reqresp_parser.c in the SIP channel driver in Asterisk Open Source 1.8.x before 1.8.4.3 allows remote attackers to cause a denial of service (NULL pointer dereference and daemon crash) via a SIP packet with a Contact header that lacks a < (less than) character.
http://www.debian.org/security/2011/dsa-2276
http://secunia.com/advisories/45239
http://secunia.com/advisories/45201
http://secunia.com/advisories/45048
http://lists.fedoraproject.org/pipermail/package-announce/2011-July/062628.html
http://downloads.asterisk.org/pub/security/AST-2011-009.html
http://downloads.asterisk.org/pub/security/AST-2011-009-1.8.diff