Facebook Messenger bug let attacker listen through your phone

A Facebook Messenger bug discovered earlier this year was revealed this week by a security researcher. This bug worked with the Facebook Messenger call system, taking advantage of a loophole that let an attacker make a phone call and auto-answer said call on the receiver's end. With this bug, an attacker could effectively listen to you through your phone without your knowledge.

Facebook Messenger sets up calls in WebRTC, slightly different from how a call is made with your phone's normal phone app. Normally a set of checks are required for the caller and the receiver to connect. Ideally, the caller wouldn't be able to connect to the receiver unless the receiver tapped a button to accept a call.

The security researcher "Natashenka" (N. Silvanovich) discovered a way to bypass the system that's normally required to connect the call. The caller uses a special sort of message to force the receiver's phone to connect audio.

Per the researcher's note: "If this message is sent to the callee device while it is ringing, it will cause it to start transmitting audio immediately, which could allow an attacker to monitor the callee's surroundings."