Date: Thu, 27 Oct 2011 17:01:30 +0200
From: Benjamin Renaut <benml@...idev.fr>
To: full-disclosure@...ts.grok.org.uk
Subject: Re: Symlink vulnerabilities

I just wrote a quick PoC for this (warning: didn't test the code a lot):

http://pastebin.com/FaaEsXRW

(compile that with -O3).
successfully tried it on my machine (Debian stable, amd64, high-end laptop).
It probably has more chances of success on low-end hardware, or if the 
system is busy.

If you really really want it to work the first time, bzexe a binary 
(say, ls) and then open the resulting file.
Increment the value of the "skip" variable at the top and add:

sleep 1

right after the "/bin/ln $tmpfile..." line. This will slow down the 
bzexe shell script so the flaw is more visible.

The example shellcode in the PoC will simply execute "id" and write the 
output into /tmp/stdout.bzexe_poc.

Note that this'll work not only with root: you can execute something 
with the rights of any user launching a bzexe-d binary (as long as you 
know what they're gonna run beforehand).

Regards,
Benjamin Renaut.

On 27/10/11 16:08, Benjamin Renaut wrote:
> Imagine the following scenario:
>
> - You create /tmp/<prog name>  (a directory)
> - Root is launching a bzexe-d binary (<prog name>).
> - The ln done by bzexe results in the link being created inside
> /tmp/<prog name>  (your directory), as explained by Vlad.
> - Before the bzexe shell script executes /tmp/<prog name>, you remove
> your directory (/tmp/<prog name>) and replace it by an evil shell script.
> - The bzexe shell code executes *your* shell script, with root rights.
>
> =>  So *yes*, this seems exploitable, given the following requirement:
> root has to execute a bzexe binary and you have to know it beforehand.
>
> The race condition if a little hard to exploit but it's doable.
>
> As for a POC, just add some sleep commands inside the bzexe shell script
> and it becomes easy to prove.
>
> On 27/10/11 15:43, xD 0x41 wrote:
>> Vlad,
>>       I wont repeat myself, again, your PoC will NOT work.
>> it will NOT get root anything!
>> please, understand it, and maybe, make a working poc, then see why...
>> It was shown clearly by 2 people or more that, it cannot work.
>> If you can do better, and, sure, do what you said wich is exact area
>> where it wont listen tho... wich is here:
>>
>> This means that right after the "ln" command AND before "/tmp/dd" is
>> launched, the user can replace the directory "/tmp/dd" by a shell script
>> with the same name ("/tmp/dd").
>>
>> You try to change and fiddle here, it would need alot better than just
>> the current shell scripting, and, even then, i dnt think it would win
>> the race conditiobn.
>> I have made a poc, i know atleast 2 others have, and, I guess those
>> people have already shown where it goes wrong for them and, it is no
>> different, because, for whatever reason, if you say it is not aslr,
>> thats fine... but, i am just saying what my opinion is.
>> You can dream all you want about this being so leet but, dude, they
>> would have bothered to patch it, if indeed it led to anything *root*
>> or pwnage, wich is when they suually act.
>> So, I know for fact, there is easier methods than this to be had, and
>> telling you, am very experinced with these scripts, and this
>> particular one, i wanted to use, ofcourse, to root some kernels! Hell,
>> i am a blackhat...so, i wanted to remake but, nothing, and when
>> exchanged notes on it, I got nothing but told, aslr/memory will shut
>> it down, etc etc..  but, i stfu and kept writing my poc, then, when i
>> thought it was perfect, it would not allow me to occupy that space,
>> maybe if i could be bothered to occupy the EIP or, overwrite it
>> somehow, then i would perhaps bother for race, but seriusly in the
>> form your saying, well, hey, kets just tar anything, and, rely on the
>> internals to fail...surely this would never give us root...
>>
>>
>> as i said.. you dont need to even look as far as you are, but, this is
>> a flawed and, sorry but, it is prroven to fail.
>> if it were any decent, like kcopes sh scripts usually are, then, it
>> would be PATHCED :)
>> Sorry, no banana i said :)
>> I mean it.
>> you will NOT make any PoC here in bash script.
>> Sorry, but, get out of those delusional state, and, skip to real
>> world, it is 2011, and it is vanilllllla :)
>> anyhow, have a good day but, please, i have said and prooven whatever
>> i wanted, so, i dont need any more tech talk, it is a failed poc, when
>> u will understand even how secteams like ubuntu work, then maybe, you
>> will see that, it is indeed flawed and, thus de prioritised and face
>> it, i know many hax0rs and, they never once mentioned it, and believe
>> me, they sfrape these lists with 3x finetooth combs :P
>> This is NOT HAPPENIN!
>> Ok, that is enough now, have a nice day, and, no, i dont want any
>> replys saying how "yes yes but it can thru this and that..." , coz, IT
>> CANT so please, have a nice day, and, go read up on git or
>> sumthin...do sumthin useful, your wasting time pushing on a dead pioc.
>> either fkn make it properly, or stfu and simply accept it cannot b
>> done, now, i know howmany lines it took me, and, if you cannot make
>> the failure poc wich you outlined, then, id be dissapintedm, as
>> someone alredy has done, exactly what you did, and, showed it
>> executing, and FAILING.
>> That was end of it for the whole list i believe.
>> A poc, wich was perfectly coded to meet your standards, and, it failed
>> dude. you cannot just chmod any file and make it *suid*... seriously,
>> whatever ebooks your reading (GNY??) get off them, theyre bad for your
>> health like nicotine is to me :)
>> ciao baby
>> xdab
>>
>>
>>
>>
>> On 27 October 2011 06:31, vladz<vladz@...zero.fr>   wrote:
>>> This vulnerability is trivial and I don't even know why it is making so
>>> much noises as bzexe is almost never used and the exploit would only
>>> work under certain circumstances.  It quoted it because it was an
>>> example of insecure uses of "/tmp", thats all!
>>>
>>> Note for "xD 0x41":  before you say something about "ASLR", know that it
>>> has nothing to see with it.
>>>
>>> I will explain it shortly (even if Tavis was very clear), and hope this
>>> will end this conversation! lol
>>>
>>> Imagine the "dd" command has been compressed by root using bzexe:
>>>
>>>    # bzexe /bin/dd
>>>      /bin/dd:  1.996:1,  4.008 bits/byte, 49.90% saved, 49168 in, 24635 out.
>>>
>>>    # ls -l /bin/dd*
>>>    -rwxr-xr-x 1 root root 25286 26 oct.  20:38 /bin/dd
>>>    -rwxr-xr-x 1 root root 49168 28 avril  2010 /bin/dd~
>>>
>>>    # cat -n /bin/dd | head -15
>>>         1        #!/bin/sh
>>>        [...]
>>>        10          prog="`echo $0 | /bin/sed 's|^.*/||'`"
>>>        11          if /bin/ln $tmpfile "/tmp/$prog" 2>/dev/null; then
>>>        12            trap '/bin/rm -f $tmpfile "/tmp/$prog"; exit $res' 0
>>>        13            (/bin/sleep 5; /bin/rm -f $tmpfile "/tmp/$prog") 2>/dev/null&
>>>        14            /tmp/"$prog" ${1+"$@"}; res=$?
>>>        [...]
>>>
>>> If a user creates a directory "/tmp/dd", look what happens when root
>>> calls "dd":
>>>
>>>    # bash -x /bin/dd
>>>    [...]
>>>    + /usr/bin/tail -n +23 /bin/dd
>>>    + umask 0022
>>>    + /bin/chmod 700 /tmp/gztmpIfsTrk
>>>    ++ /bin/sed 's|^.*/||'
>>>    ++ echo /bin/dd
>>>    + prog=dd
>>>    + /bin/ln /tmp/gztmpIfsTrk /tmp/dd<<   ln succeeded!
>>>    + trap '/bin/rm -f $tmpfile "/tmp/$prog"; exit $res' 0
>>>    + /tmp/dd<<   /tmp/dd is launched!
>>>    /bin/dd: line 14: /tmp/dd: is a directory
>>>
>>> This means that right after the "ln" command AND before "/tmp/dd" is
>>> launched, the user can replace the directory "/tmp/dd" by a shell script
>>> with the same name ("/tmp/dd").
>>>
>>> Is this clear enough?
>>>
>>> Cheers,
>>> vladz.
>>> --
>>> http://vladz.devzero.fr
>>> PGP key 8F7E2D3C from pgp.mit.edu
>>>
>>>
>>> On Tue, Oct 25, 2011 at 08:42:38PM -0400, bugs@....dhs.org wrote:
>>>> Aw, Even if you loop and copy a binary continuously into that directory
>>>> say bash is bzexe'd.
>>>>
>>>> and our exploit does the following
>>>>
>>>> #!/bin/sh
>>>> chmod 777 /etc/shadow
>>>>
>>>> You'll get,
>>>>
>>>> kemical:~# bzexe bash
>>>>     bash:     2.214:1,  3.614 bits/byte, 54.83% saved, 700492 in, 316442 out.
>>>> kemical:~# ./bash
>>>> ./bash: line 14: /tmp/bash: is a directory
>>>> /bin/rm: cannot remove `/tmp/bash': Is a directory
>>>>
>>>> kemical:~# ls -l /etc/shadow
>>>> -rw-r----- 1 root shadow 1174 2010-12-07 16:49 /etc/shadow
>>>>
>>>>
>>>> + /bin/rm -f /tmp/gztmpYCf11e /tmp/bash
>>>> /bin/rm: cannot remove `/tmp/bash': Is a directory
>>>>
>>>>
>>>>
>>>>> -----BEGIN PGP SIGNED MESSAGE-----
>>>>> Hash: SHA256
>>>>>
>>>>> Race condition != Memory corruption...
>>>>>
>>>>> (and therefore ASLR has NOTHING to do with it...)
>>>>>
>>>>> http://i.imgur.com/l1l3o.gif<= me after reading this.
>>>>>
>>>>> On 10/25/2011 06:56 PM, xD 0x41 wrote:
>>>>>> ln actually succeeds, but created /tmp/foo/foo instead. The attacker
>>>>>> still
>>>>>> owns /tmp/foo, so he quickly rename()s it and replaces /tmp/foo with his
>>>>>> exploit.
>>>>>>
>>>>>> You can make it bypass Aslr ?
>>>>>> This is what im talking about tavis, not the well known ln and other
>>>>>> bugs you have pleasured us all with :)
>>>>>> THIS one, cannot be won.
>>>>>> proove it, it is a shitty poc, i cannot get passed the break when it
>>>>>> symlinks across using ln, it triggers something, and shuts whatever
>>>>>> down..
>>>>>> Your audit and kcopes audit bugs, work alittle differently..
>>>>>> This PoC is a *fail* Tavis, you would otherwise have made it into a real
>>>>>> poc that actually spawns root , yes even in cron if what your saying is
>>>>>> right , no?
>>>>>> Im saying, Kernel will shut your PoC down, your saying it wont.
>>>>>> Proove me wrong , coz sofar, many have tried and many have failed.
>>>>>> it does not even need be disclosed, i dont mind.
>>>>>> i would be happy thelp fix a bug within the kernel but, we both know
>>>>>> this is not within kernel land,it is a bug in another area,
>>>>>> It still must bypass atleast ASLR on vanilla to be called a real poc,and
>>>>>> be treated as such by the secteam of Ubuntu and debian, of wich, they
>>>>>> dont seem to be in any hurry atall about this one, where, your ones, and
>>>>>> kcopes, they were VERY prompt to jump on.
>>>>>> i believe many have recreated it, but simply cannnot get it to spawn a
>>>>>> stable enough root shell.
>>>>>> Your the brains in bash, i wont deny you this, but i do not se this one
>>>>>> working Tavis :s
>>>>>> Please, by all means, proove it and Vladz name is clear. Otherwise to me
>>>>>> is just another exposed failed poc wich is screaming for ubuntu devteam
>>>>>> to give a crap :s.
>>>>>> My outlook is bleak, yes, but i was part of one of such teams years ago,
>>>>>> altho, i wont go into that now, it is not even part of this OS, so, I do
>>>>>> know how secteams somewhat work, they prioritise things.
>>>>>> if a bug is being used like crazy to exploit, they will simply implant
>>>>>> some new binarys, along with theyre kernel..and possibly update bzexe
>>>>>> and bunzip etc, all of wich have had many flaws, i just dont think a
>>>>>> race condition can be won in this case.
>>>>>> Thats from actual hard code exploits not running because of aslr, on the
>>>>>> simplest of setups even.
>>>>>> Its already out, this infos, so, if you think it also leads to root,
>>>>>> then i would expect YOU of all people to be alot more proactive about
>>>>>> it.
>>>>>> Your not though.
>>>>>> I appreciate the time you have taken but, i believe you wont win this
>>>>>> race :).
>>>>>> Have a nice day.
>>>>>> xd
>>>>>>
>>>>>>
>>>>>> On 25 October 2011 21:06, Tavis Ormandy<taviso@...xchg8b.com
>>>>>> <mailto:taviso@...xchg8b.com>>   wrote:
>>>>>>
>>>>>>       xD 0x41<secn3t@...il.com<mailto:secn3t@...il.com>>   wrote:
>>>>>>
>>>>>>       >   Hello,
>>>>>>       >        Your 'race condition possibly leading to root'is a myth...
>>>>>>       >   Yes thats maybe because race condition or not, it is ASLR wich
>>>>>> will
>>>>>>       >   prevent from ANY rootshell,and Yes, it has bveen tried... You can
>>>>>> do
>>>>>>       >   better, go right ahed ;-) I am betting you thats why it aint being
>>>>>>       ptached
>>>>>>       >   in any hurry, because obv if you read some notes about it in the
>>>>>>       committs,
>>>>>>       >   you will see they must have reproduced the said bugs, in and with,
>>>>>>       more
>>>>>>       >   than JUST bzexe even... but anyhow, your PoC is bs.
>>>>>>
>>>>>>       I think you misunderstood, he's not talking about memory corruption,
>>>>>> his
>>>>>>       attack sounds like a legitimate filesystem race. I'll try to
>>>>>>       explain, the
>>>>>>       bzexe utility compresses executables and then decompresses them at
>>>>>>       runtime
>>>>>>       by prepending a decompression stub.
>>>>>>
>>>>>>       His attack is against the stub, which is a bourne shell script. It
>>>>>>       basically
>>>>>>       does this:
>>>>>>
>>>>>>          1. Safely decompress the original executable inside /tmp using
>>>>>>       tempfile.
>>>>>>          2. Create a hardlink to the decompressed executable with the same
>>>>>>       name
>>>>>>       of the original input (this is a trick to maintain argv[0], which is
>>>>>>       not as
>>>>>>       easy in bourne as it is in modern shells).
>>>>>>          3. Execute the hardlink with the requested parameters.
>>>>>>
>>>>>>       His attack is against stage 2, he points out that although it is
>>>>>>       safe to use
>>>>>>       the link() system call in /tmp, the ln(1) utility does some
>>>>>> convenience
>>>>>>       processing if you pass it a directory name.
>>>>>>
>>>>>>       So, the attack scenario would be that root executed a bzexe
>>>>>> compressed
>>>>>>       executable called foo, and then he creates the directory /tmp/foo,
>>>>>>       and makes
>>>>>>       it 777.
>>>>>>
>>>>>>       ln actually succeeds, but created /tmp/foo/foo instead. The attacker
>>>>>>       still
>>>>>>       owns /tmp/foo, so he quickly rename()s it and replaces /tmp/foo with
>>>>>> his
>>>>>>       exploit.
>>>>>>
>>>>>>       Now root executes it, and gives him a root shell.
>>>>>>
>>>>>>       Vladz suggests using -F, which will solve this problem by telling ln
>>>>>>       to use
>>>>>>       the directory name instead. This will work nicely.
>>>>>>
>>>>>>       >   Make it then ill
>>>>>>       >   believe it, ask others, you wont beat aslr on even vanilla,. So,
>>>>>> stop
>>>>>>       >   complaining you did not get into patch- halll of flame.. it was
>>>>>>       not really
>>>>>>       >   going to be ever exploited, or you would surely not be the one
>>>>>> posting
>>>>>>       >   this ;) Anyhow, nice try but no banana. xd
>>>>>>
>>>>>>       I think it's quite a nice example, and a nice simple solution.
>>>>>> Imagine a
>>>>>>       system where crond executes a bzexe utility at regular intervals,
>>>>>> Vladz'
>>>>>>       attack will eventually succeed.
>>>>>>
>>>>>>       Tavis.
>>>>>>
>>>>>>       >
>>>>>>       >
>>>>>>       >   On 24 October 2011 05:55, vladz<vladz@...zero.fr
>>>>>>       <mailto:vladz@...zero.fr>>   wrote:
>>>>>>       >
>>>>>>       >   >   On Fri, Oct 21, 2011 at 07:59:59PM -0400, bugs@....dhs.org
>>>>>>       <mailto:bugs@....dhs.org>   wrote:
>>>>>>       >   >   >   bzexe utility:
>>>>>>       >   >   >
>>>>>>       >   >   >   /bin/bzexe:tmp=gz$$ /bin/bzexe:rm -f zfoo[12]$$
>>>>>>       >   >
>>>>>>       >   >   I reported this one several months ago (in some conditions it
>>>>>>       could lead
>>>>>>       >   >   to a root exploit) and provided an easy solution, but no
>>>>>> updates:
>>>>>>       >   >
>>>>>>       >   >     http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=632862
>>>>>>       >   >
>>>>>>       >   >   -- http://vladz.devzero.fr PGP key 8F7E2D3C from pgp.mit.edu
>>>>>>       <http://pgp.mit.edu>
>>>>>>       >   >
>>>>>>       >   >   _______________________________________________ Full-Disclosure
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>>>>>>       >
>>>>>>       >
>>>>>>
>>>>>>
>>>>>>       --
>>>>>>       -------------------------------------
>>>>>>       taviso@...xchg8b.com<mailto:taviso@...xchg8b.com>   | pgp encrypted
>>>>>>       mail preferred
>>>>>>       -------------------------------------------------------
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