Apple has stopped signing a recent version of its mobile operating system, iOS 17.5.1, preventing iPhone users from downgrading to that specific version. The update has been unsigned for a while now. Since the release of iOS 17.6, it has been impossible to downgrade the firmware.
A common practice at Apple
Apple often stops signing a version of iOS, usually a week or two after a new update is released, and sometimes even the same day, as in this case.
When an update is no longer signed, it cannot be installed on an iPhone due to a server-side software check. Apple systematically validates the installation of a firmware in order to avoid any risk.
This policy encourages users to keep their operating system up to date, to ensure they have the latest security improvements. It also allows Apple to have the most up-to-date mobile fleet possible, reaching as many people as possible with its new features. It also does this to counter jailbreaking.
The latest iOS news
As a reminder, iOS version 17.5.1 fixed a corruption issue that could cause deleted photos to reappear on an iPhone. The iOS 17.6 update doesn’t have any notable features, with Apple adding unspecified bug fixes and security updates.