other session-based attacks, as for the attacker to be able to reuse a valid
session ID and hijack the associated session, it must still be active.
Remember for secure coding one of our goals is to reduce the attack surface of our application.
.Net ASPX
ASPX the developer can change the default time out for a session. This code in the web.cong le sets the
timeout session to 15 minutes. The default timeout for an aspx session is 30 minutes.
Java
PHP
Does not have a session timeout mechanism. PHP developers will need to create their own custom session timeout.
Session Logout/Ending.
Web applications should provide mechanisms that allow security aware users to actively close their session once they
have nished using the web application.
.Net ASPX Session.Abandon() method destroys all the objects stored in a Session object and releases their resources.
If you do not call the Abandon method explicitly, the server destroys these objects when the session times out. You
should use it when the user logs out. Session.Clear() Removes all keys and values from the session. Does not change
session ID. Use this command if you if you don’t want the user to relogin and reset all the session specic data.
Session Attacks
Generally three sorts of session attacks are possible:
• Session Hijacking: stealing someone’s session-id, and using it to impersonate that user.
• Session Fixation: setting someone’s session-id to a predened value, and impersonating them using that
known value
• Session Elevation: when the importance of a session is changed, but its ID is not.
<system.web>
<sessionState
mode=”InProc”
cookieless=”true”
timeout=”15” />
</system.web>
Sample 8.5
<session-cong>
<session-timeout>1</session-timeout>
</session-cong>
Sample 8.6
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68
Session Hijacking
• Mostly done via XSS attacks, mostly can be prevented by HTTP-Only session cookies (unless Javascript code
requires access to them).
• It’s generally a good idea for Javascript not to need access to session cookies, as preventing all avors of XSS
is usually the toughest part of hardening a system.
• Session-ids should be placed inside cookies, and not in URLs. URL information are stored in browser’s history,
and HTTP Referrers, and can be accessed by attackers.
• As cookies can be accessed by default from javascript and preventing all avors of XSS is usually the toughest
part of hardening a system, there is an attribute called “HTTPOnly”, that forbids this access. The session cookie
should has this attribute set. Anyway, as there is no need to access a session cookie from the client, you
should get suspicious about client side code that depends on this access.
• Geographical location checking can help detect simple hijacking scenarios. Advanced hijackers use the
sameIP (or range) of the victim.
• An active session should be warned when it is accessed from another location.
• An active users should be warned when s/he has an active session somewhere else (if the policy allows
multiple sessions for a single user).
Session Fixation
• If the application sees a new session-id that is not present in the pool, it should be rejected and a new
session-id should be advertised. This is the sole method to prevent xation.
• All the session-ids should be generated by the application, and then stored in a pool to be checked later for.
Application is the sole authority for session generation.
Session Elevation
• Whenever a session is elevated (login, logout, certain authorization), it should be rolled.
• Many applications create sessions for visitors as well (and not just authenticated users). They should
denitely roll the session on elevation, because the user expects the application to treat them securely after
they login.
• When a down-elevation occurs, the session information regarding the higher level should be ushed.
• Sessions should be rolled when they are elevated. Rolling means that the session-id should be changed, and
the session information should be transferred to the new id.
Server-Side Defenses for Session Management
.NET ASPX
Generating new session Id’s helps prevent, session rolling, xation, hijacking.
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69
Java
PHP.INI
References
• https://wwww.owasp.org/index.php/SecureFlag
public class GuidSessionIDManager : SessionIDManager {
public override string CreateSessionID(HttpContext context){
return Guid.NewGuid().ToString();
}
public override bool Validate(string id) {
try{
Guid testGuid = new Guid(id);
if (id == testGuid.ToString())
return true;
}catch(Exception e) { throw e }
return false;
}
}
Sample 8.7
request.getSession(false).invalidate();
//and then create a new session with
getSession(true) (getSession())
Sample 8.8
session.use_trans_sid = 0
session.use_only_cookies
Sample 8.9
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70
A3
71
9.1 Overview
What is Cross-Site Scripting (XSS)?
Cross-site scripting (XSS) is a type of coding vulnerability. It is usually found in web applications. XSS enables attack-
ers to inject malicious into web pages viewed by other users. XSS may allow attackers to bypass access controls such
as the same-origin policy may. This is one of the most common vulnerabilities found accordingly with OWASP Top
10. Symantec in its annual threat report found that XSS was the number two vulnerability found on web servers. The
severity/risk of this vulnerability may range from a nuisance to a major security risk, depending on the sensitivity of
the data handled by the vulnerable site and the nature of any security mitigation implemented by the site’s organi-
zation.
Description
There are three types of XSS, Reected XSS (Non-Persistent), Stored XSS(Persistent), and DOM based XSS. Each of
these types has dierent means to deliver a malicious payload to the server. The important takeaway is the conse-
quences are the same.
What to Review
Cross-site scripting vulnerabilities are dicult to identify and remove from a web application
Cross-site scripting aws can be dicult to identify and remove from a web application. The best practice to search
for aws is to perform an intense code review and search for all places where user input through a HTTP request
could possibly make its way into the HTML output.
Code reviewer needs to closely review.
1. That untrusted data is not transmitted in the same HTTP responses as HTML or JavaScript.
2. When data is transmitted from the server to the client, untrusted data must be properly encoded and the HTTP
response. Do not assume data from the server is safe. Best practice is to always check data.
3. When introduced into the DOM, untrusted data MUST be introduced using one of the following APIs:
a. Node.textContent
b. document.createTextNode
c. Element.setAttribute (second parameter only)
Code reviewer should also be aware of the HTML tags (such as <img src…>, <iframe…>, <bgsound src…> etc.) can
be used to transmit malicious JavaScript.
Web application vulnerability automated tools/scanners can help to nd Cross-Site scripting aws. However they
cannot nd all XSS vulnerabilities, hence manual code reviews are important. Manual code reviews wont catch
all either but a defense in depth approach is always the best approach based on your level of risk.
OWASP Zed Attack Proxy(ZAP) is an easy to use integrated penetration-testing tool for nding vulnerabilities in web
applications. ZAP provides automated scanners as well as a set of tools that allow you to nd security vulnerabilities
manually. It acts as a web proxy that you point your browser to so it can see the trac going to a site and allows you
to spider, scan, fuzz, and attack the application. There are other scanners available both open source and commercial.
CROSS-SITE SCRIPTING (XSS)
A3
A3 - Cross-Site Scripting (XSS)
72
Use Microsft’s Anti-XSS library
Another level of help to prevent XSS is to use an Anti-XSS library.
Unfortunately, HtmlEncode or validation feature is not enough to deal with XSS, especially if the user input
needs to be added to JavaScript code, tag attributes, XML or URL. In this case a good option is the Anti-XSS
libray
.NET ASPX
1. On ASPX .Net pages code review should check to make sure web cong le does not turn o page
validation. <pages validateRequest=”false” />
2. .Net framework 4.0 does not allow page validation to be turned o. Hence if the programmer wants to turn of
page validation the developer will need to regress back to 2.0 validation mode. <httpRuntime requestValidation-
Mode=”2.0” />
3. Code reviewer needs to make sure page validation is never turned o on anywhere and if it is understand
why and the risks it opens the organization to. <%@ Page Language=”C#” ValidationRequest=”false”
.NET MVC
When MVC web apps are exposed to malicious XSS code, they will throw an error like the following one:
To avo id th is vu ln era bi lity, m ake s ure t he foll ow in g co de i s in cl ude d:
<%server.HtmlEncode(stringValue)%>
The HTMLEncode method applies HTML encoding to a specied string. This is useful as a quick method of
encoding form data and other client request data before using it in your Web application. Encoding data con-
verts potentially unsafe characters to their HTML-encoded equivalent.(MSDN,2013)
JavaScript and JavaScript Frameworks
Both Javascript and Javascript frameworks are now widely use in web applications today. This hinders the
code reviewer in knowing what frameworks do a good job on preventing XSS aws and which ones don’t.
Code reviewer should check to see what to see if any CVE exists for the framework being used and also check
that the javascript framework is the latest stable version.
Figure 6: Example .Net XSS Framework Error
73
OWASP References
• OWASP XSS Prevention Cheat Sheet
• OWASP XSS Filter Evasion Cheat Sheet
• OWASP DOM based XSS Prevention Cheat Sheet
• Testing Guide: 1st 3 chapters on Data Validation Testing
• OWASP Zed Attack Proxy Project
External References
• https://www4.symantec.com/mktginfo/whitepaper/ISTR/21347932_GA-internet-security-threat-re-
port-volume-20-2015-social_v2.pdf
• https://cwe.mitre.org/data/denitions/79.html
• http://webblaze.cs.berkeley.edu/papers/scriptgard.pdf
• http://html5sec.org
• https://cve.mitre.org
9.2 HTML Attribute Encoding
HTML attributes may contain untrusted data. It is important to determine if any ot the HTML attributes on a
given page contains data from outside the trust boundary.
Some HTML attributes are considered safer than others such as align, alink, alt, bgcolor, border, cellpadding,
cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth,
multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabin-
dex, title, usemap, valign, value, vlink, vspace, width.
When reviewing code for XSS we need to look for HTML attributes such as the following reviewing code for
XSS we need to look for HTML attributes such as the following:
<input type=”text” name=”fname” value=”UNTRUSTED DATA”>
Attacks may take the following format:
“><script>/* bad stu */</script>
What is Attribute encoding?
HTML attribute encoding replaces a subset of characters that are important to prevent a string of characters
from breaking the attribute of an HTML element.
This is because the nature of attributes, the data they contain, and how they are parsed and interpreted by
a browser or HTML parser is dierent than how an HTML document and its elements are read; OWASP XSS
Prevention Cheat Sheet. Except for alphanumeric characters, escape all characters with ASCII values less than
256 with the &#xHH; format (or a named entity if available) to prevent switching out of the attribute. The rea-
son this rule is so broad is that developers frequently leave attributes unquoted. Properly quoted attributes
can only be escaped with the corresponding quote. Unquoted attributes can be broken out of with many
characters, including [space]% * + , - /; < = > ^ and |.
Attribute encoding may be performed in a number of ways. Two resources are:
1. HttpUtility.HtmlAttributeEncode
74
http://msdn.microsoft.com/en-us/library/wdek0zbf.aspx
2. OWASP Java Encoder Project
https://www.owasp.org/index.php/OWASP_Java_Encoder_Project
HTML Entity
HTML elements which contain user controlled data or data from untrusted sourced should be reviewed for
contextual output encoding. In the case of HTML entities we need to help ensure HTML Entity encoding is
performed:
Example HTML Entity containing untrusted data:
HTML Entity Encoding is required
It is recommended to review where/if untrusted data is placed within entity objects. Searching the source
code fro the following encoders may help establish if HTML entity encoding is being done in the application
and in a consistent manner.
OWASP Java Encoder Project
https://www.owasp.org/index.php/OWASP_Java_Encoder_Project
OWASP ESAPI
http://code.google.com/p/owasp-esapi-java/source/browse/trunk/src/main/java/org/owasp/esapi/codecs/
HTMLEntityCodec.java
JavaScript Parameters
Untrusted data, if being placed inside a JavaScript function/code requires validation. Invalidated data may
break out of the data context and wind up being executed in the code context on a users browser.
Examples of exploitation points (sinks) that are worth reviewing for:
HTML Body Context <span>UNTRUSTED DATA</span> OR <body>...UNTRUSTED DATA </body> OR
<div>UNTRUSTED DATA </div>
& --> &amp; < --> &lt; > --> &gt; “ --> &quot; ‘ --> &#x27;
<input type=”text” name=”data” value=”<%=Encode.forHtmlAttribute(dataValue)%>” />
String safe = ESAPI.encoder().encodeForHTML( request.getParameter( “input” ) );
A3 - Cross-Site Scripting (XSS)
75
<script>var currentValue=’UNTRUSTED DATA’;</script> <script>someFunction(‘UNTRUSTED DATA’);</
script> attack: ‘);/* BAD STUFF */
Potential solutions:
OWASP HTML Sanitizer ProjecT
OWASP JSON Sanitizer Project
ESAPI JavaScript escaping can be call in this manner:
Please note there are some JavaScript functions that can never safely use untrusted data as input - even if
javascript escaped!
For example (note this is an example of how NOT to use JavaScript):
eval()
jquery
Safe usage (use text, not html)
<script> window.setInterval(‘...EVEN IF YOU ESCAPE UNTRUSTED DATA YOU ARE XSSED HERE...’); </script>
String safe = ESAPI.encoder().encodeForJavaScript( request.getParameter( “input” ) );
$(“#userInput”)
var txtField = “A1”; var txtUserInput = “’test@google.ie’;alert(1);”; eval( “document.forms[0].” + txtField +
“. va l u e =” + A1 ) ;
Sample 9.1
var txtAlertMsg = “Hello World: “; var txtUserInput = “test<script>alert(1)<\/script>”; $(“#message”).html(
txtAlertMsg +”” + txtUserInput + “”);
Sample 9.2
A3 - Cross-Site Scripting (XSS)
76
.text
Nested Contexts
Best to avoid such nested contexts: an element attribute calling a JavaScript function etc. these contexts can
really mess with your mind.
When the browser processes this it will rst HTML decode the contents of the onclick attribute. It will pass the
results to the JavaScript Interpreter. So we have 2 contextx here...HTML and Javascript (2 browser parsers). We
need to apply “layered” encoding in the RIGHT order:
1. JavaScript encode
2. HTML Attribute Encode so it “unwinds” properly and is not vulnerable.
<div onclick=”showError(‘<%=request.getParameter(“errorxyz”)%>’)” >An error occurred </div>
Here we have a HTML attribute(onClick) and within a nested Javascript function call (showError).
<div onclick=”showError (‘<%= Encoder.encodeForHtml(Encoder.encodeForJavaScript( request.
getParameter(“error”)%>’)))” >An error occurred ....</div>
( “test<script>alert(1)<\/script>”);<-- treat user input as text
A3 - Cross-Site Scripting (XSS)
77
A4
78 A4 - Insecure Direct Object Reference
INSECURE DIRECT OBJECT REFERENCE
A4
10.1 Overview
Insecure Direct Object Reference is a commonplace vulnerability with web applications that provide varying
levels of access or exposes an internal object to the user. Examples of what can be exposed are database re-
cords, URLs, les, account numbers, or allowing users to bypass web security controls by manipulating URLs.
The user may be authorized to access the web application, but not a specic object, such as a database record,
specic le or even an URL. Potential threats can come from an authorized user of the web application who al-
ters a parameter value that directly points to an object that the user isn’t authorized to access. If the application
doesn’t verify the user for that specic object, it can result in an insecure direct object reference aw.
10.2 Description
The source of the problem of this risk is based on the manipulation or updating of the data generated previ-
ously at server side.
What to Review
SQL Injection
An example of an attack making use of this vulnerability could be a web application where the user has already
been validated. Now the user wants to view open his open invoices via another web page. The application
passes the account number using the URL string. The an application uses unveried data in a SQL call that is
accessing account information:
The attacker simply modies the ‘acct’ parameter in their browser to send whatever account number they
want. If the application does not perform user verication, the attacker can access any user’s account, instead
of only the intended customer’s account.
HTTP POST requests
A Cyber Security Analyst (Ibrahim Raafat) found an insecure direct object reference vulnerability with Yahoo!
Suggestions by using Live HTTP Headers check the content in the POST request he could see:
Where parameter ‘d’ is the topic id and ‘cid’ is the respective comment ID. While testing, he found changing
prop=addressbook&d=367443&crumb=Q4.PSLBfBe.&cid=1236547890&cmd= delete_comment
String query = “SELECT * FROM accts WHERE account = ?”; PreparedStatement pstmt = connection.
prepareStatement(query , ... );
pstmt.setString( 1, request.getParameter(“acct”));
ResultSet results = pstmt.executeQuery();
Sample 10.1
79
A4 - Insecure Direct Object Reference
the d and cid parameter values allow him to delete other comments from the forum, that are actually posted
by another user.
Next, he used the same method to test post deletion mechanism and found a similar vulnerability in that. A
normal HTTP Header POST request of deleting a post is:
He found that, appending the d (topic id) variable to the URL allows him to delete the respective post of other
users:
After futher analysis he found an attacker could modify the parameters in HTTP POST requests to delete 1.5
million records entered by Yahoo users
Indirect Reference Maps
Moreover, the attackers may nd out the internal naming conventions and infer the method names for oper-
ation functionality. For instance, if an application has URLs for retrieving detail information of an object like:
Attackers will try to use the following URLS to perform modication on the object:
Also if web application is returning an object listing part of the directory path or object name an attacker can
modify these.
Data Binding Technique
Another popular feature seen in most of the design frameworks (JSP/Structs, Spring) is data binding, where HTTP
GET request parameters or HTTP POST variables get directly bound to the variables of the corresponding business/
command object. Binding here means that the instance variables of such classes get automatically initialize with
the request parameter values based on their names. Consider a sample design given below; observe that the busi-
ness logic class binds the business object class binds the business object with the request parameters.
The aw in such design is that the business objects may have variables that are not dependent on the request pa-
rameters. Such variables could be key variables like price, max limit, role, etc. having static values or dependent on
some server side processing logic. A threat in such scenarios is that an attacker may supply additional parameters
in request and try to bind values for unexposed variable of business object class.
POST cmd=delete_item&crumb=SbWqLz.LDP0
POST cmd=delete_item&crumb=SbWqLz.LDP0&d=xxxxxxxx
xyz.com/Customers/View/2148102445 or xyz.com/Customers/ViewDetails.aspx?ID=2148102445
xyz.com/Customers/Update/2148102445 or xyz.com/Customers/Modify.aspx?ID=2148102445
Or xyz.com/Customers/admin
80
Secure Design Recommendation:
• An important point to be noted here is that the business/form/command objects must have only those
instance variables that are dependent on the user inputs.
• If additional variables are present those must not be vital ones like related to the business rule for the feature.
• In any case the application must accept only desired inputs from the user and the rest must be rejected or left
unbound. And initialization of unexposed of variables, if any must take place after the binding logic.
Review Criteria
Review the application design and check if it incorporates a data binding logic. In case it does, check if business
objects/beans that get bound to the request parameters have unexposed variables that are meant to have static
values. If such variables are initialized before the binding logic this attack will work successfully.
What the Code Reviewer needs to do:
Code reviewer needs to “map out all locations in the code being reviewed where user input is used to reference
objects directly”. These locations include where user input is used to access a database row, a le, application pag-
es, etc. The reviewer needs to understand if modication of the input used to reference objects can result in the
retrieval of objects the user is not authorized to view.
If untrusted input is used to access objects on the server side, then proper authorization checks must be employed
to ensure data cannot be leaked. Proper input validation will also be required to ensure the untrusted input is
properly understood and used by the server side code. Note that this authorization and input validation must be
performed on the server side; client side code can be bypassed by the attacker.
Binding issues in MVC .NET
A.K.A Over-Posting A.K.A Mass assignments
In MVC framework, mass assignments are a mechanism that allows us to update our models with data coming in a
request in HTTP form elds. As the data that needs to be updated comes in a collection of form elds, a user could
send a request and modify other elds in the model that may not be in the form and the developer didn’t intend
to be updated.
Depending on the models you create, there might be sensitive data that you would not like to be modied. The
vulnerability is exploited when a malicious user modifys a model’s elds, which are not exposed to the user via the
view, and the malicious user to change hidden model values adds additional model parameters.
public class user
{
public int ID { get; set; } <- exposed via view
public string Name { get; set; } <- exposed via view
public bool isAdmin{ get; set; } <-hidden from view
}
Sample 10.2
A4 - Insecure Direct Object Reference
81
Corresponding view (HTML)
The corresponding HTML for this model contain 2 elds: ID and Name
If an attacker adds the isAdmin parameter to the form and submits they can change the model object above.
So a malicious attacker may change isAdmin=true
Recommendations:
1 - Use a model which does not have values the user should not edit.
2 - Use the bind method and whitelist attributes which can be updated.
3 - Use the controller.UpdateModel method to exclude certain attribute updates.
References
• OWASP Top 10-2007 on Insecure Dir Object References
• ESAPI Access Reference Map API
• ESAPI Access Control API (See isAuthorizedForData(), isAuthorizedForFile(), isAuthorizedForFunction() )
• https://www.owasp.org/index.php/Category:OWASP_Application_Security_Verication_Standard_Project
• https://cwe.mitre.org/data/denitions/639.html
• https://cwe.mitre.org/data/denitions/22.html
ID: <%= Html.TextBox(“ID”) %> <br>
Name: <%= Html.TextBox(“Name”) %> <br>
<-- no isAdmin here!
Sample 10.3
A4 - Insecure Direct Object Reference
82
A5
83
SECURITY MISCONFIGURATION
A5
Many modern applications are developed on frameworks. These frameworks provide the developer less work
to do, as the framework does much of the “housekeeping”. The code developed will extend the functionality
of the framework. It is here that the knowledge of a given framework, and language in which the application
is implemented, is of paramount importance. Much of the transactional functionality may not be visible in the
developer’s code and handled in “parent” classes. The reviewer should be aware and knowledgeable of the
underlying framework.
Web applications do not execute in isolation, they typically are deployed within an application server frame-
work, running within an operating system on a physical host, within a network.
Secure operating system conguration (also called hardening) is not typically within the scope of code review.
For more information, see the Center for Internet Security operating system benchmarks.
Networks today consist of much more than routers and switches providing transport services. Filtering switch-
es, VLANs (virtual LANs), rewalls, WAFs (Web Application Firewall), and various middle boxes (e.g. reverse
proxies, intrusion detection and prevention systems) all provide critical security services when congured to
do so. This is a big topic, but outside the scope of this web application code review guide. For a good summary,
see the SANS (System Administration, Networking, and Security) Institute Critical Control 10: Secure Congu-
rations for Network Devices such as Firewalls, Routers, and Switches.
Application server frameworks have many security related capabilities. These capabilities are enabled and con-
gured in static conguration les, commonly in XML format, but may also be expressed as annotations within
the code.
11.1 Apache Struts
In struts the struts-cong.xml and the web.xml les are the core points to view the transactional functionality
of an application. The struts-cong.xml le contains the action mappings for each HTTP request while the
web.xml le contains the deployment descriptor.
The struts framework has a validator engine, which relies on regular expressions to validate the input data.
The beauty of the validator is that no code has to be written for each form bean. (Form bean is the Java object
which received the data from the HTTP request). The validator is not enabled by default in struts. To enable the
validator, a plug-in must be dened in the <plug-in> section of struts-cong.xml. The property dened tells
the struts framework where the custom validation rules are dened (validation.xml) and a denition of the
actual rules themselves (validation-rules.xml).
Without a proper understanding of the struts framework, and by simply auditing the Java code, one would not
see any validation being executed, and one does not see the relationship between the dened rules and the
Java functions.
The action mappings dene the action taken by the application upon receiving a request. Here, in sample
11.1, we can see that when the URL contains “/login” the LoginAction shall be called. From the action map-
pings we can see the transactions the application performs when external input is received.
84
11.2 Java Enterprise Edition Declarative Conguration
Some security capabilities are accessible from within a Java program. Programmatic security is done within the
web application using framework specic or standard Java Enterprise Edition (JEE) framework APIs. JEE has the
JEE security model which, uses a role-based security model in which, access to application resources is granted
based on the security role. The security role is a logical grouping of principals (authenticated entities, usually a
user), and access is declared by specifying a security constraint on the role.
The constraints and roles are expressed as deployment descriptors expressed as XML elements. Dierent types
of components use dierent formats, or schemas, for their deployment descriptors:
• Web components may use a web application deployment descriptor in the le web.xml
• Enterprise JavaBeans components may use an EJB deployment descriptor named META-INF/ejb-jar.xml
The deployment descriptor can dene resources (e.g. servlets accessible via a specic URL), which roles are
authorized to access the resource, and how access is constrained (e.g. via GET but not POST).
The example web component descriptor in sample 11.2, (included in the “web.xml” le) denes a Catalog
servlet, a “manager” role, a SalesInfo resource within the servlet accessible via GET and POST requests, and
species that only users with “manager” role, using SSL and successfully using HTTP basic authentication
should be granted access.
A5 - Security Misconguration
<?xml version=”1.0” encoding=”ISO-8859-1” ?>
<!DOCTYPE struts-cong PUBLIC
“-//Apache Software Foundation//DTD Struts Conguration 1.0//EN”
“http://jakarta.apache.org/struts/dtds/struts-cong_1_0.dtd”>
<struts-cong>
<form-beans>
<form-bean name=”login” type=”test.struts.LoginForm” />
</form-beans>
<global-forwards>
</global-forwards>
<action-mappings>
<action
path=”/login”
type=”test.struts.LoginAction” >
<forward name=”valid” path=”/jsp/MainMenu.jsp” />
<forward name=”invalid” path=”/jsp/LoginView.jsp” />
</action>
</action-mappings>
<plug-in className=”org.apache.struts.validator.ValidatorPlugIn”>
<set-property property=”pathnames”
value=”/test/WEB-INF/validator-rules.xml, /WEB-INF/validation.xml”/>
</plug-in>
</struts-cong>
Sample 11.1
85
A5 - Security Misconguration
<?xml version=”1.0” encoding=”ISO-8859-1”?>
<web-app xmlns=”http://java.sun.com/xml/ns/j2ee”
xmlns:xsi=”http://www.w3.org/2001/XMLSchema-instance”
xsi:schemaLocation=”http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_5.xsd” version=?2.5?>
<display-name>A Secure Application</display-name>
<servlet>
<servlet-name>catalog</servlet-name>
<servlet-class>com.mycorp.CatalogServlet</servlet-class>
<init-param>
<param-name>catalog</param-name>
<param-value>Spring</param-value>
</init-param>
<!-- Dene Security Roles -->
<security-role-ref>
<role-name>MGR</role-name>
<role-link>manager</role-link>
</security-role-ref>
</servlet>
<security-role>
<role-name>manager</role-name>
</security-role>
<servlet-mapping>
<servlet-name>catalog</servlet-name>
<url-pattern>/catalog/*</url-pattern>
</servlet-mapping>
<!-- Dene A Security Constraint -->
<security-constraint>
<!-- Specify the Resources to be Protected -->
<web-resource-collection>
<web-resource-name>SalesInfo</web-resource-name>
<url-pattern>/salesinfo/*</url-pattern>
<http-method>GET</http-method>
<http-method>POST</http-method>
</web-resource-collection>
<!-- Specify which Users Can Access Protected Resources -->
<auth-constraint>
<role-name>manager</role-name>
</auth-constraint>
Sample 11.2
86
Security roles can also be declared for enterprise Java beans in the “ejb-jar.xml” le as seen in gure sample 11.3.
For beans, however, rather than specifying access to resources within servlets, access to bean methods is
specied. The example in sample 11.4 illustrates several types of method access constraints for several beans.
<!-- Specify Secure Transport using SSL (condential guarantee) -->
<user-data-constraint>
<transport-guarantee>CONFIDENTIAL</transport-guarantee>
</user-data-constraint>
</security-constraint>
<!-- Specify HTTP Basic Authentication Method -->
<login-cong>
<auth-method>BASIC</auth-method>
<realm-name>le</realm-name>
</login-cong>
</web-app>
<ejb-jar>
<assembly-descriptor>
<security-role>
<description>The single application role</description>
<role-name>TheApplicationRole</role-name>
</security-role>
</assembly-descriptor>
</ejb-jar>
Sample 11.3
<ejb-jar>
<assembly-descriptor>
<method-permission>
<description>The employee and temp-employee roles may access any
method of the EmployeeService bean </description>
<role-name>employee</role-name>
<role-name>temp-employee</role-name>
<method>
<ejb-name>EmployeeService</ejb-name>
<method-name>*</method-name>
</method>
</method-permission>
Sample 11.4
A5 - Security Misconguration
87
<method-permission>
<description>The employee role may access the ndByPrimaryKey,
getEmployeeInfo, and the updateEmployeeInfo(String) method of
the AardvarkPayroll bean </description>
<role-name>employee</role-name>
<method>
<ejb-name>AardvarkPayroll</ejb-name>
<method-name>ndByPrimaryKey</method-name>
</method>
<method>
<ejb-name>AardvarkPayroll</ejb-name>
<method-name>getEmployeeInfo</method-name>
</method>
<method>
<ejb-name>AardvarkPayroll</ejb-name>
<method-name>updateEmployeeInfo</method-name>
<method-params>
<method-param>java.lang.String</method-param>
</method-params>
</method>
</method-permission>
<method-permission>
<description>The admin role may access any method of the
EmployeeServiceAdmin bean </description>
<role-name>admin</role-name>
<method>
<ejb-name>EmployeeServiceAdmin</ejb-name>
<method-name>*</method-name>
</method>
</method-permission>
<method-permission>
<description>Any authenticated user may access any method of the
EmployeeServiceHelp bean</description>
<unchecked/>
<method>
<ejb-name>EmployeeServiceHelp</ejb-name>
<method-name>*</method-name>
</method>
</method-permission>
<exclude-list>
<description>No reTheCTO methods of the EmployeeFiring bean may be
used in this deployment</description>
<method>
<ejb-name>EmployeeFiring</ejb-name>
<method-name>reTheCTO</method-name>
</method>
</exclude-list>
A5 - Security Misconguration
88
If XML deployment descriptors are used to secure the application, code review should include the “web.xml”
and “ejb-jar.xml” les to ensure that access controls are properly applied to the correct roles, and authentica-
tion methods are as expected.
11.3 JEE Annotations
JEE annotations for security are dened in the Java Docs [2]. The available annotations are:
• @DeclareRoles
• @DenyAll - no roles may invoke the method.
• @PermitAll - all roles may invoke the method.
• @RolesAllowed - roles permitted to invoke the method.
• @RunAs - dynamically run the method as a particular role.
For example the code in sample 11.5 allows employees and managers to add movies to the persistent store,
anyone to list movies, but only managers may delete movies.
Code review should look for such annotations. If present, ensure they reect the correct roles and permissions,
and are consistent with any declared role permissions in the “ejb-jar.xml” le.
</assembly-descriptor>
</ejb-jar>
public class Movies {
private EntityManager entityManager;
@RolesAllowed({“Employee”, “Manager”})
public void addMovie(Movie movie) throws Exception {
entityManager.persist(movie);
}
@RolesAllowed({“Manager”})
public void deleteMovie(Movie movie) throws Exception {
entityManager.remove(movie);
}
@PermitAll
public List<Movie> getMovies() throws Exception {
Query query = entityManager.createQuery(“SELECT m from Movie as m”);
return query.getResultList();
}
}
Sample 11.5
A5 - Security Misconguration
89
11.4 Framework Specic Conguration
Apache Tomcat
The server.xml[3] le should be reviewed to ensure security related parameters are congured as expected. The tom-
cat server.xml le denes many security related parameters.
Filters are especially powerful, and a code review should validate they are used unless there is a compelling reason
not to.
Jetty
Jetty adds several security enhancements:
• Limiting form content
• Obfuscating passwords
The maximum form content size and number of form keys can be congured at server and web application level
in the “jetty-web.xml” le.
<Congure class=”org.eclipse.jetty.webapp.WebAppContext”>
…
<Set name=”maxFormContentSize”>200000</Set>
<Set name=”maxFormKeys”>200</Set>
</Congure>
<congure class=”org.eclipse.jetty.server.Server”>
...
<Call name=”setAttribute”>
<Arg>org.eclipse.jetty.server.Request.maxFormContentSize</Arg>
<Arg>100000</Arg>
</Call>
<Call name=”setAttribute”>
<Arg>org.eclipse.jetty.server.Request.maxFormKeys</Arg>
<Arg>2000</Arg>
</Call>
</congure>
Sample 11.6
Parameter Description
Server This is the shutdown port.
Connectors maxPostSize, maxParameterCount, server, SSLEnabled, secure, ciphers.
Host
Context
Filter
autoDeploy, deployOnStartup, deployXML
crossContext, privileged, allowLinking
Tomcat provides a number of lters which may be congured to incoming requests
Table 12: Apache Tomcat Security Parameters
A5 - Security Misconguration
90
Jetty also supports the use of obfuscated passwords in jetty XML les where a plain text password is usually
needed. sample 11.7 shows example code setting the password for a JDBC Datasource with obfuscation (the
obfuscated password is generated by Jetty org.eclipse.jetty.util.security.Password utility).
JBoss AS
JBoss Application Server, like Jetty, allows password obfuscation (called password masking in JBoss) in its XML
conguration les. After using JBoss password utility to create password mask, replace any occurrence of a
masked password in XML conguration les with the following annotation.
See Masking Passwords in XML Conguration in the JBoss Security Guide.
<New id=”DSTest” class=”org.eclipse.jetty.plus.jndi.Resource”>
<Arg></Arg>
<Arg>jdbc/DSTest</Arg>
<Arg>
<New class=”com.jolbox.bonecp.BoneCPDataSource”>
<Set name=”driverClass”>com.mysql.jdbc.Driver</Set>
<Set name=”jdbcUrl”>jdbc:mysql://localhost:3306/foo</Set>
<Set name=”username”>dbuser</Set>
<Set name=”password”>
<Call class=”org.eclipse.jetty.util.security.Password” name=”deobfuscate”>
<Arg>OBF:1ri71v1r1v2n1ri71shq1ri71shs1ri71v1r1v2n1ri7</Arg>
</Call>
</Set>
<Set name=”minConnectionsPerPartition”>5</Set>
<Set name=”maxConnectionsPerPartition”>50</Set>
<Set name=”acquireIncrement”>5</Set>
<Set name=”idleConnectionTestPeriod”>30</Set>
</New>
</Arg>
</New>
Sample 11.7
<annotation>@org.jboss.security.integration.password.Password
(securityDomain=MASK_NAME,methodName=setPROPERTY_NAME)
</annotation>
Sample 11.8
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91
Oracle WebLogic
WebLogic server supports additional deployment descriptors in the “weblogic.xml” le as shown in table 13.
More information on WebLogic additional deployment descriptors may be found at weblogic.xml Deployment
Descriptors.
For general guidelines on securing web applications running within WebLogic, see the Programming WebLog-
ic Security guide and the NSA’s BEA WebLogic Platform Security Guide.
11.5 Programmatic Conguration: J2EE
The J2EE API for programmatic security consists of methods of the EJBContext interface and the HttpServle-
tRequest interface. These methods allow components to make business-logic decisions based on the security
role of the caller or remote user (there are also methods to authenticate users, but that is outside the scope of
secure deployment conguration).
The J2EE APIs that interact with J2EE security conguration include:
• getRemoteUser, which determines the user name with which the client authenticated
• isUserInRole, which determines whether a remote user is in a specic security role.
• getUserPrincipal, which determines the principal name of the current user and returns a java.security.
Use of these programmatic APIs should be reviewed to ensure consistency with the conguration. Specically,
the security-role-ref element should be declared in the deployment descriptor with a role-name subelement
containing the role name to be passed to the isUserInRole method.
The code in sample 11.9 demonstrates the use of programmatic security for the purposes of programmatic
login and establishing identities and roles. This servlet does the following:
• displays information about the current user.
• prompts the user to log in.
• prints out the information again to demonstrate the eect of the login method.
Parameter Description
externally-dened Role to principal mappings are externally dened in WebLogic Admin Console
run-as-principal-name Assign a principal to a role when running as that role
run-as-role-assignment
security-permission
Filter
security-permission-spec
security-role-assignment
Contains the run-as-principal-name descriptor
Contains security-permission-spec descriptor
Tomcat provides a number of lters which may be congured to incoming requests
Specify application permissions [6]
Explicitly assign principals to a role
Table 13: Weblogic Security Parameters
A5 - Security Misconguration
92
• Iogs the user out.
• prints out the information again to demonstrate the eect of the logout method.
package enterprise.programmatic_login;
import java.io.*;
import java.net.*;
import javax.annotation.security.DeclareRoles;
import javax.servlet.*;
import javax.servlet.http.*;
@DeclareRoles(“javaee6user”)
public class LoginServlet extends HttpServlet {
/**
* Processes requests for both HTTP GET and POST methods.
* @param request servlet request
* @param response servlet response
*/
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType(“text/html;charset=UTF-8”);
PrintWriter out = response.getWriter();
try {
String userName = request.getParameter(“txtUserName”);
String password = request.getParameter(“txtPassword”);
out.println(“Before Login” + “<br><br>”);
out.println(“IsUserInRole?..”
+ request.isUserInRole(“javaee6user”)+”<br>”);
out.println(“getRemoteUser?..” + request.getRemoteUser()+”<br>”);
out.println(“getUserPrincipal?..”
+ request.getUserPrincipal()+”<br>”);
out.println(“getAuthType?..” + request.getAuthType()+”<br><br>”);
try {
request.login(userName, password);
} catch(ServletException ex) {
out.println(“Login Failed with a ServletException..”
+ ex.getMessage());
return;
}
out.println(“After Login...”+”<br><br>”);
out.println(“IsUserInRole?..”
Sample 11.9
A5 - Security Misconguration
93
More detailed information can be found in the Java EE Tutorial: Using Programmatic Security with Web Appli-
cations.
11.6 Microsoft IIS
ASP.NET / IIS applications use an optional XML-based conguration le, named web.cong, to maintain ap-
plication conguration settings. This covers issues such as authentication, authorization, error pages, HTTP
settings, debug settings, web service settings, etc. Without knowledge of these les, a transactional analysis
would be very dicult and not accurate.
In IIS 7 there is a conguration system which aects the hierarchy level and how one le can inherit from
another. The following gure shows how this will work and the location of each le (Aguilar, 2006)
+ request.isUserInRole(“javaee6user”)+”<br>”);
out.println(“getRemoteUser?..” + request.getRemoteUser()+”<br>”);
out.println(“getUserPrincipal?..”
+ request.getUserPrincipal()+”<br>”);
out.println(“getAuthType?..” + request.getAuthType()+”<br><br>”);
request.logout();
out.println(“After Logout...”+”<br><br>”);
out.println(“IsUserInRole?..”
+ request.isUserInRole(“javaee6user”)+”<br>”);
out.println(“getRemoteUser?..” + request.getRemoteUser()+”<br>”);
out.println(“getUserPrincipal?..”
+ request.getUserPrincipal()+”<br>”);
out.println(“getAuthType?..” + request.getAuthType()+”<br>”);
} nally {
out.close();
}
}
...
}
machine.cong (root) web.cong
ApplicationHost.cong
windows\system32\intsrv
web.cong
c:\inetpub\wwwroot
http://localhost
web.cong
d:\MyApp
http://localhost/MyApp
Figure 7: IIS Conguration Files
A5 - Security Misconguration
94
It is possible to provide a le web.cong at the root of the virtual directory for a web application. If the le is
absent, the default conguration settings in machine.cong will be used. If the le is present, any settings in
web.cong will override the default settings.
Many of the important security settings are not set in the code, but in the framework conguration les.
Knowledge of the framework is of paramount importance when reviewing framework-based applications.
Some examples of framework specic parameters in the web.cong le are shown in table 14.
11.7 Framework Specic Conguration: Microsoft IIS
Security features can be congured in IIS using the Web.cong (application level) or ApplicationHost.cong (server
level) le, in the <system.webServer><security> section. The types of features that may be congured include:
• Permitted authentication methods
• Authorization rules
• Request lters and limits
• Use of SSL
• Source IP address ltering
• Error handling
The Web.cong and ApplicationHost.cong les should be included in code review. The <system.web-
<authentication mode=”Forms”>
<forms name=”name”
loginUrl=”url”
protection=”Encryption”
timeout=”30” path=”/”
requireSSL=”true|”
slidingExpiration=”false”>
<credentials passwordFormat=”Clear”>
<user name=”username” password=”password”/>
</credentials>
</forms>
<passport redirectUrl=”internal”/>
</authentication>
Sample 11.10
Parameter Description
authentication mode The default authentication mode is ASP.NET forms-based authentication.
loginUrl Species the URL where the request is redirected for login if no valid authentication cookie is found.
protection
timeout
Species that the cookie is encrypted using 3DES or DES but DV is not performed on the cookie. Beware of plaintext
attacks.
Cookie expiry time in minutes.
Table 14: Parameters In The Web.cong File
A5 - Security Misconguration
95
Server><security> sections should be reviewed to ensure all security conguration is as expected.
For guidelines on securing the overall conguration of Microsoft IIS, see the IIS supports basic, client certicate,
digest, IIS client certicate, and Windows authentication methods. They are congured in the <system.web-
Server><security><authentication> section.
The example in sample 11.11 disables anonymous authentication for a site named MySite, then enables both
basic authentication and windows authentication for the site.
IIS authorization conguration allows specication of users access to the site or server and is congured in the
<system.webServer><security><authorization> section.
The conguration in sample 11.12 removes the default IIS authorization settings, which allows all users access
to Web site or application content, and then congures an authorization rule that allows only users with ad-
ministrator privileges to access the content.
IIS supports ltering, including enforcing limits, on incoming HTTP requests. Table 15 shows many of the IIS
security parameters that can be set.
<location path=”MySite”>
<system.webServer>
<security>
<authentication>
<anonymousAuthentication enabled=”false” />
<basicAuthentication enabled=”true” defaultLogonDomain=”MySite” />
<windowsAuthentication enabled=”true” />
</authentication>
</security>
</system.webServer>
</location>
Sample 11.11
<conguration>
<system.webServer>
<security>
<authorization>
<remove users=”*” roles=”” verbs=”” />
<add accessType=”Allow” users=”” roles=”Administrators” />
</authorization>
</security>
</system.webServer>
</conguration
Sample 11.12
A5 - Security Misconguration
96
These parameters are congured in the <system.webServer><security><requestFiltering> section. The ex-
ample in sample 11.13:
• Denies access to two URL sequences. The rst sequence prevents directory transversal and the second se-
quence prevents access to alternate data streams.
• Sets the maximum length for a URL to 2KB and the maximum length for a query string to 1KB.
<conguration>
<system.webServer>
<security>
<requestFiltering>
<denyUrlSequences>
<add sequence=”..” />
<add sequence=”:” />
</denyUrlSequences>
<leExtensions allowUnlisted=”false” />
<requestLimits maxUrl=”2048” maxQueryString=”1024” />
<verbs allowUnlisted=”false” />
</requestFiltering>
</security>
</system.webServer>
</conguration>
Sample 11.13
Parameter Function
denyUrlSequences A list of prohibited URL patterns
leExtensions Allowed or prohibited le extensions
hiddenSegments
requestLimits
verbs
alwaysAllowedUrls
alwaysAllowedQueryStrings
denyQueryStringSequences
lteringRules
URLs that cannot be browsed
URL, content, query string, and HTTP header length limits
Allowed or prohibited verbs
URLs always permitted
Query strings always allowed
Prohibited query strings
Custom ltering rules
Table 15: IIS Security Parameters
A5 - Security Misconguration
97
• Denies access to unlisted le name extensions and unlisted HTTP verbs.
IIS allows specifying whether SSL is supported, is required, whether client authentication is supported or re-
quired, and cipher strength. It is congured in the <system.webServer><security><access> section. The ex-
ample in gure A5.13 species SSL as required for all connections to the site MySite.
IIS allows restrictions on source IP addresses or DNS names. It is congured in the <system.webServer><se-
curity><ipSecurity> section as shown in sample 11.15 where the example conguration denies access to the
IP address 192.168.100.1 and to the entire 169.254.0.0 network:
Detailed information on IIS security conguration can be found at IIS Security Conguration. Specic security
feature conguration information can be found at Authentication, Authorization, SSL, Source IP, Request Filter-
ing, and Custom Request Filtering[12].
11.8 Programmatic Conguration: Microsoft IIS
Microsoft IIS security conguration can also be programmatically set from various languages:
• appcmd.exe set cong
• C#
• Visual Basic
• JavaScript
For example, disabling anonymous authentication for a site named MySite, then enabling both basic authenti-
cation and windows authentication for the site (as done via conguration in the section above) can be accom-
<location path=”MySite”>
<system.webServer>
<security>
<access sslFlags=”ssl”>
</security>
</system.webServer>
</location>
Sample 11.14
<location path=”Default Web Site”>
<system.webServer>
<security>
<ipSecurity>
<add ipAddress=”192.168.100.1” />
<add ipAddress=”169.254.0.0” subnetMask=”255.255.0.0” />
</ipSecurity>
</security>
</system.webServer>
</location>
Sample 11.15
A5 - Security Misconguration
98
plished from the command line using the commands in gure sample 11.16.
Alternatively the same authentication setup can be coded programmatically as in sample 11.17.
When reviewing source code, special attention should be paid to conguration updates in security sections.
11.9 Further IIS Congurations
Filtering Requests and URL Rewriting
appcmd.exe set cong “MySite” -section:system.webServer/security/authentication
/anonymousAuthentication /enabled:”False” /commit:apphost
appcmd.exe set cong “MySite” -section:system.webServer/security/authentication
/basicAuthentication /enabled:”True” /commit:apphost
appcmd.exe set cong “MySite” -section:system.webServer/security/authentication
/windowsAuthentication /enabled:”True” /commit:apphost
Sample 11.16
using System;
using System.Text;
using Microsoft.Web.Administration;
internal static class Sample {
private static void Main() {
using(ServerManager serverManager = new ServerManager()) {
Conguration cong = serverManager.GetApplicationHostConguration();
CongurationSection anonymousAuthenticationSection =
cong.GetSection(“system.webServer/security/authentication
/anonymousAuthentication”, “MySite”);
anonymousAuthenticationSection[“enabled”] = false;
CongurationSection basicAuthenticationSection =
cong.GetSection(“system.webServer/security/authentication
/basicAuthentication”, “MySite”);
basicAuthenticationSection[“enabled”] = true;
CongurationSection windowsAuthenticationSection =
cong.GetSection(“system.webServer/security/authentication
/windowsAuthentication”, “MySite”);
windowsAuthenticationSection[“enabled”] = true;
serverManager.CommitChanges();
}
}
}
Sample 11.17
A5 - Security Misconguration
99
Request Filtering was introduced in IIS7 and it has replaced the functionality UrlScan add-on for IIS 6.0. This
built-in security feature allows to lter undesired URL request but it is also possible to congure dierent kinds
of ltering. To begin with, it is important to understand how the IIS pipeline works when a request is done. The
following diagram shows the order in these modules
HTTP
RESPONSE
BEGIN REQUEST
AUTHENTICATE REQUEST
AUTHORIZE REQUEST
RESOLVE CACHE
END REQUEST
REQUEST FILTERING
(HIGH PRIORITY)
URL REWRITE MODULE
(MEDIUM PRIORITY)
HTTP
REQUEST
(Yakushev, 2008)
Figure 8: IIS Request FilteringFiles
A5 - Security Misconguration
100
Request ltering can be setup through the IIS interface or on the web.cong le. Example:
(Yakushev, 2008)
<conguration>
<system.webServer>
<security>
<requestFiltering>
<denyUrlSequences>
<add sequence=”..” />
<add sequence=”:” />
</denyUrlSequences>
<leExtensions allowUnlisted=”false” />
<requestLimits maxUrl=”2048” maxQueryString=”1024” />
<verbs allowUnlisted=”false” />
</requestFiltering>
</security>
</system.webServer>
</conguration>
Sample 11.18
using System;
using System.Text;
using Microsoft.Web.Administration;
internal static class Sample
{
private static void Main()
{
using (ServerManager serverManager = new ServerManager())
{
Conguration cong = serverManager.GetWebConguration(“Default Web Site”);
CongurationSection requestFilteringSection = cong.GetSection(“system.webServer/security
/requestFiltering”);
CongurationElementCollection denyUrlSequencesCollection =
requestFilteringSection.GetCollection(“denyUrlSequences”);
CongurationElement addElement = denyUrlSequencesCollection.CreateElement(“add”);
addElement[“sequence”] = @”..”;
denyUrlSequencesCollection.Add(addElement);
CongurationElement addElement1 = denyUrlSequencesCollection.CreateElement(“add”);
addElement1[“sequence”] = @”:”;
denyUrlSequencesCollection.Add(addElement1);
CongurationElement addElement2 = denyUrlSequencesCollection.CreateElement(“add”);
addElement2[“sequence”] = @”\”;
denyUrlSequencesCollection.Add(addElement2);
Sample 11.19
A5 - Security Misconguration
101
This can also be done through the application code, for example:
(Yakushev, 2008)
Filtering Double - Encoded Requests
This attack technique consists of encoding user request parameters twice in hexadecimal format in order to
bypass security controls or cause unexpected behavior from the application. It’s possible because the web-
server accepts and processes client requests in many encoded forms.
By using double encoding it’s possible to bypass security lters that only decode user input once. The second
decoding process is executed by the backend platform or modules that properly handle encoded data, but
don’t have the corresponding security checks in place.
Attackers can inject double encoding in pathnames or query strings to bypass the authentication schema and
security lters in use by the web application.
There are some common character that are used in Web applications attacks. For example, Path Traversal at-
tacks use “../” (dot-dot-slash) , while XSS attacks use “<” and “>” characters. These characters give a hexadeci-
mal representation that diers from normal data.
For example, “../” (dot-dot-slash) characters represent %2E%2E%2f in hexadecimal representation. When the
% symbol is encoded again, its representation in hexadecimal code is %25. The result from the double encod-
ing process ”../”(dot-dot-slash) would be %252E%252E%252F:
• The hexadecimal encoding of “../” represents “%2E%2E%2f”
• Then encoding the “%” represents “%25”
• Double encoding of “../” represents “%252E%252E%252F”
If you do not want IIS to allow doubled-encoded requests to be served, use the following (IIS Team,2007):
serverManager.CommitChanges();
}
}
}
<conguration>
<system.webServer>
<security>
<requestFiltering
allowDoubleEscaping=”false”>
</requestFiltering>
</security>
</system.webServer>
</conguration>
Sample 11.20
A5 - Security Misconguration
102
Filter High Bit Characters
This allows or rejects all requests to IIS that contain non-ASCII characters . When this occurs error code 404.12.
is displayed to the user . The UrlScan (IIS6 add-on) equivalent is AllowHighBitCharacters.
Filter Based on File Extensions
Using this lter you can allow IIS to a request based on le extensions, the error code logged is 404.7. The Al-
lowExtensions and DenyExtensions options are the UrlScan equivalents.
Filter Based on Request Limits
When IIS rejects a request based on request limits, the error code logged is:
• 404.13 if the content is too long.
• 404.14 if the URL is too large.
• 404.15 if the query string is too long.
This can be used to limit a long query string or too much content sent to an application which you cannot
change the source code to x the issue.
<conguration>
<system.webServer>
<security>
<requestFiltering
allowHighBitCharacters=”true”>
</requestFiltering>
</security>
</system.webServer>
</conguration>
Sample 11.21
<conguration>
<system.webServer>
<security>
<requestFiltering>
<leExtensions allowUnlisted=”true” >
<add leExtension=”.asp” allowed=”false”/>
</leExtensions>
</requestFiltering>
</security>
</system.webServer>
</conguration>
Sample 11.22
A5 - Security Misconguration
103
Filter by Verbs
When IIS reject a request based on this feature, the error code logged is 404.6. This corresponds to the UseAl-
lowVerbs, AllowVerbs, and DenyVerbs options in UrlScan.
In case you want the application to use only certain type of verb, it is necessary to rt set the allowUnlisted to
‘false’ and then set the verbs that you would like to allow (see example)
Filter Based on URL Sequences
This feature denes a list of sequences that IIS can reject when it is part of a request. When IIS reject a request
for this feature, the error code logged is 404.5.This corresponds to the DenyUrlSequences feature in UrlScan.
This is a very powerful feature. This avoids a given character sequence from ever being attended by IIS:
<conguration>
<system.webServer>
<security>
<requestFiltering>
<requestLimits
maxAllowedContentLength=”30000000”
maxUrl=”260”
maxQueryString=”25”
/>
</requestFiltering>
</security>
</system.webServer>
</conguration>
Sample 11.23
<conguration>
<system.webServer>
<security>
<requestFiltering>
<verbs allowUnlisted=”false”>
<add verb=”GET” allowed=”true” />
</verbs>
</requestFiltering>
</security>
</system.webServer>
</conguration>
Sample 11.24
<conguration>
<system.webServer>
Sample 11.25
A5 - Security Misconguration
104
Filter Out Hidden Segments
In case you want IIS to serve content in binary directory but not the binary, you can apply this conguration.
Password protection and sensitive information
The web.cong les might include sensitive information in the connection strings such as database passwords,
mail server user names among others.
Sections that are required to be encrypted are:
• <appSettings>. This section contains custom application settings.
• <connectionStrings>. This section contains connection strings.
• <identity>. This section can contain impersonation credentials.
• <sessionState>. This section contains the connection string for the out-of-process session state provider.
Passwords and user names contained in a <connectionstring> section should be encrypted. ASP.NET allows
you to encrypt this information by using the functionality aspnet_regiis .This utility is found in the installed
.NET framework under the folder
%windows%\Microsoft.NET\Framework\v2.0.50727
You can specify the section you need to encrypt by using the command:
aspnet_regiis -pef sectiontobeencryoted
Encrypting sections in Web.Cong le
Even though encrypting sections is possible, not all sections can be encrypted, specically sections that are
<security>
<requestFiltering>
<denyUrlSequences>
<add sequence=”..”/>
</denyUrlSequences>
</requestFiltering>
</security>
</system.webServer>
</conguration>
<conguration>
<system.webServer>
<security>
<requestFiltering>
<hiddenSegments>
<add segment=”BIN”/>
</hiddenSegments>
</requestFiltering>
</security>
</system.webServer>
</conguration>
Sample 11.26
A5 - Security Misconguration
105
read before user code is run. The following sections cannot be encrypted:
• <processModel>
• <runtime>
• <mscorlib>
• <startup>
• <system.runtime.remoting>
• <congProtectedData>
• <satelliteassemblies>
• <cryptographySettings>
• <cryptoNameMapping>
• <cryptoClasses>
Machine-Level RSA key container or User-Level Key Containers
Encrypting a single le using machine-level RSA key has its disadvantages when this le is moved to other
servers. In this case, user-level RSA key container is strongly advised. The RSAProtectedCongurationProvider
supports machine-level and user-level key containers for key storage.
RSA machine key containers are stored in the following folder:
\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\MachineKeys
User Key Container
When the application that needs to be protected is in a shared hosting environment and protection of sensi-
tive data cannot be accessible to other applications, the user key container is strongly recommended.
In this case each application should have a separate identity.
RSA user-level key containers are stored in the following folder:
\Documents and Settings\{UserName}\Application Data\Microsoft\Crypto\RSA
IIS congurations
Depending on the version of IIS that must be congured, it is important to revise some of its settings which
can comprise security in the server.
Trust level
The trust level is a set of Code Access Security permissions granted to an application within a hosting environ-
ment. These are dened using policy les. Depending on the trust level that must be congured, it is possible
to grant FULL, HIGH, MEDIUM, LOW or MINIMAL level. The ASP.NET host does not apply any additional policy
to applications that are running at the full-trust level.
Example:
<system.web>
<securityPolicy>
<trustLevel name=”Full” policyFile=”internal”/>
</securityPolicy>
</system.web>
Sample 11.27
A5 - Security Misconguration
106
Lock Trust Levels
In the .NET framework web.cong le is possible to lock applications from changing their trust level
This le is found at:
C:\Windows\Microsoft.NET\Framework\{version}\CONFIG
The following example shows how to lock 2 dierent application conguration trust levels (MSDN, 2013)
References
• Yakushev Ruslan , 2008 “IIS 7.0 Request Filtering and URL Rewriting “ available at http://www.iis.net/learn/
extensions/url-rewrite-module/iis-request-ltering-and-url-rewriting (Last accessed on 14 July, 2013)
• OWASP, 2009 “Double Encoding” available at https://www.owasp.org/index.php/Double_Encoding (Last ac-
cessed on 14 July, 2013)
• IIS Team, 2007 “Use Request Filtering “ available at http://www.iis.net/learn/manage/conguring-security/
use-request-ltering (Last accessed on 14 July, 2013)
• Aguilar Carlos ,2006 “The new Conguration System in IIS 7” available at http://blogs.msdn.com/b/carlosag/
archive/2006/04/25/iis7congurationsystem.aspx (Last accessed on 14 July, 2013)
• MSDN, 2013 . How to: Lock ASP.NET Conguration Settings available at http://msdn.microsoft.com/en-us/
library/ms178693.aspx (Last accessed on 14 July, 2013)
11.10 Strongly Named Assemblies
During the build process either QA or Developers are going to publish the code into executable formats. Usu-
ally this consists of an exe or and one or several DLL’s. During the build/publish process a decision needs to be
made to sign or not sign the code.
Signing your code is called creating “strong names” by Microsoft. If you create a project using Visual Studio and
use Microsofts “Run code analysis” most likely your will encounter a Microsoft design error if the code is not
strong named; “Warning 1 CA2210 : Microsoft.Design : Sign ‘xxx.exe’ with a strong name key.”
<conguration>
<location path=”application1” allowOverride=”false”>
<system.web>
<trust level=”High” />
</system.web>
</location>
<location path=”application2” allowOverride=”false”>
<system.web>
<trust level=”Medium” />
</system.web>
</location>
</conguration>
Sample 11.28
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Code review needs to be aware if strong naming is being used, benets and what threat vectors strong naming
helps prevent or understand the reasons for not using strong naming.
A strong name is a method to sign an assembly’s identity using its text name, version number, culture informa-
tion, a public key and a digital signature. (Solis, 2012)
• Strong naming guarantees a unique name for that assembly.
• Strong names protect the version lineage of an assembly. A strong name can ensure that no one can pro-
duce a subsequent version of your assembly. Users can be sure that a version of the assembly they are loading
comes from the same publisher that created the version the application was built with.
The above two points are very important if you are going to use Global Assembly Cache (GAC).
• Strong names provide a strong integrity check and prevent spoong. Passing the .NET Framework security
checks guarantees that the contents of the assembly have not been changed since it was built.
Note, however, that strong names in and of themselves do not imply a level of trust like that provided, for
example, by a digital signature and supporting certicate. If you use the GAC assemblies remember the assem-
blies are not veried each time they load since the GAC by design is a locked-down, admin-only store.
What strong names can’t prevent is a malicious user from stripping the strong name signature entirely, modi-
fying the assembly, or re-signing it with the malicious user’s key.
The code reviewer needs to understand how the strong name private key will be kept secure and managed.
This is crucible if you decide strong name signatures are a good t for your organization.
If principle of least privilege is used so code is not or less susceptible to be access by the hacker and the GAC is
not being used strong names provides less benets or no benets at all.
How to use Strong Naming
Signing tools
In order to create a strong name assembly there are a set of tools and steps that you need to follow
Using Visual Studio
In order to use Visual Studio to create a Strongly Named Assembly, it is necessary to have a copy of the public/
private key pair le. It is also possible to create this pair key in Visual Studio
In Visual Studio 2005, the C#, Visual Basic, and Visual J# integrated development environments (IDEs) allow you
to generate key pairs and sign assemblies without the need to create a key pair using Sn.exe (Strong Name
Tool).
These IDEs have a Signing tab in the Project Designer. . The use of the AssemblyKeyFileAttribute to identify key
le pairs has been made obsolete in Visual Studio 2005.
The following gure illustrates the process done by the compiler
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Using Strong Name tool
The Sign Tool is a command-line tool that digitally signs les, veries signatures in les, or time stamps les.
The Sign Tool is not supported on Microsoft Windows NT, Windows Me, Windows 98, or Windows 95.
In case you aren’t using the “Visual Studio Command Prompt” (Start >> Microsoft Visual Studio 2010 >> Visual
Studio Tools >> Visual Studio Command Prompt (2010)) you can locate sn.exe at %ProgramFiles%\Microsoft
SDKs\Windows\v7.0A\bin\sn.exe
The following command creates a new, random key pair and stores it in keyPair.snk.
sn -k keyPair.snk
The following command stores the key in keyPair.snk in the container MyContainer in the strong name CSP.
sn -i keyPair.snk MyContainer
The following command extracts the public key from keyPair.snk and stores it in publicKey.snk.
sn -p keyPair.snk publicKey.snk
The following command displays the public key and the token for the public key contained in publicKey.snk.
sn -tp publicKey.snk
The following command veries the assembly MyAsm.dll.
sn -v MyAsm.dll
The following command deletes MyContainer from the default CSP.
sn -d MyContainer
Using the Assembly Linker(AI.exe)
C#VB
COMPILER
• ASSEMBLY BINARY
• SIMPLE NAME
• VERSION NUMBER
• CULTURE INFO
PUBLIC KEY
PRIVATE KEY
ASSEMBLY
Manifest
Version Number
Culture Info
Public Key
Simple Name
Metadata
CIL
Digital Signature
Figure 9: C# Strong Naming
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This tool is automatically installed with Visual Studio and with the Windows SDK. To run the tool, we recom-
mend that you use the Visual Studio Command Prompt or the Windows SDK Command Prompt (CMD Shell).
These utilities enable you to run the tool easily, without navigating to the installation folder. For more informa-
tion, see Visual Studio and Windows SDK Command Prompts.
If you have Visual Studio installed on your computer:
On the taskbar, click Start, click All Programs, click Visual Studio, click Visual Studio Tools, and then click Visual
Studio Command Prompt.
-or-
If you have the Windows SDK installed on your computer:
On the taskbar, click Start, click All Programs, click the folder for the Windows SDK, and then click Command
Prompt (or CMD Shell).
At the command prompt, type the following:
al sources options
Remarks
All Visual Studio compilers produce assemblies. However if you have one or more modules (metadata without
a manifest) you can use Al.exe to create an assembly with the manifest in a separate le.
To install assemblies in the cache, remove assemblies from the cache, or list the contents of the cache, use the
Global Assembly Cache Tool (Gacutil.exe).
The following command creates an executable le t2a.exe with an assembly from the t2.netmodule module.
The entry point is the Main method in MyClass.
al t2.netmodule /target:exe /out:t2a.exe /main:MyClass.Main
Use Assembly attributes
You can insert the strong name information in the code directly. For this, depending on where the key le is
located you can use AssemblyKeyFileAttribute or AssemblyKeyNameAttribute
Use Compiler options :use /keyle or /delaysign
Safeguarding the key pair from developers is necessary to maintain and guarantee the integrity of the assem-
blies. The public key should be accessible, but access to the private key is restricted to only a few individuals.
When developing assemblies with strong names, each assembly that references the strong-named target as-
sembly contains the token of the public key used to give the target assembly a strong name. This requires that
the public key be available during the development process.
You can use delayed or partial signing at build time to reserve space in the portable executable (PE) le for the
strong name signature, but defer the actual signing until some later stage (typically just before shipping the
assembly).
You can use /keyle or /delaysign in C# and VB.NET (MSDN)
References
• http://msdn.microsoft.com/en-us/library/wd40t7ad(v=vs.80).aspx
• http://msdn.microsoft.com/en-us/library/c405shex(v=vs.110).aspx
• http://msdn.microsoft.com/en-us/library/k5b5tt23(v=vs.80).aspx
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110
• http://msdn.microsoft.com/en-us/library/t07a3dye(v=vs.80).aspx
• http://msdn.microsoft.com/en-us/library/t07a3dye(v=vs.110).aspx
11.10 Round Tripping
Round Tripping is a reverse engineering technique that allows and attacker to decompile an assembly from a
certain application. Ildasm.exe can be used for this purpose, and ILAsm is used to recompiled the assembly.
The MSIL Disassembler( Ildasm.exe) is a companion tool to the MSIL Assembler (Ilasm.exe). Ildasm.exe takes a
portable executable (PE) le that contains Microsoft intermediate language (MSIL) code and creates a text le
suitable as input to Ilasm.exe. This tool is automatically installed with Visual Studio and with the Windows SDK.
The importance of Obfuscation
As mentioned before, Round Tripping is a technique used to reverse engineer assemblies. Therefore, if you
want to avoid your assemblies being reversed engineered or even worse, that the code is victim of malicious
manipulation using the Ildasm and Ilasm tools, then it is advisable to apply it. There are dierent kinds of prod-
ucts that can be used for this purpose such as DeepSea, Crypto or Dotfuscator.
Using Obfuscation
The most eective technique used to avoid reverse engineering and tampering of assemblies is the use of
Obfuscation. Visual Studio contains a version of Dotfuscator. This program is accessible by choosing on the VS
menu, Tools Dotfuscator(Community Edition menu command). Note: This tools is not available in Express
versions
To obfuscate your assemblies:
• Build the project in VS Studio
• Tools--> Dotfuscator Community Edition
• A screen prompts asking for which project type, choose ‘Creat New Project’ and click OK
• On the Input tab of the Dotfuscator interface, click ‘Browse and Add assembly to list’
Browse for the compiled application
ASPNetCongs
Introduction
Securing resources in ASP.NET applications is a combination of conguration settings in the Web.cong le
but also, it’s important to remember that the IIS congurations play also a big part on this. It’s an integrated
approach which provides a total framework of security.
The following highlights the most important aspects of ASP.NET conguration settings within the web.cong
le. For a total overview see chapter ASP.NET security (https://www.owasp.org/index.php/CRV2_Framework-
SpecIssuesASPNet)
Secure Conguration Values
Sensitive Information saved in cong les should be encrypted. Encryption keys stored in the machineKey
element for example or connectionstrings with username and passwords to login to database.
Lock ASP.NET Conguration settings
You can lock conguration settings in ASP.NET conguration les (Web.cong les) by adding an allowOver-
ride attribute to a location element
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111
Congure directories using Location Settings
Through the <location> element you can establish settings for specic folders and les. The Path attribute is
used to specify the le or subdirectory. This is done in the Web.cong le example:
Congure exceptions for Error Code handling
Showing and handling the correct error code when a user sends a bad request or invalid parameters is an im-
portant conguration subject. Logging these errors are also an excellent help when analyzing potential attacks
to the application.
It is possible to congure these errors in the code or in the Web.Cong le
The HttpException method describes an exception that occurred during the processing of HTTP requests.For
example:
or in the Web.cong le:
<location path=”.” >
<section1 .../>
<section2 ... />
</location>
<location path=”Default Web Site” >
<section1 … />
<section2 … />
</location
<location path=”Default Web Site/MyApplication/Admin/xyz.html” >
<section1 ... />
<section2 ... />
</location>
Sample 11.29
if (string.IsNullOrEmpty(Request[“id”]))
throw new HttpException(400, “Bad request”);
Sample 11.30
<conguration>
<system.web>
<customErrors mode=”On” defaultRedirect=”ErrorPage.html”
redirectMode=”ResponseRewrite”>
Sample 11.31
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112
Input validation
Anything coming from external sources can be consider as input in a web application. Not only the user in-
serting data through a web form, but also data retrieved from a web service or database, also headers sent
from the browsers fall under this concept. A way of dening when input is safe can be done through outlining
a trust boundary.
Dening what is known as trust boundary can help us to visualize all possible untrusted inputs. One of those
are user input.ASP.NET has dierent types of validations depending on the level of control to be applied. By
default, web pages code is validated against malicious users. The following is a list types of validations used
(MSDN, 2013):
References
MSDN, 2013 “Securing ASP.NET Congurations” available at
http://msdn.microsoft.com/en-us/library/ms178699%28v=vs.100%29.aspx (Last Viewed, 25th July 2013)
11.11 .NET Authentication Controls
In the .NET, there are Authentication tags in the conguration le. The <authentication> element congures
the authentication mode that your applications use. The appropriate authentication mode depends on how
your application or Web service has been designed. The default Machine.cong setting applies a secure Win-
dows authentication default as shown below
authentication Attributes:mode=”[Windows|Forms|Passport|None]”
<authentication mode=”Windows” />
<error statusCode=”400” redirect=”BadRequest.html” />
<error statusCode=”404” redirect=”FileNotFound.html” />
</customErrors>
</system.web>
</conguration>
Type of validation Control to use
Required entry RequiredFieldValidator
Comparison to a value CompareValidator
Description
Ensures that the user does not skip an entry.
Compares a user’s entry against a constant value, against the value of another
control (using a comparison operator such as less than, equal, or greater than),
or for a specic data type.
Range checking RangeValidator Checks that a user’s entry is between specied lower and upper boundaries.
You can check ranges within pairs of numbers, alphabetic characters, and dates.
Pattern matching RegularExpressionValidator Checks that the entry matches a pattern dened by a regular expression. This
type of validation enables you to check for predictable sequences of characters,
such as those in e-mail addresses, telephone numbers, postal codes, and so on.
User-dened CustomValidator Checks the user’s entry using validation logic that you write yourself. This type
of validation enables you to check for values derived at run time.
Figure 10: IIS Input Validation
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Forms Authentication Guidelines
To use Forms authentication, set mode=“Forms” on the <authentication> element. Next,
congure Forms authentication using the child <forms> element. The following fragment shows a secure
<forms> authentication element conguration:
Use the following recommendations to improve Forms authentication security:
• Partition your Web site.
• Set protection=“All”.
• Use small cookie time-out values.
• Consider using a xed expiration period.
• Use SSL with Forms authentication.
• If you do not use SSL, set slidingExpiration = “false”.
• Do not use the <credentials> element on production servers.
• Congure the <machineKey> element.
• Use unique cookie names and paths.
Classic ASP
For classic ASP pages, authentication is usually performed manually by including the user information in ses-
sion variablesafter validation against a DB, so you can look for something like:
Session (“UserId”) = UserName
Session (“Roles”) = UserRoles
Code Review .Net Manage Code
.NET Managed code is less vulnerable to common vulnerabilities found in unmanaged code such as Buer
Overows and memory corruption however there could be issues in the code that can aect performance and
security. The following is a summary of the recommended practices to look for during the code review. Also, it
is worth mentioning some tools that can make the work easier on this part and they can help you understand
and pin point aws in your code
Code Access Security
This supports the execution of semi-trusted code, preventing several forms of security threats. The following is
a summary of possible vulnerabilities due to improper use of Code Access security:
<authentication mode=”Forms”>
<forms loginUrl=”Restricted\login.aspx” Login page in an SSL protected folder
protection=”All” Privacy and integrity
requireSSL=”true” Prevents cookie being sent over http
timeout=”10” Limited session lifetime
name=”AppNameCookie” Unique per-application name
path=”/FormsAuth” and path
slidingExpiration=”true” > Sliding session lifetime
</forms>
</authentication>
Sample 11.32
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114
Declarative security
Use declarative security instead of imperative whenever possible.
Example of declarative syntax(MSDN[2], 2013):
Unmanaged code
Even though C# is a strong type language, it is possible to use unmanaged code calls by using the ‘unsafe’
code. “Check that any class that uses an unmanaged resource, such as a database connection across method
calls, implements the IDisposable interface. If the semantics of the object are such that a Close method is more
logical than a Dispose method, provide a Close method in addition to Dispose”.
Exception handling
Manage code should use exception handling for security purposes among other reasons. Make sure that you
follow these recommendations:
*Avoid exception handling in loops, use try/catch block if it is necessary.
*Identify code that swallows exceptions
*Use exceptions handling for unexpected conditions and not just to control the ow in the application
Tools
FxCop
FxCop is an analysis tool that analyses binary assemblies, not source code. The tool has a predened set of
rules and it is possible to congure and extend them.
Some of the available rules regarding security are (CodePlex, 2010):
Vulnerability
Improper use of link demands or asserts
Code allows untrusted callers
Implications
The code is susceptible to luring attacks
Malicious code can use the code to perform sensitive operations and access resources
Table 16: Code Access Security Vulnerabilities
[MyPermission(SecurityAction.Demand, Unrestricted = true)]
public class MyClass
{
public MyClass()
{
//The constructor is protected by the security call.
}
public void MyMethod()
{
//This method is protected by the security call.
}
public void YourMethod()
{
//This method is protected by the security call.
}
}
Sample 11.33
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115
Rule Description
EnableEventValidationShouldBeTrue Veries if the EnableEventValidation directive is disabled on a certain page.
ValidateRequestShouldBeEnabled Veries if the ValidateRequest directive is disabled on a certain page.
ViewStateEncryptionModeShouldBeAlways
EnableViewStateMacShouldBeTrue
EnableViewStateShouldBeTrue
ViewStateUserKeyShouldBeUsed
EnableCrossAppRedirectsShouldBeTrue
FormAuthenticationProtectionShouldBeAll
FormAuthenticationRequireSSLShouldBeTrue
FormAuthenticationShouldNotContainFormAuthentica
tionCredentials
CustomErrorPageShouldBeSpecied
DebugCompilationMustBeDisabled
Veries if the ViewStateEncryptionMode directive is not set to Never on a certain page.
Veries if the EnableViewStateMac directive is not set to false on a certain page.
Veries if the EnableViewState directive is not set to false on a certain page.
Veries if the Page.ViewStateUserKey is being used in the application to prevent CSRF.
Veries that system.web.authentication.forms enableCrossAppRedirects is set to true. The
settings indicate if the user should be redirected to another application url after the authen-
tication process. If the setting is false, the authentication process will not allow redirection to
another application or host. This helps prevent an attacker to force the user to be redirected
to another site during the authentication process. This attack is commonly called Open redi-
rect and is used mostly during phishing attacks.
Veries that the protection attribute on the system.web.authentication.forms protection is
set to All which species that the application use both data validation and encryption to help
protect the authentication cookie.
Veries that the requireSSL attribute on the system.web.authentication.forms conguration
element is set to True which forces the authentication cookie to specify the secure attribute.
This directs the browser to only provide the cookie over SSL.
Veries that no credentials are specied under the form authentication conguration.
Veries that the CustomErrors section is congured to have a default URL for redirecting uses
in case of error.
Veries that debug compilation is turned o. This eliminates potential performance and se-
curity issues related to debug code enabled and additional extensive error messages being
returned.
Table 17: FxCop Flags
HttpCookiesRequireSSLShouldBeTrue
TraceShouldBeDisabled
Veries that the system.web.httpCookies requireSSL conguration is set to True which forces
all cookies to be sent with the secure attribute. This indicates the browser to only provide the
cookie over SSL.
Veries that the system.web.trace enabled setting is set to false which disables tracing. It is
recommended to disable tracing on production servers to make sure that an attacker cannot
gain information from the trace about your application. Trace information can help an attack-
er probe and compromise your application.
FormAuthenticationSlidingExpirationShouldBeFalse
HttpCookiesHttpOnlyCookiesShouldBeTrue
Veries that system.web.authentication.forms slidingExpiration is set to false when the site
is being served over HTTP. This will force the authentication cookie to have a xed timeout
value instead of being refreshed by each request. Since the cookie will traverse over clear text
network and could potentially be intercepted, having a xed timeout value on the cookie
will limit the amount of time the cookie can be replayed. If the cookie is being sent only over
HTTPS, it is less likely to be intercepted and having the slidingExpiration setting to True will
cause the timeout to be refreshed after each request which gives a better user experience.
Veries that the system.web.httpCookies httpOnlyCookies conguration setting is set to True
which forces all cookies to be sent with the HttpOnly attribute.
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116
Rule Description
AnonymousAccessIsEnabled
PagesEnableViewStateMacShouldBeTrue
PagesValidateRequestShouldBeEnabled
PagesViewStateEncryptionModeShouldBeAlways
CustomErrorsModeShouldBeOn
MarkVerbHandlersWithValidateAntiforgeryToken
PagesEnableEventValidationMustBeTrue
RoleManagerCookieProtectionShouldBeAll
RoleManagerCookieRequireSSLShouldBeTrue
RoleManagerCookieSlidingExpirationShouldBeTrue
HttpRuntimeEnableHeaderCheckingShouldBeTrue
Looks in the web.cong le to see if the authorization section allows anonymous access.
Veries that the viewstate mac is enabled.
Verify that validateRequest is enabled.
Veries that the viewstate encryption mode is not congured to never encrypt.
Veries that the system.web.customErrors mode is set to On or RemoteOnly. This disable de-
tailed error message returned by ASP.NET to remote users.
ValidateAntiforgeryTokenAttribute is used to protect against potential CSRF attacks against
ASP.NET MVC applications.
Veries that event validation is enabled.
Veries that the system.web.rolemanager cookieProtection is set to All which enforces the
cookie to be both encrypted and validated by the server.
Veries that the system.web.rolemanager cookieRequireSSL attribute is set to True which
forces the role manager cookie to specify the secure attribute. This directs the browser to only
provide the cookie over SSL.
Veries that the system.web.rolemanager cookieSlidingExpiration is set to false when the site
is being served over HTTP. This will force the authentication cookie to have a xed timeout
value instead of being refreshed by each request. Since the cookie will traverse over clear text
network and could potentially be intercepted, having a xed timeout value on the cookie
will limit the amount of time the cookie can be replayed. If the cookie is being sent only over
HTTPS, it is less likely to be intercepted and having the slidingExpiration setting to True will
cause the timeout to be refreshed after each request which gives a better user experience.
Veries that the system.web.httpRuntime enableHeaderChecking attribute is set to true. The
setting indicates whether ASP.NET should check the request header for potential injection
attacks. If an attack is detected, ASP.NET responds with an error. This forces ASP.NET to apply
the ValidateRequest protection to headers sent by the client. If an attack is detected the ap-
plication throws HttpRequestValidationException.
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A6
118 A6 - Sensitive Data Exposure
SENSITIVE DATA EXPOSURE
A6
Many web applications do not properly protect sensitive data, such as credit cards, tax IDs, and authentication
credentials. Attackers may steal or modify such weakly protected data to conduct credit card fraud, identity
theft, or other crimes. Sensitive data deserves extra protection such as encryption at rest or in transit, as well
as special precautions when exchanged with the browser.
12.1 Cryptographic Controls
Software developers, architects and designers are at the forefront of deciding which category a particular
application resides in. Cryptography provides for security of data at rest (via encryption), enforcement of data
integrity (via hashing/digesting), and non-repudiation of data (via signing). To ensure this cryptographic code
adequately protections the data, all source code must use a standard (secure) algorithms with strong key sizes.
Common aws when implementing cryptographic code includes the use of non-standard cryptographic algo-
rithms, custom implementation of cryptography (standard & non-standard) algorithms, use of standard algo-
rithms which are cryptographically insecure (e.g. DES), and the implementation of insecure keys can weaken
the overall security posture of any application. Implementation of the aforementioned aws enable attackers
to use cryptanalytic tools and techniques to decrypt sensitive data.
12.2 Description
Many companies handle sensitive information for their customers, for instance medical details or credit card numbers,
and industry regulations dictate this sensitive information must be encrypted to protect the customers’ information.
In the medical industry the HIPAA regulations advise businesses what protections must be applied to medical data,
in the nancial industry many regulations cover PII (personally identiable information) controls.
Regardless of the nancial impact of regulatory penalties, there are many business reasons to protect (though en-
cryption or hashing) the information processed by an application, including privacy and fraud detection/protection.
All sensitive data that the application handles should be identied and encryption should be enforced. Simi-
larly a decision should be made as to whether sensitive data must be encrypted in transit (i.e. being sent from
one computer to another) and/or at rest (i.e. stored in a DB, le, keychain, etc.):
1) Protection in transit; this typically means using the SSL/TLS layer to encrypt data travelling on the HTTP
protocol, although it can also include FTPS, or even SSL on TCP. Frameworks such as IIS and Apache Struts
come with SSL/TLS functionality included, and thus the developer will not be coding the actual TLS encryp-
tion, but instead will be conguring a framework to provide TLS security.
However the decisions made here, even at an architectural level, need to be well informed, and a discussion on
TLS design decisions is covered in section 1.3.
2) Protection at rest; this can include encryption of credit cards in the database, hashing of passwords,
producing message authentication codes (MACs) to ensure a message has not been modied between com-
puters. Where TLS code will come with a framework, code to encrypt or hash data to be stored will typically
need to use APIs provided by cryptographic libraries.
The developer will not be writing code to implement the AES algorithm (OpenSSL or CryptoAPI will do that), the
developer will be writing modules to use an AES implantation in the correct way. Again the correct decisions need to
be made regarding up-to-date algorithms, key storage, and other design decisions, which are covered in section 1.4.
119
A6 - Sensitive Data Exposure
Cryptography Denitions
Before diving into discussions on encrypting trac and data, some terminology used in the realm of cryptog-
raphy is dened in table 18.
12.3 What to Review: Protection in Transit
The terms Secure Socket Layer (SSL) and Transport Layer Security (TLS) are often used interchangeably. In
fact, SSL v3.1 is equivalent to TLS v1.0. However, dierent versions of SSL and TLS are supported by modern
web browsers and by most modern web frameworks and platforms. Note that since developments in attacks
against the SSL protocol have shown it to be weaker against attacks, this guide will use the term TLS to refer to
transport layer security over the HTTP or TCP protocols.
The primary benet of transport layer security is the protection of web application data from unauthorized
disclosure and modication when it is transmitted between clients (web browsers) and the web application
server, and between the web application server and back end and other non-browser based enterprise com-
ponents.
In theory, the decision to use TLS to protect computer to computer communication should be based on the
nature of the trac or functionality available over the interface. If sensitive information is passing over the
Hashing
Entropy
Encoding
Salt
Encryption
Symmetric Encryption
Public-Key Encryption
(PKI)
Certicate
Non-reversible transformation of data into what is called a ‘ngerprint’ or ‘hashvalue’. Input of any size can be taken
and always results in the same size of output (for the algorithm). The aim is not to convert the ngerprint back into the
source data at a later time, but to run the hash algorithm over two sets of data to determine if they produce the same
ngerprint. This would show that data has not been tampered with.
Essentially this is randomness. Cryptographic functions will need to work with some form of randomness to allow the
source data to be encrypted in such a way that an attacker cannot reverse the encryption without the necessary key.
Having a good source of entropy is essential to any cryptographic algorithm.
Transforming data from one form into another, typically with the aim of making the data easier to work with. For ex-
ample encoding binary data (which could not be printed to a screen into printable ASCII format which can be copy/
pasted. Note that encoding does not aim to hide the data, the method to return the encoded data back to its original
form will be publically known.
A non-secret value that can be added to a hashing algorithm to modify the ngerprint result. One attack against
hashing algorithms is a ‘rainbow table attack’ where all source values are pre-computed and a table produced. The
attacker can then take a ngerprint, look it up in the table, and correspond it to the original data. Using a unique salt
value for each data to be hashed protects against rainbow tables, as a rainbow table for each salt value would need to
be created, which would greatly extend the time taken by an attacker. The salt is not a secret and can be stored or sent
with the ngerprint.
Transformation of source data into an encrypted form that can be reversed back to the original source. Typically the
algorithms used to encrypt are publically known, but rely on a secret ‘key’ to guide the transformation. An attacker
without the key should not be able to transform the data back into the original source.
A form of encryption where the same key is known to both the sender and the receiver. This is a fast form of encryption,
however requires a secure, out-of-band method to pass the symmetric key between the sender and receiver.
A form of encryption using two keys, one to encrypt the data, and one to decrypt the data back to its original form. This
is a slower method of encryption however one of the keys can be publically known (referred to as a ‘public key’). The
other key is called a ‘private key’ and is kept secret. Any data encrypted with the public key can be decrypted back into
its original form using the private key. Similarly any data encrypted with the private key can be decrypted back to its
original form using the public key.
An association between an entity (e.g. person, company) and a public key. Typically this forms part of a public-key
infrastructure where certain trusted entities (e.g. Certicate Authorities in internet TLS) perform validation of an entitles
credentials and assert (using their own certicate) that a declared public key belongs to the entity.
Term Description
Table 18: Cryptographic Denitions
120
interface, TLS will prevent eavesdroppers from being able to view or modify the data. Likewise if the interface
allows money to be transferred, or sensitive functions to be initiated, then TLS will protect the associated login
or session information authorizing the user to perform those functions. However with the price of certicates
dropping, and TLS conguration within frameworks becoming easier, TLS protection of an interface is not a
large endeavor and many web sites are using TLS protections for their entire site (i.e. there are only HTTPS
pages, no HTTP pages are available).
The server validation component of TLS provides authentication of the server to the client. If congured to re-
quire client side certicates, TLS can also play a role in client authentication to the server. However, in practice
client side certicates are not often used in lieu of username and password based authentication models for
clients.
Using Validated Implementations
The US government provides a list of software that has been validated to provide a strong and secure imple-
mentation of various cryptographic functions, including those used in TLS. This list is referred to as the FIPS
140-2 validated cryptomodules.
A cryptomodule, whether it is a software library or a hardware device, implements cryptographic algorithms
(symmetric and asymmetric algorithms, hash algorithms, random number generator algorithms, and message
authentication code algorithms). The security of a cryptomodule and its services (and the web applications
that call the cryptomodule) depend on the correct implementation and integration of each of these three
parts. In addition, the cryptomodule must be used and accessed securely. In order to leverage the benets of
TLS it is important to use a TLS service (e.g. library, web framework, web application server) which has been
FIPS 140-2 validated. In addition, the cryptomodule must be installed, congured and operated in either an
approved or an allowed mode to provide a high degree of certainty that the FIPS 140-2 validated cryptomod-
ule is providing the expected security services in the expected manner.
When reviewing designs or code that is handling TLS encryption, items to look out for include:
• Use TLS for the login pages and any authenticated pages. Failure to utilize TLS for the login landing page
allows an attacker to modify the login form action, causing the user’s credentials to be posted to an arbitrary
location. Failure to utilize TLS for authenticated pages after the login enables an attacker to view the unen-
crypted session ID and compromise the user’s authenticated session.
• Use TLS internally when transmitting sensitive data or exposing authenticated functionality. All networks,
both external and internal, which transmit sensitive data must utilize TLS or an equivalent transport layer se-
curity mechanism. It is not sucient to claim that access to the internal network is “restricted to employees”.
Numerous recent data compromises have shown that the internal network can be breached by attackers. In
these attacks, sniers have been installed to access unencrypted sensitive data sent on the internal network.
• Prefer all interfaces (or pages) being accessible only over HTTPS. All pages which are available over TLS must
not be available over a non-TLS connection. A user may inadvertently bookmark or manually type a URL to a
HTTP page (e.g. http://example.com/myaccount) within the authenticated portion of the application.
• Use the “secure” and “http-only” cookie ags for authentication cookies. Failure to use the “secure” ag
enables an attacker to access the session cookie by tricking the user’s browser into submitting a request to an
unencrypted page on the site. The “http-only” ag denies JavaScript functions access to the cookies contents.
• Do not put sensitive data in the URL. TLS will protect the contents of the trac on the wire, including the URL
when transported, however remember that URL’s are visible in browser history settings, and typically written
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to server logs.
• Prevent the caching of sensitive data. The TLS protocol provides condentiality only for data in transit but it
does not help with potential data leakage issues at the client or intermediary proxies.
• Use HTTP Strict Transport Security (HSTS) for high risk interfaces. HSTS will prevent any web clients from
attempting to connect to your web site over a non-TLS protocol. From a server-side point of view this may
seem irrelevant if no non-TLS pages are provided, however a web site setting up HSTS does protect clients
from other attacks (e.g. DNS cache poisioning).
• Use 2048 key lengths (and above) and SHA-256 (and above). The private key used to generate the cipher key
must be suciently strong for the anticipated lifetime of the private key and corresponding certicate. The
current best practice is to select a key size of at least 2048 bits. Note that attacks against SHA-1 have shown
weakness and the current best practice is to use at least SHA-256 or equivalent.
• Only use specied, fully qualied domain names in your certicates. Do not use wildcard certicates, or RFC
1918 addresses (e.g. 10.* or 192.168.*). If you need to support multiple domain names use Subject Alternate
Names (SANs) which provide a specic listing of multiple names where the certicate is valid. For example the
certicate could list the subject’s CN as example.com, and list two SANs: abc.example.com and xyz.example.
com. These certicates are sometimes referred to as “multiple domain certicates”.
• Always provide all certicates in the chain. When a user receives a server or host’s certicate, the certicate
must be validated back to a trusted root certication authority. This is known as path validation. There can
be one or more intermediate certicates in between the end-entity (server or host) certicate and root certif-
icate. In addition to validating both endpoints, the client software will also have to validate all intermediate
certicates, which can cause failures if the client does not have the certicates. This occurs in many mobile
platforms.
12.4 What to Review: Protection at Rest
As a general recommendation, companies should not create their own custom cryptographic libraries and al-
gorithms. There is a huge distinction between groups, organizations, and individuals developing cryptograph-
ic algorithms and those that implement cryptography either in software or in hardware. Using an established
cryptographic library that has been developed by experts and tested by the industry is the safest way to im-
plement cryptographic functions in a company’s code. Some common examples of libraries used in various
languages and environments are covered in the below table.
DiscussionLanguage
C# .NET
Libraries
Class libraries
within ‘Sys-
tem.Security.
Cryptogra-
phy’
For applications coded in C#.NET there are class libraries and implementations within the ‘System.Security.Cryptog-
raphy’ that should be used. This namespace within .NET aims to provide a number of warppers that do not require
procient knowledge of cryptography in order to use it.
Table 19: Popular Cryptographic implementations according to environment
C/C++
(Win32)
CryptoAPI
and DPAPI
For C/C++ code running on Win32 platforms, the CreyptoAPI and DPAPI are recommended.
C/C++
(Linux)
OpenSSL,
NSS, boringssl
For C/C++ on Linux/Unix operating systems, us OpenSSL, NSS, or one of the many forks of these libraries.
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A secure way to implement robust encryption mechanisms within source code is by using FIPS [7] compliant algo-
rithms with the use of the Microsoft Data Protection API (DPAPI) [4] or the Java Cryptography Extension (JCE) [5].
A company should identify minimum standards for the following when establishing your cryptographic code
strategy:
• Which standard algorithms are to be used by applications
• Minimum key sizes to be supported
• What types of data must be encrypted
When reviewing code handling cryptography look out for:
• Is strong enough encryption algorithms being used, and is the implementation of those algorithms FIPS-140 com-
plaint.
• Is the right type of cryptographic algorithm being used, is data being hashed that should be encrypted with a
symmetric key? If there is no way to safely transfer the symmetric key to the other party, is public key cryptographic
algorithms being employed?
• In any cryptographic system the protection of the key is the most important aspect. Exposure of the symmetric or
private key means the encrypted data is no longer private. Tightly control who has access to enter or view the keys,
and how the keys are used within applications.
• Any code implementing cryptographic processes and algorithms should be reviewed and audited against a set of
company or regulatory specications. High level decisions need to be made (and continually revisited) as to what an
organization considers ‘strong encryption’ to be, and all implementation instances should adhere to this standard.
• Cryptographic modules must be tested under high load with multithreaded implementations, and each piece of
encrypted data should be checked to ensure it was encrypted and decrypted correctly.
• In .Net check for examples of cryptography in the MSDN Library Security Practices: .NET Framework 2.0 Security
Practices at a Glance
o Check that the Data Protection API (DPAPI) is being used.
o Verify no proprietary algorithms are being used.
o Check that RNGCryptoServiceProvider is used for PRNG.
o Verify key length is at least 128 bits.
• In ASP perform all of these checks on the COM wrapper as ASP does not have direct access to cryptographic functions
o Check that the Data Protection API (DPAPI) or CryptoAPI is being used into COM object
ASP
Java
CryptoAPI
and DPAPI
Java Cryp-
tography
Extension,
BouncyCas-
tle, Spring
Security
Classis ASP pages do not have direct access to cryptographic functions, so the only way is to create COM wrappers in
Visual C++ or Visual Basic, implementing calls to CryptoAPI or DPAPI. Then call them from ASP pages using the Server.
CreateObject method.
JCE is a standard API that any cryptographic library can implement to provide cryptographic functions to the developer.
Oracle provide a list of companies that act as Cryptographic Service Providers and/or oer clean room implementations
of the JCE. BouncyCastle is on of the more popular implementations. Spring Secuirty is also popular in application
where Spring is already being utilized.
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o Verify no proprietary algorithms are being used
o Check that RNGCryptoServiceProvider is used for PRNG
o Verify key length is at least 128 bits
• For Java check that the Java Cryptography Extension (JCE) is being used
o Verify no proprietary algorithms are being used
o Check that SecureRandom (or similar) is used for PRNG
o Verify key length is at least 128 bits
Bad Practice: Use of Insecure Cryptographic Algorithms
The DES and SHA-0 algorithms are cryptographically insecure. The example in sample 12.1 outlines a cryptographic
module using DES (available per using the Java Cryptographic Extensions) which should not be used. Additionally,
SHA-1 and MD5 should be avoided in new applications moving forward.
Good Practice: Use Strong Entropy
The source code in sample 12.2 outlines secure key generation per use of strong entropy:
package org.badexample.crypto;
<snip>
try {
/** Step 1. Generate a DES key using KeyGenerator */
KeyGenerator keyGen = KeyGenerator.getInstance(“DES”);
SecretKey secretKey = keyGen.generateKey();
/** Step2. Create a Cipher by specifying the following parameters
* a. Algorithm name - here it is DES
* b. Mode - here it is CBC
* c. Padding - PKCS5Padding */
Cipher desCipher = Cipher.getInstance(“DES/CBC/PKCS5Padding”);
<snip>
Sample 12.1
package org.owasp.java.crypto;
import java.security.SecureRandom;
import java.security.NoSuchAlgorithmException;
import sun.misc.BASE64Encoder;
/**
* This program provides the functionality for Generating a Secure Random Number.
* There are 2 ways to generate a Random number through SecureRandom.
* 1. By calling nextBytes method to generate Random Bytes
Sample 12.2
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Good Practice: Use Strong Algorithms
Below illustrates the implementation of AES (available per Using the Java Cryptographic Extensions):
* 2. Using setSeed(byte[]) to reseed a Random object
*/
public class SecureRandomGen {
public static void main(String[] args) {
try {
// Initialize a secure random number generator
SecureRandom secureRandom = SecureRandom.getInstance(“SHA512”);
// Method 1 - Calling nextBytes method to generate Random Bytes
byte[] bytes = new byte[512];
secureRandom.nextBytes(bytes);
// Printing the SecureRandom number by calling secureRandom.nextDouble()
System.out.println(“ Secure Random # generated by calling nextBytes() is “ + secureRandom.
nextDouble());
// Method 2 - Using setSeed(byte[]) to reseed a Random object
int seedByteCount = 10;
byte[] seed = secureRandom.generateSeed(seedByteCount);
secureRandom.setSeed(seed);
System.out.println(“ Secure Random # generated using setSeed(byte[]) is “ + secureRandom.
nextDouble());
} catch (NoSuchAlgorithmException noSuchAlgo)
{
System.out.println(“ No Such Algorithm exists “ + noSuchAlgo);
}
}
}
package org.owasp.java.crypto;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.Cipher;
import java.security.NoSuchAlgorithmException;
import java.security.InvalidKeyException;
import java.security.InvalidAlgorithmParameterException;
import javax.crypto.NoSuchPaddingException;
Sample 12.3
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import javax.crypto.BadPaddingException;
import javax.crypto.IllegalBlockSizeException;
import sun.misc.BASE64Encoder;
/**
* This program provides the following cryptographic functionalities
* 1. Encryption using AES
* 2. Decryption using AES
*
* High Level Algorithm :
* 1. Generate a DES key (specify the Key size during this phase)
* 2. Create the Cipher
* 3. To Encrypt : Initialize the Cipher for Encryption
* 4. To Decrypt : Initialize the Cipher for Decryption
*/
public class AES {
public static void main(String[] args) {
String strDataToEncrypt = new String();
String strCipherText = new String();
String strDecryptedText = new String();
try{
/**
* Step 1. Generate an AES key using KeyGenerator
* Initialize the keysize to 128
*/
KeyGenerator keyGen = KeyGenerator.getInstance(“AES”);
keyGen.init(128);
SecretKey secretKey = keyGen.generateKey();
/**
* Step2. Create a Cipher by specifying the following parameters
* a. Algorithm name - here it is AES
*/
Cipher aesCipher = Cipher.getInstance(“AES”);
/**
* Step 3. Initialize the Cipher for Encryption
*/
aesCipher.init(Cipher.ENCRYPT_MODE,secretKey);
/**
* Step 4. Encrypt the Data
* 1. Declare / Initialize the Data. Here the data is of type String
* 2. Convert the Input Text to Bytes
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* 3. Encrypt the bytes using doFinal method
*/
strDataToEncrypt = “Hello World of Encryption using AES “;
byte[] byteDataToEncrypt = strDataToEncrypt.getBytes();
byte[] byteCipherText = aesCipher.doFinal(byteDataToEncrypt);
strCipherText = new BASE64Encoder().encode(byteCipherText);
System.out.println(“Cipher Text generated using AES is “ +strCipherText);
/**
*/ Step 5. Decrypt the Data
* 1. Initialize the Cipher for Decryption
* 2. Decrypt the cipher bytes using doFinal method
*/
aesCipher.init(Cipher.DECRYPT_MODE,secretKey,aesCipher.getParameters());
byte[] byteDecryptedText = aesCipher.doFinal(byteCipherText);
strDecryptedText = new String(byteDecryptedText);
System.out.println(“ Decrypted Text message is “ +strDecryptedText);
}
catch (NoSuchAlgorithmException noSuchAlgo)
{
System.out.println(“ No Such Algorithm exists “ + noSuchAlgo);
}
catch (NoSuchPaddingException noSuchPad)
{
System.out.println(“ No Such Padding exists “ + noSuchPad);
}
catch (InvalidKeyException invalidKey)
{
System.out.println(“ Invalid Key “ + invalidKey);
}
catch (BadPaddingException badPadding)
{
System.out.println(“ Bad Padding “ + badPadding);
}
catch (IllegalBlockSizeException illegalBlockSize)
{
System.out.println(“ Illegal Block Size “ + illegalBlockSize);
}
catch (InvalidAlgorithmParameterException invalidParam)
{
System.out.println(“ Invalid Parameter “ + invalidParam);
}
}
}
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1.1.4 References
[1] Bruce Schneier, Applied Cryptography, John Wiley & Sons, 2nd edition, 1996.
[2] Michael Howard, Steve Lipner, The Security Development Lifecycle, 2006, pp. 251 - 258
[3] .NET Framework Developer’s Guide, Cryptographic Services, http://msdn2.microsoft.com/en-us/library/93bsk-
f9z.aspx
[4] Microsoft Developer Network, Windows Data Protection, http://msdn2.microsoft.com/en-us/library/
ms995355.aspx
[5] Sun Developer Network, Java Cryptography Extension, http://java.sun.com/products/jce/
[6] Sun Developer Network, Cryptographic Service Providers and Clean Room Implementations, http://java.sun.
com/products/jce/jce122_providers.html
[7] Federal Information Processing Standards, http://csrc.nist.gov/publications/ps/
12.5 Encryption, Hashing & Salting
A cryptographic hash algorithm, also called a hash function, is a computer algorithm designed to provide a random
mapping from an arbitrary block of data (string of binary data) and return a xed-size bit string known as a “message
digest” and achieve certain security.
Cryptographic hashing functions are used to create digital signatures, message authentication codes (MACs), other
forms of authentication and many other security applications in the information infrastructure. They are also used
to store user passwords in databases instead of storing the password in clear text and help prevent data leakage in
session management for web applications. The actual algorithm used to create a cryptology function varies per im-
plementation (SHA-256, SHA-512, etc.)
Never accept in a code review an algorithm created by the programmer for hashing. Always use cryptographic func-
tions that are provided by the language, framework, or common (trusted) cryptographic libraries. These functions are
well vetted and well tested by experience cryptographers.
In the United States in 2000, the department of Commerce Bureau of Export revised encryption export regulations.
The results of the new export regulations it that the regulations have been greatly relaxed. However if the code is to
be exported outside of the source country current export laws for the export and import counties should be reviewed
for compliance.
Case in point is if the entire message is hashed instead of a digital signature of the message the National Security
Agency (NSA) considers this a quasi-encryption and State controls would apply.
It is always a valid choice to seek legal advice within the organization if the code review is being done to ensure legal
compliance.
With security nothing is secure forever. This is especially true with cryptographic hashing functions. Some hashing
algorithms such as Windows LanMan hashes are considered completely broken. Others like MD5, which in the past
were considered safe for password hash usage, have known issues like collision attacks (note that collision attacks do
not aect password hashes). The code reviewer needs to understand the weaknesses of obsolete hashing functions
as well as the current best practices for the choice of cryptographic algorithms.
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Working with Salts
The most common programmatic issue with hashing is:
• Not using a salt value
• Using a salt the salt value is too short
• Same salt value is used in multiple hashes.
The purpose of a salt is to make it harder for an attacker to perform pre-computed hashing attack (e.g., using rainbow
tables). Take for example that the SHA512 has of ‘password’ is as shown in row 1 of table 20, and any attacker with a
rainbow table will spot the hash value corresponding to ‘password’. Taking into consideration it takes days or weeks
to compute a rainbow table to values up to around 8 or 10 characters, the eort to produce this table is worth it when
an application is not using any salts.
Now take a scenario where an application adds a salt of ‘WindowCleaner’ to all passwords entered. Now the hash of
‘password’ becomes the hash of ‘passwordWindowCleaner’, which is shown in row 2 of table 20. This is unlikely to be
in the attackers rainbow table, however the attacker can now spend the next 10 days (for example) computing a new
rainbow table with ‘WindowCleaner’ on the end of every 8 to 10 character string and they again can now decode our
hashed database of passwords.
At a last step, an application can create a random salt for each entry, and store that salt in the DB with the hashed
password. Now for user1, the random salt is ‘a0w8hsdfas8ls587uas87’, meaning the password to be hashed is ‘pass-
worda0w8hsdfas8ls587uas87’, shown in row 3 of table 20, and for user2, the random salt is ‘8ash87123klnf9d8dq3w’,
meaning the password to be hashed is ‘password8ash87123klnf9d8dq3w’, shown in row 4 of table X, and repeat for
all users.
Now an attacker would need a rainbow table for each users’ password they mean to decrypt – whereas before it took
10 days to decrypt all of the DB passwords using the same salt, now it takes 10 days to create a rainbow table for
user1’s password, and another 10 days for user2’s password, etc. If there were 100,000 users, that’s now 100,000 x 10
days = 1,000,000 days or 2738 years, to create rainbow tables for all the users.
As can be seen, the salt does not need to be secret, as it is the fact that unique salts are used for each user that
slows down an attacker.
FingerprintMethod to hash ‘password’
No salt B109F3BBBC244EB82441917ED06D618B9008DD09B3
BEFD1B5E07394C706A8BB980B1D7785E5976EC049B
46DF5F1326AF5A2EA6D103FD07C95385FFAB0CACBC86
Salt = ‘WindowCleaner’
Salt = ‘a0w8hsdfas8ls587uas87’
Salt = ‘8ash87123klnf9d8dq3w’
E6F9DCB1D07E5412135120C0257BAA1A27659D41DC7
7FE2DE4C345E23CB973415F8DFDFFF6AA7F0AE0BDD
61560FB028EFEDF2B5422B40E5EE040A0223D16F06F
5AA762E7C83CFF223B5A00ADA939FBD186C4A2CD01
1B0A7FE7AF86B8CA5420C7A47B52AFD2FA6B9BB172
22ACF32B3E13F8C436447C36364A5E2BE998416A103A
8058D43195B1CF2794D012A86AC809BFE73254A82C8C
E6C10256D1C46B9F45700D040A6AC6290746058A63E5
0AAF8C87ABCD5C3AA00CDBDB31C10BA6D12A1A7
Table 20: Salt usages and associated ngerprints
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One way to generate a salt value is using a pseudo-random number generator, as shown in sample 12.4 below.
Note that a salt value does not need to possess the quality of a cryptographically secure randomness.
Best practices is to use a cryptographically function to create the salt, create a salt value for each hash value,
and a minimum value of 128 bits (16 characters). The bits are not costly so don’t save a few bits thinking you
gain something back in performance instead use a value of 256-bit salt value. It is highly recommended.
Best Practices
Industry leading Cryptographer’s are advising that MD5 and SHA-1 should not be used for any applications.
The United State FEDERAL INFORMATION PROCESSING STANDARDS PUBLICATION (FIPS) species seven cryp-
tographic hash algorithms — SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, and SHA-512/256
are approved for federal use.
The code reviewer should consider this standard because the FIPS is also widely adopted by the information
technology industry.
The code reviewer should raise a red ag if MD5 and SHA-1 are used and a risk assessment be done to under-
stand why these functions would be used instead of other better-suited hash functions. FIPS does allow that
MD5 can be used only when used as part of an approved key transport scheme where no security is provided
by the algorithm.
sample 12.5 below shows an example function which could implement a generic hash feature for an applica-
tion.
private int minSaltSize = 8;
private int maxSaltSize = 24;
private int saltSize;
private byte[] GetSalt(string input) {
byte[] data;
byte[] saltBytes;
RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
saltBytes = new byte[saltSize];
rng.GetNonZeroBytes(saltBytes);
data = Encoding.UTF8.GetBytes(input);
byte[] dataWithSaltBytes =
new byte[data.Length + saltBytes.Length];
for (int i = 0; i < data.Length; i++)
dataWithSaltBytes[i] = data[i];
for (int i = 0; i < saltBytes.Length; i++)
dataWithSaltBytes[data.Length + i] = saltBytes[i];
return dataWithSaltBytes;
}
Sample 12.4
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Line 1 lets us get our hashing algorithm we are going to use from the cong le. If we use the machine cong
le our implementation would be server wide instead of application specic.
Line 3 allows us to use the cong value and set it according as our choice of hashing function. ComputHash
could be SHA-256 or SHA-512.
References
http://valerieaurora.org/hash.html (Lifetimes of cryptographic hash functions)
http://docs.oracle.com/javase/6/docs/api/java/security/SecureRandom.html
http://msdn.microsoft.com/en-us/library/system.security.cryptography.rngcryptoserviceprovider.aspx
http://csrc.nist.gov/publications/ps/ps180-4/ps-180-4.pdf
Ferguson and Schneier (2003) Practical Cryptography (see Chapter 6; section 6.2 Real Hash Functions)
12.6 Reducing the attack surface
The Attack Surface of an application is a description of the entry/exit points, the roles/entitlements of the users, and
the sensitivity of the data held within the application. For example, entry points such as login screens, HTML forms,
le upload screens, all introduce a level of risk to the application. Note that the code structure also forms part of
the Attack Surface, in that the code checking authentication, or crypto, etc., is exercised by critical functions on the
application.
Description
The attack surface of a software environment is a description of the entry points where an attacker can try to manipu-
late an application, it typically takes the form of a systems diagram where all entry points (interfaces) are pointed out.
App Code File:
<add key=”HashMethod” value=”SHA512”/>
C# Code:
1: preferredHash = HashAlgorithm.Create((string)CongurationManager.AppSettings[“HashMethod”]);
2:
3: hash = computeHash(preferredHash, testString);
4:
5: private string computeHash(HashAlgorithm myHash, string input) {
6: byte[] data;
7: data = myHash.ComputeHash(Encoding.UTF8.GetBytes(input));
8: sb = new StringBuilder();
9: for (int i = 0; i < data.Length; i++) {
10: sb.Append(data[i].ToString(“x2”));
11: }
12: return sb.ToString();
13: }
Sample 12.5
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Michael Howard (at Microsoft) and other researchers have developed a method for measuring the Attack Surface of
an application, and to track changes to the Attack Surface over time, called the Relative Attack Surface Quotient (RSQ).
It is assumed that the application Attack Surface is already known, probably through some previous threat modeling
exercise, or Architectural Risk Analysis. Therefor the entry and exit points are known, the sensitivity of the data within
the application is understood, and the various users of the system, and their entitlements, have been mapped in
relation to the functions and data.
From a code review point of view, the aim would be to ensure the change being reviewed is not unnecessarily in-
creasing the Attack Surface. For example, is the code change suddenly using HTTP where only HTTPS was used
before? Is the coder deciding to write their own hash function instead of using the pre-existing (and well exercised/
tested) central repository of crypto functions? In some development environments the Attack Surface changes can
be checked during the design phase if such detail is captured, however at code review the actual implementation is
reected in the code and such Attack Surface exposures can be identied.
You can also build up a picture of the Attack Surface by scanning the application. For web apps you can use a tool like
the OWASP Zed Attack Proxy Project (ZAP), Arachni, Skipsh, w3af or one of the many commercial dynamic testing
and vulnerability scanning tools or services to crawl your app and map the parts of the application that are accessible
over the web. Once you have a map of the Attack Surface, identify the high risk areas, then understand what com-
pensating controls you have in place.
Note that backups of code and data (online, and on oine media) are an important but often ignored part of a sys-
tem’s Attack Surface. Protecting your data and IP by writing secure software and hardening the infrastructure will all
be wasted if you hand everything over to bad guys by not protecting your backups.
What to Review
When reviewing code modules from an Attack Surface point of view, some common issues to look out for include:
• Does the code change modify the attack surface? By applying the change to the current Attack Surface of the ap-
plication does it open new ports or accept new inputs? If it does could the change be done in a way that does not
increase the attack surface? If a better implementation exists then that should be recommended, however if there is
no way to implement the code without increasing the Attack Surface, make sure the business knows of the increased
risk.
• Is the feature unnecessarily using HTTP instead of HTTPS?
• Is the function going to be available to non-authenticated users? If no authentication is necessary for the function
to be invoked, then the risk of attackers using the interface is increased. Does the function invoke a backend task that
could be used to deny services to other legitimate users?
o E.g. if the function writes to a le, or sends an SMS, or causes a CPU intensive calculation, could an attacker write a
script to call the function many times per second and prevent legitimate users access to that task?
• Are searches controlled? Search is a risky operation as it typically queries the database for some criteria and returns
the results, if attacker can inject SQL into query then they could access more data than intended.
• Is important data stored separately from trivial data (in DB, le storage, etc). Is the change going to allow unauthen-
ticated users to search for publicly available store locations in a database table in the same partition as the username/
password table? Should this store location data be put into a dierent database, or dierent partition, to reduce the
risk to the database information?
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• If le uploads are allowed, are they be authenticated? Is there rate limiting? Is there a maximum le size for each
upload or aggregate for each user? Does the application restrict the le uploads to certain types of le (by checking
MIME data or le sux). Is the application is going to run virus checking?
• If you have administration users with high privilege, are their actions logged/tracked in such a way that they a) can’t
erase/modify the log and b) can’t deny their actions?
o Are there any alarms or monitoring to spot if they are accessing sensitive data that they shouldn’t be? This could
apply to all types of users, not only administrators.
• Will changes be compatible with existing countermeasures, or security code, or will new code/countermeasures
need to be developed?
• Is the change attempting to introduce some non-centralized security code module, instead of re-using or extending
an existing security module?
• Is the change adding unnecessary user levels or entitlements that will complicate the attack surface.
• If the change is storing PII or condential data, is all of the new information absolutely necessary? There is little value
in increasing the risk to an application by storing the social security numbers of millions of people, if the data is never
used.
• Does application conguration cause the attack surface to vary greatly depending on conguration settings, and is
that conguration simple to use and alert the administrator when the attack surface is being expanded?
• Could the change be done in a dierent way that would reduce the attack surface, i.e instead of making help items
searchable and storing help item text in a database table beside the main username/password store, providing static
help text on HTML pages reduces the risk through the ‘help’ interface.
• Is information stored on the client that should be stored on the server?
1.2.3 References
https://www.owasp.org/index.php/Attack_Surface_Analysis_Cheat_Sheet
https://www.owasp.org/index.php/OWASP_Zed_Attack_Proxy_Project
http://www.cs.cmu.edu/~wing/publications/Howard-Wing03.pdf
A6 - Sensitive Data Exposure
133
A7
134 A7 - Missing Function Level Access Control
Most web applications verify function level access rights before making that functionality visible in the UI. However,
applications need to perform the same access control checks on the server when each function is accessed. If requests
are not veried, attackers will be able to forge requests in order to access functionality without proper authorization.
13.1 Authorization
Authorization is as important as authentication. Access to application functionality and access to all data should be
authorized. For data access authorization, application logic should check if the data belongs to the authenticated
user, or if the user should be able to access that data.
Placement of security checks is a vital area of review in an application design. Incorrect placement can render the
applied security controls useless, thus it is important to review the application design and determine the correctness
of such checks. Many web application designs are based on the concept of Model-View-Controller (MVC) that have a
central controller which listens to all incoming request and delegates control to appropriate form/business process-
ing logic. Ultimately the user is rendered with a view. In such a layered design, when there are many entities involved
in processing a request, developers can go wrong in placing the security controls in the incorrect place, for example
some application developers feel “view” is the right place to have the authorization checks.
Authorization issues cover a wide array of layers in a web application; from the functional authorization of a user to
gain access to a particular function of the application at the application layer, to the Database access authorization
and least privilege issues at the persistence layer.
In most of the applications the request parameters or the URL’s serve as sole factors to determine the processing logic.
In such a scenario the elements in the request, which are used for such identications, may be subject to manipula-
tion attacks to obtain access to restricted resources or pages in the application.
There are two main design methods to implements authorization: Role Base Access Control (RBAC) and Access Con-
trol Lists (ACLs). RBAC is used when assigning users to roles, and then roles to permissions. This is a more logical
modeling of actual system authorization. It also allows administrators to ne-grain and re-check role-permission as-
signments, while making sure that every role has the permissions it is supposed to have (and nothing more or less).
Thus assigning users to roles should reduce the chance of human-error. Many web frameworks allow roles to be
assigned to logged in users, and custom code can check the session information to authorize functionality based on
the current users role.
13.2 Description
It seems logical to restrict the users at the page/view level they won’t be able to perform any operation in the appli-
cation. But what if instead of requesting for a page/view an unauthorized user tries to request for an internal action
such as to add/modify any data in the application? It will be processed but the resultant view will be denied to the
user; because the aw lies in just having a view based access control in the applications. Much of the business logic
processing (for a request) is done before the “view” is executed. So the request to process any action would get pro-
cessed successfully without authorization.
In an MVC based system given this issue is shown in gure 11 below, where the authentication check is pres-
ent in the view action.
MISSING FUNCTION LEVEL ACCESS CONTROL
A7
135
A7 - Missing Function Level Access Control
In this example neither the controller servlet (central processing entity) nor the action classes have any access
control checks. If the user requests an internal action such as add user details, without authentication, it will
get processed, but the only dierence is that the user will be shown an error page as resultant view will be
disallowed to the user. A similar aw is observed in ASP.NET applications where the developers tend to mix the
code for handling POSTBACK’s and authentication checks. Usually it is observed that the authentication check
in the ASP.NET pages are not applied for POSTBACKs, as indicated below. Here, if an attacker tries to access the
page without authentication an error page will be rendered. Instead, if the attacker tries to send an internal
POSTBACK request directly without authentication it would succeed.
Unused or undeclared actions or functionality can present in the conguration les. Such congurations that
are not exposed (or tested) as valid features in the application expand the attack surface and raise the risk to
the application. An unused conguration present in a conguration le is shown in sample 13.1, where the
‘TestAction’ at the end of the le has been left over from the testing cycle and will be exposed to external users.
It is likely this action would not be checked on every release and could expose a vulnerability.
CONTROLLER
SERVIET
ACTION
CLASS
EXECUTE{}
1. REQUEST
7. RESPONSE
Cong.xml
2READ CONFIG FILE
3RETURN ACTION MAPPING
URL = ACTION
Authentication
check built
inside the view
4INTANTIATE ACTION CLASS AND
CALLS EXECUTE METHOD
5RETURN DATA TO BE VIEWED/
EDITED
6
RENDER VIEW AND DATA
AUTH
CHECK
VIEW
Figure 11: MVC Access Control
<mapping>
<url>/InsecureDesign/action/AddUserDetails</url>
<action>Action.UserAction</action>
Sample 13.1
136
Another popular feature seen in most of the design frameworks today is data binding, where the request
parameters get directly bound to the variables of the corresponding business/command object. Binding here
means that the instance variables of such classes get automatically initialized with the request parameter val-
ues based on their names. The issue with this design is that the business objects may have variables that are
not dependent on the request parameters. Such variables could be key variables like price, max limit, role etc.
having static values or dependent on some server side processing logic. A threat in such scenarios is that an at-
tacker may supply additional parameters in request and try to bind values for unexposed variable of business
object class. In this case the attacker can send an additional “price” parameter in the request which binds with
the unexposed variable “price” in business object, thereby manipulating business logic.
What to Review
It is imperative to place all validation checks before processing any business logic and in case of ASP.NET applications
independent of the POSTBACKs. The security controls like authentication check must be place before processing any
request.
The use of lters is recommended when authorization is being implemented in MVC 3 and above as .NET MVC 3
introduced a method in global.asax called RegisterGlobalFilters which can be used to default deny access to URL’s in
the application.
It is recommended when reviewing MVC3/4 .NET to take a look at how authorization is being implemented. The line
above, “lters.Add(new System.Web.Mvc.AuthorizeAttribute());” default denies access to any request without a valid
<success>JSP_WithDesign/Success.jsp</success>
</mapping>
<mapping>
<url>/InsecureDesign/action/ChangePassword</url>
<action>Action.ChangePasswordAction</action>
<success>JSP_WithDesign/Success.jsp</success>
</mapping>
<mapping>
<url>/InsecureDesign/action/test</url>
<action>Action.TestAction</action>
<success>JSP_WithDesign/Success.jsp</success>
</mapping>
public static void RegisterGlobalFilters(GlobalFilterCollection lters)
{
lters.Add(new HandleErrorAttribute());
lters.Add(new System.Web.Mvc.AuthorizeAttribute());
}
Sample 13.2
A7 - Missing Function Level Access Control
137
session. If this is implemented we may need to provide unauthorized access to certain pages such as a registration
page, public welcome page or a login page.
The directive “AllowAnonymous” is used to provide access to public pages with no valid session required. The code
may look like this:
When reviewing code for authorization, the following considerations can be checked for:
• Every entry point should be authorized. Every function should be authorized.
• Authorization checks should be ecient, and implemented in a central code base such that it can be applied
consistently.
• In cases where authorization fails, a HTTP 403 not authorized page should be returned.
• When using RBAC, there must be some way for the application to report on the currently provisioned users of the
system and their associated roles. This allows the business to periodically audit the user access to the system and
ensure it is accurate. For example, if a user is provisioned as an admin on the system, then that user changes job to
another department, it could be the case that the admin role is no longer appropriate.
• There should be an easy method to change or remove a user’s role (in RBAC systems). Adding, modifying or remov-
ing a user from a role should result in audit logs.
• For roles that are higher risk, addition, modication and deletion of those roles should involve multiple levels of
authorization (e.g. maker/checker), this may be tracked within the application itself, or through some centralized
role application. Both the functionality and code of the system controlling roles should be part of the review scope.
• At a design level attempt to keep the range of roles simple. Applications with multiple permission levels/roles often
increases the possibility of conicting permission sets resulting in unanticipated privileges.
• In application architectures with thick clients (i.e. mobile apps or binaries running on a PC) do not attempt to
perform any authorization in the client code, as this could be bypassed by an attacker. In browser based applications
do not perform any authorization decisions in JavaScript.
• Never base authorization decisions on untrusted data. For example do not use a header, or hidden eld, from the
client request to determine the level of authorization a user will have, again this can be manipulated by an attacker.
• Follow the principle of ‘complete mediation’, where authorization is checked at every stage of a function. For exam-
ple, if an application has four pages to browse through to purchase an item (browse.html, basket.html, inputPay-
ment.html, makePayment.html) then check user authorization at every page, and stage within pages, instead of only
performing a check in the rst page.
[AllowAnonymous]
public ActionResult LogMeIn(string returnUrl)
Sample 13.3
A7 - Missing Function Level Access Control
138
• By default deny access to any page, and then use authorization logic to explicitly allow access based on roles/
ACL rules.
• Where to hold authorization data, DB or session?
• Remove all redundant/test/unexposed business logic congurations from the le
• The business/form/command objects must have only those instance variables that are dependent on the
user inputs.
A7 - Missing Function Level Access Control
139
A8
140 A8 - Cross-Site Request Forgery (CSRF)
A CSRF attack forces a logged-on victim’s browser to send a forged HTTP request, including the victim’s session
cookie and any other automatically included authentication information, to a vulnerable web application. This
allows the attacker to force the victim’s browser to generate requests the vulnerable application thinks are
legitimate requests from the victim.
14.1 Description
CSRF is an attack which forces an end user to execute unwanted actions on a web application in which they are
currently authenticated. With a little help of social engineering (like sending a link via email/chat), an attacker
may force the users of a web application to execute actions of the attacker’s choosing. A successful CSRF ex-
ploit can compromise end user data, and protected functionality, in the case of a normal privileged user. If the
targeted end user is the administrator account, this can compromise the entire web application.
The impact of a successful cross-site request forgery attack is limited to the capabilities exposed by the vul-
nerable application. For example, this attack could result in a transfer of funds, changing a password, or pur-
chasing an item in the user’s context. In eect, CSRF attacks are used by an attacker to make a target system
perform a function (funds Transfer, form submission etc.) via the target’s browser without knowledge of the
target user, at least until the unauthorized function has been committed.
CSRF is not the same as XSS (Cross Site Scripting), which forces malicious content to be served by a trusted
website to an unsuspecting victim. Cross-Site Request Forgery (CSRF, a.k.a C-SURF or Confused-Deputy) at-
tacks are considered useful if the attacker knows the target is authenticated to a web based system. They only
work if the target is logged into the system, and therefore have a small attack footprint. Other logical weak-
nesses also need to be present such as no transaction authorization required by the user. In eect CSRF attacks
are used by an attacker to make a target system perform a function (Funds Transfer, Form submission etc..) via
the target’s browser without the knowledge of the target user, at least until the unauthorized function has
been committed. A primary target is the exploitation of “ease of use” features on web applications (One-click
purchase).
Impacts of successful CSRF exploits vary greatly based on the role of the victim. When targeting a normal user,
a successful CSRF attack can compromise end-user data and their associated functions. If the targeted end
user is an administrator account, a CSRF attack can compromise the entire Web application. The sites that are
more likely to be attacked are community Websites (social networking, email) or sites that have high dollar
value accounts associated with them (banks, stock brokerages, bill pay services). This attack can happen even
if the user is logged into a Web site using strong encryption (HTTPS). Utilizing social engineering, an attacker
will embed malicious HTML or JavaScript code into an email or Website to request a specic ‘task url’. The task
then executes with or without the user’s knowledge, either directly or by utilizing a Cross-site Scripting aw
(ex: Samy MySpace Worm).
How They Work
CSRF attacks work by sending a rogue HTTP request from an authenticated user’s browser to the application,
which then commits a transaction without authorization given by the target user. As long as the user is au-
thenticated and a meaningful HTTP request is sent by the user’s browser to a target application, the applica-
tion does not know if the origin of the request is a valid transaction or a link clicked by the user (that was, say,
in an email) while the user is authenticated to the application. The request will be authenticated as the request
from the users browser will automatically include the ‘Cookie’ header, which is the basis for authentication. So
CROSS-SITE REQUEST FORGERY (CSRF)
A8
141
A8 - Cross-Site Request Forgery (CSRF)
an attacker makes the victim perform actions that they didn’t intend to, such as purchase an item. Sample
14.1 shows an example an HTTP POST to a ticket vendor to purchase a number of tickets.
What to Review
This issue is simple to detect, but there may be compensating controls around the functionality of the application
which may alert the user to a CSRF attempt. As long as the application accepts a well formed HTTP request and the
request adheres to some business logic of the application CSRF shall work.
By checking the page rendering we need to see if any unique identiers are appended to the links rendered by the
application in the user’s browser. If there is no unique identier relating to each HTTP request to tie a HTTP request to
the user, we are vulnerable. Session ID is not enough, as the session ID shall be sent automatically if a user clicks on a
rogue link, as the user is already authenticated.
Prevention Measures That Do NOT Work
Examples of attempted CSRF prevent techniques which attackers can bypass are listed in table 21, these measures
should not be used in sensitive applications and should fail code review.
POST http://TicketMeister.com/Buy_ticket.htm HTTP/1.1
Host: ticketmeister
User-Agent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O;) Firefox/1.4.1
Cookie: JSPSESSIONID=34JHURHD894LOP04957HR49I3JE383940123K
ticketId=ATHX1138&to=PO BOX 1198 DUBLIN 2&amount=10&date=11042008
The response of the vendor is to acknowledge the purchase of the tickets:
HTTP/1.0 200 OK
Date: Fri, 02 May 2008 10:01:20 GMT
Server: IBM_HTTP_Server
Content-Type: text/xml;charset=ISO-8859-1
Content-Language: en-US
X-Cache: MISS from app-proxy-2.proxy.ie
Connection: close
<?xml version=”1.0” encoding=”ISO-8859-1”?>
<pge_data> Ticket Purchased, Thank you for your custom.
</pge_data>
Sample 14.1
Measure Description
Using a Secret Cookie Remember that all cookies, even the secret ones, will be submitted with every request. All authentication tokens
will be submitted regardless of whether or not the end-user was tricked into submitting the request. Further-
more, session identiers are simply used by the application container to associate the request with a specic
session object. The session identier does not verify that the end-user intended to submit the request.
Table 21: Unsuccessful Countermeasures For Csrf Attacks
142
Password, transferring funds, purchasing, the application may not simply execute the request, but should re-
spond to the request with another request for the user’s password, or an out-of-band (i.e. two-factor) authen-
tication item.
Preventing CSRF
Checking if the request has a valid session cookie is not enough, the application needs to have a unique iden-
tier assocaited with every HTTP request sent to the application. CSRF requests (from an attackers e-mail) will
not have this valid unique identier. The reason CSRF requests won’t have this unique request identier is the
unique ID is rendered as a hidden eld, or within the URL, and is appended to the HTTP request once a link/
button press is selected. The attacker will have no knowledge of this unique ID, as it is random and rendered
dynamically per link, per page.
Application logic to prevent CSRF then include:
1. A list of unique IDs is compiled prior to delivering the page to the user. The list contains all valid unique IDs
generated for all links on a given page. The unique ID could be derived from a secure random generator such
as SecureRandom for J2EE and could be stored in the session or another centralized cache.
2. A unique ID is appended to each link/form on the requested page prior to being displayed to the user.
3. The application checks if the unique ID passed with the HTTP request is valid for a given request. If the
unique ID passed with the HTTP request is valid for a given request.
4. If the unique ID is not present, terminate the user session and display an error to the user.
General Recommendation: Synchronizer Token Pattern
In order to facilitate a “transparent but visible” CSRF solution, developers are encouraged to adopt the Syn-
chronizer Token Pattern http://www.corej2eepatterns.com/Design/PresoDesign.htm. The synchronizer
token pattern requires the generating of random “challenge” tokens that are associated with the user’s current
session. These challenge tokens are then inserted within the HTML forms and links associated with sensitive
server-side operations. When the user wishes to invoke these sensitive operations, the HTTP request should
include this challenge token. It is then the responsibility of the server application to verify the existence and
correctness of this token. By including a challenge token with each request, the developer has a strong control
to verify that the user actually intended to submit the desired requests. Inclusion of a required security token in
HTTP requests associated with sensitive business functions helps mitigate CSRF attacks as successful exploita-
tion assumes the attacker knows the randomly generated token for the target victim’s session. This is analo-
gous to the attacker being able to guess the target victim’s session identier. The following synopsis describes a
general approach to incorporate challenge tokens within the request.
When a Web application formulates a request (by generating a link or form that causes a request when submitted or
A8 - Cross-Site Request Forgery (CSRF)
Applications can be developed to only accept POST requests for the execution of business logic. The misconcep-
tion is that since the attacker cannot construct a malicious link, a CSRF attack cannot be executed. Unfortunate-
ly, this logic is incorrect. There are numerous methods in which an attacker can trick a victim into submitting a
forged POST request, such as a simple form hosted in an attacker’s website with hidden values. This form can be
triggered automatically by JavaScript or can be triggered by the victim who thinks the form will do something
else.
Salt = ‘a0w8hsdfas8ls587uas87’
URL Rewriting
Multi-Step transactions are not an adequate prevention of CSRF. As long as an attacker can predict or deduce
each step of the completed transaction, then CSRF is possible.
This might be seen as a useful CSRF prevention technique as the attacker cannot guess the victim’s session ID.
However, the user’s credential is exposed over the URL.
Only Accepting POST Requests
143
clicked by the user), the application should include a hidden input parameter with a common name such as “CSRFTo-
ken”. The value of this token must be randomly generated such that it cannot be guessed by an attacker. Consider
leveraging the java.security.SecureRandom class for Java applications to generate a suciently long random token.
Alternative generation algorithms include the use of 256-bit BASE64 encoded hashes. Developers that choose this
generation algorithm must make sure that there is randomness and uniqueness utilized in the data that is hashed to
generate the random token.
Depending on the risk level of the product, it may only generate this token once for the current session. After initial
generation of this token, the value is stored in the session and is utilized for each subsequent request until the session
expires. When a request is issued by the end-user, the server-side component must verify the existence and validity of
the token in the request as compared to the token found in the session. If the token was not found within the request
or the value provided does not match the value within the session, then the request should be aborted, token should
be reset and the event logged as a potential CSRF attack in progress.
To further enhance the security of this proposed design, consider randomizing the CSRF token parameter name and
or value for each request. Implementing this approach results in the generation of per-request tokens as opposed to
per-session tokens. Note, however, that this may result in usability concerns. For example, the “Back” button browser
capability is often hindered as the previous page may contain a token that is no longer valid. Interaction with this
previous page will result in a CSRF false positive security event at the server. Regardless of the approach taken, devel-
opers are encouraged to protect the CSRF token the same way they protect authenticated session identiers, such
as the use of SSLv3/TLS.
Disclosure of Token in URL
Many implementations of this control include the challenge token in GET (URL) requests as well as POST requests.
This is often implemented as a result of sensitive server-side operations being invoked as a result of embedded links
in the page or other general design patterns. These patterns are often implemented without knowledge of CSRF and
an understanding of CSRF prevention design strategies. While this control does help mitigate the risk of CSRF attacks,
the unique per-session token is being exposed for GET requests. CSRF tokens in GET requests are potentially leaked at
several locations: browser history, HTTP log les, network appliances that make a point to log the rst line of an HTTP
request, and Referer headers if the protected site links to an external site.
In the latter case (leaked CSRF token due to the Referer header being parsed by a linked site), it is trivially easy for the
linked site to launch a CSRF attack on the protected site, and they will be able to target this attack very eectively,
since the Referer header tells them the site as well as the CSRF token. The attack could be run entirely from javascript,
so that a simple addition of a script tag to the HTML of a site can launch an attack (whether on an originally malicious
site or on a hacked site). This attack scenario is easy to prevent, the referer will be omitted if the origin of the request
<form action=”/transfer.do” method=”post”>
<input type=”hidden” name=”CSRFToken”
value=”OWY4NmQwODE4ODRjN2Q2NTlhMmZlYWE...
wYzU1YWQwMTVhM2JmNGYxYjJiMGI4MjJjZDE1ZDZ...
MGYwMGEwOA==”>
…
</form>
Sample 14.2
A8 - Cross-Site Request Forgery (CSRF)
144
is HTTPS. Therefore this attack does not aect web applications that are HTTPS only.
The ideal solution is to only include the CSRF token in POST requests and modify server-side actions that have state
changing aect to only respond to POST requests. This is in fact what the RFC 2616 requires for GET requests. If sen-
sitive server-side actions are guaranteed to only ever respond to POST requests, then there is no need to include the
token in GET requests.
Viewstate (ASP.NET)
ASP.NET has an option to maintain your ViewState. The ViewState indicates the status of a page when submitted to
the server. The status is dened through a hidden eld placed on each page with a <form runat=”server”> control.
Viewstate can be used as a CSRF defense, as it is dicult for an attacker to forge a valid Viewstate. It is not impossible
to forge a valid Viewstate since it is feasible that parameter values could be obtained or guessed by the attacker.
However, if the current session ID is added to the ViewState, it then makes each Viewstate unique, and thus immune
to CSRF.
To use the ViewS tateUserKey proper ty wi thin the Viewstate to protec t ag ainst spo ofed pos t ba cks add the followi ng
in the OnInit virtual method of the page-derived class (This property must be set in the Page.Init event)
To key the Viewstate to an individual using a unique value of your choice use “(Page.ViewStateUserKey)”. This must
be applied in Page_Init because the key has to be provided to ASP.NET before Viewstate is loaded. This option has
been available since ASP.NET 1.1. However, there are limitations on this mechanism. Such as, ViewState MACs are only
checked on POSTback, so any other application requests not using postbacks will happily allow CSRF.
Double Submit Cookies
Double submitting cookies is dened as sending a random value in both a cookie and as a request parameter, with
the server verifying if the cookie value and request value are equal.
When a user authenticates to a site, the site should generate a (cryptographically strong) pseudorandom value and
set it as a cookie on the user’s machine separate from the session id. The site does not have to save this value in any
way. The site should then require every sensitive submission to include this random value as a hidden form value (or
other request parameter) and also as a cookie value. An attacker cannot read any data sent from the server or modify
cookie values, per the same-origin policy. This means that while an attacker can send any value he wants with a mali-
cious CSRF request, the attacker will be unable to modify or read the value stored in the cookie. Since the cookie value
and the request parameter or form value must be the same, the attacker will be unable to successfully submit a form
unless he is able to guess the random CSRF value.
Direct Web Remoting (DWR) Java library version 2.0 has CSRF protection built in as it implements the double cookie
submission transparently.
The above CSRF prevents rely on the use of a unique token and the Same-Origin Policy to prevent CSRF by maintain-
ing a secret token to authenticate requests. The following methods can prevent CSRF by relying upon similar rules
protected override OnInit(EventArgs e) {
base.OnInit(e);
if (User.Identity.IsAuthenticated)
ViewStateUserKey = Session.SessionID; }
Sample 14.3
A8 - Cross-Site Request Forgery (CSRF)
145
that CSRF exploits can never break.
Checking The Referer Header
Although it is trivial to spoof the referer header on your own browser, it is impossible to do so in a CSRF attack. Check-
ing the referer is a commonly used method of preventing CSRF on embedded network devices because it does
not require a per-user state. This makes a referer a useful method of CSRF prevention when memory is scarce. This
method of CSRF mitigation is also commonly used with unauthenticated requests, such as requests made prior to
establishing a session state which is required to keep track of a synchronization token.
However, checking the referer is considered to be a weaker from of CSRF protection. For example, open redirect
vulnerabilities can be used to exploit GET-based requests that are protected with a referer check and some organiza-
tions or browser tools remove referrer headers as a form of data protection. There are also common implementation
mistakes with referer checks. For example if the CSRF attack originates from an HTTPS domain then the referer will be
omitted. In this case the lack of a referer should be considered to be an attack when the request is performing a state
change. Also note that the attacker has limited inuence over the referer. For example, if the victim’s domain is “site.
com” then an attacker have the CSRF exploit originate from “site.com.attacker.com” which may fool a broken referer
check implementation. XSS can be used to bypass a referer check.
In short, referer checking is a reasonable form of CSRF intrusion detection and prevention even though it is not a
complete protection. Referer checking can detect some attacks but not stop all attacks. For example, if the HTTP
referrer is from a dierent domain and you are expecting requests from your domain only, you can safely block that
request.
Checking The Origin Header
The Origin HTTP Header standard was introduced as a method of defending against CSRF and other Cross-Domain
attacks. Unlike the referer, the origin will be present in HTTP request that originates from an HTTPS URL. If the origin
header is present, then it should be checked for consistency.
Challenge-Response
Challenge-Response is another defense option for CSRF. As mentioned before it is typically used when the func-
tionality being invoked is high risk. While challenge-response is a very strong defense to CSRF (assuming proper
implementation), it does impact user experience. For applications in need of high security, tokens (transparent) and
challenge-response should be used on high risk functions.
The following are some examples of challenge-response options:
• CAPTCHA
• Re-Authentication (password)
• One-time Token
No Cross-Site Scripting (XSS) Vulnerabilities
Cross-Site Scripting is not necessary for CSRF to work. However, any cross-site scripting vulnerability can be used to
defeat token, Double-Submit cookie, referer and origin based CSRF defenses. This is because an XSS payload can
simply read any page on the site using a XMLHttpRequest and obtain the generated token from the response, and in-
clude that token with a forged request. This technique is exactly how the MySpace (Samy) worm defeated MySpace’s
anti CSRF defenses in 2005, which enabled the worm to propagate. XSS cannot defeat challenge-response defenses
such as Captcha, re-authentication or one-time passwords. It is imperative that no XSS vulnerabilities are present to
ensure that CSRF defenses can’t be circumvented.
A8 - Cross-Site Request Forgery (CSRF)
146
A9
147
A9 - Using Components with Known Vulnerabilities
Components, such as libraries, frameworks, and other software modules, almost always run with full privileg-
es. If a vulnerable component is exploited, such an attack can facilitate serious data loss or server takeover.
Applications using components with known vulnerabilities may undermine application defenses and enable
a range of possible attacks and impacts.
15.1 Description
Today it would be rare for an application or software component to be developed without the re-use of some
open source or paid-for library or framework. This makes a lot of sense as these frameworks and libraries are
already developed and working, and have had a good degree of testing applied. However these third party
components can also be a source of security vulnerabilities when an attacker nds a aw in the components
code, in fact this aw has added attraction since the attacker knows the exploit will work on everyone who is
using the component.
This issue has matured to such a state that aws/exploits for popular frameworks, libraries and operating sys-
tems are sold on underground markets for large sums of money.
What to Review
There is really no code to review for this topic, unless your organization has taken it upon itself to review the code of
the component (assuming its open source and not a closed source third party library), in which case the code review
would be similar to any other audit review. However code review can be used within the larger company-wide track-
ing or audit mechanisms that lets the organization know what third party code it is using.
Regardless of the size of company, the use of third party components, and their versions, should be tracked to en-
sure the organization can be alerted when any security vulnerabilities are agged. For smaller companies with 1 or
2 products this tracking could be as easy as a spreadsheet or wiki page, however for larger companies with 100s of
applications or products, the task of tracking developer use of third party frameworks and libraries is equally as large
as the risk posed by those libraries.
If a company has 20 products and each of those products use 5 or 6 third party components (e.g. Apache web servers,
OpenSSL crypto libraries, Java libraries for regex and DB interactions, etc.) that leaves the company with over 100
external sources where security vulnerabilities can come from. If the company suddenly hears of a heartbleed type
vulnerability, it has to be able to react and upgrade those aected applications, or take other countermeasures, to
protect itself and its customers.
Controlling the Ingredients
One method used by larger companies to limit their exposure to third party vulnerabilities is to control which librar-
ies can be used by their developers. For example they could specify that developers should use a certain version of
OpenSSL as the crypto library instead of other options.
This allows management and risk controllers to know their risk prole to vulnerabilities on the market, if a bug ap-
pears in bouncycastle, they know they are not exposed (i.e. some developer didn’t use bouncycastle on one of the
products, because it’s not on the list of crypto libraries to use). On the other hand, if there is a bug in OpenSSL, all their
eggs are in that basket and they need to upgrade immediately.
USING COMPONENTS WITH KNOWN
VULNERABILITIES
A9
148 A9 - Using Components With Known Vulnerabilities
There will obviously be technical challenges to limiting the choices of third party components, and such a policy
could be unpopular with developers who’ll want to use the latest and greatest framework, but the rst step to secur-
ing a product is knowing what ingredients you’ve made it with.
How can such a policy be tracked or enforced? At some point the library or framework, in the form of .dll/.so or as
source code, will be integrated into the codeline.
Such integrations should be subject to code review, and as a task of this code review the reviewer can check:
1. The library is one that can be used in the product suite (or maybe is already used and the developer is simply
unaware, in which case the review should be rejected and the original integration used)
2. Any tracking or auditing software (even a basic spread sheet) is updated to reect that the product is using the
third party library. This allows for rapid remediation if a vulnerability appears, meaning the product will be patched.
SledgeHammer to Crack a Nut
One last responsibility of the reviewer will be to ensure the correct third party library is being used for the function-
ality needed. Many libraries come with vast amounts of functionality which may not be used. For example, is the
developer including a library to perform their regex’s, but which also will include other functionality not used/needed
by the application? This increases the attack surface of the application and can cause unexpected behavior when
that extra code opens a port and communicates to the internet.
If the reviewer thinks too much functionality/code is being introduced they can advise to turn o non-used function-
ality, or better still nd a way to not include that functionality in the product (e.g. by stripping out code, or hardcoding
branches so unused functions are never used).
The OWASP project “OWASP Dependency Check” can provide a measure of automation for library checking
(https://www.owasp.org/index.php/OWASP_Dependency_Check)
149
A10
150
Web applications frequently redirect and forward users to other pages and websites, and use untrusted data to de-
termine the destination pages. Without proper validation, attackers can redirect victims to phishing or malware sites,
or use forwards to access unauthorized pages.
16.1 Description
Unvalidated redirects and forwards are possible when a web application accepts untrusted input that could cause
the web application to redirect the request to a URL contained within untrusted input. By modifying untrusted URL
input to a site, an attacker may successfully launch a phishing scam and steal user credentials.
As the server name in the modied link is identical to the original site, phishing attempts may have a more trust-
worthy appearance. Invalidated redirect and forward attacks can also be used to maliciously craft a URL that would
pass the application’s access control check and then forward the attacker to privileged functions that they would
normally not be able to access.
Redirects
Redirect functionality on a web site allows a user’s browser to be told to go to a dierent page on the site. This can be
done to improve user interface or track how users are navigating the site.
To p rovide th e redi rec t fu nc tion ality a site may have a s pecic U RL to pe rform the re direc t:
• http://www.example.com/utility/redirect.cgi
This page will take a parameter (from URL or POST body) of ‘URL’ and will send back a message to the user’s browser
to go to that page, for example:
• http://www.example.com/utility/redirect.cgi?URL=http://www.example.com/viewtxn.html
However this can be abused as an attacker can attempt to make a valid user click on a link that appears to be for
www.example.com but which will invoke the redirect functionality on example.com to cause the users browser to go
to a malicious site (one that could look like example.com and trick the user into entering sensitive or authentication
information:
• http://www.example.com/utiltiy/redirect cgi?URL=http://attacker.com/fakelogin.html
Forwards
Forwards are similar to redirects however the new page is not retrieved by the users browser (as occurred with the
redirect) but instead the server framework will obtain the forwarded page and return it to the users browser. This is
achieved by ‘forward’ commands within Java frameworks (e.g. Struts) or ‘Server.Transfer’ in .Net. As the forward is per-
formed by the server framework itself, it limits the range of URLs the attacker can exploit to the current web site (i.e.
attacker cannot ‘forward’ to attacker.com), however this attack can be used to bypass access controls. For example,
where a site sends the forwarded page in the response:
• If purchasing, forward to ‘purchase.do’
• If cancelling, forward to ‘cancelled.do’
This will then be passed as a parameter to the web site:
• http://www.example.com/txn/acceptpayment.html?FWD=purchase
UNVALIDATED REDIRECTS AND FORWARDS
A10
A10 - Unvalidated Redirects And Forwards
151
A10 - Unvalidated Redirects And Forwards
If instead an attacker used the forward to attempt to access to a dierent page within the web site, e.g. admin.do, then
they may access pages that they are not authorized to view, because authorization is being applied on the ‘accept-
payment’ page, instead of the forwarded page.
What to Review
If any part of the URL being forwarded, or redirected, to is based on user input, then the site could be at risk. Ensure:
• All redirects/forwards are constructed based on a whitelist, or
• All redirtects/forwards use reletive paths to ensure they stay on the trusted site
Redirects
The following examples demonstrate unsafe redirect and forward code. The following Java code receives the URL
from the ‘url’ parameter and redirects to that URL.
The following PHP code obtains a URL from the query string and then redirects the user to that URL.
A similar example of C# .NET Vulnerable Code:
The above code is vulnerable to an attack if no validation or extra method controls are applied to verify the
certainty of the URL. This vulnerability could be used as part of a phishing scam by redirecting users to a ma-
licious site. If user input has to be used as part of the URL to be used, then apply strict validation to the input,
ensuring it cannot be used for purposes other than intended.
Note that vulnerable code does not need to explicitly call a ‘redirect’ function, but instead could directly modi-
fy the response to cause the client browser to go to the redirected page. Code to look for is shown in table 22.
response.sendRedirect(request.getParameter(“url”));
Sample 16.1
$redirect_url = $_GET[‘url’];
header(“Location: “ . $redirect_url);
Sample 16.2
string url = request.QueryString[“url”];
Response.Redirect(url);
Sample 16.3
152
Where an attacker has posted a redirecting URL on a forum, or sends in an e-mail, the web site can check the
referer header to ensure the user is coming from a page within the site, although this countermeasure will not
apply if the malicious URL is contained within the site itself.
Consider creating a whitelist of URLs or options that a redirect is allowed to go to, or deny the ability for the
user input to determine the scheme or hostname of the redirect. A site could also encode (or encrypt) the
URL value to be redirected to such that an attacker cannot easily create a malicious URL parameter that, when
unencoded (or unencypted), will be considered valid.
Forwards
The countermeasure for forwards is to either whitelist the range of pages that can be forwarded to (similar to
redirects) and to enforce authentication on the forwarded page as well as the forwarding page. This means
that even if an attacker manages to force a forward to a page they should not have access to, the authentica-
tion check on the forwarded page will deny them access.
Note on J2EE
There is a noted aw related to the “sendRedirect” method in J2EE applications. For example:
• response.sendRedirect(“home.html”);
This method is used to send a redirection response to the user who then gets redirected to the desired web
component whose URL is passed an argument to the method. One such misconception is that execution ow
in the Servlet/JSP page that is redirecting the user stops after a call to this method. Note that if there is code
present after the ‘If’ condition it will be executed.
The fact that execution of a servlet or JSP continues even after sendRedirect() method, also applies to Forward
method of the RequestDispatcher Class. However, <jsp:forward> tag is an exception, it is observed that the
execution ow stops after the use of <jsp:forward> tag.
After issue a redirect or forward, terminate code ow using a “return” statement.
References
• OWASP Article on Open Redirects https://www.owasp.org/index.php/Open_redirect
Method of Redirection Description
Redirect Response
(note 301 and 307 responses will
also cause a redirect)
HTTP/1.1 302 Found
Location: http://www.attacker.com/page.html
Meta Tag <html><head>
<meta http-equiv=”Refresh”
content=”0;url=http://attacker.com/page.html”>
JavaScript
Refresh Header
<script type=”text/javascript”>
<!—
window.location=”http://attacker.com/page.html”
//-->
HTTP/1.1 200 OK
Refresh=0; url=http://attacker.com/page.html
Table 22: Redirect Risks
A10 - Unvalidated Redirects And Forwards
153
• CWE Entry 601 on Open Redirects http://cwe.mitre.org/data/denitions/601.html
• WASC Article on URL Redirector Abuse http://projects.webappsec.org/w/page/13246981/URL%20Redi-
rector%20Abuse
• Google blog article on the dangers of open redirects http://googlewebmastercentral.blogspot.
com/2009/01/open-redirect-urls-is-your-site-being.html
• Preventing Open Redirection Attacks (C#) http://www.asp.net/mvc/tutorials/security/preventing-open-re-
direction-attacks
A10 - Unvalidated Redirects And Forwards
154 HTML 5
HTML5 was created to replace HTLML4, XHTML and the HTML DOM Level 2. The main purpose of this new stan-
dard is to provide dynamic content without the use of extra proprietary client side plugins. This allows designers
and developers to create exceptional sites providing a great user experience without having to install any addi-
tional plug-ins into the browser.
17.1 Description
Ideally users should have the latest web browser installed but this does not happens as regularly as security ex-
perts advice, therefore the website should implement 2 layer controls, one layer independent from browser type,
second, as an additional control.
What to Review: Web Messaging
Web Messaging (also known as Cross Domain Messaging) provides a means of messaging between documents
from dierent origins in a way that is generally safer than the multiple hacks used in the past to accomplish this
task. The communication API is as follows:
However, there are still some recommendations to keep in mind:
• When posting a message, explicitly state the expected origin as the second argument to ‘postMessage’ rather
than ‘*’ in order to prevent sending the message to an unknown origin after a redirect or some other means of the
target window’s origin changing.
• The receiving page should always:
o Check the ‘origin’ attribute of the sender to verify the data is originating from the expected location.
o Perform input validation on the ‘data’ attribute of the event to ensure that it’s in the desired format.
• Don’t assume you have control over the ‘data’ attribute. A single Cross Site Scripting aw in the sending page
allows an attacker to send messages of any given format.
• Both pages should only interpret the exchanged messages as ‘data’. Never evaluate passed messages as code (e.g.
via ‘eval()’) or insert it to a page DOM (e.g. via ‘innerHTML’), as that would create a DOM-based XSS vulnerability.
• To assign the data value to an element, instead of using an insecure method like ‘element.innerHTML = data’, use
the safer option: ‘element.textContent = data;’
• Check the origin properly exactly to match the FQDN(s) you expect. Note that the following code: ‘ if(message.
orgin.indexOf(“.owasp.org”)!=-1) { /* ... */ }’ is very insecure and will not have the desired behavior as ‘www.owasp.
org.attacker.com’ will match.
• If you need to embed external content/untrusted gadgets and allow user-controlled scripts (which is highly
discouraged), consider using a JavaScript rewriting framework such as Google’s Caja or check the information
on sandboxed frames.
HTML5
155
HTML 5
What to Review: Cross Origin Resource Sharing
Cross Origin Resource Sharing or CORS is a mechanism that enables a web browser to perform “cross-domain”
requests using the XMLHttpRequest L2 API in a controlled manner. In the past, the XMLHttpRequest L1 API
only allowed requests to be sent within the same origin as it was restricted by the same origin policy.
Cross-Origin requests have an Origin header that identies the domain initiating the request and is automat-
ically included by the browser in the request sent to the server. CORS denes the protocol between a web
browser and a server that will determine whether a cross-origin request is allowed. In order to accomplish
this goal, there are HTTP headers that provide information on the messaging context including: Origin, Ac-
cess-Control-Request-Method, Access-Control-Request-Headers, Access-Control-Allow-Origin, Access-Con-
trol-Allow-Credentials, Access-Control-Allow-Methods, Access-Control-Allow-Headers.
The CORS specication mandates that for non simple requests, such as requests other than GET or POST or requests
that uses credentials, a pre-ight OPTIONS request must be sent in advance to check if the type of request will have
a negative impact on the data. The pre-ight request checks the methods, headers allowed by the server, and if
credentials are permitted, based on the result of the OPTIONS request, the browser decides whether the request is
allowed or not.
Items to note when reviewing code related to CORS includes:
• Ensure that URLs responding with ‘Access-Control-Allow-Origin: *’ do not include any sensitive content or informa-
tion that might aid attacker in further attacks. Use the ‘Access-Control-Allow-Origin’ header only on chosen URLs that
need to be accessed cross-domain. Don’t use the header for the whole domain.
• Allow only selected, trusted domains in the ‘Access-Control-Allow-Origin’ header. Prefer whitelisting domains over
blacklisting or allowing any domain (do not use ‘*’ wildcard nor blindly return the ‘Origin’ header content without any
checks).
• Keep in mind that CORS does not prevent the requested data from going to an unauthenticated location. It’s still
important for the server to perform usual Cross-Site Request Forgery prevention.
• While the RFC recommends a pre-ight request with the ‘OPTIONS’ verb, current implementations might not per-
form this request, so it’s important that “ordinary” (‘GET’ and ‘POST’) requests perform any access control necessary.
• Discard requests received over plain HTTP with HTTPS origins to prevent mixed content bugs.
• Don’t rely only on the Origin header for Access Control checks. Browser always sends this header in CORS requests,
but may be spoofed outside the browser. Application-level protocols should be used to protect sensitive data.
What to Review: WebSockets
Traditionally the HTTP protocol only allows one request/response per TCP connection. Asynchronous JavaS-
cript and XML (AJAX) allows clients to send and receive data asynchronously (in the background without a
page refresh) to the server, however, AJAX requires the client to initiate the requests and wait for the server
responses (half-duplex). HTML5 WebSockets allow the client/server to create a ‘full-duplex’ (two-way) commu-
nication channels, allowing the client and server to truly communicate asynchronously. WebSockets conduct
their initial ‘upgrade’ handshake over HTTP and from then on all communication is carried out over TCP chan-
nels.
The following is sample code of an application using Web Sockets:
156
When reviewing code implementing websockets, the following items should be taken into consideration:
• Drop backward compatibility in implemented client/servers and use only protocol versions above hybi-00. Popular
Hixie-76 version (hiby-00) and older are outdated and insecure.
• The recommended version supported in latest versions of all current browsers is rfc6455 RFC 6455 (supported by
Firefox 11+, Chrome 16+, Safari 6, Opera 12.50, and IE10).
• While it’s relatively easy to tunnel TCP services through WebSockets (e.g. VNC, FTP), doing so enables access to these
tunneled services for the in-browser attacker in case of a Cross Site Scripting attack. These services might also be
called directly from a malicious page or program.
• The protocol doesn’t handle authorization and/or authentication. Application-level protocols should handle that
separately in case sensitive data is being transferred.
• Process the messages received by the websocket as data. Don’t try to assign it directly to the DOM nor evaluate as
code. If the response is JSON, never use the insecure eval() function; use the safe option JSON.parse() instead.
• Endpoints exposed through the ‘ws://’ protocol are easily reversible to plain text. Only ‘wss://’ (WebSockets over SSL/
TLS) should be used for protection against Man-In-The-Middle attacks.
• Spoong the client is possible outside a browser, so the WebSockets server should be able to handle incorrect/
malicious input. Always validate input coming from the remote site, as it might have been altered.
• When implementing servers, check the ‘Origin:’ header in the Websockets handshake. Though it might be spoofed
[Constructor(in DOMString url, optional in DOMString protocol)]
interface WebSocket
{ readonly attribute DOMString URL;
// ready state const unsigned short CONNECTING = 0;
const unsigned short OPEN = 1;
const unsigned short CLOSED = 2;
readonly attribute unsigned short readyState;
readonly attribute unsigned long bueredAmount;
// networking
attribute Function onopen;
attribute Function onmessage;
attribute Function onclose;
boolean send(in DOMString data);
void close();
};
WebSocket implements EventTarget;
var myWebSocket = new WebSocket(“ws://www.websockets.org”);
myWebSocket.onopen = function(evt) { alert(“Connection open ...”); };
myWebSocket.onmessage = function(evt) { alert( “Received Message: “ + evt.data); };
myWebSocket.onclose = function(evt) { alert(“Connection closed.”); };
Sample 17.1
HTML 5
157
outside a browser, browsers always add the Origin of the page that initiated the Websockets connection.
• As a WebSockets client in a browser is accessible through JavaScript calls, all Websockets communication can be
spoofed or hijacked through Cross Site Scripting. Always validate data coming through a WebSockets connection.
5.1.5 What to Review: Server-Sent Events
Server sent events seem similar to WebSockets, however they do not use a special protocol (they re-used HTTP) and
they allow the client browser to solely listen for updates (messages) from the server, thereby removing the need for
the client to send any polling or other messages up to the server.
When reviewing code that is handling server sent events, items to keep in mind are:
• Validate URLs passed to the ‘EventSource’ constructor, even though only same-origin URLs are allowed.
• As mentioned before, process the messages (‘event.data’) as data and never evaluate the content as HTML or script
code.
• Always check the origin attribute of the message (‘event.origin’) to ensure the message is coming from a
trusted domain. Use a whitelist.
HTML 5
158
Same Origin Policy (SOP), also called Single Origin Policy is a part of web application security model. Same Origin
Policy has vulnerabilities that the code reviewer needs to take into consideration. SOP covers three main areas of
web development, Trust, Authority, and Policy. Same Origin Policy is made of the combination of three components
(Scheme, Hostname and Port).
18.1 Description
Internet Explorer has two major exceptions when it comes to same origin policy:
1. Trust Zones: if both domains are in highly trusted zone e.g, corporate domains, then the same origin limita-
tions are not applied.
2. Port: IE doesn’t include port into Same Origin components, therefore http://yourcompany.com:81/index.
html and http://yourcompany.com/index.html are considered from same origin and no restrictions are applied.
These exceptions are non-standard and not supported in any of other browser but would be helpful if developing an
app for Windows RT (or) IE based web application.
The following gure displays the various parts of the URL:
SAME ORIGIN POLICY
foo://username:password@example.com:8042/over/there/index.dtb?type=animal&name=narwhal#nose
userinfo
pathauthority
hierarchical part
interpretable as keys
interpretable as values
scheme
name
hostname queryport
interpretable as lenamepath
interpretable as lename
fragment
Figure 12
Same Origin Policy
159
• foo://username:password@example.com:8042/over/there/index.dtb?type=animal&name =narwhal#nose
18.2 What to Review
• If application allows user-supplied data in the URL then the code reviewer needs to make sure the path,
query or Fragment Id Code data is validated.
• Make sure user-supplied scheme name or authority section has good input validation. This is a major code
injection and phishing risk. Only permit prexes needed by the application. Do not use blacklisting. Code re-
viewer should make sure only whitelisting is used for validation.
• Make sure authority section should only contain alphanumerics, “-“, and “.” And be followed by “/”, “?”,”#”. The
risk here an IDN homograph attack.
• Code reviewer needs to make sure the programmer is not assuming default behavior because the program-
mers browser properly escapes a particular character or browser standard says the character will be escaped
properly before allowing any URL-derived values are put inside a database query or the URL is echoed back
to the user.
• Resources with a MIME type of image/png are treated as images and resources with MIME type of text/html
are treated as HTML documents. Web applications can limit that content’s authority by restricting its MIME
type. For example, serving user-generated content as image/png is less risky than serving user-generated
content as text/html.
• Privileges on document and resources should grant or withhold privileges from origins as a whole (rath-
er than discriminating between individual documents within an origin). Withholding privileges is ineective
because the document without the privilege can usually obtain the privilege anyway because SOP does not
isolate documents within an origin.
Same Origin Policy
160 Reviewing Logging Code
Applications log messages of varying intensity and to varying sinks. Many logging APIs allow you to set the
granularity of log message from a state of logging nearly all messages at level ‘trace’ or ‘debug’ to only logging
the most important messages at level ‘critical’. Where the log message is written to is also a consideration,
sometimes it can be written to a local le, other times to a database log table, or it could be written over a
network link to a central logging server.
The volume of logging has to be controlled since the act of writing messages to the log uses CPU cycles, thus
writing every small detail to a log will use up more resources (CPU, network bandwidth, disk space). Couple
that with the fact that the logs have to be parsed or interpreted by a tool or human in order for them to be use-
ful, the consumer of the log could have to parse through thousands of lines to nd a message of consequence.
19.1 Description
Logs can vary by type; for example a log may simply contain application state or process data, allowing sup-
port or development track what the system is doing when a bug has occurred. Other logs may be specic to
security, only logging important information that a central security system will have interest in. Further logs
could be used for business purposes, such as billing.
Application logs can be powerful as the application business logic has the most information about the user
(e.g. identity, roles, permissions) and the context of the event (target, action, outcomes), and often this data
is not available to either infrastructure devices, or even closely-related applications. Application logging is an
important feature of a production system, especially to support personnel and auditors, however it is often
forgotten and is rarely described in sucient detail in design/requirement documentation. The level and con-
tent of security monitoring, alerting and reporting needs to be set during the requirements and design stage
of projects, and should be proportionate to the information security risks. Application logging should also be
consistent within the application, consistent across an organization’s application portfolio and use industry
standards where relevant, so the logged event data can be consumed, correlated, analyzed and managed by
a wide variety of systems.
All types of applications may send event data to remote systems, either directly over a network connection, or asyn-
chronously though a daily/weekly/monthly secure copy of the log to some centralized log collection and manage-
ment system (e.g. SIEM or SEM) or another application elsewhere.
If the information in the log is important, and could possibly be used for legal matters, consider how the source (log)
can be veried, and how integrity and non-repudiation can be enforced. Log data, temporary debug logs, and back-
ups/copies/extractions, must not be destroyed before the duration of the required data retention period, and must
not be kept beyond this time. Legal, regulatory and contractual obligations may impact on these periods.
Server applications commonly write event log data to the le system or a database (SQL or NoSQL), however logging
could be required on client devices such as applications installed on desktops and on mobile devices may use local
storage and local databases. Consider how this client logging data is transferred to the server.
What to Review
When reviewing code modules from a logging point of view, some common issues to look out for include:
REVIEWING LOGGING CODE
161
Reviewing Logging Code
• When using the le system, it is preferable to use a separate partition than those used by the operating system,
other application les and user generated content
• For le-based logs, apply strict permissions concerning which users can access the directories, and the permissions
of les within the directories
• In web applications, the logs should not be exposed in web-accessible locations, and if done so, should have
restricted access and be congured with a plain text MIME type (not HTML)
• When using a database, it is preferable to utilize a separate database account that is only used for writing log
data and which has very restrictive database, table, function and command permissions
• Consider what types of messages should be logged:
o Input validation failures e.g. protocol violations, unacceptable encodings, invalid parameter names and values
o Output validation failures e.g. database record set mismatch, invalid data encoding
o Authentication successes and failures
o Authorization (access control) failures
o Session management failures e.g. cookie session identication value modication
o Connection timings
• Consider what each log message should contain:
o Date and time, in some common format (also makes sense to ensure all nodes of an application are synced
through something like NTP
o User performing the action
o Action being performed/attempted
o Information on the client, e.g. IP address, source port, user-agent
o External classications e.g. NIST Security Content Automation Protocol (SCAP), Mitre Common Attack Pat-
tern Enumeration and Classication (CAPEC)
o Perform sanitization on all event data to prevent log injection attacks e.g. carriage return (CR), line feed (LF)
and delimiter characters (and optionally to remove sensitive data)
• If writing to databases, read, understand and apply the SQL injection cheat sheet
• Ensure logging is implemented and enabled during application security, fuzz, penetration and performance testing
162
• Ensure logging cannot be used to deplete system resources, for example by lling up disk space or exceeding
database transaction log space, leading to denial of service
• The logging mechanisms and collected event data must be protected from mis-use such as tampering in
transit, and unauthorized access, modication and deletion once stored
• Store or copy log data to read-only media as soon as possible
• Consider what should not be logged:
o Session identication values (consider replacing with a hashed value if needed to track session specic events)
o Sensitive personal data and some forms of personally identiable information (PII)
o Authentication passwords (successful or unsuccessful)
o Database connection strings
o Keys
o Data of a higher security classication than the logging system is allowed to store
References
• See NIST SP 800-92 Guide to Computer Security Log Management for more guidance.
• Mitre Common Event Expression (CEE)
• PCISSC PCI DSS v2.0 Requirement 10 and PA-DSS v2.0 Requirement 4
• Other Common Log File System (CLFS), Microsoft
Reviewing Logging Code
163
Error Handling
Proper error handling is important in two ways:
1. It may aect the state of the application. The initial failure to prevent the error may cause the application to traverse
into an insecure state. This covers the key premise of ‘failing securely’, errors induced should not leave the application
in an insecure state. Resources should be locked down and released, sessions terminated (if required), and calculations
or business logic should be halted (depending on the type of error, of course).
2. It may leak system information to a user. An important aspect of secure application development is to prevent
information leakage. Error messages give an attacker great insight into the inner workings of an application. Weak
error handling also aids the attacker, as the errors returned may assist them in constructing correct attack vectors.
A generic error page for most errors is recommended, this approach makes it more dicult for attackers to identify
signatures of potentially successful attacks such as blind SQL injection using booleanization ‘’<should include
reference>’’ or analysis of response time characteristics.
20.1 Description
The purpose of reviewing the Error Handling code is to assure that the application fails safely under all possible
error conditions, expected and unexpected. No sensitive information is presented to the user when an error occurs.
A company coding guidelines should include sections on Error Handling and how it should be controlled by an
application suite, this will allow developers to code against this guidelines as well as review against them.A company
coding guidelines should include sections on Error Handling and how it should be controlled by an application suite,
this will allow developers to code against this guidelines as well as review against them.
For example, SQL injection is much tougher to successfully execute without some healthy error messages. It lessens
the attack footprint, and an attacker would have to resort to using “blind SQL injection” which is more dicult and
time consuming.
A well-planned error/exception handling guideline is important in a company for three reasons:
1. Good error handling does not give an attacker any information which is a means to an end, attacking the application
2. A proper centralised error strategy is easier to maintain and reduces the chance of any uncaught errors “bubbling
up” to the front end of an application.
3. Information leakage could lead to social engineering exploits, for example if the hosting companies name is
returned, or some employees name can be seen.
Regardless of whether the development language provide checked exceptions or not, reviewers should
remember:
• Not all errors are exceptions. Don’t rely on exception handling to be your only way of handling errors, handle all
case statement ‘default’ sections, ensure all ‘if’ statements have their ‘else’ clauses covered, ensure that all exits from
a function (e.g. return statements, exceptions, etc.) are covered. RAII concepts (e.g. auto pointers and the like) are
an advantage here. In languages like Java and C#, remember that errors are dierent from exceptions (dierent
hierarchy) and should be handled.
ERROR HANDLING
164 Error Handling
• Catching an exception is not automatically handling it. You’ve caught your exception, so how do you handle
it? For many cases this should be obvious enough, based on your business logic, but for some (e.g. out of
memory, array index out of bounds, etc.) the handling many not be so simple.
• Don’t catch more that you can handle. Catch all clauses (e.g. ‘catch(Exception e)’ in Java & C# or ‘catch(...)
in C++) should be avoided as you will not know what type of exception you are handling, and if you don’t
know the exception type, how do you accurately handle it? It could be that the downstream server is not
responding, or a user may have exceeded their quota, or you may be out of memory, these issues should be
handled in dierent ways and thus should be caught in exception clauses that are specic.
When an exception or error is thrown, we also need to log this occurrence. Sometimes this is due to bad
development, but it can be the result of an attack or some other service your application relies on failing. This
has to be imagined in the production scenario, if your application handles ‘failing securely’ by returning an
error response to the client, and since we don’t want to leak information that error will be generic, we need to
have some way of identifying why the failure occurred. If your customer reports 1000’s of errors occurred last
night, you know that customer is going to want to know why. If you don’t have proper logging and traceability
coded into your application then you will not be able to establish if those errors were due to some attempted
hack, or an error in your business logic when handling a particular type of error.
All code paths that can cause an exception to be thrown should check for success in order for the exception
not to be thrown. This could be hard to impossible for a manual code review to cover, especially for large
bodies of code. However if there is a debug version of the code, then modules/functions could throw relevant
exceptions/errors and an automated tool can ensure the state and error responses from the module is as
expected. This then means the code reviewer has the job of ensuring all relevant exceptions/errors are tested
in the debug code.
What to Review
When reviewing code it is recommended that you assess the commonality within the application from an
error/exception handling perspective. Frameworks have error handling resources which can be exploited to
assist in secure programming, and such resources within the framework should be reviewed to assess if the
error handling is “wired-up” correctly. A generic error page should be used for all exceptions if possible as this
prevents the attacker from identifying internal responses to error states. This also makes it more dicult for
automated tools to identify successful attacks.
For JSP struts this could be controlled in the struts-cong.xml le, a key le when reviewing the wired-up struts
environment:
Specication can be done for JSP in web.xml in order to handle unhandled exceptions. When unhandled
exceptions occur, but are not caught in code, the user is forwarded to a generic error page:
<exception key=”bank.error.nowonga”
path=”/NoWonga.jsp”
type=”mybank.account.NoCashException”/>
Sample 20.1
165
Also in the case of HTTP 404 or HTTP 500 errors during the review you may nd:
For IIS development the ‘Application_Error()’ handler will allow the application to catch all uncaught exceptions and
handle them in a consistent way. Note this is important or else there is a chance your exception informaiton could be
sent back to the client in the response.
For Apache development, returning failures from handlers or modules can prevent an further processing by the
Apache engine and result in an error response from the server. Response headers, body, etc can be set by by the
handler/module or can be congured using the “ErrorDocument” conguration. We should use a localized description
string in every exception, a friendly error reason such as “System Error – Please try again later”. When the user sees an
error message, it will be derived from this description string of the exception that was thrown, and never from the
exception class which may contain a stack trace, line number where the error occurred, class name, or method name.
Do not expose sensitive information like exception messages. Information such as paths on the local le system is
considered privileged information; any internal system information should be hidden from the user. As mentioned
before, an attacker could use this information to gather private user information from the application or components
that make up the app.
Don’t put people’s names or any internal contact information in error messages. Don’t put any “human” information,
which would lead to a level of familiarity and a social engineering exploit.
What to Review: Failing Securely
There can be many dierent reasons why an application may fail, for example:
• The result of business logic conditions not being met.
• The result of the environment wherein the business logic resides fails.
• The result of upstream or downstream systems upon which the application depends fail.
• Tech nical hardware / physica l failure.
Error Handling
<error-page>
<exception-type>UnhandledException</exception-type>
<location>GenericError.jsp</location>
</error-page>
Sample 20.2
<error-page>
<error-code>500</error-code>
<location>GenericError.jsp</location>
</error-page>
Sample 20.3
166
Failures are like the Spanish Inquisition; popularly nobody expected the Spanish Inquisition (see Monty Python) but in
real life the Spanish knew when an inquisition was going to occur and were prepared for it, similarly in an application,
though you don’t expect errors to occur your code should be prepared for them to happen. In the event of a failure,
it is important not to leave the “doors” of the application open and the keys to other “rooms” within the application
sitting on the table. In the course of a logical workow, which is designed based upon requirements, errors may occur
which can be programmatically handled, such as a connection pool not being available, or a downstream server
returning a failure.
Such areas of failure should be examined during the course of the code review. It should be examined if resources
should be released such as memory, connection pools, le handles etc.
The review of code should also include pinpointing areas where the user session should be terminated or invalidated.
Sometimes errors may occur which do not make any logical sense from a business logic perspective or a technical
standpoint, for example a logged in user looking to access an account which is not registered to that user. Such
conditions reect possible malicious activity. Here we should review if the code is in any way defensive and kills
the user’s session object and forwards the user to the login page. (Keep in mind that the session object should be
examined upon every HTTP request).
What to Review: Potentially Vulnerable Code
Java
In Java we have the concept of an error object; the Exception object. This lives in the Java package java.lang and is
derived from the Throwable object. Exceptions are thrown when an abnormal occurrence has occurred. Another
object derived from Throwable is the Error object, which is thrown when something more serious occurs. The Error
object can be caught in a catch clause, but cannot be handled, the best you can do is log some information about
the Error and then re-throw it.
Information leakage can occur when developers use some exception methods, which ‘bubble’ to the user UI due to a
poor error handling strategy. The methods are as follows:
• printStackTrace()
• getStackTrace()
Also important to know is that the output of these methods is printed in System console, the same as System.out.
println(e) where there is an Exception. Be sure to not redirect the outputStream to PrintWriter object of JSP, by
convention called “out”, for example:
• printStackTrace(out);
Note it is possible to change where system.err and system.out write to (like modifying fd 1 & 2 in bash or C/C++),
using the java.lang.system package:
• setErr() for the System.err eld, and
• setOut() for the System.out eld.
This could be used on a process wide basis to ensure no output gets written to standard error or standard out (which
can be reected back to the client) but instead write to a congured log le.
C#.NET
In .NET a System.Exception object exists and has commonly used child objects such as ApplicationException and
SystemException are used. It is not recommended that you throw or catch a SystemException this is thrown by
runtime.
Error Handling
167
When an error occurs, either the system or the currently executing application reports it by throwing an exception
containing information about the error, similar to Java. Once thrown, an exception is handled by the application or
by the default exception handler. This Exception object contains similar methods to the Java implementation such as:
• StackTrace
• Source
• Message
• HelpLink
In .NET we need to look at the error handling strategy from the point of view of global error handling and the handling
of unexpected errors. This can be done in many ways and this article is not an exhaustive list. Firstly, an Error Event is
thrown when an unhandled exception is thrown.
This is part of the TemplateControl class, see reference:
• http://msdn.microsoft.com/library/default.asp?url=/library/enus/cpref/html/
frlrfSystemWebUITemplateControlClassErrorTopic.asp
Error handling can be done in three ways in .NET, executed in the following order:
• On the aspx or associated codebehind page in the Page_Error.
• In the global.asax le’s Application_Error (as mentioned before).
• In the web.cong le’s customErrors section.
It is recommended to look in these areas to understand the error strategy of the application.
Classic ASP
Unlike Java and .NET, classic ASP pages do not have structured error handling in try-catch blocks. Instead they have a
specic object called “err”. This makes error handling in a classic ASP pages hard to do and prone to design errors on
error handlers, causing race conditions and information leakage. Also, as ASP uses VBScript (a subtract of Visual Basic),
sentences like “On Error GoTo label” are not available. In classic ASP there are two ways to do error handling, the rst
is using the err object with a “On Error Resume Next” and “On Error GoTo 0”.
The second is using an error handler on an error page (http://support.microsoft.com/kb/299981).
Public Function IsInteger (ByVal Number)
Dim Res, tNumber
Number = Trim(Number)
tNumber=Number
On Error Resume Next ‘If an error occurs continue execution
Number = CInt(Number) ‘if Number is a alphanumeric string a Type Mismatch error will occur
Res = (err.number = 0) ‘If there are no errors then return true
On Error GoTo 0 ‘If an error occurs stop execution and display error
re.Pattern = “^[\+\-]? *\d+$” ‘only one +/- and digits are allowed
IsInteger = re.Test(tNumber) And Res
End Function
Sample 20.4
Error Handling
168
C++
In the C++ language, any object or built in type can be thrown. However there is a STL type std::exception
which is supposed to be used as the parent of any user dened exception, indeed this type is used in the
STL and many libraries as the parent type of all exceptions. Having std::exception encourages developers to
create a hierarchy of exceptions which match the exception usage and means all exceptions can be caught by
catching a std::exception object (instead of a ‘catch (...)’ block).
Unlike Java, even errors that you can’t recover from (e.g. std::bad_alloc which means your out of memory)
derive from std::exception, so a ‘catch( std::exception& e)’ is similar to ‘catch (...)’ except that it allows you access
to the exception so you can know what occurred and possibly print some error information using e.what().
There are many logging libraries for C++, so if your codebase uses a particular logging class look for usages of
that logger for anywhere sensitive information can be written to the logs.
What to Review: Error Handling in IIS
Page_Error is page level handling which is run on the server side in .NET. Below is an example but the error
information is a little too informative and hence bad practice.
The text in the example above has a number of issues. Firstly, it displays the HTTP request to the user in the
form of Request.Url.ToString().
Assuming there has been no data validation prior to this point, we are vulnerable to cross site scripting attacks.
Secondly, the error message and stack trace is displayed to the user using Server.GetLastError().ToString()
which divulges internal information regarding the application.
After the Page_Error is called, the Application_Error sub is called.
When an error occurs, the Application_Error function is called. In this method we can log the error and redirect
to another page. In fact catching errors in Application_Error instead of Page_Error would be an example of
centralizing errors as described earlier.
Dim ErrObj
set ErrObj = Server.GetLastError()
‘Now use ErrObj as the regular err object
Sample 20.5
<script language=”C#” runat=”server”>
Sub Page_Error(Source As Object, E As EventArgs)
Dim message As String = Request.Url.ToString()& Server.GetLastError().ToString()
Response.Write(message) // display message
End Sub
</script>
Sample 20.6
Error Handling
169
Above is an example of code in Global.asax and the Application_Error method. The error is logged and then the user
is redirected. Non-validated parameters are being logged here in the form of Request.Path. Care must be taken
not to log or display non-validated input from any external source. ‘’<link to XSS>’’
Web.cong has custom error tags which can be used to handle errors. This is called last and if Page_error or
Application_error is called and has functionality, that functionality shall be executed rst. If the previous two
handling mechanisms do not redirect or clear (Response.Redirect or a Server.ClearError), this will be called and
you shall be forwarded to the page dened in web.cong in the customErrors section, which is congured as
follows:
The “mode” attribute value of “On” means that custom errors are enabled whilst the “O” value means that
custom errors are disabled. The “mode” attribute can also be set to “RemoteOnly” which species that custom
errors are shown only to remote clients and ASP.NET errors are shown to requests coming from the the local
host. If the “mode” attribute is not set then it defaults to “RemoteOnly”.
When an error occurs, if the status code of the response matches one of the error elements, then the relevent
‘redirect’ value is returned as the error page. If the status code does not match then the error page from the
‘defaultRedirect’ attribute will be displayed. If no value is set for ‘defaultRedirect’ then a generic IIS error page
is returned.
An example of the customErrors section completed for an application is as follows:
<%@ Import Namespace=”System.Diagnostics” %>
<script language=”C#” runat=”server”>
void Application_Error(Object sender, EventArgs e) {
String Message = “\n\nURL: http://localhost/” + Request.Path
+ “\n\nMESSAGE:\n “ + Server.GetLastError().Message
+ “\n\nSTACK TRACE:\n” + Server.GetLastError().StackTrace;
// Insert into Event Log
EventLog Log = new EventLog();
Log.Source = LogName;
Log.WriteEntry(Message, EventLogEntryType.Error);
Server.Redirect(Error.htm) // this shall also clear the error
}
</script>
Sample 20.7
<customErrors mode=”<On|O|RemoteOnly>” defaultRedirect=”<default redirect page>”>
<error statusCode=”<HTTP status code>” redirect=”<specic redirect page for listed status code>”/>
</customErrors>
Sample 20.8
Error Handling
170
What to Review: Error Handling in Apache
In Apache you have two choices in how to return error messages to the client:
1. You can write the error status code into the req object and write the response to appear the way you want,
then have you handler return ‘DONE’ (which means the Apache framework will not allow any further handlers/
lters to process the request and will send the response to the client.
2. Your handler or lter code can return pre-dened values which will tell the Apache framework the result of
your codes processsing (essentially the HTTP status code). You can then congure what error pages should be
returned for each error code.
In the interest of centralizing all error code handling, option 2 can make more sense. To return a specic pre-
dened value from your handler, refer to the Apache documentation for the list of values to use, and then
return from the handler function as shown in the following example:
In the httpd.conf le you can then specify which page should be returned for each error code using the
‘ErrorDocument’ directive. The format of this directive is as follows:
• ErrorDocument <3-digit-code> <action>
... where the 3 digit code is the HTTP response code set by the handler, and the action is a local or external URL
to be returned, or specic text to display. The following examples are taken from the Apache ErrorDocument
documentation (https://httpd.apache.org/docs/2.4/custom-error.html) which contains more information
and options on ErrorDocument directives:
<customErrors mode=”On” defaultRedirect=”error.html”>
<error statusCode=”500” redirect=”err500.aspx”/>
<error statusCode=”404” redirect=”notHere.aspx”/>
<error statusCode=”403” redirect=”notAuthz.aspx”/>
</customErrors>
Sample 20.9
static int my_handler(request_rec *r)
{
if ( problem_processing() )
{
return HTTP_INTERNAL_SERVER_ERROR;
}
... continue processing request ...
}
Sample 20.10
Error Handling
171
What to Review: Leading Practice for Error Handling
Code that might throw exceptions should be in a try block and code that handles exceptions in a catch block.
The catch block is a series of statements beginning with the keyword catch, followed by an exception type and
an action to be taken.
Example: Java Try-Catch:
.NET Try–Catch
ErrorDocument 500 “Sorry, our script crashed. Oh dear”
ErrorDocument 500 /cgi-bin/crash-recover
ErrorDocument 500 http://error.example.com/server_error.html
ErrorDocument 404 /errors/not_found.html
ErrorDocument 401 /subscription/how_to_subscribe.html
Sample 20.11
public class DoStu {
public static void Main() {
try {
StreamReader sr = File.OpenText(“stu.txt”);
Console.WriteLine(“Reading line {0}”, sr.ReadLine());
}
catch(MyClassExtendedFromException e) {
Console.WriteLine(“An error occurred. Please leave to room”);
logerror(“Error: “, e);
}
}
}
Sample 20.12
public void run() {
while (!stop) {
try {
// Perform work here
} catch (Throwable t) {
// Log the exception and continue
WriteToUser(“An Error has occurred, put the kettle on”);
logger.log(Level.SEVERE, “Unexception exception”, t);
Sample 20.13
Error Handling
172
C++ Try–Catch
In general, it is best practice to catch a specic type of exception rather than use the basic catch(Exception) or
catch(Throwable) statement in the case of Java.
What to Review: The Order of Catching Exceptions
Keep in mind that many languages will attempt to match the thrown exception to the catch clause even if it means
matching the thrown exception to a parent class. Also remember that catch clauses are checked in the order they are
coded on the page. This could leave you in the situation where a certain type of exception might never be handled
correctly, take the following example where ‘non_even_argument’ is a subclass of ‘std::invalid_argument’:
}
}
}
void perform_fn() {
try {
// Perform work here
} catch ( const MyClassExtendedFromStdException& e) {
// Log the exception and continue
WriteToUser(“An Error has occurred, put the kettle on”);
logger.log(Level.SEVERE, “Unexception exception”, e);
}
}
Sample 20.14
class non_even_argument : public std::invalid_argument {
public:
explicit non_even_argument (const string& what_arg);
};
void do_fn()
{
try
{
// Perform work that could throw
}
catch ( const std::invalid_argument& e )
{
Sample 20.15
Error Handling
173
The problem with this code is that when a ‘non_even_argument is thrown, the catch branch handling
‘std::invalid_argument’ will always be executed as it is a parent of ‘non_even_argument’ and thus the runtime
system will consider it a match (this could also lead to slicing). Thus you need to be aware of the hierarchy of
your exception objects and ensure that you list the catch for the more specic exceptions rst in your code.
If the language in question has a nally method, use it. The nally method is guaranteed to always be called.
The nally method can be used to release resources referenced by the method that threw the exception. This is
very important. An example would be if a method gained a database connection from a pool of connections,
and an exception occurred without nally, the connection object shall not be returned to the pool for some
time (until the timeout). This can lead to pool exhaustion. nally() is called even if no exception is thrown.
A Java example showing nally() being used to release system resources.
What to Review: Releasing resources and good housekeeping
RAII is Resource Acquisition Is Initialization, which is a way of saying that when you rst create an instance of
a type, it should be fully setup (or as much as possible) so that it’s in a good state. Another advantage of RAII
is how objects are disposed of, eectively when an object instance is no longer needed then it resources are
automatically returned when the object goes out of scope (C++) or when it’s ‘using’ block is nished (C# ‘using’
directive which calls the Dispose method, or Java 7’s try-with-resources feature)
RAII has the advantage that programmers (and users to libraries) don’t need to explicitly delete objects, the
objects will be removed themselves, and in the process of removing themselves (destructor or Dispose)
For Classic ASP pages it is recommended to enclose all cleaning in a function and call it into an error handling
// Perform generic invalid argument processing and return failure
}
catch ( const non_even_argument& e )
{
// Perform specic processing to make argument even and continue processing
}
}
void perform_fn() {
try {
// Perform work here
} catch ( const MyClassExtendedFromStdException& e) {
// Log the exception and continue
WriteToUser(“An Error has occurred, put the kettle on”);
logger.log(Level.SEVERE, “Unexception exception”, e);
}
}
Sample 20.16
Error Handling
174
statement after an “On Error Resume Next”.
Building an infrastructure for consistent error reporting proves more dicult than error handling. Struts
provides the ActionMessages and ActionErrors classes for maintaining a stack of error messages to be reported,
which can be used with JSP tags like <html: error> to display these error messages to the user.
To report a dierent severity of a message in a dierent manner (like error, warning, or information) the
following tasks are required:
1. Register, instantiate the errors under the appropriate severity
2. Identify these messages and show them in a consistent manner.
Struts ActionErrors class makes error handling quite easy:
Now that we have added the errors, we display them by using tags in the HTML page.
References
• For classic ASP pages you need to do some IIS conguration, follow http://support.microsoft.com/
kb/299981 for more information.
• For default HTTP error page handling in struts (web.xml) see https://software-security.sans.org/
blog/2010/08/11/security-miscongurations-java-webxml-les
ActionErrors errors = new ActionErrors()
errors.add(“fatal”, new ActionError(“....”));
errors.add(“error”, new ActionError(“....”));
errors.add(“warning”, new ActionError(“....”));
errors.add(“information”, new ActionError(“....”));
saveErrors(request,errors); // Important to do this
Sample 20.17
<logic:messagePresent property=”error”>
<html:messages property=”error” id=”errMsg” >
<bean:write name=”errMsg”/>
</html:messages>
</logic:messagePresent >
Sample 20.18
Error Handling
175
How will your code and applications react when something has gone wrong? Many companies that follow
secure design and coding principals do so to prevent attackers from getting into their network, however
many companies do not consider designing and coding for the scenario where an attacker may have found
a vulnerability, or has already exploited it to run code within a companies rewalls (i.e. within the Intranet).
Many companies employ SIEM logging technologies to monitor network and OS logs for patterns that detect
suspicions activity, this section aims to further encourage application layers and interfaces to do the same.
21.1 Description
This section concentrates on:
1. Design and code that allows the user to react when a system is being attacked.
2. Concepts allowing applications to ag when they have been breached.
When a company implements secure design and coding, it will have the aim of preventing attackers from
misusing the software and accessing information they should not have access to. Input validation checks for
SQL injections, XSS, CSRF, etc. should prevent attackers from being able to exploit these types of vulnerabilities
against the software. However how should software react when an attacker is attempting to breach the
defenses, or the protections have been breached?
For an application to alert to security issues, it needs context on what is ‘normal’ and what constitutes a
security issue. This will dier based on the application and the context within which it is running. In general
applications should not attempt to log every item that occurs as the excessive logging will slow down the
system, ll up disk or DB space, and make it very hard to lter through all the information to nd the security
issue.
At the same time, if not enough information is monitored or logged, then security alerting will be very hard to
do based on the available information. To achieve this balance an application could use its own risk scoring
system, monitoring at a system level what risk triggers have been spotted (i.e. invalid inputs, failed passwords,
etc.) and use dierent modes of logging. Take an example of normal usage, in this scenario only critical items
are logged. However if the security risk is perceived to have increased, then major or security level items
can be logged and acted upon. This higher security risk could also invoke further security functionality as
described later in this section.
Take an example where an online form (post authentication) allows a user to enter a month of the year. Here
the UI is designed to give the user a drop down list of the months (January through to December). In this case
the logged in user should only ever enter one of 12 values, since they typically should not be entering any text,
instead they are simply selecting one of the pre-dened drop down values.
If the server receiving this form has followed secure coding practices, it will typically check that the form eld
matches one of the 12 allowed values, and then considers it valid. If the form eld does not match, it returns
an error, and may log a message in the server. This prevents the attacker from exploiting this particular eld,
however this is unlikely to deter an attacker and they would move onto other form elds.
REVIEWING SECURITY ALERTS
Reviewing Security Alerts
176 Reviewing Security Alerts
In this scenario we have more information available to us than we have recorded. We have returned an error
back to the user, and maybe logged an error on the server. In fact we know a lot more; an authenticated user
has entered an invalid value which they should never have been able to do (as it’s a drop down list) in normal
usage.
This could be due to a few reasons:
• There’s a bug in the software and the user is not malicious.
• An attacker has stolen the users login credentials and is attempting to attack the system.
• A user has logged in but has a virus/trojan which is attempting to attack the system.
• A user has logged in but is experiencing a man-in-the-middle attack.
• A user is not intending to be malicious but has somehow changed the value with some browser plugin, etc.
If it’s the rst case above, then the company should know about it to x their system. If it’s case 2, 3 or 3 then the
application should take some action to protect itself and the user, such as reducing the functionality available
to the user (i.e. no PII viewable, can’t change passwords, can’t perform nancial transactions) or forcing further
authentication such as security questions or out-of-band authentication. The system could also alert the user
to the fact that the unexpected input was spotted and advise them to run antivirus, etc., thus stopping an
attack when it is underway.
Obviously care must be taken in limiting user functionality or alerting users encase it’s an honest mistake, so
using a risk score or noting session alerts should be used. For example, if everything has been normal in the
browsing session and 1 character is out of place, then showing a red pop-up box stating the user has been
hacked is not reasonable, however if this is not the usual IP address for the user, they have logged in at an
unusual time, and this is the 5th malformed entry with what looks like an SQL injection string, then it would be
reasonable for the application to react. This possible reaction would need to be stated in legal documentation.
In another scenario, if an attacker has got through the application defenses extracted part of the applications
customer database, would the company know? Splitting information in the database into separate tables
makes sense from an eciency point of view, but also from a security view, even putting condential
information into a separate partition can make it harder for the attacker. However if the attacker has the
information it can be hard to detect and applications should make steps to aid alerting software (e.g. SIEM
systems). Many nancial institutions use risk scoring systems to look at elements of the user’s session to give
a risk score, if Johnny always logs in at 6pm on a Thursday from the same IP, then we have a trusted pattern.
If suddenly Johnny logs in at 2:15am from an IP address on the other side of the world, after getting the
password wrong 7 times, then maybe he’s jetlagged after a long trip, or perhaps his account has been hacked.
Either way, asking him for out-of-band authentication would be reasonable to allow Johnny to log in, or to
block an attacker from using Johnny’s account.
If the application takes this to a larger view, it can determine that on a normal day 3% of the users log on
in what would be considered a riskier way, i.e. dierent IP address, dierent time, etc. If on Thursday it sees
this number rise to 23% then has something strange happened to the user base, or has the database been
hacked? This type of information can be used to enforce a blanket out-of-band authentication (and internal
investigation of the logs) for the 23% of ‘riskier’ users, thereby combining the risk score for the user with the
overall risk score for the application.
177
Another good option is ‘honey accounts’ which are usernames and passwords that are never given out to
users. These accounts are added just like any other user, and stored in the DB, however they are also recorded
in a special cache and checked on login. Since they are never given to any user, no user should ever logon with
them, however if one of those accounts are used, then the only way that username password combination
could be known is if an attacker got the database, and this information allows the application to move to a
more secure state and alert the company that the DB has been hacked.
What to Review
When reviewing code modules from a security alerting point of view, some common issues to look out for
include:
• Will the application know if it’s being attacked? Does it ignore invalid inputs, logins, etc. or does it log them
and monitor this state to capture a cumulative perception of the current risk to the system?
• Can the application automatically change its logging level to react to security threats? Is changing security
levels dynamic or does it require a restart?
• Does the SDLC requirements or design documentation capture what would constitute a security alert? Has
this determination been peer reviewed? Does the testing cycle run through these scenarios?
• Does the system employ ‘honey accounts’ such that the application will know if the DB has been compromised?
• Is there a risk based scoring system that records the normal usage of users and allows for determination or
reaction if the risk increases?
• If a SIEM system is being used, have appropriate triggers been identied? Has automated tests been created
to ensure those trigger log messages are not accidentally modied by future enhancements or bug xes?
• Does the system track how many failed login attempts a user has experienced? Does the system react to this?
• Does certain functionality (i.e. transaction initiation, changing password, etc) have dierent modes of
operation based on the current risk score the application is currently operating within?
• Can the application revert back to ‘normal’ operation when the security risk score drops to normal levels?
• How are administrators alerted when security risk score rises? Or when a breach has been assumed? At an
operational level, is this tested regularly? How are changes of personnel handled?
Reviewing Security Alerts
178
Attack detection undertaken at the application layer has access to the complete context of an interaction
and enhanced information about the user. If logic is applied within the code to detect suspicious activity
(similar to an application level IPS) then the application will know what is a high-value issue and what is noise.
Input data are already decrypted and canonicalized within the application and therefore application-specic
intrusion detection is less susceptible to advanced evasion techniques. This leads to a very low level of attack
identication false positives, providing appropriate detection points are selected.
The fundamental requirements are the ability to perform four tasks:
1. Detection of a selection of suspicious and malicious events.
2. Use of this knowledge centrally to identify attacks.
3. Selection of a predened response.
4. Execution of the response.
22.1 Description
Applications can undertake a range of responses that may include high risk functionality such as changes to
a user’s account or other changes to the application’s defensive posture. It can be dicult to detect active
defense in dynamic analysis since the responses may be invisible to the tester. Code review is the best method
to determine the existence of this defense.
Other application functionality like authentication failure counts and lock-out, or limits on rate of le uploads
are ‘localized’ protection mechanisms. This sort of standalone logic is ‘not’ active defense equivalents in
the context of this review, unless they are rigged together into an application-wide sensory network and
centralized analytical engine.
It is not a bolt-on tool or code library, but instead oers insight to an approach for organizations to specify
or develop their own implementations – specic to their own business, applications, environments, and risk
prole – building upon existing standard security controls.
What to Review
In the case where a code review is being used to detect the presence of a defense, its absence should be noted
as a weakness. Note that active defense cannot defend an application that has known vulnerabilities, and
therefore the other parts of this guide are extremely important. The code reviewer should note the absence
of active defense as a vulnerability.
The purpose of code review is not necessarily to determine the ecacy of the active defense, but could simply
be to determine if such capability exists.
Detection points can be integrated into presentation, business and data layers of the application. Application-
specic intrusion detection does not need to identify all invalid usage, to be able to determine an attack. There
is no need for “innite data” or “big data” and therefore the location of “detection points” may be very sparse
within source code.
REVIEW FOR ACTIVE DEFENSE
Reviewing for Active Defense
179
A useful approach for identifying such code is to nd the name of a dedicated module for detecting suspicious
activity (such as OWASP AppSensor). Additionally a company can implement a policy of tagging active defense
detection points based on Mitre’s Common Attack Pattern Enumeration and Classifcation (CAPEC), strings
such as CAPEC-212, CAPEC-213, etc.
The OWASP AppSensor detection point type identiers and CAPEC codes will often have been used in
conguration values (e.g. [https://code.google.com/p/appsensor/source/browse/trunk/AppSensor/src/test/
resources/.esapi/ESAPI.properties?r=53 in ESAPI for Java]), parameter names and security event classication.
Also, examine error logging and security event logging mechanisms as these may be being used to collect
data that can then be used for attack detection. Identify the code or services called that perform this logging
and examine the event properties recorded/sent. Then identify all places where these are called from.
An examination of error handling code relating to input and output validation is very likely to reveal the
presence of detection points. For example, in a whitelist type of detection point, additional code may have
been added adjacent, or within error handling code ow:
In some situations attack detection points are looking for blacklisted input, and the test may not exist otherwise,
so brand new code is needed. Identication of detection points should also have found the locations where
events are recorded (the “event store”). If detection points cannot be found, continue to review the code for
execution of response, as this may provide insight into the existence of active defense.
The event store has to be analysed in real time or very frequently, in order to identify attacks based on
predened criteria. The criteria should be dened in conguration settings (e.g. in conguration les, or read
from another source such as a database). A process will examine the event store to determine if an attack is in
progress, typically this will be attempting to identify an authenticated user, but it may also consider a single
IP address, range of IP addresses, or groups of users such as one or more roles, users with a particular privilege
or even all users.
Once an attack has been identied, the response will be selected based on predened criteria. Again an
examination of conguration data should reveal the thresholds related to each detection point, groups of
detection points or overall thresholds.
The most common response actions are user warning messages, log out, account lockout and administrator
notication. However, as this approach is connected into the application, the possibilities of response actions
are limited only by the coded capabilities of the application.
Search code for any global includes that poll attack identication/response identied above. Response actions
(again a user, IP address, group of users, etc) will usually be initiated by the application, but in some cases other
applications (e.g. alter a fraud setting) or infrastructure components (e.g. block an IP address range) may also
be involved.
Examine conguration les and any external communication the application performs.
if ( var !Match this ) {
Error handling
Record event for attack detection
}
Reviewing for Active Defense
180
The following types of responses may have been coded:
• Logging increased
• Administrator notication
• Other notication (e.g. other system)
• Proxy
• User status change
• User notication
• Timing change
• Process terminated (same as traditional defenses)
• Function disabled
• Account log out
• Account lock out
• Collect data from user.
Other capabilities of the application and related system components can be repurposed or extended, to
provide the selected response actions. Therefore review the code associated with any localised security
measures such as account lock out.
References
• The guidance for adding active response to applications given in theOWASP_AppSensor_Project
• Category: OWASP Enterprise Security API
• https://code.google.com/p/appsensor/ AppSensor demonstration code
Reviewing for Active Defense
181
Race Conditions occur when a piece of code does not work as it is supposed to (like many security issues). They
are the result of an unexpected ordering of events, which can result in the nite state machine of the code to
transition to a undened state, and also give rise to contention of more than one thread of execution over the
same resource. Multiple threads of execution acting or manipulating the same area in memory or persisted
data which gives rise to integrity issues.
23.1 Description
With competing tasks manipulating the same resource, we can easily get a race condition as the resource is
not in step-lock or utilises a token based system such as semaphores. For example if there are two processes
(Thread 1, T1) and (Thread 2, T2). The code in question adds 10 to an integer X. The initial value of X is 5.
X = X + 10
With no controls surrounding this code in a multithreaded environment, the code could experience the
following problem:
T1 places X into a register in thread 1
T2 places X into a register in thread 2
T1 adds 10 to the value in T1’s register resulting in 15
T2 adds 10 to the value in T2’s register resulting in 15
T1 saves the register value (15) into X.
T1 saves the register value (15) into X.
The value should actually be 25, as each thread added 10 to the initial value of 5. But the actual value is 15 due
to T2 not letting T1 save into X before it takes a value of X for its addition.
This leads to undened behavior, where the application is in an unsure state and therefore security cannot be
accurately enforced.
What to Review
• In C#.NET look for code which used multithreaded environments:
o Thread
o System.Threading
o ThreadPool
o System.Threading.Interlocked
• In Java code look for
o java.lang.Thread
o start()
o stop()
o destroy()
o init()
o synchronized
RACE CONDITIONS
Race Conditions
182 Race Conditions
o wait()
o notify()
o notifyAll()
• For classic ASP multithreading is not a directly supported feature, so this kind of race condition could be
present only when using COM objects.
• Static methods and variables (one per class, not one per object) are an issue particularly if there is a shared
state among multiple threads. For example, in Apache, struts static members should not be used to store
information relating to a particular request. The same instance of a class can be used by multiple threads, and
the value of the static member cannot be guaranteed.
• Instances of classes do not need to be thread safe as one is made per operation/request. Static states must
be thread safe.
o References to static variables, these must be thread locked.
o Releasing a lock in places other then nally{} may cause issues.
o Static methods that alter static state.
References
• http://msdn2.microsoft.com/en-us/library/f857xew0(vs.71).aspx
183
A buer is an amount of contiguous memory set aside for storing information. For example if a program
has to remember certain things, such as what your shopping cart contains or what data was inputted prior
to the current operation. This information is stored in memory in a buer. Languages like C, C++ (which
many operating systems are written in), and Objective-C are extremely ecient, however they allow code to
access process memory directly (through memory allocation and pointers) and intermingle data and control
information (e.g. in the process stack). If a programmer makes a mistake with a buer and allows user input
to run past the allocated memory, the user input can overwrite program control information and allow the
user to modify the execution of the code.
Note that Java, C#.NET, Python and Ruby are not vulnerable to buer overows due to the way they store their
strings in char arrays, of which the bounds are automatically checked by the frameworks, and the fact that
they do not allow the programmer direct access to the memory (the virtual machine layer handles memory
instead). Therefore this section does not apply to those languages. Note however that native code called
within those languages (e.g. assembly, C, C++) through interfaces such as JNI or ‘unsafe’ C# sections can be
susceptible to buer overows.
24.1 Description
To al lo cat e a b uer t he co de de cl are s a va ria bl e of a pa rt ic ul ar si ze :
• char myBuer[100]; // large enough to hold 100 char variables
• int myIntBuf[5]; // large enough to hold 5 integers
• Widget myWidgetArray[17]; // large enough to hold 17 Widget objects
As there is no automatic bounds checking code can attempt to add a Widget at array location 23 (which does
not exist). When the code does this, the complier will calculate where the 23rd Widget should be placed in
memory (by multiplying 23 x sizeof(Widget) and adding this to the location of the ‘myWidgetArray’ pointer).
Any other object, or program control variable/register, that existed at this location will be overwritten.
Arrays, vectors, etc. are indexed starting from 0, meaning the rst element in the container is at ‘myBuer[0]’,
this means the last element in the container is not at array index 100, but at array index 99. This can often lead
to mistakes and the ‘o by one’ error, when loops or programming logic assume objects can be written to the
last index without corrupting memory.
In C, and before the C++ STL became popular, strings were held as arrays of characters:
• char nameString[10];
This means that the ‘nameString’ array of characters is vulnerable to array indexing problems described above,
and when many of the string manipulation functions (such as strcpy, strcat, described later) are used, the
possibility of writing beyond the 10th element allows a buer overrun and thus memory corruption.
As an example, a program might want to keep track of the days of the week. The programmer tells the
computer to store a space for 7 numbers. This is an example of a buer. But what happens if an attempt to add
BUFFER OVERRUNS
Buer Overruns
184 Buer Overruns
8 numbers is performed? Languages such as C and C++ do not perform bounds checking, and therefore if the
program is written in such a language, the 8th piece of data would overwrite the program space of the next
program in memory, and would result in data corruption. This can cause the program to crash at a minimum
or a carefully crafted overow can cause malicious code to be executed, as the overow payload is actual code.
What to Review: Buer Overruns
C library functions such as strcpy (), strcat (), sprintf () and vsprintf () operate on null terminated strings and perform
no bounds checking. gets() is another function that reads input (into a buer) from stdin until a terminating newline
or EOF (End of File) is found. The scanf () family of functions also may result in buer overows.
Using strncpy(), strncat() and snprintf() functions allows a third ‘length’ parameter to be passed which determines the
maximum length of data that will be copied/etc. into the destination buer. If this is correctly set to the size of the
buer being written to, it will prevent the target buer being overowed. Also note fgets() is a replacement for gets().
Always check the bounds of an array before writing it to a buer. The Microsoft C runtime also provides additional
versions of many functions with an ‘_s’ sux (strcpy_s, strcat_s, sprintf_s). These functions perform additional checks
for error conditions and call an error handler on failure.
The C code below is not vulnerable to buer overow as the copy functionality is performed by ‘strncpy’ which
species the third argument of the length of the character array to be copied, 10.
void copyData(char *userId) {
char smallBuer[10]; // size of 10
strcpy (smallBuer, userId);
}
int main(int argc, char *argv[]) {
char *userId = “01234567890”; // Payload of 12 when you include the ‘\n’ string termination
// automatically added by the “01234567890” literal
copyData (userId); // this shall cause a buer overload
}
Sample 24.1
void copyData(char *userId) {
char smallBuer[10]; // size of 10
strncpy(smallBuer, userId, sizeof(smallBuer)); // only copy rst 10 elements
smallBuer[10] = 0; // Make sure it is terminated.
}
int main(int argc, char *argv[]) {
char *userId = “01234567890”; // Payload of 11
copyData (userId);
}
Sample 24.2
185
Modern day C++ (C++11) programs have access to many STL objects and templates that help prevent security
vulnerabilities. The std::string object does not require the calling code have any access to underlying pointers,
and automatically grows the underlying string representation (character buer on the heap) to accommodate
the operations being performed. Therefore code is unable to cause a buer overow on a std::string object.
Regarding pointers (which can be used in other ways to cause overows), C++11 has smart pointers which
again take away any necessity for the calling code to user the underlying pointer, these types of pointers are
automatically allocated and destroyed when the variable goes out of scope. This helps to prevent memory leaks and
double delete errors. Also the STL containers such as std::vector, std::list, etc., all allocate their memory dynamically
meaning normal usage will not result in buer overows. Note that it is still possible to access these containers
underlying raw pointers, or reinterpret_cast the objects, thus buer overows are possible, however they are more
dicult to cause.
Compliers also help with memory issues, in modern compilers there are ‘stack canaries’ which are subtle elements
placed in the complied code which check for out-of-bound memory accesses. These can be enabled when compiling
the code, or they could be enabled automatically. There are many examples of these stack canaries, and for some
system many choices of stack canaries depending on an organizations appetite for security versus performance.
Apple also have stack canaries for iOS code as Objective-C is also susceptible to buer overows.
In general, there are obvious examples of code where a manual code reviewer can spot the potential for overows
and o-by-one errors, however other memory leaks (or issues) can be harder to spot. Therefore manual code review
should be backed up by memory checking programs available on the market.
What to Review: Format Function Overruns
A format function is a function within the ANSI C specication that can be used to tailor primitive C data types to
human readable form. They are used in nearly all C programs to output information, print error messages, or process
strings.
Some format parameters:
The %s in this case ensures that value pointed to by the parameter ‘abc’ is printed as an array of characters.
For example:
Format String Relevant Input
%x Hexadecimal values (unsigned int)
%d
%u
Decimal
Unsigned decimal (unsigned int)
%n Integer
%s Strings ((const) (unsigned) char*)
Table 23: Format Function Overruns
char* myString = “abc”;
printf (“Hello: %s\n”, abc);
Buer Overruns
186
Through supplying the format string to the format function we are able to control the behaviour of it. So
supplying input as a format string makes our application do things it’s not meant to. What exactly are we able
to make the application do?
If we supply %x (hex unsigned int) as the input, the ‘printf’ function shall expect to nd an integer relating to
that format string, but no argument exists. This cannot be detected at compile time. At runtime this issue shall
surface.
For every % in the argument the printf function nds it assumes that there is an associated value on the stack.
In this way the function walks the stack downwards reading the corresponding values from the stack and
printing them to the user.
Using format strings we can execute some invalid pointer access by using a format string such as:
• printf (“%s%s%s%s%s%s%s%s%s%s%s%s”);
Worse again is using the ‘%n’ directive in ‘printf()’. This directive takes an ‘int*’ and ‘writes’ the number of bytes
so far to that location.
Where to look for this potential vulnerability. This issue is prevalent with the ‘printf()’ family of functions,
‘’printf(),fprintf(), sprintf(), snprintf().’ Also ‘syslog()’ (writes system log information) and setproctitle(const char
*fmt, ...); (which sets the string used to display process identier information).
What to Review: Integer Overows
Data representation for integers will have a nite amount of space, for example a short in many languages is
16 bits twos complement number, which means it can hold a maximum number of 32,767 and a minimum
number of -32,768. Twos complement means that the very rst bit (of the 16) is a representation of whether
the number of positive or negative. If the rst bit is ‘1’, then it is a negative number.
The representation of some boundary numbers are given in table 24.
If you add 1 to 32,766, it adds 1 to the representation giving the representation for 32,767 shown above.
However if you add one more again, it sets the rst bit (a.k.a. most signicant bit), which is then interpreted by
the system as -32,768.
If you have a loop (or other logic) which is adding or counting values in a short, then the application could
experience this overow. Note also that subtracting values below -32,768 also means the number will wrap
around to a high positive, which is called underow.
Number Representation
32,766 0111111111111110
32,767 0111111111111111
-32,768
-1
1000000000000000
1111111111111111
Table 24: Integer Overows
Buer Overruns
187
The binary representation of 0x7f is 1111111111111111111111111111111; this integer is initialized with
the highest positive value a signed long integer can hold.
Here when we add 1 to the hex value of 0x7f the value of the integer overows and goes to a negative
number (0x7f + 1 = 80000000) In decimal this is (-2147483648). Think of the problems this may cause.
Compilers will not detect this and the application will not notice this issue.
We get these issues when we use signed integers in comparisons or in arithmetic and also when comparing
signed integers with unsigned integers.
Here if v2 is a massive negative number the “if” condition shall pass. This condition checks to see if v2 is bigger
than the array size.
If the bounds check was not performed the line “myArray[v2] = v1” could have assigned the value v1 to a
location out of the bounds of the array causing unexpected results.
References
• See the OWASP article on buer overow attacks.
#include <stdio.h>
int main(void){
int val;
val = 0x7f; /* 2147483647*/
printf(“val = %d (0x%x)\n”, val, val);
printf(“val + 1 = %d (0x%x)\n”, val + 1 , val + 1); /*Overow the int*/
return 0;
}
Sample 24.3
int myArray[100];
int llArray(int v1, int v2){
if(v2 > sizeof(myArray) / sizeof(int) -1 ){
return -1; /* Too Big */
}
myArray [v2] = v1;
return 0;
}
Sample 24.4
Buer Overruns
188
• See the OWASP Testing Guide on how to test for buer overow vulnerabilities.
• See Security Enhancements in the CRT: http://msdn2.microsoft.com/en-us/library/8ef0s5kh(VS.80).aspx
JavaScript has several known security vulnerabilities, with HTML5 and JavaScript becoming more prevalent in
web sites today and with more web sites moving to responsive web design with its dependence on JavaScript
the code reviewer needs to understand what vulnerabilities to look for. JavaScript is fast becoming a signif-
icant point of entry of hackers to web application. For that reason we have included in the A1 Injection sub
section.
The most signicant vulnerabilities in JavaScript are cross-site scripting (XSS) and Document Object Model,
DOM-based XSS.
Detection of DOM-based XSS can be challenging. This is caused by the following reasons.
• JavaScript is often obfuscated to protect intellectual property.
• JavaScript is often compressed out of concerned for bandwidth.
In both of these cases it is strongly recommended the code review be able to review the JavaScript before it
has been obfuscated and or compressed. This is a huge point of contention with QA software professionals
because you are reviewing code that is not in its production state.
Another aspect that makes code review of JavaScript challenging is its reliance of large frameworks such as
Microsoft .NET and Java Server Faces and the use of JavaScript frameworks, such as JQuery, Knockout, Angular,
Backbone. These frameworks aggravate the problem because the code can only be fully analyzed given the
source code of the framework itself. These frameworks are usually several orders of magnitude larger then the
code the code reviewer needs to review.
Because of time and money most companies simple accept that these frameworks are secure or the risks are
low and acceptable to the organization.
Because of these challenges we recommend a hybrid analysis for JavaScript. Manual source to sink validation
when necessary, static analysis with black-box testing and taint testing.
First use a static analysis. Code Reviewer and the organization needs to understand that because of event-driv-
en behaviors, complex dependencies between HTML DOM and JavaScript code, and asynchronous commu-
nication with the server side static analysis will always fall short and may show both positive, false, false –pos-
itive, and positive-false ndings.
Black-box traditional methods detection of reected or stored XSS needs to be preformed. However this ap-
proach will not work for DOM-based XSS vulnerabilities.
Taint analysis needs to be incorporated into static analysis engine. Taint Analysis attempts to identify variables
that have been ‘tainted’ with user controllable input and traces them to possible vulnerable functions also known
Buer Overruns
CLIENT SIDE JavaScript
189
as a ‘sink’. If the tainted variable gets passed to a sink without rst being sanitized it is agged as vulnerability.
Second the code reviewer needs to be certain the code was tested with JavaScript was turned o to make sure
all client sided data validation was also validated on the server side.
Code examples of JavaScript vulnerabilities.
Explanation: An attacker can send a link such as “http://hostname/welcome.html#name=<script>alert(1)</
script>” to the victim resulting in the victim’s browser executing the injected client-side code.
Line 5 may be a false-positive and prove to be safe code or it may be open to “Open redirect attack” with taint
analysis the static analysis should be able to correctly identied if this vulnerability exists. If static analysis
relies only on black-box component this code will have agged as vulnerable requiring the code reviewer to
do a complete source to sink review.
Additional examples and potential security risks
Source: document.url
Sink: document.write()
Results: Results:document.write(“<script>malicious code</script>”);
Cybercriminal may controlled the following DOM elements including
document.url,document.location,document.referrer,window.location
Source: document.location
Sink: windon.location.href
Results: windon.location.href = http://www.BadGuysSite; - Client code open redirect.
Source: document.url
Storage: windows.localstorage.name
<html>
<script type=”text/javascript”>
var pos=document.URL.indexOf(“name=”)+5;
document.write( document.URL.substring(pos,document.URL.length));
</script>
<html>
Sample 25.1
var url = document.location.url;
var loginIdx = url.indexOf(‘login’);
var loginSux = url.substring(loginIdx);
url = ‘http://mySite/html/sso/’ + loginSux;
document.location.url = url;
Sample 25.2
Buer Overruns
190
Sink: elem.innerHTML
Results: elem.innerHTML = <value> =Stored DOM-based Cross-site Scripting
eval() is prone to security threats, and thus not recommended to be used.
Consider these points:
1. Code passed to the eval is executed with the privileges of the executer. So, if the code passed can be aect-
ed by some malicious intentions, it leads to running malicious code in a user’s machine with your website’s
privileges.
2. A malicious code can understand the scope with which the code passed to the eval was called.
3. You also shouldn’t use eval() or new Function() to parse JSON data.
The above if used may raise security threats. JavaScript when used to dynamically evaluate code will create a
potential security risk.
eval(‘alert(“Query String ‘ + unescape(document.location.search) + ‘”);’);
eval(untrusted string); Can lead to code injection or client-side open redirect.
JavaScripts “new function” also may create a potential security risk.
Three points of validity are required for JavaScript
1. Have all the logic server-side, JavaScript validation be turned o on the client
2. Check for all sorts of XSS DOM Attacks, never trust user data, know your source and sinks (i.e. look at all
variables that contain user supplied input).
3. Check for insecure JavaScript libraries and update them frequently.
References:
• http://docstore.mik.ua/orelly/web/jscript/ch20_04.html
• https://www.owasp.org/index.php/Static_Code_Analysis
• http://www.cs.tau.ac.il/~omertrip/fse11/paper.pdf
• http://www.jshint.com
Buer Overruns
191
APENDIX
192 Code Review Do’s And Dont’s
At work we are professions. But we need to make sure that even as professionals that when we do code reviews
besides the technical aspects of the code reviews we need to make sure we consider the human side of code
reviews. Here is a list of discussion points that code reviewers; peer developers need to take into consideration.
This list is not comprehensive but a suggestion starting point for an enterprise to make sure code reviews are
eective and not disruptive and a source of discourse. If code reviews become a source of discourse within
an organization the eectives of nding security, functional bugs will decline and developers will nd a way
around the process. Being a good code reviewer requires good social skills, and is a skill that requires practice
just like learning to code.
• You don’t have to nd fault in the code to do a code review. If you always ne something to criticize your
comments
will loose credibility.
• Do not rush a code review. Finding security and functionality bugs is important but other developers or team
mem-
bers are waiting on you so you need to temper your do not rush with the proper amount urgency.
• When reviewing code you need to know what is expected. Are you reviewing for security, functionality, main-
tain-
ability, and/or style? Does your organization have tools and documents on code style or are you using your
own coding style? Does your organization give tools to developers to mark unacceptable coding standards
per the organizations own coding standards?
• Before beginning a code review does your organization have a dened way to resolve any conicts that may
come
up in the code review by the developer and code reviewer?
• Does the code reviewer have a dene set of artifacts that need to be produce as the result of the code review?
• What is the process of the code review when code during the code review needs to be changed?
• Is the code reviewer knowledgeable about the domain knowledge of the code that is being reviewed? Ample
evidence abounds that code reviews are most eective if the code reviewer is knowledgeable about the do-
main of the code I.e. Compliance regularizations for industry and government, business functionality, risks, etc.
Agile Software Development Lifecycle
CODE REVIEW DO’S AND DONT’S
193
AGILE
SOFTWARE
DEVELOPMENT
LIFECYCLE
DESIGN
CODE
TEST
DEPLOY
INTERACTION
FASE
INTERACTION
FASE
INTERACTION
FASE
INTERACTION
FASE
1
3
24
DESIGN
CODE
TEST
DEPLOY
DESIGN
CODE
TEST
DEPLOY
DESIGN
CODE
TEST
DEPLOY
Code Review Do’s And Dont’s
194
Integrating security into the agile sdlc process ow is dicult. The organization will need constant involve-
ment from security team and or a dedication to security with well-trained coders on every team.
Continuous Integration and Test Driven Development
The term ‘Continuous Integration’ originated with the Extreme Programming development process. Today it is
one of the best practices of SDLC Agile. CI requires developers to check code into source control management
START
FINISH/
SIGNED
OFF
CHECK-IN CODE
SAST - AdHoc Static Analysis - Developer Initiated
CHECKMARX / SAST
SECURITY
DEVELOPMENT
RECORD METRICS
CHECK-IN
DEV
6
7
1
5
YES:
FIX CODE
YES:
FAIL
NO/FALSE POSITIVE
SAST
SAST
PASS OR
FAIL?
FALSE
POSITIVE SIGNOFF
INVOKE SAST
ADJUST RULEBASE
YES
SIGN/OFF?
ISR
NO:
REQUEST FIX/
ESCALATE
PASS/FAIL NO PASS
TUNED RULES
CONTROLLED
BY APPSEC
YES
NO
211
34
8
SAST
PASS OR
FAIL?
Code Review Do’s And Dont’s
195
application (scm) several times a day. An automated build server and application verify each check-in. The
advantage is team members can quickly detect build problems early in the software development process.
The disadvantage of CI for the Code Reviewer is while code may build properly; software security vulnerabil-
ities may still exist. Code review may be part of the check-in process but review may only to make sure the
code only meets the minimum standards of the organization. Code review is not a secure code review with risk
assessment approach on what needs to have additional time spent doing a code review.
The second disadvantage for the code review is because the organization is moving quickly with Agile process
a design aw may be introduced allowing a security vulnerabilities and the vulnerabilities may get deployed.
A red ag for the code reviewer is…
1. No user stories talk about security vulnerabilities based on risk.
2. User stories do not openly describe source and sinks.
3. No risk assessment for the application has been done.
Breaking Down Process Areas
The term ‘Test Driven Development’ like CI originated with Extreme Programming development process. To-
day like CI it is one of the best practices of SDLC Agile. TDD requires developers to rely on repetition of a very
short development cycle. First the step is the developer writes an automated test case that denes a needed
improvement or new functionality. This rst step TDD initial fails the test. Subsequent steps the developers
create a minimum amount of code to pass. Finally the developer refactors the new code to the organization
acceptable coding standard.
BREAKING DOWN PROCESS AREAS
AGILE
1
CONTINUOUS INTEGRATION
2
TEST DRIVEN DEVELOPMENT
3STORY
SPRINT
BACKLOG
1
TDD:
CREATE TEST
FIRST CODE
UNTIL IT PASSES
INTEGRATION:
AUTOMATED
BUILD AND TESTS
IMMEDIATE
FEEDBACK ON
ISSUES
FUNCTIONING
PRODUCT
PRODUCT
BACKLOG
STORY
1
STORY
2
SPRINT
DAY
(MAX)
30
CONTINOUS
INTEGRATION
PERIOD
HOUR
(MAX)
24
Code Review Do’s And Dont’s
196
CATEGORY DESCRIPTION PASS FAIL
Are there backdoor/unexposed business logic classes?General
Is the placement of authentication and authorization check correct?Authorization
Are the checks correct implemented? Is there any backdoor parameter?Authorization
Is the check applied on all the required les and folder within web root directory?Authorization
Are security checks placed before processing inputs?Authorization
Incase of container-managed authentication - Is the authentication based on web methods only?Authorization
Incase of container-managed authentication - Is the authentication based on web methods only?Authorization
Is Password Complexity Check enforced on the password?Authorization
Is password stored in an encrypted format?Cryptography
Is password disclosed to user/written to a le/logs/console?Authorization
Are there unused congurations related to business logic?Business Logic and
Design
If request parameters are used to identify business logic methods, is there a proper mapping of
user privileges and methods/actions allowed to them?
Business Logic and
Design
Check if unexposed instance variables are present in form objects that get bound to user inputs.
If present, check if they have default values.
Business Logic and
Design
Check if unexposed instance variables present in form objects that get bound to user inputs. If
present, check if they get initialized before form binding.
Business Logic and
Design
Is there execution stopped/terminated after for invalid request? I.e. when authentication/autho-
rization check fails?
Authorization
Check if unexposed instance variables are present in form objects that get bound to user inputs.
If present, check if they have default values.
Business Logic and
Design
Check if unexposed instance variables present in form objects that get bound to user inputs. If
present, check if they get initialized before form binding.
Business Logic and
Design
Is there execution stopped/terminated after for invalid request? I.e. when authentication/autho-
rization check fails?
Authorization
Are the checks correct implemented? Is there any backdoor parameter?Business Logic and
Design
Is the check applied on all the required les and folder within web root directory?Business Logic and
Design
Is there any default conguration like Access- ALL?Business Logic and
Design
Does the conguration get applied to all les and users?Business Logic and
Design
Incase of container-managed authentication - Does the authentication get applied on all
resources?
Authorization
Does the design handle sessions securely?Session
Management
CODE REVIEW CHECKLIST
Code Review Checklist
197
CATEGORY DESCRIPTION PASS FAIL
Are database credentials stored in an encrypted formatCryptography
Does the design support weak data stores like at lesBusiness Logic and
Design
Does the centralized validation get applied to all requests and all the inputs?Business Logic and
Design
Does the centralized validation check block all the special characters?Business Logic and
Design
Does are there any special kind of request skipped from validation?Business Logic and
Design
Does the design maintain any exclusion list for parameters or features from being validated?Business Logic and
Design
Are all the untrusted inputs validated?
Input data is constrained and validated for type, length, format, and range.
Imput Validation
Is the data sent on encrypted channel? Does the application use HTTPClient for making external
connections?
Cryptography
Is the data sent on encrypted channel? Does the application use HTTPClient for making external
connections?
Cryptography Does the design involve session sharing between components/modules? Is session validated
correctly on both ends?
Session
Management
Does the design use any elevated OS/system privileges for external connections/commands?Business Logic and
Design
Is there any known aw(s) in API’s/Technology used? For eg: DWRBusiness Logic and
Design
Does the design framework provide any inbuilt security control? Like <%: %> in ASP.NET MVC? Is
the application taking advantage of these controls?
Business Logic and
Design
Are privileges reduce whenever possible?Business Logic and
Design
Is the program designed to fail gracefully?Business Logic and
Design
Are logs logging personal information, passwords or other sensitive information?Logging and
Auditing
Do audit logs log connection attempts (both successful and failures)?Logging and
Auditing
Is there a process(s) in place to read audit logs for unintended/malicious behaviors?Logging and
Auditing
Is all PI and sensitive information being sent over the network encrypted form.Cryptography
Does application design call for server authentication (anti-spoong measure)?Authorization
Does application support password expiration?Authorization
Does application use custom schemes for hashing and or cryptographic?Cryptography
Code Review Checklist
198
Classes that contain security secrets (like passwords) are only accessible through protected API’sGeneral
Classes that contain security secrets (like passwords) are only accessible through protected API’sGeneral
Keys are not held in code.Cryptography
Plain text secrets are not stored in memory for extended periods of time.General
Array bounds are checked.General
All sensitive information used by application has been identiedGeneral
CATEGORY DESCRIPTION PASS FAIL
Are cryptographic functions used by the application the most recent version of these protocols,
patched and process in place to keep them updated?
Cryptography
Are external libraries, tools, plugins used by the application functions the most recent version of
these protocols, patched and process in place to keep them updated?
General
Does are there any special kind of request skipped from validation?Cryptography
User and role based privileges are documentedUser Management
and Authentication
Authentication cookies are not persistedUser Management
and Authentication
Authentication cookies are encryptedUser Management
and Authentication
Authentication credentials are not passed by HTTP GETUser Management
and Authentication
Authorization checks are granular (page and directory level)User Management
and Authentication
Authorization based on clearly dened rolesUser Management
and Authentication
Authorization works properly and cannot be circumvented by parameter manipulationUser Management
and Authentication
Authorization cannot be bypassed by cookie manipulationUser Management
and Authentication
No session parameters are passed in URLsSession
Management
Session cookies expire in a reasonable short timeSession
Management
Session cookies are encryptedSession
Management
Session data is validatedSession
Management
Session id is complexSession
Management
Session storage is secureSession
Management
Code Review Checklist
199
CATEGORY DESCRIPTION PASS FAIL
Session inactivity timeouts are enforcedSession
Management
Data is validated on server sideData Management
Is all XML input data validated against an agreed schema?Data Management
Web service endpoints address in Web Services Description Language (WSDL) is checked for validityWeb Services
Web service protocols that are unnecessary are disable (HTTP GET and HTTP POSTWeb Services
Has the correct encoding been applied to all data being output by the applicationData Management
HTTP headers are validated for each requestData Management
Are all of the entry points and trust boundaries identied by the design and are in risk analysis
report?
Business Logic and
Design
Is output that contains untrusted data supplied input have the correct type of encoding (URL
encoding, HTML encoding)?
Data Management
Web service has documentation protocol is disable if the application does not need dynamic
generation of WSDL.
Web Services
Code Review Checklist
200
THREAT MODELING EXAMPLE
Step 1 Decompose the Application
The goal of decomposing the application is to gain an understanding of the application and how it interacts
with external entities. Information gathering and documentation achieve this goal. The information gathering
process is carried out using a clearly dened structure, which ensures the correct information is collected. This
structure also denes how the information should be documented to produce the threat model.
Description
The college library website is the rst implementation of a website to provide librarians and library patrons
(students and college sta) with online services. As this is the rst implementation of the website, the function-
ality will be limited. There will be three users of the application:
1. Students
2. Sta
The rst item in the threat model is the general information relating to the threat model. This must include
the following:
1. Application Name: The name of the application
2. Application Version: The version of the application
3. Description: A high level description of the application
4. Document Owner: The owner of the threat modeling document
5. Participants: The participants involved in the threat modeling process for this application
6. Reviewer: The reviewer(s) of the threat model
General Information
Figure 13
THREAT MODEL
INFORMATION
V2.0
OWNER - DAVID LOWRY
PARTICIPANTS - DAVID ROOK
REVIEWER - EOIN KEARY
Figure 14
Threat Modeling Example
201
3. Librarians Sta and students will be able to log in and search for books, and sta members can request
books. Librarians will be able to log in, add books, add users, and search for books.
Entry points should be documented as follows:
1. ID
A unique ID assigned to the entry point. This will be used to cross reference the entry point with any threats or
vulnerabilities that are identied. In the case of layer entry points, a major, minor notation should be used.
2. Name
A descriptive name identifying the entry point and its purpose.
3. Description
A textual description detailing the interaction or processing that occurs at the entry point.
4. Trust Levels
The level of access required at the entry point is documented here. These will be cross-referenced with the trusts
levels dened later in the document.
Entry Points
Figure 15
Assets are documented in the threat model as follows:
1. ID
A unique ID is assigned to identify each asset. This will be used to cross reference the asset with any threats or
vulnerabilities that are identied.
2. Name
A descriptive name that clearly identies the asset.
3. Description
A textual description of what the asset is and why it needs to be protected.
4. Trust Levels
The level of access required to access the entry point is documented here. These will be cross-referenced with the
trust levels dened in the next step.
Assets
Figure 16
Threat Modeling Example
202
Trust levels are documented in the threat model as follows:
1. ID
A unique number is assigned to each trust level. This is used to cross reference the trust level with the entry points
and assets.
2. Name
A descriptive name that allows identication of the external entities that have been granted this trust level.
3. Description
A textual description of the trust level detailing the external entity who has been granted the trust level.
Trust Levels
By using the understanding learned on the college library website architecture and design, the data ow dia-
gram can be created as shown in gure X.
Figure 17
LIBRARIANSUSERS
WEB PAGES
ON DISK
DATABASE
FILES
WEB SERVER / DATABASE BOUNDARY
USER / WEBSERVER BOUNDARY
REQUEST REQUEST
RESPONSES RESPONSES
COLEGE
LIBRARY
DATABASE
COLEGE
LIBRARY
DATABASE
SQL QUERY CALLS
PAGES
DATA DATA
DATA
Figure 18
Threat Modeling Example
203
Specically the user login data ow diagram will appear as in gure 19.
A threat list of generic threats organized in these categories with examples and the aected security con-
trols is provided in the following table:
Stride
Threat Modeling Example: Step 2a Threat Categorization
The rst step in the determination of threats is adopting a threat categorization. A threat categorization pro-
vides a set of threat categories with corresponding examples so that threats can be systematically identied in
the application in a structured and repeatable manner.
LOGIN
PROCESS
USERS
WEB
PAGES
DATABASE
FILES
AUTHENTICATE
USER SQL
QUERY RESULT
AUTHENTICATE
USER SQL
QUERY RESULT
PAGES
WEB
SERVER
COLEGE
LIBRARY
DATABASE
LOGIN REQUEST
LOGIN RESPONSE
AUTHENTICATE USER ()
AUTHENTICATE USER RESULT
DATA
DATA
USER / WEBSERVER BOUNDARY
WEB SERVER /
DATABASE BOUNDARY
Example
application threat
model of the user
login
Figure 19
Threat Modeling Example
204
By referring to the college library website it is possible to document sample threats related to the use cases such
as:
Threat: Malicious user views condential information of students, faculty members and librarians.
1. Damage potential
Threat to reputation as well as nancial and legal liability:8
2. Reproducibility
Fully reproducible:10
3. Exploitability
Require to be on the same subnet or have compromised a router:7
4. Aected users
Aects all users:10
5. Discoverability
Can be found out easily:10
Overall DREAD score: (8+10+7+10+10) / 5 = 9
In this case having 9 on a 10 point scale is certainly an high risk threat.
Microsoft DREAD threat-risk ranking model
Threat Modeling Example: Step 2b Ranking of Threats
Threats tend to be ranked from the perspective of risk factors. By determining the risk factor posed by the
various identied threats, it is possible to create a prioritized list of threats to support a risk mitigation strategy,
such as deciding on which threats have to be mitigated rst. Dierent risk factors can be used to determine
which threats can be ranked as High, Medium, or Low risk. In general, threat risk models use dierent factors
to model risks.
Threat Modeling Example
205
This appendix gives practical examples of how to carry out code crawling in the following programming languages:
• .Net
• Java
• ASP
• C++/Apache
Searching for Code in .NET
Firstly one needs to be familiar with the tools one can use in order to perform text searching, following this one needs to know
what to look for.
One could scan through the code looking for common patterns or keywords such as “User”, “Password”, “Pswd”, “Key”, “Http”,
etc... This can be performed using the “Find in Files” tool in VS or using ndstring as follows: ndstr /s /m /i /d:c:\projects\code-
base\sec “http” *.*
HTTP Request Strings
Requests from external sources are obviously a key area of a security code review. We need to ensure that all
HTTP requests received are data validated for composition, max and min length, and if the data falls within the
realms of the parameter whitelist. Bottom-line is this is a key area to look at and ensure security is enabled.
STRING TO SEARCH
request.accesstypes request.httpmethod request.cookies request.url
request.browser request.querystring request.certicate request.urlreferrer
request.TotalBytes request.BinaryRead
request.headers request.form request.servervariables request.userlanguages
request.les request.item request.rawurl request.useragent
STRING TO SEARCH
response.write HttpUtility HtmlEncode UrlEncode
innerText innerHTML <%=
HTML Output
Here we are looking for responses to the client. Responses which go unvalidated or which echo external input
without data validation are key areas to examine. Many client side attacks result from poor response validation.
OUTPUT
CODE CRAWLING
Code Crawling
206 Code Crawling
SQL & Database
Locating where a database may be involved in the code is an important aspect of the code review. Looking at
the database code will help determine if the application is vulnerable to SQL injection. One aspect of this is
to verify that the code uses either SqlParameter, OleDbParameter, or OdbcParameter(System.Data.SqlClient).
These are typed and treat parameters as the literal value and not executable code in the database.
STRING TO SEARCH
exec sp_ select from insert update
delete from where delete execute sp_ exec xp_
.Provider System.Data.sql ADODB.recordset New OleDbConnection
ExecuteReader DataSource SqlCommand Microsoft.Jet
SqlDataReader ExecuteReader SqlDataAdapter StoredProcedure
sqloledb sql server driver Server.CreateObject
exec @ execute @ executestatement executeSQL
setlter executeQuery GetQueryResultInXML adodb
Cookies
Cookie manipulation can be key to various application security exploits, such as session hijacking/xation and
parameter manipulation. One should examine any code relating to cookie functionality, as this would have a
bearing on session security.
STRING TO SEARCH
System.Net.Cookie HTTPOnly document.cookie
HTML Tags
Many of the HTML tags below can be used for client side attacks such as cross site scripting. It is important to
examine the context in which these tags are used and to examine any relevant data validation associated with
the display and use of such tags within a web application.
TAGS
<meta> <object> <frame security <iframe security
<img> <style> <layer> <ilayer>
STRING TO SEARCH
HtmlEncode URLEncode <applet> <frameset>
<embed> <frame> <html> <iframe>
207
Code Crawling
Input Controls
The input controls below are server classes used to produce and display web application form elds. Looking for
such references helps locate entry points into the application.
webcontrols.dropdownlist
STRING TO SEARCH
htmlcontrols.htmlinputhidden webcontrols.hiddeneld webcontrols.hyperlink webcontrols.textbox
webcontrols.label webcontrols.linkbutton webcontrols.listbox webcontrols.checkboxlist
WEB.cong
The .NET Framework relies on .cong les to dene conguration settings. The .cong les are text-based XML
les. Many .cong les can, and typically do, exist on a single system. Web applications refer to a web.cong
le located in the application’s root directory. For ASP.NET applications, web.cong contains information about
most aspects of the application’s operation.
httpRuntime sessionState maxRequestLength Debug
Credentials identity impersonate timeout remote
connectionStrings authentication mode Allow Deny
forms protection appSettings CongurationSettings appSettings
webcontrols.dropdownlist CustomErrors httpCookies httpHandlers
STRING TO SEARCH
requestEncoding responseEncoding Trace authorization
compilation webcontrols.linkbutton webcontrols.listbox webcontrols.checkboxlist
global.asax
Each application has its own global.asax le if one is required. Global.asax sets the event code and values for
an application using scripts. One must ensure that application variables do not contain sensitive information,
as they are accessible to the whole application and to all users within it.
STRING TO SEARCH
Application_OnAuthenticateRequest Application_OnAuthorizeRequest Session_OnStart Session_OnEnd
208
Logging
Logging can be a source of information leakage. It is important to examine all calls to the logging subsystem and to
determine if any sensitive information is being logged. Common mistakes are logging userID in conjunction with
passwords within the authentication functionality or logging database requests which may contain sensitive data.
STRING TO SEARCH
log4net Console.WriteLine System.Diagnostics.Debug System.Diagnostics.Trace
machine.cong
It is important that many variables in machine.cong can be overridden in the web.cong le for a particular
application.
Threads and Concurrency
Locating code that contains multithreaded functions as concurrency issues can result in race conditions,
which may result in security vulnerabilities. The Thread keyword is where new threads objects are created.
Code that uses static global variables that hold sensitive security information may cause session issues. Code
that uses static constructors may also cause issues between threads. Not synchronizing the Dispose method
may cause issues if a number of threads call Dispose at the same time, this may cause resource release issues.
STRING TO SEARCH
validateRequest enableViewState enableViewStateMac
STRING TO SEARCH
Thread Dispose
Class Design
Public and Sealed relate to the design at class level. Classes that are not intended to be derived from should
be sealed. Make sure all class elds are Public for a reason. Don’t expose anything that is not necessary.
STRING TO SEARCH
Public Sealed
Code Crawling
209
Code Crawling
Reection, Serialization
Code may be generated dynamically at runtime. Code that is generated dynamically as a function of external
input may give rise to issues. If code contains sensitive data, does it need to be serialized?
MyClass
STRING TO SEARCH
Serializable AllowPartiallyTrustedCallersAttribute GetObjectData System.Reection
STRING TO SEARCH
catch nally trace enabled customErrors mode
StrongNameIdentity StrongNameIdentityPermission
Exceptions & Errors
Ensure that the catch blocks do not leak information to the user in the case of an exception. Ensure when
dealing with resources that the nally block is used. Having trace enabled is not great from an information
leakage perspective. Ensure customized errors are properly implemented
Cryptography
If cryptography is used then is a strong enough cipher used, i.e. AES or 3DES? What size key is used? The larger
the better. Where is hashing performed? Are passwords that are being persisted hashed? They should be. How
are random numbers generated? Is the PRNG “random enough”?
xor
STRING TO SEARCH
RNGCryptoServiceProvider SHA MD5 base64
DES RC2
System.Security.Cryptography
System.Random Random
Storage
If storing sensitive data in memory, it is recommend to usemy the following.
STRING TO SEARCH
SecureString ProtectedMemory
210
Authorization, Assert & Revert
Bypassing the .Net code access security permission? Not a good idea. Below is a list of potentially dangerous
permissions such as calling unmanaged code, outside the CLR.
Legacy Methods
Some standard functions that should be checked in any context include the following.
Searching for Code in Java
Input and Output Streams
These are used to read data into one’s application. They may be potential entry points into an application.
The entry points may be from an external source and must be investigated. These may also be used in path
traversal attacks or DoS attacks.
STRING TO SEARCH
printf strcpy
ControlDomainPolicy ControlPolicy
SkipVerication ControlEvidence SerializationFormatter ControlPrincipal
STRING TO SEARCH
RequestMinimum RequestOptional Assert Debug.Assert
CodeAccessPermission MemberAccess ControlAppDomain UnmanagedCode
java.io.FileOutputStream File ObjectInputStream PipedInputStream
STRING TO SEARCH
FileInputStream ObjectInputStream FilterInputStream PipedInputStream
SequenceInputStream StringBuerInputStream BueredReader ByteArrayInputStream
StreamTokenizer getResourceAsStream java.io.FileReader java.io.FileWriter
java.io.RandomAccessFile java.io.File renameTo Mkdir
Code Crawling
211
Code Crawling
Servlets
These API calls may be avenues for parameter/header/URL/cookie tampering, HTTP Response Splitting and
information leakage. They should be examined closely as many of such APIs obtain the parameters directly
from HTTP requests.
STRING TO SEARCH
javax.servlet.* getParameterNames getParameterValues getParameter
getLocalName getAttribute getAttributeNames getLocalAddr
getPrincipal getUserPrincipal isUserInRole getInputStream
HttpServletRequest getQueryString getHeaderNames getHeaders
getAuthType getRemoteUser getCookies isSecure
getServerName getRemoteAddr getRemoteHost getRealPath
getParameterMap getScheme getProtocol getContentType
getName getPath getDomain getComment
getValueNames getRequestedSessionId
getRequestURI getRequestURL getServerName getValue
getMethod getPath getReader getRealPath
setHeader setAttribute putValue javax.servlet.http.Cookie
getOutputStream getWriter addCookie addHeader
Cross Site Scripting
These API calls should be checked in code review as they could be a source of Cross Site Scripting vulnerabilities.
STRING TO SEARCH
javax.servlet.ServletOutputStream.print strcpy
Response Splitting
Response splitting allows an attacker to take control of the response body by adding extra CRLFs into headers.
In HTTP the headers and bodies are separated by 2 CRLF characters, and thus if an attackers input is used in
a response header, and that input contained 2 CRLFs, then anything after the CRLFs would be interpreted as
the response body. In code review ensure functionality is sanitizing any information being put into headers.
STRING TO SEARCH
javax.servlet.http.HttpServletResponse.sendRedirect strcpy setHeader
212
Redirection
Any time an application is sending a redirect response, ensure that the logic involved cannot be manipulated
by an attackers input. Especially when input is used to determine where the redirect goes to.
SQL & Database
Searching for Java database related code should help pinpoint classes/methods which are involved in the
persistence layer of the application being reviewed.
SSL
Looking for code which utilises SSL as a medium for point to point encryption. The following fragments
should indicate where SSL functionality has been developed.
STRING TO SEARCH
sendRedirect setStatus addHeader etHeader
STRING TO SEARCH
java.sql.Connection.prepareStatement java.sql.ResultSet.getObject select insert
java.sql.Statement.executeQuery java.sql.Statement.execute delete update
java.sql.Connection.prepareCall
createStatement java.sql.ResultSet.getString executeQuery jdbc
java.sql.Statement.executeUpdate java.sql.Statement.addBatch execute executestatement
STRING TO SEARCH
com.sun.net.ssl SSLContext SSLSocketFactory TrustManagerFactory
HttpsURLConnection KeyManagerFactory
Session Management
The following APIs should be checked in code review when they control session management.
STRING TO SEARCH
getSession invalidate getId
Code Crawling
213
Code Crawling
Logging
We may come across some information leakage by examining code below contained in one’s application.
Legacy Interaction
Here we may be vulnerable to command injection attacks or OS injection attacks. Java linking to the native
OS can cause serious issues and potentially give rise to total server compromise.
STRING TO SEARCH
java.lang.Runtime.exec java.lang.Runtime.getRuntime getId
STRING TO SEARCH
java.io.PrintStream.write log4j jLo Lumberjack
JDLabAgent
MonoLog qog just4log log4Ant
Ajax and JavaScript
Look for Ajax usage, and possible JavaScript issues:
STRING TO SEARCH
document.write eval document.cookie window.location
document.URL document.URL
Searching for Code in Classic ASP
Input APIs in ASP are commonly used to retrieve the input from the request, therefore code review should ensure
these requests (and dependent logic) cannot be manipulated by an attacker. Output APIs are used by ASP to write
the response body that will be sent to the end user, hence code review should check these requests are used in a
proper manner and no sensitive information can be returned. Cookies can also be a source of information leakage.
STRING TO SEARCH
Request Request.QueryString Request.Form Request.ServerVariables
Response.Write Response.BinaryWrite <%= .cookies
Query_String hidden include .inc
214
STRING TO SEARCH
err. Server.GetLastError On Error Resume Next On Error GoTo 0
STRING TO SEARCH
location.href location.replace method=”GET” On Error GoTo 0
Error Handling
Ensure errors in an application are handled properly, otherwise an attacker could use error conditions to
manipulate the application.
Information in URL
These APIs are used to extract information from the URL object in the request. Code review should check that
the information extracted from the URL is sanitized.
Database
These APIs can be used to interact with a database, which can lead to SQL attacks. Code review can check
these API calls use sanitized input.
STRING TO SEARCH
commandText select from update insert into
.open ADODB. Commandtype ICommand
delete from where IRowSet execute .execute
Session
These API calls can control session within ASP applications.
STRING TO SEARCH
session.timeout session.abandon session.removeall
Code Crawling
215
Code Crawling
DOS Prevention & Logging
The following ASP APIs can help prevent DOS attacks against the application. Leaking information to a log
can be of use to an attacker, hence the following API call can be checked in code review to ensure no sensitive
information is being written to logs.
Redirection
Do not allow attacker input to control when and where rejection occurs.
STRING TO SEARCH
server.ScriptTimeout IsClientConnected WriteEntry
STRING TO SEARCH
Response.AddHeader Response.AppendHeader Response.Redirect Response.Status
Response.StatusCode Server.Transfer Server.Execute
Searching for Code in Javascript and AJAX
Ajax and JavaScript have brought functionality back to the client side, which has brought a number of old security
issues back to the forefront. The following keywords relate to API calls used to manipulate user state or the control
the browser. The advent of AJAX and other Web 2.0 paradigms has pushed security concerns back to the client side,
but not excluding traditional server side security concerns. Look for Ajax usage, and possible JavaScript issues.
STRING TO SEARCH
eval document.cookie document.referrer document.attachEvent
document.open document.URL document.URLUnencoded document.write
document.create document.execCommand document.forms[0].action document.location
document.body document.body.innerHtml document.body.innerText document.close
document.writeln location.hash location.href location.search
window.alert window.attachEvent window.createRequest window.execScript
window.location window.open window.navigate window.setInterval
window.setTimeout XMLHTTP
216
Searching for Code in C++ and Apache
Commonly when a C++ developer is building a web service they will build a CGI program to be invoked by
a web server (though this is not ecient) or they will use the Apache httpd framework and write a handler
or lter to process HTTP requests/responses. To aid these developers, this section deals with generic C/C++
functions used when processing HTTP input and output, along with some of the common Apache APIs that
are used in handlers.
Legacy C/C++ Methods
For any C/C++ code interacting with web requests, code that handles strings and outputs should be checked
to ensure the logic does not have any aws.
STRING TO SEARCH
exec sprint document.referrer fprintf
cerr System popen stringstream
fstringstream Malloc free
strncpy Strcat cout cin
printf Stdio FILE strcpy
Request Processing
When coding within Apache, the following APIs can be used to obtain data from the HTTP request object.
STRING TO SEARCH
headers_in ap_read_request post_read_request
Response Processing
Depending on the type of response to be sent to the client, the following Apache APIs can be used.
STRING TO SEARCH
headers_out ap_rprintf ap_send_error_response ap_send_fd
ap_vprintf
Code Crawling
217
Code Crawling
Logging
Log messages can be implemented using custom loggers included in the module (e.g. log4cxx, etc), by using
the Apache provided logging API, or by simply writing to standard out or standard error.
STRING TO SEARCH
cout cerr ap_open_stderr_log ap_error_log2stderr
ap_log_error ap_log_perror ap_log_rerror
STRING TO SEARCH
ap_unescape_all ap_unescape_url ap_unescape_url_keep2f ap_unescape_urlencoded
ap_escape_path_segment
Cookie Processing
Cookie can be obtained from the list of request headers, or from specialized Apache functions.
STRING TO SEARCH
headers_in headers_out headers_out ap_cookie_write2
ap_cookie_read ap_cookie_check_string
HTML Encoding
When the team has got a handle for the HTML input or output in the C/C++ handler, the following methods
can be used to ensure/check HTML encoding.
HTML
218
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