The WannaCry ransomware attack was a worldwide cyberattack in May 2017 by the WannaCry ransomware cryptoworm, which targeted computers running the Microsoft Windows operating system by encrypting data and demanding ransom payments in the Bitcoin cryptocurrency. (around 300 bitcoin) It propagated through EternalBlue, an exploit developed by the United States National Security Agency (NSA) for older Windows systems. EternalBlue was stolen and leaked by a group called The Shadow Brokers at least a year prior to the attack. While Microsoft had released patches previously to close the exploit, much of WannaCry's spread was from organizations that had not applied these, or were using older Windows systems that were past their end-of-life. These patches were imperative to an organization's cyber-security but many were not applied because of neglect, ignorance, mismanagement, or a misunderstanding about their importance. Some have claimed a need for 24/7 operation, aversion to risking having formerly working applications breaking because of patch changes, lack of personnel or time to install them, or other reasons.
The attack began at 07:44 UTC on 12 May 2017 and was halted a few hours later at 15:03 UTC by the registration of a kill switch discovered by Marcus Hutchins. The kill switch prevented already infected computers from being encrypted or further spreading WannaCry. The attack was estimated to have affected more than 200,000 computers across 150 countries, with total damages ranging from hundreds of millions to billions of dollars.