The SIP implementation in Cisco IOS 12.3 and 12.4 allows remote attackers to cause a denial of service (device reload) via a malformed SIP message, aka Bug ID CSCtb93416, the "SIP Message Handling Denial of Service Vulnerability."
http://www.cisco.com/en/US/products/products_security_advisory09186a0080b20f32.shtml
http://tools.cisco.com/security/center/viewAlert.x?alertId=20063