The WannaCry ransomware attack impacts over 400,000 computers worldwide, targeting computers of the United Kingdom's National Health Services and Telefónica computers.

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.

The National Health Service (NHS) is the collective term for the publicly funded healthcare systems of the United Kingdom: the National Health Service (England), NHS Scotland, NHS Wales, and Health and Social Care which was created separately and is often referred to locally as "the NHS". The original three systems were established in 1948 as part of major social reforms following the Second World War, and officially launched at Park Hospital in Davyhulme, near Manchester, England. The founding principles were that services should be comprehensive, universal and free at the point of delivery. Each service provides a comprehensive range of health services, provided without charge for residents of the United Kingdom apart from dental treatment and optical care. In England, NHS patients have to pay prescription charges; some, such as those aged over 60, or those on certain state benefits, are exempt.

Telefónica, S.A. is a Spanish multinational telecommunications company with registered office and headquarters located in two different places, both in Madrid, Spain. It is one of the largest telephone operators and mobile network providers in the world. It provides fixed and mobile telephony, broadband, and subscription television, operating in Europe and the Americas.