Does Fuzzing really work?

[Aviram Jenik <aviram () beyondsecurity com>]

There's a lot of talk lately on whether fuzzing can actually be used to find 
vulnerabilities - and more importantly, reliably rule out the existence of 
unknown vulnerabilities. 

Since most of this talk revolves around Dave's note "There are no new
MSRPC bugs. You should give up looking for them" I thought this was the right 
forum to answer this question. 
The question was whether RPC fuzzing can really rule out vulnerabilities, and 
our experience shows it can (at least, as much as you can rule out anything 
in IT security).

Let me throw some numbers at you(*). The FTP protocol has 310 "scenarios" of 
valid FTP sessions. If you try to overflow each time a different part of the 
command in every scenario you get a little over 12M attack combinations. If 
you use some of our nifty beSTORM 2.0 optimizations you get to 70,679 attack 
vectors. Even with the lamest FTP server allowing just 5 simultaneous 
connections and taking a full second to process each session it would take 
only 4 hours to fully test the protocol.

FTP is too simple you say? With more complex protocols like SIP you have 
> 15,000 scenarios and something like 40,680,459 attack vectors after 
optimizations. Sounds scary at first, but a SIP server capable of handling 
500 requests per second would take only 22 hours to test, which means you can 
leave it running when you go home for the weekend and come back for the 
results. If you don't feel like waiting 22 hours, put it on 5 machines and 
have an answer by 4 hours. If you don't feel like waiting 4 hours... well you 
get the point.

HTTP is probably as complex as they come, but most servers can handle >100,000 
requests per second in a closed environment and a fast local network. 
Suddenly trying all HTTP combinations is not as hard as it seems.

And so on, and so on.

My point is to those people who mock fuzzers - you either tried the wrong 
kind, or you tried them a long time ago. I'm not saying that buffer overflows 
are suddenly obsolete (don't delete that ZERT bookmark just yet!). But 
nowadays there is no reason for an FTP server to come out with buffer 
overflows; there's just no excuse.



(*) Don't believe the numbers? Check the URL below and see for yourself.

-- 
Regards,
Aviram Jenik
Beyond Security
(703) 286-7725 x504

http://www.BeyondSecurity.com
http://www.SecuriTeam.com

Looking for Unknown Vulnerabilities?
http://beyondsecurity.com/beSTORM
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