WannaCry is a ransomware worm that spread rapidly through across a number of computer networks in May of 2017. After infecting a Windows computers, it encrypts files on the PC's hard drive, making them impossible for users to access, then demands a ransom payment in bitcoin in order to decrypt them.

A number of factors made the initial spread of WannaCry particularly noteworthy: it struck a number of important and high-profile systems, including many in Britain's National Health Service; it exploited a Windows vulnerability that was suspected to have been first discovered by the United States National Security Agency; and it was tentatively linked by Symantec and other security researchers to the Lazarus Group, a cybercrime organization that may be connected to the North Korean government.

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