Update 4: According to experts tracking and analyzing the worm and its spread, this could be one of the worst-ever recorded attacks of its kind. The security researcher who tweets and blogs as MalwareTech told The Intercept "I've never seen anything like this with ransomware," and "the last worm of this degree I can remember is Conficker." Conficker was a notorious Windows worm first spotted in 2008; it went on to infect over nine million computers in nearly 200 countries. As The Intercept details,

Today's WannaCry attack appears to use an NSA exploit codenamed ETERNALBLUE, a software weapon that would have allowed the spy agency's hackers to break into any of millions of Windows computers by exploiting a flaw in how certain version of Windows implemented a network protocol commonly used to share files and to print.