Race condition in LoginUIFramework in Apple Mac OS X 10.7.x before 10.7.4, when the Guest account is enabled, allows physically proximate attackers to login to arbitrary accounts by entering the account name and no password.
http://www.securityfocus.com/bid/53459
http://www.securityfocus.com/bid/53445
http://support.apple.com/kb/HT5281
http://lists.apple.com/archives/security-announce/2012/May/msg00001.html