chan_sip.c in the SIP channel driver in Asterisk Open Source 1.6.x before 1.6.2.18.1 and 1.8.x before 1.8.4.3 does not properly handle '\0' characters in SIP packets, which allows remote attackers to cause a denial of service (memory corruption) or possibly have unspecified other impact via a crafted packet.
http://downloads.asterisk.org/pub/security/AST-2011-008.diff
http://downloads.asterisk.org/pub/security/AST-2011-008.html
http://lists.fedoraproject.org/pipermail/package-announce/2011-July/062628.html
http://secunia.com/advisories/45048
http://secunia.com/advisories/45201
http://secunia.com/advisories/45239
http://securitytracker.com/id?1025706
http://www.debian.org/security/2011/dsa-2276