Multiple iPhone models have reportedly been affected by a bug in iOS 11.1.2, stated Softpedia.
The “critical bug” places devices into an infinite reboot loop, and causes them to shut down in certain cases.
The reported bug started surfacing on 2 December, and is said to be linked to a notification system issue.
Softpedia noted that a temporary fix for the problem is to set a device’s date to 1 December 2017 (Settings > General > Date & Time > Disable Set Automatically > December 1).
Users on iOS 11.1.2 are then advised to update their devices to iOS 11.2, which fixes the issue.