A vulnerability in Session Initiation Protocol (SIP) call handling in Cisco IP Phone 8800 Series devices could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition because the SIP process unexpectedly restarts. All active phone calls are dropped as the SIP process restarts. The vulnerability is due to incomplete input validation of the SIP packet header. An attacker could exploit this vulnerability by sending a malformed SIP packet to a targeted phone. An exploit could allow the attacker to cause a DoS condition because all phone calls are dropped when the SIP process unexpectedly restarts. Cisco Bug IDs: CSCvc62590.