LD_PRELOAD-rand-Hijack-Example

Code samples to go along with the blog post on using LD_PRELOAD to hijack rand() in a number guessing game.

Links to Tutorials

Blog: https://blog.secureideas.com/2020/10/not-so-random-using-ld_preload-to-hijack-the-rand-function.html

Overview

This repo is used to provide some code samples to go along side a tutorial in Secure Ideas blogs and videos. This code repo will provide you with the code for:

The number guessing game
The LD_PRELOAD hijack code
A make file to build it easily in a Kali VM.

The goal of this tutorial is to show how to use LD_PRELOAD to hijack imported functions in a binary. In this case we have a random number guessing game as a tutorial and will be using LD_PRELOAD to cheat in it by hijacking rand() and making it return a static value of 42, making the random number guessing game not-so-random and a lot easier to win.

Building The Code Using Make.

Once you clone the repo, simply change into the directory and run make or make all to build both the game and the LD_PRELOAD hijack shared object. The make file also includes a help option to show all of the build targets:

kali@kali:~$ make help
Default Target:
         all => Build the default guessing game & hijack shared object.

Game Targets:
         game => Build the default guessing game.
         game_debug => Builds a debug version of guessing game.
         game_nocolor => Build a non-colorized guessing game.
         game_debug_nocolor => Build a non-colorized debug guessing game.

Hijack Targets:
         hijack => Build standard LD_PRELOAD hijack shared object.

Maintance Targets:
         clean => delete the binaries.
kali@kali:~$

Basic Game Usage

Standard guessing game build and usage:

kali@kali:~$ make game
gcc -Wall -O3 -s src/guessing_game.c -o guessing_game
kali@kali:~$ ./guessing_game 

        ---===[ Number Guessing Game v1.0 ]===---

 [?] Guess a number between 0-31337 > 4444
 [-] You lose. :-(

kali@kali:~$

Building the debug build and usage:

kali@kali:~$ make game_debug
gcc -Wall -O3 -DDEBUG src/guessing_game.c -o guessing_game
kali@kali:~$ ./guessing_game 

        ---===[ Number Guessing Game v1.0 ]===---

 [?] Guess a number between 0-31337 > 4444
 [DEBUG] user_guess => 4444
 [DEBUG]   rand_num => 26148
 [-] You lose. :-(

kali@kali:~$

Cheating By Hijacking Random with Our LD_PRELOAD

Using the LD_PRELOAD method to inject our shared object into the game to hijack rand() and always make it return 42: