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Python3 Buffer Overflow: Fixing Return Address

G5 Cyber Security

TL;DR

This guide shows how to fix a buffer overflow exploit in Python 3 where the wrong return address is being written. We’ll cover identifying the issue, calculating the correct offset, and crafting an exploit payload.

Prerequisites

1. Understanding the Problem

A buffer overflow occurs when a program writes data beyond the allocated space for a buffer. In this case, we’re aiming to overwrite the return address on the stack so that execution jumps to our malicious code.

2. Vulnerable Program (Example)

# vulnerable.py
def function(input_string):
    buffer = bytearray(10)
    for i in range(len(input_string)):
        buffer[i % len(buffer)] = ord(input_string[i])
    print("Buffer contents: ", buffer)

if __name__ == "__main__":
    user_input = input("Enter some text: ")
    function(user_input)

This program has a fixed-size buffer of 10 bytes. If the user provides more than 10 bytes of input, it will overflow the buffer.

3. Identifying the Offset

  1. Run the program with a pattern: Use a tool like pattern_create.rb (from Metasploit) to generate a unique pattern string.
  2. Execute the vulnerable program: Run the program and provide the generated pattern as input. 
    python3 vulnerable.py < /path/to/pattern_string
  3. Examine the crash: When the program crashes, it will display a segmentation fault with an address on the stack. This is likely where the overwritten return address is located.
  4. Find the offset: Use pattern_offset.rb (from Metasploit) to determine the exact offset of the return address within the pattern. 
    /path/to/pattern_offset.rb < crash_output

For example, if pattern_offset.rb outputs ’14’, then the return address is located at offset 14.

4. Crafting the Exploit Payload

  1. Determine the target address: Find the memory address of a function you want to execute (e.g., system()). You can use tools like GDB or pwntools for this. 
    # Example using GDB:
gdb vulnerable.py
break main
run
pinfo system
  2. Create the payload: The payload will consist of:
    • Padding to fill the buffer up to the return address offset.
    • The target address (in little-endian format).
  3. Convert the address to bytes: Use Python’s struct.pack function. 
    import struct
target_address = 0x401166 # Example system() address
payload = b"A" * 14 + struct.pack(<I, target_address)

5. Executing the Exploit

  1. Run the program with the payload: Execute the vulnerable program and provide the crafted payload as input. 
    python3 vulnerable.py < /path/to/payload
  2. Verify execution: If successful, the target function (e.g., system()) should be executed.
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