Full Disclosure mailing list archives
Re: Symlink vulnerabilities
From: Tavis Ormandy <taviso () cmpxchg8b com>
Date: Tue, 25 Oct 2011 12:06:25 +0200
xD 0x41 <secn3t () gmail 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 () devzero fr> wrote:On Fri, Oct 21, 2011 at 07:59:59PM -0400, bugs () fbi 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 _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.grok.org.uk/full-disclosure-charter.html Hosted and sponsored by Secunia - http://secunia.com/
-- ------------------------------------- taviso () cmpxchg8b com | pgp encrypted mail preferred ------------------------------------------------------- _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.grok.org.uk/full-disclosure-charter.html Hosted and sponsored by Secunia - http://secunia.com/
Current thread:
- Re: Symlink vulnerabilities, (continued)
- Re: Symlink vulnerabilities Raj Mathur (राज माथुर) (Oct 21)
- Re: Symlink vulnerabilities James Condron (Oct 22)
- Re: Symlink vulnerabilities Michal Zalewski (Oct 22)
- Re: Symlink vulnerabilities dave bl (Oct 21)
- Re: Symlink vulnerabilities bugs (Oct 22)
- Re: Symlink vulnerabilities Leon Kaiser (Oct 24)
- Re: Symlink vulnerabilities bugs (Oct 24)
- Re: Symlink vulnerabilities xD 0x41 (Oct 25)
- Re: Symlink vulnerabilities Tavis Ormandy (Oct 25)
- Re: Symlink vulnerabilities bugs (Oct 25)
- Re: Symlink vulnerabilities Tavis Ormandy (Oct 25)
- Re: Symlink vulnerabilities xD 0x41 (Oct 25)
- Re: Symlink vulnerabilities Michal Zalewski (Oct 25)
- Re: Symlink vulnerabilities xD 0x41 (Oct 25)