Assignment 4 Instructions

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Buffer overflow
You are given a Linux program called grade that runs on the Rutgers iLab Linux systems. Download
the file here and unzip it.
When you run it, it prompts you with
Do you want your grade (y/n)?

If you enter a y, you will see your grade, which is an F.
Fortunately, the code for this program is sloppy and it fails to check for the end of the buffer when
reading the user’s response. You need to take advantage of this to provide an input that will display
your grade as an A+.

Groups
This is an individual assignment. You are expected to do this on your own and submit your own
version of the injection and program output.

Environment
Your submissions will be tested on Rutgers iLab Linux systems. You can most likely develop this on
any other x64 architecture Linux system but you are responsible for making sure that it will work on
the iLab systems.

What you need to do
This grade change is possible via an overflow since it happens that the grade string (the
symbol grade) is located in higher memory than the string containing the user’s answer to the prompt
(the symbol answer).
The caveat is that there is data in between the string that should not be modified.
You will need to use the gdb debugger to set a breakpoint prior to reading the input at a function
called getinput() and see what the contents are in between the two buffers so you can recreate that
data in your buffer overflow.
The data that you feed the program will be the response string (y) followed by the recreated data,
followed by the string “A+”, which represents your new grade.
Since you cannot type in most of this data, you will create a file containing the hex values for all the
data as text (e.g., 0x11 0x22 0x33 …) and then use the spitbytes program to read the file and pipe
its output to the grade program:
./spitbytes bytes.txt | ./grade

The spitbytes program simply reads whitespace-separated numbers that are represented in printable
ASCII text (e.g., 42 for decimal, 0x2ain hex) and outputs them in binary.

Hints
There is absolutely no programming that needs to be done here. Some of you will finish this
assignment in just a few minues. Some of you, however, may not have any experience with using

gdb and may feel a bit lost in dumping memory and constructing the sequence of bytes you need to
inject. Allow yourself sufficient time and realize that we have an exam coming up.

References
There are many tutorials on using gdb on the web. Here’s one but you can find hundreds of others.
All you need to know are:
•
•
•
•

the break command
the run command
the x command to examine memory
finding addresses that correspond to symbols (either the print or info commands or just
dump contents and see the address via the x command.

You’ll also need to know the ASCII codes for the charactes you need to enter and you can easily find
these on the web (e.g., wikipedia) or run man ascii on any Linux or macOS system.

What to submit
When you have completed your assignment, you will need to submit only two files via sakai:
1. bytes.txt, which is a file that contains the textual representations of the data that you will
send as input to the grade program.
2. output.txt, which contains the exact output of everything printed by running
the grade program. For example:
./spitbytes bytes.txt | ./grade >output.txt

Validate that the contents are correct before submitting. We will use a script to generate and test
your programs, and you will be penalized for any deviation. Do not submit anything else.
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