All 500,000 victims of Cryptolocker can now recover files encrypted by the malware without paying a ransom.
The malicious program encrypted files on Windows computers and demanded a substantial fee before handing over the key to the scrambled files.
Thanks to security experts, an online portal has been created where victims can get the key for free.
The portal was created after security researchers grabbed a copy of Cryptolocker's database of victims.
"This time we basically got lucky," said Michael Sandee, principal analyst at Fox-IT - one of the security firms which helped tackle the cyber-crime group behind Cryptolocker.
In late May, law enforcement agencies and security companies seized a worldwide network of hijacked home computers that was being used to spread both Cryptolocker and another strain of malware known as Gameover Zeus.
This concerted action seems to have prompted an attempt by the gang to ensure one copy of their database of victims did not fall into police hands, said Mr Sandee.
What the criminals did not know, he said, was that police forces and security firms were already in control of part of the network and were able to grab the data as it was being sent.
Now, security firms Fox-IT and FireEye - which aided the effort to shut down the Gameover Zeus group - have created a portal, called Decrypt Cryptolocker, via which any of the 500,000 victims can find out the key to unlock their files.
"All they have to do is submit a file that's been encrypted from that we can figure out which encryption key was used," said Greg Day, chief technology officer at FireEye.