The iPhone-Dev team today released an updated version of Redsn0w that can be used to jailbreak iOS 4.2.1. The new Redsn0w 0.9.6 b4 enables iOS 4.2.1 jailbreak on iPhone, iPod Touch and iPad. It is based on Geohot’s iboot exploit in Limera1n, and is a tethered jailbreak. So if you are rebooting your jailbroken idevice, you should be near a PC or Mac to take it to the jailbreak mode. However, Redsn0w 0.9.6 b4 support un-tethered jailbreak on iPhone models with old iBoot ROM and in iPod Touch 2G models. So until someone releases a complete working untethered jailbreak for iOS 4.2, Redsn0w 0.9.6 b4 is the only option to get Cydia working in your device. Follow our step by step guide to jailbreak iOS 4.2.1 using Redsn0w.
Jailbreak iOS 4.2.1
1. Download iOS 4.2 ipsw corresponding to your iPhone, iPod Touch or iPad here.
2. Download Redsn0w 0.9.6 b4 for Windows or Mac from Hotfile here.
3. Open Redsn0w and point it to the downloaded iOS 4.2.1 ipsw file.
4. If you have pointed it to the correct file, it will show something as given above. Click next.
5. At this point you need to check Install Cydia option and proceed.
6. Press Power OFF button and enter into the DFU mode. Just follow the steps given in the Redsn0w 0.9.6 b4 UI, if you are not sure about this.
7. Redsn0w 0.9.6 b4 will now install iOS 4.2.1 jailbreak files in your iPhone, iPod Touch or iPad using the Limera1n exploit.
8. If your are on an iPhone 3G, iPhone 3G S ( old iBoot ROM) or iPod Touch 2G ( model not starting with MC), steps to your untethered iOS 4.2.1 jailbreak completes here. Cydia should be there in your Springboard.
9. As said earlier, Redsn0w 0.9.6 b4 is a tethered jailbreak for iPhone 4, iPhone 3G S ( new model), iPad, iPod Touch 4G, 3G, 2G (MC) and iPad. So even a jailbroken device in this category should use Redsn0w again and boot it to the tethered jailbreak mode as shown below.
After that you can access Cydia, or SSH into the device. A non boot-tethered device will show a Grey or White Cydia icon instead of the bright one. We can expect a full, un-tethered iOS 4.2.1 jailbreak, and an Ultrasn0w unlock very soon.
Update: Now you can jailbreak iOS 4.2.1 untethered with Pwnagetool 4.2 (Mac) and Sn0wbreeze 2.2 (Windows).