A vulnerability in the Session Initiation Protocol (SIP) call-handling functionality of Cisco IP Phone 7800 Series phones and Cisco IP Phone 8800 Series phones could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected phone. The vulnerability is due to incomplete input validation of SIP Session Description Protocol (SDP) parameters by the SDP parser of an affected phone. An attacker could exploit this vulnerability by sending a malformed SIP packet to an affected phone. A successful exploit could allow the attacker to cause all active phone calls on the affected phone to be dropped while the SIP process on the phone unexpectedly restarts, resulting in a DoS condition. Cisco Bug IDs: CSCvf40066.