Ransomware is a type of malicious software that prevents or limits users from accessing their data, and demands ransom payments in order to unlock the system holding the data. It also goes by another name: cryptoviral extortion. Ransomware first emerged in 1989, but it wasn't until 2013 that it became a widespread threat, with $5 million paid out to victims.

Ransomware should not be confused with worms; the latter uses network behavior to spread itself and does not seek out files on individual computers. The original version of Petya ransomware did not encrypt files; instead, it locked the user out of their computer, displayed a message on the screen and demanded a cash payment to unlock it.

The term "ransomware" derives from the fact that it was originally used by hackers in Russia and Eastern Europe to extort money from users by preventing them from accessing their computers until they paid up.

It is now used worldwide and can target nearly any type of computer file