Bypassing Stack Protections

TL;DR

This guide explains how to bypass common stack protections like va_randomize_space and the stack protector. These protections aim to prevent buffer overflows from being exploited, but they aren’t foolproof. We’ll cover techniques including return-oriented programming (ROP) and disabling protections at runtime.

Understanding Stack Protections

Before we bypass them, let’s quickly understand what these protections do:

• Stack Protector: Adds a ‘canary’ value onto the stack before the return address. If this canary is modified during function execution (e.g., by a buffer overflow), it indicates a potential attack and terminates the program.
• va_randomize_space: Randomizes the base address of various memory regions, including the stack. This makes it harder for attackers to predict where code or data is located in memory.

Bypassing Stack Protector

1. Identify if a Stack Protector is Enabled: Check the compiled binary’s disassembly. Look for instructions that push and pop a canary value around function calls. You can also use tools like objdump or radare2 to analyze the executable.
2. Find a Leak: The goal is to find a way to read the canary value from memory. This could be through format string vulnerabilities, information leaks in other functions, or by exploiting another bug that reveals stack contents.
3. Overwrite with Leaked Canary: Once you have the canary value, include it in your overflow payload *before* overwriting the return address. This prevents the stack protector from triggering.

Example (Conceptual):

# Assume canary is 0xdeadbeef and return address offset is 40 bytes
payload = b'A'*40 + pack(0xdeadbeef) + pack(target_address)

Bypassing va_randomize_space

1. Disable Address Space Layout Randomization (ASLR): If possible, disable ASLR on the target system. This makes memory addresses predictable.
   • On Linux:

     sudo sysctl -w kernel.randomize_va_space=0

2. Information Leaks: If ASLR is enabled, you need to leak memory addresses.
   • Library Base Address: Leak the base address of a loaded library (e.g., libc). This provides a known starting point for calculating other addresses.
   • Stack Base Address: Leak the stack base address, if possible.
3. Return-Oriented Programming (ROP): ROP is the most common technique to bypass va_randomize_space.
   • Find Gadgets: Identify short sequences of instructions (‘gadgets’) in existing code that end with a ret instruction. These gadgets are used to build chains of operations. Tools like ROPgadget can help find these.
   • Build ROP Chain: Construct a chain of gadget addresses, placing them on the stack after the return address. Each gadget performs a small part of your desired operation (e.g., popping registers, performing arithmetic).
   • Execute ROP Chain: Overwrite the return address with the address of the first gadget in your ROP chain. When the function returns, it will jump to that gadget, and execution will proceed through the chain.

   Example (Conceptual – simplified):

   # Assume pop rdi; ret is at 0x401234 and system() address is at 0x7ffff7a00000
   payload = b'A'*40 + pack(0x401234) + pack('/bin/sh') + pack(0x7ffff7a00000)
   

   Important Considerations

   • Address Packing: Ensure you are packing addresses correctly for the target architecture (e.g., 32-bit or 64-bit). Use libraries like pwntools to handle this automatically.
   • NOP Sleds: Add a sequence of ‘no operation’ (NOP) instructions before your ROP chain to increase the chances of hitting it, especially if you have limited precision in address calculations.
   • Debugging: Use a debugger (e.g., GDB) to step through the execution and verify that your payload is working as expected.