After a standard system upgrade you need to restart your session to affect
the necessary changes.
USN-537-1 fixed vulnerabilities in gnome-screensaver. The fixes were
incomplete, and only reduced the scope of the vulnerability, without
fully solving it. This update fixes related problems in compiz.
Original advisory details:
Jens Askengren discovered that gnome-screensaver became confused when
running under Compiz, and could lose keyboard lock focus. A local attacker
could exploit this to bypass the user's locked screen saver.