A teen hacker known for his deftness with iPhones has figured out how to unlock models running the latest firmware versions by cracking a protection that has frustrated hackers for weeks.

The breakthrough by George Hotz, aka Geohot, means people who have bought a recent iPhone will once again be able to use it on the phone network of their choice. Apple makes as much as $400 for every handset that's activated on an approved network, so its developers have worked hard to prevent the so-called unlocking of iPhones.

Last year, 17-year-old Geohot was among the first group of hackers to break Apple's iron-fisted grasp on the iPhone, a coup that won him a Nissan 350Z and 3 8GB iPhones. Apple promptly responded by issuing updated firmware that stymied such efforts. Not only did the updates disable modified phones, effectively turning them into $400 bricks, they also prevented unlocking software from working in many cases. The arms race has persisted ever since.

The latest salvo was fired late last week, following a 24-hour hacking spree by Geohot that was broken up by only three hours of sleep. It turns out the latest firmware contained modifications to the device's memory registers to prevent unlocking. Geohot worked around those changes by finding another, much higher register that was vulnerable.

"I guess Apple thought big numbers were harder to guess," he wrote.

He then found a way to install his custom-built code by exploiting a flaw that allowed him to erase a range of memory addresses where security software is stored.

"The technique was not one that I was familiar with at all," said Kevin Finisterre, a researcher who has spent a fair amount of time dissecting Apple devices. "From the read it sounds as though the gentleman has made some significant progress. More importantly he is sharing."

This latest unlocking is no small accomplishment because iPhones are programmed to accept only approved SIM cards. Geohot's technique appears to work around this limitation by writing to certain sections of the firmware.

The hack is highly technical and by no means for the faint of heart. Those iPhone owners in need of more hand-holding should check out step-by-step instructions here from iClarified. ModMyiPhone also offers a tutorial here.

Several weeks ago, analysts at Bernstein Research estimated that 1 million iPhones, or a full 27 per cent of the handsets sold to date, are running on unauthorized networks. At that rate, Apple could lose $1bn in revenue over the next two years. Rest assured that developer drones in Cupertino are already laboring to circumvent this latest workaround. ®