I just read the Wikipedia article on this.
The vulnerability exists because the SMB version 1 (SMBv1) server in various versions of Microsoft Windows accepts specially crafted packets from remote attackers, allowing them to execute arbitrary code on the target computer.
The way this is phrased is ambiguous and leaves open two very different possibilities.
1) That there is an accidental defect in SMB that somehow "accepts" what might be called "deformed" packets, or
2) That there is a deliberate backdoor installed in SMB, that allows people that know the right "code" to access a machine remotely.
My nearest approximation to possibility #2 is the "magic packet" used to turn on a sleeping/suspended network card on a Network.
Is EternalBlue an exploit that takes advantage of a flaw in the code, or is it a backdoor allowing "just anyone" that knows the right "key" to access someone's computer. Given the close association between this "vulnerability" (I think it's a backdoor" and the NSA, I'm thinking it's not an accident. But I don't see anyone saying this openly at this point, so I'm asking.