Mac users might have expected their machines to suddenly lock them out, after all Apple locked out users of its iPhone who made the grave mistake of daring to get their product fixed by someone who was not an authorised Apple dealer.  But it turns out that problem was not a feature inspired by the Apple management.

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.This is opposed to Appel demanding a ransom for your hardware when you buy it.

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.After all who would waste time hacking a Mac if all they would get is are Coldplay and U2 albums.