Microsoft is paying out its first $100,000 (£63,000) "bounty" after a London hacking expert found security holes in Windows 8.
James Forshaw, a computer vulnerability researcher, discovered a new way to attack the flagship operating system.
Microsoft said it could not go into details until it had come up with a solution, but said it was "thrilled" to pay out the reward.
In a blog post, the company said finding out about the new "mitigation bypass technique" would help it protect users against a whole new class of attacks.
"Congratulations and well done," wrote Microsoft security expert Katie Moussoris.
"You not only made history by receiving a total of $109,400 from our bounty programs, you're also helping us make our customers safer from entire classes of attack.
"On behalf of over a billion people worldwide - Thank you and way to go!!"
Microsoft unveiled the bounty programme four months ago to try to bolster anti-hacking efforts.
It has also already paid out $28,000 (£17,000), mainly to users who found flaws in its Internet Explorer 11 web browser, with some of the winners donating their money to charity.
Budding security experts can still bag a reward of up to $100,000 for finding further security problems with Windows, and up to $50,000 for coming up with a solution.
Windows 8, the company's latest operating system, launched in October 2012 with improved security features, but is still being refined.
It received mixed reviews on launch, with some saying it is confusing and not user-friendly.
The 8.1 update is due for public release on October 17.
