The developers released the tool on Friday, as the deadline expires for computers all over the world first infected a week ago.

"This is not a perfect solution," internationally known hacker Matthieu Suiche said in a blog post after testing the fix on different versions of Windows. "But this is so far the only workable solution to help enterprises to recover their files if they have been infected and have no back-ups."

Paris-based Adrien Guinet, who works for the IT security company Quarkslab, developed a theoretical technique for decrypting the files blocked by WannaCry. Benjamin Delpy, an employee of the Banque de France, used Guinet's methods to create a free decryption tool he dubbed "wanakiwi."

Europol tested the team's new tool and said it was "found to recover data in some circumstances."

One drawback is that the fix only works if the computers were not rebooted from the moment of infection.

The malicious software has infected some 300,000 computers since last Friday, with Russia and China hit the hardest. The infection, however, targeted computers all over the world, with the worm threatening to delete files if the user does not provide $300 (267 euros) to $600 in bitcoin for ransom.

Only 309 users seem to have complied with the request by Friday, paying a total of $94,000 to the attackers. Officials across the world also urged people not to pay the ransom.

dj/gsw (AP, Reuters)