Signal Vulnerability Could Keep Calls Alive After Users Hang Up

A critical Signal flaw could keep calls active after hang-up, exposing microphone or camera. Update to a fixed version immediately.

A critical vulnerability, tracked as CVE-2026-113380 (CVSS score 9.3 of 10), in Signal ’s calling library could allow a malicious caller to interfere with call termination, potentially leaving the victim’s microphone and camera active after they believe the conversation has ended. The flaw affects versions before 2.72.0 of the library RingRTC, which the messaging app uses to provide voice and video calling.

It highlights a security problem that deserves attention beyond a single messaging application: ending a call should be an immediate control action, not something that can be delayed by traffic coming from the other participant.

According to the technical report published on GitHub, the vulnerability stems from how RingRTC handles events in the finite-state machine (FSM) that manages a call. The application places events in a queue for processing, but a malicious peer can flood that queue with events and prevent the local hang-up operation from being handled in a timely manner.

“A local hang-up is not a kill switch: it is just one more event placed at the back of the call’s finite-state-machine event queue. The remote peer is trusted with that queue — there is no rate limit on peer-originated events and no priority lane for hang-up.” reads the technical report published on GitHub, “Flood the queue with junk and the hang-up waits behind attacker noise forever, so the call (microphone, possibly camera) is never torn down.”

People expect that pressing the hang-up button ends the call and turns off the microphone and camera. But if the system is too busy processing other events, it may delay the command to end the call, leaving the audio or video active longer than expected.

The report explains that someone taking part in a call could exploit this weakness by sending enough events to overload the system’s queue. This could stop the other person from ending the call properly, potentially keeping the microphone active and possibly the camera too.

The underlying problem is not that Signal’s end-to-end encryption has been broken. Rather, it concerns the handling of call state and the reliable termination of a session. Encryption cannot protect users from every flaw in the software that controls when a communication session begins or ends.

The vulnerability concerns RingRTC versions earlier than 2.72.0 and can affect Signal clients that embed a vulnerable build of the library. According to the CSIRT Toscana advisory, the affected application versions are Signal for Android before 8.27.0, Signal for iOS before 8.30.0, and Signal Desktop before 8.29.0.

The official RingRTC fix and the comparison between versions 2.71.0 and 2.72.0 document the software changes. Users should install the latest available Signal release for their device rather than relying on the library version alone.

The technical report also includes a conceptual Python proof of concept. It models the difference between a queue in which a flood can bury a hang-up event and one in which critical events receive priority. The demonstration is synthetic and offline, with no network activity or real calls, so it illustrates the underlying logic rather than providing a working exploit.

This is an important distinction. The fact that the vulnerability has been made public does not mean attackers are already exploiting it. However, because it could expose private conversations, updating Signal promptly is a sensible precaution, especially for people who use it to discuss sensitive matters.

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Pierluigi Paganini

(SecurityAffairs – hacking, mobile)