Mac users have been exposed to the first known instance of ransomware for the OS X platform.
Those who downloaded infected versions of the Transmission BitTorrent discovered that they had also been given KeyRanger, a piece of malware that encrypts the user's hard drive after three days.
Users are then asked for payment to decrypt the disc – or, in other words, they have to pay a ransom to release their Mac.
Ransomware is commonly distributed on Windows-based systems, but this is the first recorded instance of this kind of malware reaching OS X in complete form.
According to Palo Alto Networks, Apple has already taken steps to close off such exploits through its Gatekeeper security system, which means that it's no longer possible to install the infected Transmission app.
Meanwhile, a new version of the Transmission app has been released, and users are urged to update to it as soon as possible.
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You can check for the presence of the malware by searching for the files "/Applications/Transmission.app/Contents/Resources/General.rtf" or "/Volumes/Transmission/Transmission.app/Contents/Resources/ General.rtf".
If they exist, your Mac is probably infected with KeyRanger.
As 9to5Mac points out, if your Mac has been infected by the KeyRanger malware, it's recommended that you don't pay the ransom to unlock your computer. Rather, you should restore your system to an earlier backup.