Jailbreak iOS 4.2.1 iPhone 4, 3GS, 3G using Redsn0w

In this guide you will learn how to jailbreak iOS 4.2.1 iPhone 4, 3GS, 3G using RedSn0w on Windows. Just follow the step-by-step instruction to jailbreak iOS 4.2.1. See more jailbreak guides.

Jailbreak iOS 4.2.1 iPhone 4, 3GS, 3G using Redsn0w


  • Tether jailbreak: iPhone 4, iPhone 3GS (new bootrom), iPod touch 2G (MC Model), iPod touch 3G, iPod touch 4G. Tethered jailbreak for newer models, means if you reboot the device then to boot back into jailbreak state you need to connect your device with computer and boot it using RedSn0w.
  • Untether Jailbreak: iPhone 3G, iPhone 3GS (old bootrom), iPod touch 2G (Non-MC)
  • Unlock for iOS 4.2.1 is not available yet so keep away from jailbreak if you need to unlock your device as well.


Download and restore iOS 4.2.1



Run iTunes and select your device from left pane.

Select you device in iTunes


Hold down the Shift key from keyboard and hit the Restore button.

Restore iOS 4.2.1

Select the iOS 4.2.1 firmware ipsw file that you have downloaded and hit the Open button. Let the iTunes finish the update process.

Select iOS firmware file

Jailbreak iOS 4.2.1


Extract the downloaded redsn0w_win_0.9.6b4.zip file and run redsn0w.exe file.

Run Redsn0w


Hit the “Browse” button and select the iOS 4.2.1 firmware ipsw.

Browse for iOS 4.2.1 firmware file


When IPSW successfully identified, hit the “Next” button and let the redsn0w prepare jailbreak data.

ipsw identified


Check the “Install Cydia” option and hit “Next”.

Install Cydia


Make sure your device is plugged into computer and Turned Off as well, hit the “Next” button.

Check device


Now put your device into DFU mode, redsn0w will guide you to do that. Read the on screen steps to put your device into DFU mode.

Put device into DFU mode Enter into DFU mode Enter into DFU mode

Redsn0w will start jailbreak process, wait for the Done! message and then hit the Finish button.

When your iPhone finishes rebooting, now your iPhone is Jailbroken and you can find Cydia icon on springboard.


To reboot tethered jailbreak device, Check “Just boot tethered rignt now” option and hit Next button.

boot tethered

Thanks to Dev-Team 🙂

See also:

5 comments… add one
Pepesoft December 4, 2010, 6:08 pm

I used redsn0w to JB Iphone 3G and then used ultrasn0w to unlock carrier. Worked fine but cydia kept crashin so i tried to do everything again and now my phone says Connect to Itunes and won`t boot up. I tried Itunes restore but it keeps failing (error 1015), then i tried RecBoot but it can`t kick the DFU mode off. So i`m stuck with Ibrick now… Any suggestions what to do???

MzHeartnSoul December 8, 2010, 12:34 am

I can’t get pass step 6, it’s saying that it can’t identify the IPSW

paul September 9, 2011, 2:16 am

I can’t pass step 5, it’s saying that it can’t identify the IPSW.

How can I do now.

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