Sandbox

Apple's software based Hypervisor that was introduced in iOS 2.0.

Problem's

 * 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 )

Exploits
The dependency on the CodeResources file makes it vulnerable to the Incomplete Codesign Exploit

More Info
http://iphonedevwiki.net/index.php/Seatbelt