ECE 404 Homework #10 solution


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Buffer Overflow Attacks
Included with this homework, you will find two socket programs written in C. One of them acts as
a server and the other as a client. Your first task is to launch an attack that executes the “secret”
function in the server side code by using the client program to send a carefully crafted string to
the server. Following this, your second task must show how you would fix server.c to prevent such
a buffer overflow attack. Here are the instructions for performing these two tasks:
• If you are using gcc on your Linux machine, you must compile the server and client programs
with the -fno-stack-protector option. Refer to pages 36-37 of Lecture 21 for more details.
If you wish, you can also use the Tiny C compiler tcc.
• You can test the programs with two different shell terminals: one terminal for a server and
the other terminal for a client. You can also run the server on a Purdue ECN machine using
a high numbered port like 7777 and the client on your own laptop.
• Use gdb to determine how you can develop a string to send (using the client program) to
the server program and trigger the execution of secretFunction(). Refer to Lecture 21.6
for more details on how to do this. Here are some things to keep in mind when creating this
1. While you send the data with the client program, you will have to run the server program
with gdb to determine the buffer overflow string to use.
2. When sending the string to the server program, note that you can send ASCII characters
as well as hexadecimal-format bytes. You can send, for example, the hex byte 0xAD using
the format \xAD.
3. As in the lecture notes, you will need to reverse the order of addresses sent to deal with
big endian-little endian conversion problems.
• You may need to turn off ASLR on your operating system for the attack to work properly.
• After you have completed crafting your buffer overflow string and then triggering the execution of secretFunction(), modify server.c to remove the buffer overflow vulnerability.
Your fix should allow the program to run without a buffer overflow (i.e. the program should
not simply exit to deal with the buffer overflow attack).
• Include comments in the code explaining what the vulnerability was and how you fixed it.
Useful Notes
• Make sure that the server program is running before you start the client program.
• You can use whichever high-numbered port number you like, but you will need to update
the client.c file and recompile each time you change it.
• Be mindful of any aliases you may have created for the gcc command in previous classes
(you should be able to view such aliases, if they exist, in ∼/.bashrc). You may unknowingly
be using additional compiler flags with gcc if it is in fact aliased. These additional compiler
flags may result in errors when trying to perform the buffer overflow attack.
Submission Instructions
• For this homework you will be submitting 2 files electronically. Your submission must include:
– The modified server code. You should put comments with an explanation of the vulnerability and fix in the comments.
– A PDF containing the specially-crafted buffer overflow string, an explanation of why
you chose the string, and the modified server code along with an explanation of your
fixes to the code as detailed above. Please make sure it is clear where the modifications
in your code are (e.g. circling, highlighting, or denoting the modified code blocks with
• Each file should include a homework header as described in the homework section of the
ECE 404 website.
Electronic Turn-in
turnin -c ece404 -p hw10 new_server.c hw10.pdf