Ubuntu Security Notice USN-688-1 – It was discovered that the Expo plugin for Compiz did not correctly restrict the screensaver window from being moved with the mouse. A local attacker could use the mouse to move the screensaver off the screen and gain access to the locked desktop session underneath. Default installs of Ubuntu were not vulnerable as Expo does not come pre-configured with mouse bindings.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/72780/USN-688-1.txt
Source: https://packetstormsecurity.com/files/72780/Ubuntu-Security-Notice-688-1.html