Chronic Dev-Team recently released GreenPois0n RC5 untethered jailbreak tool for iOS 4.2.1 (and iOS 4.1).
This tutorial includes instructions on how to use the GreenPois0n RC5 for Mac to jailbreak your iPhone 3Gs and iPhone 4.
Greenpois0n recommends a clean restore of your iOS before continuing for the jailbreak. Do not upgrade to iOS 4.1 or iOS 4.2.1 if you need to unlock your iPhone for using your carrier sim. Ultrasn0w don’t support the new baseband yet (only exception is iPhone 4). If you want to upgrade your iOS to 4.2.1 you can use TinyUmbrella, which will update your iOS to the latest version without upgrading your baseband. Start TSSServer > Start iTunes > connect your iPhone > Restore 4.2.1 > Error 1013 > Put iPhone on DFU Mode > Launch GreenPois0n
To jailbreak your iPhone 3Gs or iPhone 4 on iOS 4.2.1 (and 4.1) – untethered, follow these steps:
Goto www.greenpois0n.com and download the greenp0ison for mac. Extract it in a folder on your desktop.
Launch the GreenPois0n application
Power off your iPHone 3GS or iPhone 4 and connect it to the computer.
Once the iphone is connected to the computer, press the large Jailbreak button on the Green Pois0n application.
When GreenPois0n asks you to place your iPhone into the DFU mode and tells you how to do it, simply follow the on screen instructions and hold the sleep button for 3 seconds.
Continue holding the sleep button and press the home button with it for 10 seconds now.
After 10 seconds, release the power / sleep button and keep holding the home button for almost next 30 seconds. Do not release the home button.
If the iPhone entered the DFU mode successfully, the jailbreak process will begin itself.
Release the home button once GreenPois0n tells you that the jailbreak process is complete. Click the complete button to close the application on your computer. The iPhone will continue the process showing some verbose text scrolling on the screen.
The iPhone will restart itself after a while. Once you are on the home screen / springboard, you will see a Loader app. Launch this app by tapping it.
Once the loader application is launched, select Cydia in it and press the install Cydia button that appears.
Now you can remove the loader app by pressing the options button and pressing the large remove loader.app button once cydia has been installed successfully.
Restart your iPhone and run Cydia. Let cydia run at it’s own pace, doing all the updates and you’re done!
Your iPhone 3GS or iPhone 4 is now untethered jailbroken with iOS 4.2.1 using Greenpois0n RC5 for Mac OSX.