The scourge of ransomware has finally come to OS X! Researchers at the security firm Palo Alto Networks have announced that version 2.90 of the Transmission bittorrent client for Mac OS X has been adulterated with a new ransomware variant they have named KeRanger. Users on the Transmission forum and a message on the front page of the Transmission website confirm this:

Notice on the homepage of the Transmission Site