Exploiting Return to Libc (ret2libc) tutorial - pwn109 - PWN101 | TryHackMe

Introduction

This tutorial provides a comprehensive guide on exploiting a buffer overflow vulnerability using the Return to libc (ret2libc) technique. We will walk through the steps needed to hijack the execution flow of a vulnerable program, leak addresses from the Global Offset Table (GOT), and ultimately execute a shell command using the system function. This knowledge is particularly relevant for cybersecurity enthusiasts and ethical hackers looking to understand binary exploitation.

Step 1: Understanding the ret2libc Technique

• Familiarize yourself with the ret2libc concept, which allows attackers to exploit vulnerabilities without needing to inject shellcode.
• Recognize the importance of the Global Offset Table (GOT) and Procedure Linkage Table (PLT) in dynamically linked binaries.
• Review additional resources for a deeper understanding, such as:
  • Exploiting Return Oriented Programming (ROP)
  • Global Offset Table and Procedure Linkage Table
  • Endianness concepts

Step 2: Disassembling the Binary

• Use disassembly tools like Ghidra or IDA Pro to analyze the binary.
• Identify key functions and their addresses, focusing on the GOT and PLT.
• Check for protections like stack canaries, ASLR (Address Space Layout Randomization), and NX (Non-Executable) stack.

Step 3: Identifying Vulnerabilities

• Look for buffer overflow vulnerabilities within the binary.
• Spot the vulnerable function and its input handling.
• Ensure that the overflow allows you to overwrite the return address.

Step 4: Hijacking Execution Flow

• Understand how to manipulate the return address to redirect execution flow.
• Familiarize yourself with the concept of ROP chains that can be used in conjunction with ret2libc.

Step 5: Leaking Addresses

• Use the puts() function to leak addresses from the GOT.
• Set up the payload to call puts() with a GOT entry as a parameter.
• Create a function to read and print the leaked address to the console.

Step 6: Creating the Payload

• Construct a payload that includes:
  • The address of the puts() function.
  • The address of the GOT entry you wish to leak.
• Ensure the payload is properly aligned according to the calling convention (e.g., x86 or x64).

Example payload structure:

payload = b"A" * offset + p64(puts_addr) + p64(got_entry)

Step 7: Executing the Exploit

• Send the crafted payload to the vulnerable program.
• Observe the output for the leaked address, which will be used to identify the libc version.

Step 8: Finding the libc Version

• Use the leaked address to determine the specific version of libc running on the target:
  • Use tools like libc.rip, libc.blukat.me, or libc.nullbyte.cat to match the addresses against known libc versions.

Step 9: Modifying the Exploit for Second Stage

• Once the libc version is known, calculate the offsets for system() and "/bin/sh".
• Modify the payload to call system() with "/bin/sh" as a parameter.

Example of the modified payload:

payload = b"A" * offset + p64(system_addr) + p64(0) + p64(bin_sh_addr)

Step 10: Executing the Final Payload

• Again, send the modified payload to the vulnerable program.
• If successful, this will spawn a shell.

Conclusion

In this tutorial, you have learned the fundamental steps to successfully exploit a buffer overflow vulnerability using the ret2libc method. Key takeaways include understanding the importance of leaking addresses, constructing proper payloads, and executing them to gain shell access. As a next step, consider practicing this technique in a controlled environment, such as Capture The Flag (CTF) challenges or online platforms like TryHackMe, to further solidify your skills.