Algorithmic complexity vulnerability in Snort before 2.6.1, during predicate evaluation in rule matching for certain rules, allows remote attackers to cause a denial of service (CPU consumption and detection outage) via crafted network traffic, aka a "backtracking attack."
https://exchange.xforce.ibmcloud.com/vulnerabilities/31430
http://www.snort.org/pub-bin/snortnews.cgi
http://www.securityfocus.com/bid/21991
http://www.mandriva.com/security/advisories?name=MDKSA-2007:051
http://www.cs.wisc.edu/~smithr/pubs/randy_smith_acsac2006.zip
http://www.cs.wisc.edu/~smithr/pubs/acsac2006.pdf
http://www.acsac.org/2006/papers/54.pdf
http://www.acsac.org/2006/advance_program.html
http://www.acsac.org/2006/abstracts/54.html
http://securitytracker.com/id?1017508
http://security.gentoo.org/glsa/glsa-200702-03.xml
http://secunia.com/advisories/24338
http://secunia.com/advisories/24164
http://secunia.com/advisories/23716
http://lists.immunitysec.com/pipermail/dailydave/2007-January/003954.html