Buffer Overflow Exploitation Guide

TL;DR

This guide explains how to exploit a buffer overflow vulnerability. It covers identifying the problem, crafting an exploit payload, and delivering it to overwrite program execution.

1. Understanding Buffer Overflows

A buffer overflow happens when a program tries to write more data into a fixed-size memory area (the ‘buffer’) than it can hold. This overwrites adjacent memory locations, potentially changing how the program behaves – and allowing an attacker to take control.

2. Identifying Vulnerable Code

1. Look for unsafe functions: Functions like strcpy, gets, sprintf, and scanf (without length limits) are common culprits. They don’t check the size of input before copying it into a buffer.
2. Fuzzing: Send large or unexpected inputs to the program and monitor for crashes. Tools like AFL (American Fuzzy Lop) automate this process.
3. Static analysis: Use tools that scan source code for potential vulnerabilities without running the program.

3. Determining Offset

You need to find out how far into the buffer you can write before overwriting critical data, like the return address on the stack.

1. Debugging: Use a debugger (like GDB) to step through the vulnerable code and observe memory changes.
2. Pattern creation: Create a unique pattern of characters (e.g., using msf-pattern_create from Metasploit).
3. Send the pattern as input: Run the program with this pattern. When it crashes, examine the stack to find where the pattern overwrites the return address. The offset is the length of the valid data before the overwritten address.

msf-pattern_create -l 100

4. Crafting the Exploit Payload

The payload will overwrite the return address with a new address, redirecting execution to your malicious code.

1. Shellcode: This is machine code that performs the desired action (e.g., spawning a shell). You can find pre-made shellcode online or create your own using tools like msfvenom.
2. NOP sled: A sequence of ‘no operation’ instructions (0x90) placed before the shellcode. This increases the chances of successful execution, even if the exact address isn’t perfectly aligned.
3. Return Address: The address you want to jump to – typically the start of your shellcode or a function that calls it.
4. Payload Structure: [Padding] + [Shellcode] + [NOP sled] + [Return Address]

Example using Metasploit to generate shellcode:

msfvenom -p linux/x86/shell_reverse_tcp LHOST=127.0.0.1 LPORT=4444 -f raw

5. Delivering the Payload

1. Direct Input: If the vulnerability is in a command-line program, you can directly provide the crafted payload as input.
2. Network Exploitation: For network services, send the payload through the vulnerable network connection.
3. File Manipulation: If the vulnerability involves processing files, create a malicious file containing the payload.

6. Bypassing Protections

Modern systems have protections to make buffer overflows harder to exploit.

1. Address Space Layout Randomization (ASLR): Randomizes memory addresses, making it difficult to predict the location of shellcode or functions. Techniques like information leaks can help bypass ASLR.
2. Data Execution Prevention (DEP/NX): Prevents execution of code from data sections. Return-oriented programming (ROP) is a common technique to bypass DEP/NX.
3. Stack Canaries: A random value placed on the stack before the return address. Overwriting it will cause the program to crash. Leaking or bypassing the canary is required for exploitation.

7. Return-Oriented Programming (ROP)

If DEP/NX prevents executing shellcode directly, ROP allows you to chain together existing code snippets (‘gadgets’) in the program’s memory to achieve your goal.

1. Find Gadgets: Use tools like ROPgadget to find useful gadgets (small sequences of instructions ending with a ‘ret’ instruction).
2. Chain Gadgets: Construct a chain of gadget addresses on the stack, carefully controlling program execution.

Example finding gadgets:

ROPgadget --binary vulnerable_program | grep pop