Apple News
Saurik releases Cydia Impactor to “unjailbreak” your iPhone without updating iOS
The most notable thing about this tool is that it allows you to return to stock without needing to upgrade your device. Say, for example, you wanted to sell your iPhone, but wanted to retain its jailbreak-ability to increase resale value. Normally, if you wanted to perform a full system restore, you’d be forced to upgrade to the latest firmware, and thus lose your jailbreak. With Cydia Impacter, you can restore your device to stock without upgrading.
Cydia Impactor version 0.9.15 is currently in beta for iOS 8.3 and iOS 8.4 only. Keep in mind that before running this tool you should backup your data first via iCloud or iTunes. Cydia Impactor will remove all data, as that is what it’s designed to do.
saurik also has explained how the tool works:
Impactor uses the firmware archives provided by Apple for its “over the air” update feature. These files are ~2GB large, so Impactor goes to extreme lengths to minimize the amount of data it needs to download: most recoveries will require less than 10MB of transfer.
First, the update’s “bill of materials” is compared against the files on your device, generating a list of changes.
Then, all files on your device that have been modified or otherwise destroyed are downloaded directly from Apple.
In a series of carefully calculated file migrations, all new files are moved to the user partition, and all system data is moved back to the system partition.
Up until this point, the idea is that no changes to your system have been made that are “unsafe”: at any point, if Impactor fails/crashes or your device reboots, you can just run it again later.
Finally, all of the staged changes to the filesystem are “committed”, all user data is deleted, and iOS is told to run its “reset all content and settings”.
The idea is that this “critical window” lasts only the final few seconds of a process that lasts for multiple minutes.
(That said, I still highly recommend not at all “messing with” the app while it is running, and maybe if you have a lot of data this will take longer than I expect.)
When the device reboots, it will look as it did when you first turned it on; it will also no longer be “jailbroken”: if you want to jailbreak it, you will need to do so using a desktop jailbreaking tool.
Please note that Cydia Impactor is currently in beta and is available on Cydia. You should take a backup of your device using iTunes or iCloud before using it.
This tool works with all iOS devices except iPod Touch 6th generation…