Cisco IP Phone 7940 and 7960 with P0S3-08-6-00 firmware, and other SIP firmware before 8.7(0), allows remote attackers to cause a denial of service (device reboot) via (1) a certain sequence of 10 invalid SIP INVITE and OPTIONS messages; or (2) a certain invalid SIP INVITE message that contains a remote tag, followed by a certain set of two related SIP OPTIONS messages.