The Unity Settings Daemon before 14.04.0+14.04.20150825-0ubuntu2 and 15.04.x before 15.04.1+15.04.20150408-0ubuntu1.2 does not properly detect if the screen is locked, which allows physically proximate attackers to mount removable media while the screen is locked as demonstrated by inserting a USB thumb drive.
https://bugs.launchpad.net/ubuntu/%2Bsource/unity/%2Bbug/1438870