https://www.vice.com/en_us/article/3kx7n8/signal-bug-could-have-let-hackers-listen-to-android-users-via-microphone

All software has bugs. But there's an extra sting when that software is something you use to communicate securely.

On Friday, a researcher at Google's elite vulnerability hunting team Project Zero published details about an issue in the Android version of Signal. The bug allowed a hacker to phone a target device, and the call would be answered without the recipient needing to even accept the call, essentially letting the hacker listen-in on the victim.