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. Once activated, the ransomware connects to a Command & Control server over the TOR network and will then begin to encrypt certain types of files. It will then demand a ransom of 1 bitcoin, or about $400 USD, to receive a decryptor. Apple has already released a signature update for their XProtect antimalware software, and due to the revoked certificate, OS X will refuse to execute malicious installers signed by it.
Source: https://www.bleepingcomputer.com/news/security/keranger-the-first-apple-ransomware-discovered-in-hacked-installer-for-transmission/