[Snort-devel] Changes to PCRE

Phelps Ed (Ed) ** % ** edphelps at ...3412...
Thu Jul 11 09:49:38 EDT 2013

Hello All,


I've spent a good chunk of time in recent months reading papers and
trying to "break" snort. In particular, two papers from 2006
(http://www.acsac.org/2006/papers/54.pdf) and 2010
have kind of been my guiding literary forces. From a high-level, these
papers focus on "evil" regular expressions and the time that it takes
for snort to process them, forcing dropped packets when snort cannot
keep up. 


When I attempt similar techniques on more modern versions of snort
( - 2.9.5), I cannot replicate their results as snort barely bats
an eye to the crafted packets. Even when I implement rules with regular
expressions that no sane snort owner would even consider implementing
(pcre:"/(a+a+)+b/" or pcre:"/(.)*(a)(.)*=(.)*b\2 /") and packets that I
know should create an exponential search time, snort makes the correct
alert/allow decision in every instance with no delays. Surely this is a
testament to snort developers and the community making the necessary
changes in a timely manner to avoid dropped packets. 


So what changed? I can't seem to find any documentation on changes made
to solve the "evil regex" problem, but it seems to be very efficiently
solved. I was hoping someone could point me in the right direction,
whether it is an algorithm tweak, a change from NFAs to DFAs, or
something I haven't thought of (the most likely answer). 





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