Last month, the WannaCry ransomware attack hit more than150 countries and infected tens of thousands of systems worldwide. Among those victimized were England’s National Health Service, automobile manufacturers, and government systems. The worm’s ominous red ransom screen, informing the user that all files have been encrypted, was found not only on users’ desktops, but also on ATM screens, parking meters, digital billboards, and industrial control systems.

College textbooks on computer security have a table of terms for malicious software, including “virus,” “worm,” “Trojan horse,” and (more recently) “ransomware.” Neatly-defined categories are useful when a professor wants to give a multiple-choice test, but the real world is no longer so well-defined. WannaCry combines the defining characteristics of both ransomware and worms.

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