The Security - Keychain component in Apple OS X before 10.9.4 does not properly implement keystroke observers, which allows physically proximate attackers to bypass the screen-lock protection mechanism, and enter characters into an arbitrary window under the lock window, via keyboard input.
http://www.securitytracker.com/id/1030505
http://support.apple.com/kb/HT6296
http://archives.neohapsis.com/archives/bugtraq/2014-06/0172.html