Ars Technica:

A security research firm announced Sunday its discovery of what is believed to be the world’s first ransomware that specifically goes after OS X machines.

“This is the first one in the wild that is definitely functional, encrypts your files and seeks a ransom,” Ryan Olson, of Palo Alto Networks, told Reuters.

The KeRanger malware, which imposes a 72-hour lockout window unless the victim pays 1 bitcoin ($410 as of this writing), appears to have been first discovered via a rogue version of Transmission, a popular BitTorrent client.

If you have version 2.90 of Transmission, upgrade to the latest version as soon as possible.