Ubuntu Security Notice 898-1 – It was discovered that gnome-screensaver did not correctly handle monitor hotplugging. An attacker with physical access could cause gnome-screensaver to crash and gain access to the locked session.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/86138/USN-898-1.txt
Source: https://packetstormsecurity.com/files/86138/Ubuntu-Security-Notice-898-1.html