That’s the NSA code name for an exploit against the Windows operating system, used by the NSA for at least five years before 2017, when the Russians stole it from that agency. EternalBlue exploits a vulnerability in Microsoft’s implementation of the Server Message Block (SMB) protocol, which controls client–server communication. Because of the manner in which the SMB was coded, sending a carefully crafted data packet over the Internet to a Windows computer allowed an attacker to execute arbitrary code on the receiving computer, and thereby gain control over it. Basically, the NSA was able to exploit EternalBlue to remotely commandeer any Windows computer on the Internet.