An issue was discovered in channels/chan_sip.c in Sangoma Asterisk through 13.29.1, through 16.6.1, and through 17.0.0; and Certified Asterisk through 13.21-cert4. A SIP request can be sent to Asterisk that can change a SIP peer's IP address. A REGISTER does not need to occur, and calls can be hijacked as a result. The only thing that needs to be known is the peer's name; authentication details such as passwords do not need to be known. This vulnerability is only exploitable when the nat option is set to the default, or auto_force_rport.