The SIP channel driver (chan_sip) in Asterisk before 1.2.18 and 1.4.x before 1.4.3 does not properly parse SIP UDP packets that do not contain a valid response code, which allows remote attackers to cause a denial of service (crash).
bugs.digium.com/view.php?id=9313
secunia.com/advisories/25582
securityreason.com/securityalert/2644
www.asterisk.org/files/ASA-2007-011.pdf
www.debian.org/security/2007/dsa-1358
www.novell.com/linux/security/advisories/2007_34_asterisk.html
www.securityfocus.com/archive/1/466882/100/0/threaded
www.securityfocus.com/bid/24359
www.securitytracker.com/id?1017954
exchange.xforce.ibmcloud.com/vulnerabilities/33892