Apple Stops Signing iOS 18.3.1
Apple today stopped signing iOS 18.3.1, so iPhone users who have upgraded to iOS 18.3.2 are no longer able to downgrade to the prior version of iOS 18. Apple released iOS 18.3.2 on March 10.

It is not unusual for Apple to stop signing older versions of iOS after releasing an update. Software that is not signed cannot be installed on an iPhone due to a server-side verification check, and it keeps iPhone users from installing outdated versions of iOS. iPhone models compatible with iOS 18 are only eligible to download and run iOS 18.3.2 at this time.
By preventing customers from installing older versions of iOS or downgrading, Apple ensures that devices have the latest security improvements.
iOS 18.3.2 introduced important security fixes, addressing a vulnerability that may have been actively exploited.
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