Download greenpois0n RC5
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You can download greenpois0n from https://greenpois0n.com/
OR
You can download greenpois0n from Download greenpois0n RC5 Mac version from iPhoners.org as well (MIRROR).
If you have greenpois0n decompression issue, download greenpois0n from this mirror: Download greenpois0n without decompression error.
It’s currently only available for Mac, Windows version will be released soon. Be sure to subscribe or follow @iPhonersxOrg on Twitter to get instant updates.
It doesn’t support hacktivation, untethered jailbreak for iOS 4.2.1
Chronic 
