Ubuntu Security Notice 866-1 – It was discovered that gnome-screensaver did not always re-enable itself after applications requested it to ignore idle timers. This may result in the screen not being automatically locked after the inactivity timeout is reached, permitting an attacker with physical access to gain access to an unlocked session.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/83553/USN-866-1.txt
Source: https://packetstormsecurity.com/files/83553/Ubuntu-Security-Notice-866-1.html