res_pjsip_t38 in Sangoma Asterisk 15.x before 15.7.4 and 16.x before 16.5.1 allows an attacker to trigger a crash by sending a declined stream in a response to a T.38 re-invite initiated by Asterisk. The crash occurs because of a NULL session media object dereference.
http://seclists.org/fulldisclosure/2021/Mar/5
http://packetstormsecurity.com/files/161671/Asterisk-Project-Security-Advisory-AST-2021-006.html
http://packetstormsecurity.com/files/154371/Asterisk-Project-Security-Advisory-AST-2019-004.html
http://downloads.asterisk.org/pub/security/AST-2019-004.html