Name: CS6035 Project 7 Binary Exploitation
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2026 SUMMER SOLUTION LINK: CLICK HERE

2025 SOLUTION LINK: CLICK HERE

FALL 2024 SOLUTION LINK -VIEW

Welcome to the “Binary Exploitation” assignment! This course will be a “Capture the Flag” style project, wherein you will solve some exploits on binary programs. A correct solution will output a ‘flag’ or ‘key’.

There are 11 tasks to complete for 11 total flags. Successful completion of all these flags will result in a 100% Project grade, totaling 14% of your class grade!

You will submit these flags in json format to Gradescope for grading. Detailed directions along with VM download links are included in the writeup below!

This project uses the same VM that has been used for all the other projects

Username: binexp
Password: Mount-Titlis

Go here for project details on the course Github Pages site:

https://github.gatech.edu/pages/cs6035-tools/cs6035-tools.github.io/Projects/Binexp/

Head over to Ed Discussion and navigate to the Binary Exploit project for discussions on this project.

Good luck!

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Binary Exploitation

Learning Goals of this project

Students will learn introductory level concepts about binary exploitation. This lab develops understanding of control flow hijacking through different tasks/challenges designed to show certain vulnerabilities or weaknesses in a C program. A python library pwntools will be used to show some exploitation techniques and automation to successfully hack a program

The final deliverables:

A single json formatted file will be submitted to Gradescope. This file should be named project_binexp.json. A template can be found in the Home directory. See Submission Details for more information

Important Reference Material :

– If you’re an absolute beginner with no Linux experience, start with these three links

This Writeup explaining some rudimentary basics of Computer Architecture
This Lecture I’m Developing on how the Stack and Function Calls work in C
This Website (PicoCtf) may be able to help with some more basics from an external source

– Main Course Material References Here:

This Intro to pwntools/pwndbg video showing how to automate some exploits and use our exploit framework on the VM
pwntools Documentation
GDB command cheat sheet

Submission Details

This project is autograded from a single JSON file, see Submission Details page for comprehensive instructions

Virtual Machine

(Note: downloads can be very slow when project first releases due to very high traffic in first few hours/day) This project runs in a Virtual Machine with all the binaries/tools required, and must be used to generate the correct flags. Do not move any files from their original locations, you can install any text editors, debuggers, or anything else you may want to use that aren’t included in the VM

VirtualBox Software (Recommend Latest Version)
(No need to download new VM unless you deleted the VM for the prior tasks!)
VM Download Link
Username: to be announced at project open in Canvas

Rubric

See Rubric Page for extensive breakdown of the points possible for this project

Step 0: IMPORTANT!!

You will need to run a few commands to get the course material, as well as install/update two packages to complete this project.

You can copy and pase this entire block into a terminal or run them one at a time, if you want to do that for some reason:

cd ~/

pip install –upgrade capstone

pip install ropper

wget https://cs6035.s3.amazonaws.com/binexp/spring_2024/binexp.sh && chmod 777 binexp.sh && ./binexp.sh

The final command can be re-used at any point in this project and will allow you to re-download a fresh copy of the project material, and make a backup of your existing project_binep.json and ~/binexp/ folder.

Step 1:

Open a terminal and cd into the project directory ~/binexp/00_intro.

$ cd ~/binexp/00_intro

There are a few files in this directory if you type the ls command, detailed below:

e.py – this is a python script that uses the Pwntools library, and will be used to exploit the binary file. Named ‘e’ short for ‘exploit’

flag – this is the COMPILED BINARY that you will target for the various challenges, the e.py file targets this binary with both standard process I/O or GDB depending on which option you provide to the run command

flag.c – this is the SOURCE CODE for the COMPILED BINARY. This is a non-executable text file that is simply provided to show you the code that was compiled to be run and is PURELY FOR REFERENCE

user.txt – this file needs your 9-digit GTID number (looks like 901234567)

note, this file is a symbolic link that will be modified for all other directories, so you only need to set this file once unless you pull a fresh version of the files from our server with the restore command.

Now open e.py and follow the comments in the file to supply your GTID (9 digit numeric school ID number that looks like 901234567) where it specifies, and afterwards execute the script ./e.py to get your first flag!

Your output will look like this. Copy this submission hash and place in the json file in your home directory ~/project_binexp.json

SUBMIT YOUR FIRST FLAG TO MAKE SURE IT WORKS BEFORE CONTINUING

Also, it is a very good idea to submit each flag you get to make sure it works before moving on, in case of any issues

(Applicable for all flags): If for whatever reason you don’t get a flag and you’re positive you should, try running the exploit once or twice. The flag generator can have some unexpected behaviors. When in doubt, make a post in Ed Discussion to ‘All Instructors’ and we will assist you if possible

01_bb_steps

(watch the intro video first please)

This is a test on using GDB to read a few things that would be difficult to calculate (you can if you want to), but the main point of this task is to try running a few things in gdb and get comfortable with it first.

cd ~/binexp/01_bb_steps/

you will be met with a pwndbg prompt which waits for you to run the program (or set breakpoints, etc) which we’ll do now:

b main

sets a breakpoint at the first executable statement of the main() function.

runs the program and you will be met with some text/blocks showing various things like stack traces, register printouts, code prints, etc. Now next you will want to run the STEP command to go into the function

After that, you will notice we are now in the bb_steps() function and can traverse all the code with the NEXT command

Which will execute each following instruction. This is useful if you don’t want to go into every single function call, and rather want to just execute those calls/instructions.

Pay attention to the register window which will show each register value as the instructions are executed.

At the end of the ASM instructions, you will be prompted to enter in the answers for the two registers RBX and R15. You can enter these into GDB if you want to, however to get the official/valid flag for submission you will have to save your answers for the two registers, and then enter them into the non-debug binary run, e.g.:

binexp@cs6035:~/binexp/01_bb_steps$ ./flag

What value is currently in RBX?: 800008135

Upon correctly answering the questions, you will see your flag printed out, which you can copy into the json file!

01_basic_overflow_1

(watch the intro video first please)

This task is a very simple buffer overflow that, upon inspection, will check if a variable is non-zero. Using the information you have gathered from reading and the videos, it is your task to get this program to get to the call_me() function, and get the flag printed.

Here are some steps I suggest you follow for the remainder of the project, that will set yourself up for success when debugging/analyzing/reverse engineering the binaries.

Read the source code! You have the source code for each of these tasks saved in the same directory named flag.c which will help you understand what the program does
Look for where the input is taken! All of these programs take input in a few different ways, but they all are similar in the fact that this is how you get your exploit/solution to work! Check for things like read() or scanf in the binaries.
Once you have identified where the input is taken, look at what variables are also going to be on the stack, everything is fair game!
- Note that all of these binaries are pretty much specially written so that you will need to use the adjacent variables to pass certain checks by overflowing (not all of them though) so just try to focus on the bigger picture when you are analyzing the program
- In this instance you need to overflow the target buffer to change the initialized value of the variable make_me_not_zero. Although this is a trivial example, it is just designed to show you how unprotected buffers can result in changing other adjacent values in memory

Note: you are free to use GDB if you need to for this project but you need to run the program on the command line (i.e. ./e.py) in order to get the real flag for submission and submit it!

Task 01_basic_overflow_2

In this task you will learn details about binaries compiled from C code (with gcc) in a Linux environment, and how some basic things can be exploited such as process redirection or control flow hijacking. The steps in this flag are discussed in-depth in the intro video.

In this directory you have an executable binary named ‘flag’ which is vulnerable to a buffer overflow in one of its functions. We will be using an exploitation library called pwntools to automate some of the overflow techniques and get the binary to call a function it otherwise wouldn’t have. This function called ‘call_me’ generates a key using your Gradescope User ID to get a valid flag that will pass the autograder.

Now we will run the binary just to see what the program is doing by running the executable

$ cd ~/binexp/01_basic_overflow_2

$./flag

We see the binary is asking for a string, input any text you want or just press enter, and you’ll see that the program does nothing and just exits. That’s just to simplify the code so we can focus on the exploit.

The binary is statically linked to a shared object which has a lot of methods that construct the key and has a simple function called ‘call_me()’ which will print out your key.

This is where we will start learning about binary file formats. Without going into a deep dive about program structure, operating systems, compilers, assembly language, machine code, etc. you will still be able to understand that there are two aspects that are key in binary exploitation

Data

is simple enough, it is just any collection of bits that represent some kind of data element (like an ASCII character, integer value, pointer, etc)

Addresses

At this scope we can just think of addresses as fully unique identifiers of specific data elements. These are logical locations the computer understands.

A buffer overflow occurs when too much data is fed into an unprotected (or poorly protected) data buffer. The way that 64-bit C programs work is, a few bytes before the start of the stack frame (this is the function entry point), there is an address stored in memory called the Return Address. This Return Address is read into a register upon function return (when the function ends and intends to return to its caller), andthen process redirection to that address happens. If we override this location with another valid address, we can manipulate the control flow of the program and have it execute arbitrary (or otherwise unintended) code, with a well-formed attack. Starting off easy, we are going to modify e.py and learn a few basics of the pwntools library, which will build up into a successful attack at the end.

Open e.py with your favorite text editor and analyze the content and comments.

Once you understand what they do, proceed to fill in the cyclic size (this number is up to you, based on your understanding of the program and what would break it) to get a segmentation fault message by running

$ ./e.py dbg

This will open up a gdb terminal with a breakpoint set at main()

Type ‘c’ to continue from the breakpoint

(sometimes you might need to press ‘c’ twice if you don’t see the error, this is an issue with how gdb attaches to processes)

We see the program received an interrupt signal for a SEGMENTATION FAULT (SIGSEV, or an invalid access to memory). This happens when the program tries to access memory at a certain location that it either isn’t allowed to access, or doesn’t exist. In this case the return address for the function was overwritten by cyclic()’s data in the form of long strings of characters. Pay attention to the bottom of the screenshot where the instruction pointer is currently trying to ‘ret’ (return) to 0x6561……616b which is just a string of ascii characters in hexadecimal form. Now we know how to break the binary, let’s figure out how to purposefully break it. Using a pwntools method called ‘cyclic_find()’ we enter in the bottom 32 bits (4 bytes) of the return string (in this example is 0x6561616b) which will give the number of characters before reaching that value. This is important because we are now going to reach our first step of control flow hijacking by overflowing enough data that we can place a value and change the course of the program’s normal path.

In e.py, on the commented line below your cyclic command, we are now going to use cyclic_find() which will automate our buffer length calculation, and feed that number into cyclic(). Place in your 4 character bytes (preceded by a 0x, like 0x6561616b). Uncomment either of the lines beneath our original cyclic() call (one uses hex value and the other uses the ASCII values), and fill in the hex or ascii value described above This will fill the buffer with our calculated buffer length, appended by the ASCII byte equivalent of the variable by using another pwntools method p64(number).

After you have done that, rerun

./e.py dbg

And hit ‘c’

If done correctly, you should see something like this screenshot, where if you check the ‘ret’ instruction, we are now failing on an invalid access to our dummy address.

Stepping away from the pwntools library for a moment, we now need to find something usable within the binary that will allow us to actually call a function or do something other than just crashing the program.

Now we will use a linux command ‘objdump’ which takes a binary file and will output a dump of the binary which will give some key information about the binary. The -D flag will output binary addresses, machine code, and assembly code of the binary into a file.

objdump -D flag > flag.asm

Then open flag.asm

You will see a bunch of (likely) confusing information that at a high level translates to the code that you can see in the ‘flag.c’ file. You aren’t going to have to go through this file in any extreme expanse (unless you want to?) we are just going to focus on finding an address within the binary file that holds the machine code responsible for making a function call to ‘call_me()’.

Search for the string ‘call_me’ in flag.asm and keep looking until you find the assembly instruction: call <some address> <call_me>

Note down the highlighted address showing the call (it will be different in your binary):

Now open e.py and adjust the line (see the commented useful commands section) payload += p64( 0xdeadbeefdeadbeef )

With the hexadecimal value of the address above (prepend 0x to the value highlighted)

Now run ./e.py again from the command line (without dbg) and check the terminal output.

Did you get it? Awesome! Submit your first flag to gradescope (follow APPENDIX for more details) If not, retrace your steps in this task and also make sure you used the call call_me address in the earlier step and not the address of the actual function call_me()

Task 02_assemble_the_assembly

This task will get you to determine which assembly instructions will properly construct a call using the address of the call_me() function (the actual address of the function, as opposed to task 1 which needed the call to a function). Analyze the different instructions and look up the usage/behavior of them to figure out which one will construct the address.

You can use objdump or gdb to find the address of call_me() and figure out how you calculate it.

For debugging, I highly recommend using gdb, setting a breakpoint on the gadget function, and stepping through the options once you think you know the correct path to get to the function call.

(FYI:: you don’t have to use pwntools for this one)

Task 02_bad_rand0

This Program (very conveniently) leaks out part of the libc base address this address is randomized via ASLR so it will change a little bit every time the program is launched run the program a few times and notice what bytes are different and which ones aren’t

Next step will be analyzing the C file and see what we are comparing against in order to get to call_me – system() is a libc function, use GDB to get the address of system using command p system – run ./flag multiple times, it will ask you for input and your goal is to guess an address. Put in any random guess and try it a few times to see if you can notice a pattern versus what is leaked and what is being expected.

Fortunately there’s only one byte that is missing from our formula, so we can do some scripting in python to try out the remaining values.

– pwntools has a function called recv<line|until|all>() that will let us do some manipulation with the string returned (before we send the payload) and allow that to coerce the input we send in. – the recv functions will return a BYTES object, so you will need to do some clever manipulation of said strings that are returned, this will probably take a few iterations and permutations to get the value in the right format – note that the C file is using scanf to read in a hexadecimal number, meaning you don’t need to use p64(), you are sending in the STRING REPRESENTATION of a hex number, that means WITHOUT the ‘0x’ in the beginning, and you send the string directly on the command line like ‘ffaabbccdd’ or ‘f701234abcd’ etc!

Your task is going to be: – get the value leaked from the program – modify it with the offset of the system() function – fill in the remaining byte with a random value – send to the process – (repeat until you get a flag) note: i recommend using recvall() after you send in each payload, and write your loop logic around the output (see other flags for what kind of string output you can expect) to see if you got the right value!

Task 02_p4s5w0rd

STRINGS!

Now it’s time to learn a really useful technique to find all the available strings in a program.

And by strings, we mean any collection of printable characters that exist in the binary. So things like variable names, hardcoded paths, debug messages, or eeeeevenn…. passwords? Hopefully not in a real program but you would be surprised.

This binary has zero debugging information and you do not have the source code available, but guess what? The program is written terribly and is very unsafe, with passwords stored in plain text that can easily be dumped/searched in the binary!

I would recommend running the program once or twice to see what it’s doing (checking a series of responses to questions) and if you get every question right, then you will get the flag!

To get the strings for the program, run the command:

$ strings flag

This will output it all to the terminal which isn’t super helpful, so would suggest redirecting output to a file like:

$ strings flag > flag_str

Now you will be able to grep/search/navigate the file in a new terminal and will (hopefully) be able to figure out what the correct responses would be for the given questions.

(hint, strings are stored in the binary in the order that they’re written in the C code, might be a good idea to search for the questions they’re asking and it should be pretty easy to determine the answer from there!)

Good luck!

Task 02_the_server_client_one

This flag shows a communication between a server and a client. The client binary (flag) will send data to the server, and the server appends some (very conveniently structured) data to that message and sends it back to the client. Your goal for this task is to have the server return the ideal data to overwrite the instruction pointer with the data that is returned from the server.

Follow the same steps in previous tasks (buffer_overflow_2, more specifically) to break the program in gdb, and then figure out your buffer size, and try to fill in the response to correctly hit this function call!

If you use the pwntools e.py file, it will start the server for you so there is no need to explicitly start the server.

If you are running the program on the command line to experiment, then you must start the server each time you run the binary. You can either open a new terminal, and run ./server

Or in the same terminal, each time you run the binary, run

./server &

Your task is to figure out the breaking point, and heavily inspect the last bytes that are returned from the server in order to get the right return and get the flag!

Task 03_XORbius

Time to rev up those Reverse Engineering motors, because you need to unravel the logic that this program is checking against in order to get to the call_me() function!

No buffer overflow this time, you just ‘simply’ need to input the right values that will correctly decode the logic and pass the checks.

If you’re unfamiliar with C operators, this TUTORIAL has all the necessary operations detailed.

Suggest pen and paper for this one to work through the logic by hand, or do a ton of experimentation to get the right value!

XOR TRUTH TABLE

XOR Operations are REVERSIBLE, meaning that performing the same XOR operation on a number twice will end up with the original number!

Example:

0b11110000 ^ 0b00001111 = 0b11111111=>0b11111111 ^ 0b00001111 = 0b11110000

in Hex notation

0xF0 ^ 0x0F = 0xFF=>0xFF ^ 0x0F = 0xF0

Task 03_pointy_pointy_point

We see there is an unsafe() function which has some checks for different local variables. The positioning of these variables is important because they are declared before the input buffer which means that a buffer overflow will cause data to be overwritten.

You will find additional details on this flag in the readme file of the folder. This program is a Buffer Overflow, however you will not be changing the control flow to a specific binary address, rather you will need to enter in the right values to trick the pointer arithmetic logic and get to the call_me() function.

(psst, the math is easy, dont overthink it, it’s just addition)

Task 03_hunt_then_rop

You’ve made it! You are now on your final task. In this directory is the entire contents of /usr/bin, a collection of binary files that make up a lot of common linux uses. One of these files has been overwritten by a vulnerable program. It is your task to figure out which one. You are given a list of checksum values generated with the linux shasum command, that are known good, so your first task will be determining the sha1 hash of all of the files in this directory, and then finding the one that does not match. You are free to do this however you would like. NOTE: in your scripting method, ignore the files ‘checksums’ and ‘user.txt’. They will likely report a mismatch but you can be certain neither are not the file in question

Once you find the file it is time to begin our exploit of that file. This is a bit more complex than the other flags and will require a full ROP (return oriented programming) exploit to chain calls together, and we will also need a new tool called Ropper to find a ‘gadget’ in order to supply a function argument and pass a specific check.

In 64-bit programs, the function gets arguments through registers, in the case of intel architecture the RDI register supplies the first function argument.

So we need to find a gadget (a piece of code that we can override the instruction pointer with, that will perform a certain action and then continue with the control flow hijack) that will pop a value from the stack into the RDI register.

Let’s use ropper like this

$ ropper –file flag | grep “pop”

This will give you all gadgets within the binary that have a keyword ‘pop’ (spoiler, there’s a LOT of them). An objective for this task is to figure out what gadget will likely work best to get the required argument passed into the function you are trying to call. This Writeup is a helpful reference to understand how calling convention works for x86_64 cpu’s

Note the addresses that are output for each gadget. Once you find a gadget you think will work, we will need that as our first override value in pwntools

Pictorially, this is what our crafted exploit needs to look like (remember stack grows down)

Now we will need to supply the argument, which will be on the stack immediately after our pop gadget, figure out what that value needs to be, and add it as p64() after the pop gadget

Then we need to put the address of the function as the next call, use objdump or gdb to find the addresses (you should probably get the second function address while you’re at it). The call to our pop gadget will ‘ret’ and then hit this second function call to enter one of the unsafe functions

Finally, we need to finish our execution chain by calling the second function which will allow for exploitation. Append that address to your chain and see if you get a flag!

File submission instructions:

The contents of the submission file should be the following. There is a ~/project_binexp.json file in your vm with a template set up, or you can copy-paste this to your newly created project_binexp.json file elsewhere and replace the placeholders with the flags you retrieve from each relevant task. (the name of the file doesn’t matter, it is just named that for clarity)

Note: You can use TextEdit or Vim to create and edit this file. Do not use LibreOffice or any Word Document editor. It must be in proper JSON format with no special characters in order to pass the autograder and these Word Document editors are likely to introduce special characters.

If you can’t find the file in the VM just copy this format below:

{

“00_intro”: “<copy flag here>”,

“01_bb_steps”: “<copy flag here>”,

“01_basic_overflow_1”: “<copy flag here>”,

“01_basic_overflow_2”: “<copy flag here>”,

“02_assemble_the_assembly”: “<copy flag here>”,

“02_bad_rando”: “<copy flag here>”,

“02_p4s5w0rd”: “<copy flag here>”,

“02_the_server_client_one”: “<copy flag here>”,

“03_XORbius”: “<copy flag here>”,

“03_pointy_pointy_point”: “<copy flag here>”,