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Using the hack dubbed "greenpois0n" to jailbreak Apple's iOS 4.2.1 triggers a "jailbreak check" built into the mobile operating system since version 4.0, according to Social Apples. That "check" reportedly prevents some users from opening content in the latest version iBooks.
"There is a problem with the configuration of your iPhone," the error message in iBooks 1.2.1 reads. "Please restore with iTunes and reinstall iBooks."
Hacker "Comex" of the iPhone Dev Team explained via Twitter how the new anti-jailbreak measure works: "It seems that before opening a DRMed book, iBooks drops an improperly signed binary, tries to execute it, and if it works concludes that the device is jailbroken and refuses to open the book."
In December it was claimed that a jailbreaking application programming interface found in iOS 4 was disabled with the release of iOS 4.2. But the newly discovered security measure apparently only applies to the iBooks software.
Because the change applies only to the iBooks application downloaded from the App Store and is not a system-wide issue, it's likely that Apple's interest is to curb potential piracy of e-books. Jailbreaking is a process that allows iOS device users to run unauthorized code, and can also be used to pirate software and content from the App Store and elsewhere.
But according to Social Apples, the security measure also prevents users from accessing legally purchased e-books through the iBooks application on a jailbroken device. Though it is a warranty voiding process, the practice of jailbreaking to run unauthorized code was deemed legal by the U.S. government last July.