A security consultant based in New Zealand has released a tool that can unlock Windows computers in seconds without the need for a password.
Adam Boileau first demonstrated the hack, which affects Windows XP computers but has not yet been tested with Windows Vista, at a security conference in Sydney in 2006, but Microsoft has yet to develop a fix.
Interviewed in ITRadio's Risky Business podcast, Boileau said the tool, released to the public today, could "unlock locked Windows machines or login without a password ... merely by plugging in your Firewire cable and running a command".
Boileau, a consultant with Immunity Inc., said he did not release the tool publicly in 2006 because "Microsoft was a little cagey about exactly whether Firewire memory access was a real security issue or not and we didn't want to cause any real trouble".
But now that a couple of years have passed and the issue has not resolved, Boileau decided to release the tool on his website.
To use the tool, hackers must connect a Linux-based computer to a Firewire port on the target machine. The machine is then tricked into allowing the attacking computer to have read and write access to its memory.
With full access to the memory, the tool can then modify Windows' password protection code, which is stored there, and render it ineffective.
Older desktop computers do not come equipped with Firewire ports, which are needed for the hack to work, but many recent models do. Most laptops made in the last few years include Firewire ports.
Paul Ducklin, head of technology for security firm Sophos, said the security hole found by Boileau was not a vulnerability or bug in the traditional sense, because the ability to use the Firewire port to access a computer's memory was actually a feature of Firewire.
"If you have a Firewire port, disable it when you aren't using it," Ducklin said.
"That way, if someone does plug into your port unexpectedly, your side of the Firewire link is dead, so they can't interact with your PC, legitimately or otherwise."
Ducklin also advised people to be careful when giving others physical access to their computer.
"I know people who'd think three times about asking passing strangers to take their photo in front of the Opera House in case they did a runner with the camera, yet who are much more casual with their laptop PC, as long as it's software-locked, even though the hardware alone is worth five times as much as the camera," he said.
Microsoft was unavailable for comment at the time of publication.