[1] reported that Apple deactivated the Jailbreak API on iPhones using iOS 4.2. This API (Application Programmer’s Interface – it’s the interface or “framework” deveopers use to access the underlaying software) is used by application developers and enterprises to check if the iPhone was “modded.”

A modded iPhone can run custom firmware that allows usages Apple does not aim the user to. So they can play games without paying for for instance. Jailbreaks and other “mods” are very prestigious at developers, experts and script kiddies. These software is provided by third parties and often are not be checked for security because the source code is either not available or the code is just available in the “underground” and security application developers do not check them for malware.

Therefor these “mods” can cause some security problems: if it’s not clear whether the provider of the Jailbreak is trustworthy or not. There can be some key loggers, worms and other kind of malware included that spies the victim. So a jailbreaked iPhone can cause the vulnerability of the enterprises IT network if the data the iPhone can access and modifiy are important.

Apple deactived the API to recognize Jailbreaks for unknown reasons. There is to mention that there are other ways to check whether iPhones are still running Apple’s iOS or another one.