Identity Services in Apple iOS before 6.1 does not properly handle validation failures of AppleID certificates, which might allow physically proximate attackers to bypass authentication by leveraging an incorrect assignment of an empty string value to an AppleID.
http://support.apple.com/kb/HT5642
http://lists.apple.com/archives/security-announce/2013/Mar/msg00002.html
http://lists.apple.com/archives/security-announce/2013/Jan/msg00000.html