The GreenPois0n untethered jailbreak for iOS 4.2.1 was released a few minutes ago and I already took it for a spin on my older iPhone 3G sporting iOS 4.2.1.

This step by step guide will show you how to use GreenPois0n to jailbreak your iPhone iOS 4.2.1. It’s worth noting that this version will not hacktivate your iPhone and that so far, there is only a Mac version…

Note: Steps to jailbreak the Verizon iPhone are exactly the same.

Step 1: Make sure your iPhone is on iOS 4.2.1 as GreenPois0n will not work with any other firmware.

Step 2: Download GreenPois0n from our iPhone downloads section.

Step 3: Plug your iPhone to your computer via the USB cable then launch GreenPois0n. Click on the “Jailbreak” button.

Step 4: As soon as you hit jailbreak, you will have to follow instructions for putting your iPhone into DFU mode:

  • Press and hold the sleep button for 3 seconds
  • Continue holding the sleep button while pressing the home button for 10 seconds
  • Release the sleep button while still holding the home button for 10 seconds

Step 5: Your device will now be ready for the jailbreak. Just wait a little bit while GreenPois0n works its magic.

Step 6: Once the jailbreak has been injected in your iPhone, you will see a new icon on your springboard for Loader. Launch Loader and install Cydia from there. Finally, reboot your iPhone and you should have Cydia installed

Step 7: Done!

Enjoy your newly untethered iPhone jailbreak for iOS 4.2.1.