"Apple Inc (AAPL.O) customers were targeted by hackers over the weekend in the first campaign against Macintosh computers using a pernicious type of software known as ransomware, researchers with Palo Alto Networks Inc (PANW.N) told Reuters on Sunday.

Ransomware, one of the fastest-growing types of cyber threats, encrypts data on infected machines, then typically asks users to pay ransoms in hard-to-trace digital currencies to get an electronic key so they can retrieve their data.

Security experts estimate that ransoms total hundreds of millions of dollars a year from such cyber criminals, who typically target users of Microsoft Corp's (MSFT.O) Windows operating system.

Palo Alto Threat Intelligence Director Ryan Olson said the "KeRanger" malware, which appeared on Friday, was the first functioning ransomware attacking Apple's Mac computers."

http://www.reuters.com/article/us-apple-ransomware-idUSKCN0W80VX

KeRanger was delivered via the Bit Torrent client called Transmission. It was apparently only downloaded for a period of about 48 hours from 4 March.

If you are using Transmission and the version is 2.90, it is essential that you immediately update to 2.92. That version is clean and it also checks for KeRanger and removes it if it is present.

https://www.transmissionbt.com

Sorry, got that wrong. Downloads between 8AM US ET on March 4, and 4PM US ET March 5 may have been struck by the malware.