Hackers have infected a number of Macs with “KeRanger” malware which demands owners pay a bitcoin (about £280) for their files to be unencrypted.
Users began unwittingly downloading the malicious programme as they tried to install popular software called Transmission, which is used to transfer data on BitTorrent.
The “ransomware” stays quiet for three days after infecting each computer – and then starts to make documents, photographs, videos and other precious files inaccessible.
Cyber security experts believe the “KeRanger” virus was loaded onto the Transmission website on and Apple users could start receiving ransom demands from today unless they immediately install an updated version of the software.
Ryan Olson from Palo Alto Network said that this is the first example of Mac ransomware which is functional, encrypts your files and seeks a ransom.
Apple has revoked a digital certificate which had enabled the ransomware to be installed onto Macs in the first place although it is unclear how it ever got one in the first place.
Macs are not normal targets for virus writers, despite being used by the content industry. This is not because they are particularly secure, but because the sort of data they contain is rarely worth stealing or ransoming.