A vulnerability in Cisco SIP IP Phone Software for Cisco IP Phone 7800 Series and 8800 Series could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected phone. The vulnerability is due to insufficient validation of input Session Initiation Protocol (SIP) packets. An attacker could exploit this vulnerability by altering the SIP replies that are sent to the affected phone during the registration process. A successful exploit could allow the attacker to cause the phone to reboot and not complete the registration process.