Facebook fixed a dangerous flaw in the Android version of Facebook Messenger which gave attackers the ability to spy on users and potentially identify their surroundings without them knowing.

In a normal scenario, audio from the person making the call would not be transmitted until the person on the other end accepts the call clicking the accept button. But the recently patched vulnerability of Facebook Messenger for Android stipulated that SdpUpdate message could cause audio call to connect before callee has answered the call.

Natalie Silvanovich, the security researcher at Google Project Zero who discovered this vulnerability, explained: 

“However, there is a message type that is not used for call set-up, SdpUpdate, that causes setLocalDescription to be called immediately,” she explained. “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.”

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