WannaCry ransomware attack was a worm that infected many Windows computers around the world on May 2017. The worm had spread malware that encrypted the user's computer data (i.e. scrambled the user's computer data into meaningless information) and demanded affected users to pay $300 Bitcoin within 3 days or $600 Bitcoin within 7 days before all of the affected computer's data is destroyed. The malware had cost the world millions to billions of dollars to fix their servers and computers.