Following the release of a public jailbreak last month, Apple has stopped signing iOS 12.4 today. This means that users can no longer downgrade from iOS 12.4.1 to iOS 12.4.
A week after the jailbreak was released, Apple released iOS 12.4.1 to close the vulnerability. The jailbreak was released after it was determined that Apple had inadvertently fixed a vulnerability addressed in iOS 12.3.
The error in question was first reported to Apple by Google's Project Zero team. Apple has described the update in the security updates for iOS 12.3. The jailbreak took advantage of this gap.
When Apple no longer signs an iOS version, users can no longer downgrade to that version via iTunes. Downgrading is often done by users waiting for jailbreak updates, especially in this case given the public availability of a jailbreak for iOS 12.4.
The ability to downgrade is sometimes useful if significant errors occur after upgrading to the latest version of iOS. For this reason, Apple usually waits several weeks before older versions of iOS are no longer signed.
The release of a jailbreak for modern iPhones is becoming increasingly rare. Generally, jailbreak details are kept secret so Apple can not patch them. In addition, jailbreaks are sought after by security researchers for potential payouts on the security market.
Apple will release iOS 13 on September 19, followed by iOS 13.1 on September 30.
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