The iOS 4.2.1 Gold Master firmware was released by Apple for the all the iDevices. But as this is a beta release, it is only available for the developers. Cydia is not yet working on the iOS 4.2.1 and it crashes when you open it. But an iOS 4.2.1 compatible Cydia is ready and will be available when the official release of iOS 4.2.1 is up.
If you want to jailbreak iOS 4.2.1, then you could do it using Redsn0w 0.9.6 beta 2. Here are the steps to do it.
NOTE : This is applicable only for iPhone 3G, 3GS, 4 and iPod Touch 2G, 3G and 4G.
1. Download Redsn0w 0.9.6 beta 2.
3. Update your device to iOS 4.2.1 using iTunes.
4. Start Redsn0w and browse to iOS 4.1 firmware ISPW and not iOS 4.2.1 and go forward.
5. Check the “ Install Cydia “ box and click “ Next “.
6. Now switch off your device but keep it plugged in to the computer.
7. Now go into recovery mode by pressing the power and buttons for 10 seconds and then release the power button, but keep holding the home button until the iOS 4.2.1 jailbreak process begins.
8. Now wait till Redsn0w finish the process.
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