The CoreProcesses component in Apple Mac OS X 10.7 before 10.7.2 does not prevent a system window from receiving keystrokes in the locked-screen state, which might allow physically proximate attackers to bypass intended access restrictions by typing into this window.
http://www.securityfocus.com/bid/50085
http://support.apple.com/kb/HT5002
http://lists.apple.com/archives/Security-announce/2011//Oct/msg00003.html