Apple today stopped signing iOS 18.4.1, a week after releasing the iOS 18.5 update. Because iOS 18.4.1 is no longer being signed, iPhone users who upgraded to iOS 18.5 are no longer able to downgrade to the previous version of iOS.

"Signing" is a server-side verification check that Apple does when iOS is downloaded onto an ‌iPhone‌. An update can't be downloaded and installed unless it passes the verification check, and the unsigning process prevents customers from installing outdated versions of iOS that are not as secure.

Apple typically stops signing older software about a week after releasing an update, and doing so ensures that customers have the latest security improvements and are not vulnerable to known attacks.

iOS 18.5 added new wallpaper, a Screen Time password change for parents, and support for carrier-provided satellite features on ‌iPhone‌ 13 models. It also addressed more than 30 security vulnerabilities.

Apple has also stopped signing iPadOS 17.7.6, because it released iPadOS 17.7.8 earlier today as a replacement for the iPadOS 17.7.7 update that was causing issues for some people.

Source: Macrumors