Apple's software based mandatory access control that was introduced in iOS 2.0. It is a higher level layer on top of the XNU implementation of the TrustedBSD MAC framework.
- In addition to the new set of frameworks in 2.0, applications had to be tweaked to use the new sandbox.
- For applications to run, they need to have a folder called _Codesign with a file inside called CodeResources
- If a userland exploit was discovered in an App Store app, the CodeResources would become invalid when a file was injected.
- The checks are not as hard on built in applications in the firmware  (only version.plist is checked to prevent a soft upgrade )
The dependency on the CodeResources file makes it vulnerable to the Incomplete Codesign Exploit