Microsoft issued an alert for a remotely exploitable software flaw in mid-May. Security firms drew analogies between the vulnerability and the destruction caused by the ETERNAL BLUE exploit. The lack of a public exploit is a major reason, as is the difficulty of writing one from scratch, says David Aitel, chief security technical officer at Cyxtera. The WannaCry worm, attributed to North Korea, occurred only after the exploit had been publicly released. In 2012, Symantec analyzed malware for the use of previously unknown “zero days””]