Buffer Overflow: A Pen Testing Guide

TL;DR

A buffer overflow happens when a program tries to write more data into a memory area (the ‘buffer’) than it’s allowed. This can overwrite other important parts of the program, letting attackers run their own code. Pen testers use this to gain control of systems by crafting specific inputs that exploit these vulnerabilities.

Understanding Buffer Overflows

Buffers are temporary storage areas in a computer’s memory used to hold data while it’s being processed. A buffer overflow occurs when more data is written into the buffer than its allocated size, potentially corrupting adjacent memory locations. This corruption can lead to crashes or, more seriously, allow an attacker to execute arbitrary code.

How to Implement a Buffer Overflow in a Pen Testing Environment

1. Identify Vulnerable Code: The first step is finding code susceptible to buffer overflows. Common culprits include functions that copy data without checking the input size, such as strcpy, gets, and sprintf in C/C++.
2. Set up a Test Environment: It’s crucial to work in a safe environment. Use a virtual machine (VM) or isolated network to prevent accidental damage to production systems. A vulnerable application is needed; many are available online for practice (e.g., Metasploitable).
3. Disassemble the Target Program: Use a disassembler like objdump or radare2 to examine the program’s assembly code. This helps identify the buffer and the return address on the stack.

   objdump -d vulnerable_program | less

4. Determine Buffer Size: Find out how large the buffer is. Debuggers like gdb are invaluable for this. Set a breakpoint before and after the data copy operation to examine memory contents.

   gdb vulnerable_program

   break main

   run

   info frame

5. Craft the Exploit Payload: This is where you create the malicious input. The payload typically consists of:
   • Padding: Fill the buffer to reach the return address.
   • Return Address: Overwrite the return address with the address of your shellcode or a useful function (e.g., system).
   • Shellcode: The actual code you want to execute. This could be anything from opening a shell to creating a backdoor.
6. Send the Exploit Payload: Deliver the crafted payload to the vulnerable program. This might involve providing it as command-line input, through a network connection, or via a file.
7. Verify Exploitation: Check if your exploit worked. If successful, you should be able to execute your shellcode or gain control of the program’s execution flow.

Example (Simplified)

Let’s say a vulnerable C program has this code:

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[]) {
  char buffer[64];
  if (argc > 1) {
    strcpy(buffer, argv[1]);
    printf("Buffer contents: %sn", buffer);
  }
  return 0;
}

This code is vulnerable because strcpy doesn’t check the length of argv[1]. If you provide an input longer than 63 characters, it will overflow the buffer.

Tools

• GDB: A powerful debugger for examining program execution and memory contents.
• objdump/radare2: Disassemblers to analyze assembly code.
• Metasploit Framework: Provides pre-built exploits and tools for buffer overflow attacks.
• pwntools: A Python library for writing exploit scripts.

Important Considerations

• Address Space Layout Randomization (ASLR): ASLR randomizes the memory addresses of key program components, making it harder to predict the return address. Techniques like Return-Oriented Programming (ROP) can bypass ASLR.
• Data Execution Prevention (DEP)/NX Bit: DEP prevents code execution from data segments. ROP is also used to circumvent DEP/NX.
• Stack Canaries: Stack canaries are values placed on the stack to detect buffer overflows. Overwriting a canary will cause the program to terminate.