There were various strains of CryptoWall over the course of the year and a half, although most of them are no longer a threat to computer users these days. When CryptoWall was first discovered, however, it had security researchers from all over the world quite concerned.
This particular type of Bitcoin ransomware spread through malicious website advertisements, spread by the Zedo ad network. As one would come to expect from these evil intentions, some of the world’s biggest websites were targeted by the Zedo ad network in an attempt to spread CryptoWall to as many computers as possible.
http://themerkle.com/news/bitcoin-ransomware-education-cryptowall/
