The Untold Secrets of WiFi-Calling Services: Vulnerabilities, Attacks, and Countermeasures

Since 2016, all of four major U.S. operators have rolled out Wi-Fi calling services. They enable mobile users to place cellular calls over Wi-Fi networks based on the 3GPP IMS technology. Compared with conventional cellular voice solutions, the major difference lies in that their traffic traverses untrusted Wi-Fi networks and the Internet. This exposure to insecure networks can cause the Wi-Fi calling users to suffer from security threats. Its security mechanisms are similar to the VoLTE, because both of them are supported by the IMS. They include SIM-based security, 3GPP AKA, IPSec, etc. However, are they sufficient to secure Wi-Fi calling services? Unfortunately, our study yields a negative answer. We conduct the first security study on the operational Wi-Fi calling services in three major U.S. operators networks using commodity devices. We disclose that current Wi-Fi calling security is not bullet-proof and uncover three vulnerabilities. By exploiting the vulnerabilities, we devise two proof-of-concept attacks: telephony harassment or denial of voice service and user privacy leakage; both of them can bypass the existing security defenses. We have confirmed their feasibility using real-world experiments, as well as assessed their potential damages and proposed a solution to address all identified vulnerabilities.