Mac users have been hit with what's being called the first-ever incident of ransomware attacks in which victims are being instructed to pony up $400 if they want to get their files back.

First detected on Friday, the ransomware dubbed "KeRanger" is confirmed to be the first fully functional ransomware seen on the OS X platform.

Users at risk are those who use the program Transmission to transfer data through the BitTorrent peer-to-peer sharing network.

Attackers infected Transmission version 2.90, which was released for download over the weekend.

The ransomware manifests itself after three days, and demands that victims pay one bitcoin (about $400) to a specific address to retrieve their files, reports network security firm Palo Alto Networks.