The American software supplier Kaseya has found a solution for the large-scale hack at the beginning of this month.
The company says it has a decryption key with which computer systems can be unlocked.
Kaseya provides software packages to companies that allow them to remotely maintain and manage their customers' computer systems.
The cyber attack caused files and systems of Kaseya customers to be encrypted.
About 1,500 companies worldwide were victims of the hack.
Kaseya reports that it received the key on Wednesday and that it has since been tested by a cybersecurity company.
The company says it is actively helping customers who are still experiencing problems unlock their computer systems.
It does not say how exactly Kaseya got the decryption key.
The company says it received the key from a "third party".
The Russian hacker group REvil previously claimed the cyber attack and demanded 70 million dollars (59 million euros) in exchange for the release of all data of affected companies.