Fixing Buffer Overflow Exploit Space

TL;DR

Your buffer overflow exploit isn’t working because the crash doesn’t leave enough space for your payload. This guide shows how to find the correct offset and adjust your exploit code accordingly.

Understanding the Problem

A buffer overflow happens when a program writes data beyond the allocated memory for a buffer. Exploiting this usually involves overwriting parts of the stack, including the return address, to redirect execution to your malicious code (the payload). If the crash doesn’t provide enough space *after* the overwritten buffer and before the return address, your payload will be truncated or corrupted.

Step-by-Step Solution

1. Confirm the Crash: Make sure you can reliably trigger a segmentation fault (segfault) with your input. This confirms the overflow is happening.
   • Run your program with a long string as input.
   • Use a debugger (like GDB) to see where it crashes.
2. Find the Offset: Determine how many bytes you need to write before overwriting the return address on the stack.
   • Pattern Creation: Generate a unique, non-repeating pattern of characters (e.g., using msf-pattern_create from Metasploit). A good length is usually around 50-100 bytes.

   msf-pattern_create -l 64

   • Run with Pattern: Feed this pattern as input to your program.
   • Identify the Offset: When the program crashes, examine the stack in the debugger. Find the point where your pattern appears in the return address location. Use msf-pattern_offset to calculate the offset.

     msf-pattern_offset -q 

     Replace <crash_address> with the hexadecimal value of the overwritten return address from your debugger.

3. Adjust Payload Length: Calculate the total space available for your payload. This is usually:
   • Total stack buffer size – offset to return address = Space for payload
4. Craft Your Exploit Code: Build your exploit code, ensuring it fits within the calculated payload length.
   • Shellcode: Prepare your shellcode (the actual malicious instructions). Keep it as small as possible.
   • NOP Sled (Optional): Add a NOP sled before your shellcode to increase reliability. This provides some leeway if the exact return address isn’t perfect.

     x90

   • Return Address: Include the correct address of your payload (or the start of your NOP sled) in the overwritten return address location.
5. Test and Refine: Run your exploit again. If it still crashes, double-check:
   • The offset calculation is correct.
   • Your payload fits within the available space.
   • The return address points to a valid location in memory (where your shellcode resides).

Example

Let’s say you found an offset of 24 bytes and have a stack buffer size of 64 bytes. Your payload space is then 64 – 24 = 40 bytes.

Your exploit code should be no longer than 40 bytes, including the shellcode, NOP sled (if used), and return address.