dyld in Apple iOS before 6.1.3 and Apple TV before 5.2.1 does not properly manage the state of file loading for Mach-O executable files, which allows local users to bypass intended code-signing requirements via a file that contains overlapping segments.
http://support.apple.com/kb/HT5704
http://support.apple.com/kb/HT5702
http://lists.apple.com/archives/security-announce/2013/Mar/msg00005.html
http://lists.apple.com/archives/security-announce/2013/Mar/msg00004.html