CryptoWall

Definition
CryptoWall (also CryptoWall2) is a

"virus [that] encrypts files on a computer's hard drive and any external or shared drives to which the computer has access."

Overview
The malware "directs the user to a personalized victim ransom page that contains the initial ransom amount (anywhere from $200 to $5,000), detailed instructions about how to purchase Bitcoins, and typically a countdown clock to notify victims how much time they have before the ransom doubles. Victims are infected with CryptoWall by clicking on links in malicious e-mails that appear to be from legitimate businesses and through compromised advertisements on popular websites. According to the U.S. CERT, these infections can be devastating and recovery can be a difficult process that may require the services of a reputable data recovery specialist."