The Apple iPhone 4S‘ super-tough A5 processor has had hard-working hackers stumped. After 10 months of efforts, however, they’ve released a free jailbreaking download.

The Chronic Dev Team, a group of iOS hackers, has unveiled its GreenPois0n toolkit. The download came at the cost of “thousands of hours of brain power and effort from a legion of world-renowned hackers,” the group said in a celebratory blog post.

Apple dictates which applications can run on its devices and would very much like its operating systems to go un-manipulated. Jailbreaking is the process by which the system and user interface are opened up to user changes.
The group released the jailbreak software earlier this month.

Some caution that jailbreaking makes devices vulnerable to security issues such as viruses. Information Week pointed to a blog post by Sophos security expert Paul Ducklin, who wrote that “the only iPhone viruses ever seen in the wild (Ikee and Duh) were for jailbroken phones.”

While it’s legal in the United States to jailbreak a phone, Apple asserts that doing so invalidates its devices’ warranties.

“Hack the world!” Chester Winsniewski, also with Sophos, has blogged. “Just remember that you are on your own if you thumb your nose at the manufacturer of your device.” In December 2011, the Electronic Frontier Foundation submitted a formal application to the U.S. Copyright Office requesting that jailbreaking and circumventing digital rights management on mobile devices and video game consoles be legal.

A jailbreak, for example, is currently available enabling Apple iPhone 3GS and iPhone 4 owners to install Siri, the voice-activated personal assistant that Apple offers exclusively on the iPhone 4S.

Earlier this week the Chronic Dev team posted a video of a jailbroken iPhone 4S that loaded properly, with Siri still intact.To run the new jailbreak, interested parties can download the zip file, attach their iPhone or iPad to a computer and click “Jailbreak.” The Greenpois0n software will do its thing.

“After a few magical seconds,” writes the team, “Greenpois0n will display a ‘Complete’ message. At this point, the greenpois0n injection is complete—just wait for your device to reboot automatically.”

When the device reboots and has an Internet connection, press the Loader icon, which will download and install a Cydia app. Reboot one more time, adds the team, and then enjoy.