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.