Not even a week after iOS 4.2.1 was released, and following the update of the RedSn0w jailbreak, the Dev Team delivered today an update to their UltraSn0w unlock for iOS 4.2.1.

This unlock will work for the iPhone 3G or iPhone 3GS with baseband 04.26.08, 05.11.07, 05.12.01, or 05.13.04. It will also work with iPhone 4 with baseband 01.59.00…

For most of you, this unlock will be a little “special” and won’t be as simple as downloading UltraSn0w from Cydia. Unless you’re on one of the above mentioned basebands, you will first have to update your baseband to 06.15.00, which is the same baseband used by iPad iOS 3.2.2. This can be done using an updated version of PwnageTool (see our tutorial), or RedSn0w (see our guide).

This is for iPhone 3G or 3GS only. You will not be able to update your iPhone 4 to baseband 06.15.00. Only an iPhone 4 on baseband 01.59.00 can be unlocked so far.

Once updated to 06.15.00, you will then be able to download UltraSn0w and unlock your iPhone.

This unlock comes with a few downsides though:

WARNING — YOU DO THIS AT YOUR OWN RISK!  PLEASE UNDERSTAND THE CONSEQUENCES OF UPDATING TO 06.15.

  1. There is no way to come back down from 06.15, and there’s no hiding the baseband version from Apple.  You’ll be voiding your warranty in a very obvious way.
  2. If some future baseband comes out with a critical fix, you won’t be able to update to it if it remains down in the 05.xx sequence (then again, you wouldn’t update to it if you wanted to keep your unlock anyway).
  3. Starting with FW 4.2.1 if you have 06.15 on your iPhone you won’t ever be able to restore to stock firmware (it will fail).  You’ll need to only restore to custom IPSWs (then again, if you’re unlocker you should already be doing that).

Certainly don’t update to 06.15 if you don’t need to!  Only do this if you need the unlock and you’re stuck on 05.14 or 05.15, and you’re willing to assume the above risks.

I highly suggest you read this blog post on the Dev Team’s blog to learn everything about this unlock.

In short:

  • You can only unlock an iPhone 4 that has baseband 01.59.00. You can unlock using UltraSn0w. If your baseband is different from this, you will not be able to unlock at all, since you cannot downgrade or update the baseband.
  • If you have an iPhone 3G, you can unlock iOS 4.2.1 using PwnageTool.
  • If you have an iPhone 3GS with an old bootrom and are already unlockable but want to update to 4.2.1, please wait for the Dev Team to release an “unofficial” bundle for you that will get you to 4.2.1 but preserves your baseband.
  • If you have an iPhone 3GS with an old bootrom and already on 4.2.1, use RedSn0w.
  • iPhone 3GS users with old bootroms who want to go to 4.2.1 should not use PwnageTool. You should update to stock 4.2.1 first, then use RedSn0w to update your baseband.

Check out our PwnageTool tutorial or RedSn0w guide to show you how to update your baseband.