Understanding Android VoIP Security: A System-Level Vulnerability Assessment

VoIP is a class of new technologies that deliver voice calls over the packet-switched networks, which surpasses the legacy circuit-switched telecom telephony. Android provides the native support of VoIP, including the recent VoLTE and VoWiFi standards. While prior works have analyzed the weaknesses of VoIP network infrastructure and the privacy concerns of third-party VoIP apps, no efforts were attempted to investigate the (in)security of Android’s VoIP integration at the system level. In this paper, we first demystify Android VoIP’s protocol stack and all its four attack surfaces. We then propose a novel vulnerability assessment approach that assembles on-device Intent/API fuzzing, network-side packet fuzzing, and targeted code auditing. By testing Android from version 7.0 to the recent 9.0, we have discovered 8 zero-day Android VoIP vulnerabilities, all of which were confirmed by Google with bug bounty awards. The security consequences are serious, including denying voice calls, caller ID spoofing, unauthorized call operations, and remote code execution. To mitigate these vulnerabilities and further improve Android VoIP security, we uncover a new root cause that requires developers’ attention during their design and implementation.