A type of malware that locks computer files and demands a fee for their release has successfully targeted Apple computers.
The security researchers from Palo Alto Networks believe it is the first time ransomware has appeared on Macs.
The KeRangers malware was hidden in a version of a BitTorrent client called Transmission.
The makers of Transmission and Apple say they have taken steps to stop the malware spreading.
It is not clear how many people were affected.
Transmission is a program that lets people download and share BitTorrent files – often music or movies – with other users' computers.
After being informed of the malware, Apple revoked the developers' certificate on the infected software to prevent it from being installed if people downloaded it.
Transmission's developers released a new version of their software and added a notice on their website, urging customers to upgrade immediately.
Apple's Mac operating system – Mac OS X – has traditionally been considered to be a secure system because it is more locked down than the myriad of devices running Windows operating systems.
While writing malware code for Macs is more difficult because there is less of it around, as the computers become more commonly used criminals are working harder to target them, Greg Day, Palo Alto Networks' chief security officer for Europe, the Middle East and Africa told the BBC.
"In all honesty unfortunately this is a likely outcome," he said of his team's discovery.
"There is a simple reality which is criminals will follow us as users of IT to generate ransom.
"We've seen more Mac threats in the last few years – it's a very good reminder that there is no environment which is risk free from cyber attack."