An issue was discovered in channels/chan_sip.c in Sangoma Asterisk 13.x, 16.x, and 17.x, and Certified Asterisk 13.21, because of an incomplete fix for CVE-2019-18351. 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.