The WannaCry ransomware attack affected more than 400,000 computers worldwide, including systems used by the National Health Service and Telefónica.

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 form of bitcoin cryptocurrency. It was propagated using EternalBlue, an exploit developed by the United States National Security Agency (NSA) for Microsoft Windows systems. EternalBlue was stolen and leaked by a group called The Shadow Brokers (TSB) a month 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 patches, or were using older Windows systems that were past their end of life. These patches were imperative to cyber security, but many organizations did not apply them, citing a need for 24/7 operation, the risk of formerly working applications breaking because of the changes, lack of personnel or time to install them, or other reasons.