SIPConfusion: Exploiting SIP Semantic Ambiguities for Caller ID and SMS Spoofing

—Session Initiation Protocol (SIP) is a cornerstone of modern real-time communication systems, powering voice calls, text messaging, and multimedia sessions across services such as VoIP, VoLTE, and RCS. While SIP provides mechanisms for authentication and identity assertion, its inherent flexibility poses the risk of semantic ambiguity among implementations that can be exploited by attackers. In this paper, we present S IP C HIMERA , a novel black-box fuzzing framework designed to systematically identify ambiguity-based identity spoofing vulnerabilities across SIP implementations. We evaluated S IP C HIMERA against six widely used open-source SIP servers—including Asterisk and OpenSIPS—and nine popular user agents, uncovering that attackers could spoof their identity via manipulating identity headers and circumvent authentication. We demonstrate the real-world impact of these vulnerabilities by evaluating five VoIP devices, seven commercial SIP deployments, and three carrier-grade RCS-based SMS platforms. Our experiments show that attackers can exploit these vulnerabilities to perform caller ID spoofing in VoIP calls and send spoofed SMS messages over RCS, impersonating arbitrary users or services. We have responsibly disclosed our findings to affected vendors and received positive acknowledgments. We finally propose remedies to mitigate those issues.