Buffer Overflow: Shell Access

TL;DR

This guide shows you how to exploit a simple buffer overflow vulnerability to gain shell access on a target system. We’ll cover identifying the vulnerability, crafting an exploit payload, and running it.

Prerequisites

• A vulnerable program (we’ll assume one exists for demonstration).
• Basic understanding of assembly language (helpful but not essential).
• A debugger like GDB.
• A Linux environment (Kali is ideal).

1. Identifying the Vulnerability

Buffer overflows happen when a program writes data beyond the allocated space for a buffer. This can overwrite important parts of memory, including the return address on the stack.

• Static Analysis: Examine the source code (if available) for functions that copy user input without checking its length (e.g., strcpy, gets).
• Dynamic Analysis: Use a debugger to monitor the program’s execution while providing long inputs. Look for crashes or unexpected behavior when the buffer overflows.

2. Determining the Offset

We need to find out how many bytes of input are needed before we start overwriting the return address.

1. Set a breakpoint: Set a breakpoint just before the function returns (e.g., using GDB).
2. Provide a pattern: Send a unique, repeating pattern as input to the program. Tools like msf-pattern_create can generate these patterns. For example:

   msf-pattern_create -l 50

3. Examine the stack: When the program crashes, examine the stack in GDB to find where the pattern overwrites the return address.

   info frame

4. Find the offset: Use msf-pattern_offset to determine the exact offset of the return address within the pattern. For example:

   msf-pattern_offset -q <crash_address>

3. Crafting the Exploit Payload

The payload will overwrite the return address with the address of a shellcode or a function that executes a shell.

1. Shellcode: Shellcode is machine code designed to execute a specific task, like spawning a shell. You can find pre-made shellcode online (e.g., from Metasploit).
2. Address of system or similar: If you cannot use shellcode directly, identify the address of a function that executes commands (like system) in the program’s memory.

   info functions system

3. Payload Structure: The payload will typically consist of:
   • Padding: Bytes to fill the buffer up to the return address.
   • Return Address: The address of the shellcode or system function.
   • Shellcode (if applicable): The machine code for your desired action.

4. Running the Exploit

1. Send the payload: Send the crafted payload as input to the vulnerable program. This can be done through standard input, a file, or network connection.
2. Monitor execution: Use GDB to monitor the program’s execution and verify that the return address is overwritten correctly.

   run <payload>

3. Shell Access: If successful, you should gain shell access on the target system.

5. Example (Simplified)

Let’s assume the offset to the return address is 20 bytes and we want to execute shellcode at address 0xbffff7e4.

payload = b"A" * 20 + b"xe4xf7ffbf"  # Padding + Return Address (little-endian) + Shellcode

Send this payload to the program. If everything is set up correctly, you should get a shell.

Important Considerations

• Address Space Layout Randomization (ASLR): ASLR randomizes memory addresses, making it harder to predict the address of functions like system. Techniques like information leaks or Return-Oriented Programming (ROP) are needed to bypass ASLR.
• Non-Executable Stack: Modern systems often prevent code execution from the stack. This requires techniques like ROP to execute shellcode indirectly.
• Security Measures: Many programs have built-in security measures to detect and prevent buffer overflows.