Squid 3.0 through 3.0.STABLE16 and 3.1 through 3.1.0.11 allows remote attackers to cause a denial of service via malformed requests including (1) "missing or mismatched protocol identifier," (2) missing or negative status value," (3) "missing version," or (4) "missing or invalid status number," related to (a) HttpMsg.cc and (b) HttpReply.cc.
http://www.vupen.com/english/advisories/2009/2013
http://www.squid-cache.org/Versions/v3/3.1/changesets/b9661.patch
http://www.squid-cache.org/Advisories/SQUID-2009_2.txt
http://www.securitytracker.com/id?1022607
http://www.securityfocus.com/bid/35812
http://www.mandriva.com/security/advisories?name=MDVSA-2009:178
http://www.mandriva.com/security/advisories?name=MDVSA-2009:161