Mac users who've long assumed they're safer than Windows users - beware: Some will find out later today that they have been hit with ransomware in the first attack campaign against Mac computers.

Palo Alto Networks researchers discovered this so-called "KeRanger" malware on Friday. They warn that victims could start losing access to their data on Monday because it encrypts data three days after it is installed.

They say KeRanger will then demand that users pay one bitcoin, a digital currency worth around $400, if they want their data back.

Palo Alto says this was the first ever case of a complete ransomware attacking Apple's Mac computers. Its intelligence director, Ryan Olson, said, "This is the first one in the wild that is definitely functional, encrypts your files and seeks a ransom."

Palo Alto says hackers infected Macs through a tainted copy of the popular open source data transfer program called Transmission. Transmission removed the malicious version of its software from its website. And yesterday, it released a version that automatically removes the ransomware from infected Macs.