Users just have to update the operating system in order to fix the problem
On February 12, Canonical published in a security notice details about a gnome-screensaver vulnerability for its Ubuntu 12.10 (Quantal Quetzal) operating system.According to Canonical, gnome-screensaver would allow unintended access to the user session.
It was discovered that that gnome-screensaver did not start automatically after logging in. 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.
The security flaws can be fixed if you upgrade your system(s) to the latest gnome-screensaver package for Ubuntu 12.10 (Quantal Quetzal) the operating system. To apply the update, run the Update Manager application.
In general, a standard system update will make all the necessary changes. A system restart won't be necessary to implement the changes, but users have to restart the virtual machine.