The Private Browsing feature in CFNetwork in Apple Mac OS X before 10.8.4 does not prevent storage of permanent cookies upon exit from Safari, which might allow physically proximate attackers to bypass cookie-based authentication by leveraging an unattended workstation.
Apple Security Advisory 2013-06-04-1 - OS X Mountain Lion version 10.8.4 and Security Update 2013-002 is now available and addresses over 30 security issues.