PJSIP is a free and open source multimedia communication library written in C. In 2.17 and earlier, the OpenSSL and GnuTLS backends in pjlib/src/pj/sslsockossl.c and pjlib/src/pj/sslsockgtls.c copy DNS SubjectAltName values with string functions that recalculate their length and truncate an embedded NUL byte. With server verification enabled through --tls-verify-server for the PJSIP TLS/SIPS transport, a certificate containing a DNS SubjectAltName formed from the target hostname prefix followed by an embedded NUL and an attacker-controlled suffix can therefore be accepted for the prefix hostname. An attacker who possesses such a certificate from a trusted issuer and can intercept the connection can impersonate the target server, complete the SIP session, and receive REGISTER credentials. The mbedTLS backend is not affected because it preserves the explicit string length. No fixed version is available as of this review.