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A hidden feature in iOS 18.1 will automatically reboot a locked iPhone if it's in sleep mode but hasn't been unlocked in a while, presumably as a security measure.
The feature has been identified as “idle reboot” and is similar to a feature found on Mac computers. The Mac version, known as “hibernate mode,” saves the state of the device to disk when it goes to sleep in case the power goes out or the battery dies before the user can return to the machine.
By resetting the device to its last known state, iPhone users are better protected from forensic searches by law enforcement or other organizations. The change also makes it more difficult to hack the device using brute force or other methods.
The reboot timer is independent of charging or network functionality and is tied only to inactivity since the last unlock. This means that the reboot will occur after a certain period of time, even if network connections are not supported.
Preventing Personal Security Compromises
Christopher Vance, a forensic scientist at Magnet Forensics, was quoted in the Experts Group chat as saying, “We have discovered code in iOS 18 and above that is an inactivity timer. This timer will cause devices in AFU state to reboot to BFU state after a set period of time, which we have also defined.”
Synonyms refer to the state of the device after the first unlock and before the first unlock, when there are no apps or content open on the iPhone. Before the first unlock makes it difficult for attackers or law enforcement to use forensic tools to “jailbreak” the device.
The time between a user putting the device to sleep and the automatic reboot appears to be about four days, or 96 hours. Magnet Forensics recently acquired Grayshift, the company behind the GrayKey jailbreak tool.
“Remember, the real threat here is not the police,” Johns Hopkins University cryptographer Matthew Green told 404 Media. “These are the people who would steal your iPhone with malicious intent. This feature means that if your phone is stolen, thieves won't be able to babysit it for months until they develop the technology to crack it.”
“I would argue that rebooting after a reasonable period of inactivity probably won’t inconvenience anyone, but it will make your phone a lot more secure,” Green added. “So it seems like a pretty good idea.”
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