There is no way in the world that North Korea developed the expertise to affect this kind of attack in-house, even if it is a relatively simple piece of malware.

My guess is that the Chinese have been supporting them in a limited way. China doesn't really have a motivation to encourage North Korea's ridiculous antics and belligerent behavior of late, but it does have a great deal of interest in testing out its own cyber-attack vectors on real-world targets. If they were to directly attack the computers of another nation themselves the world will go into an uproar and their trade will suffer. On the other hand, if North Korea does it we've gotten to the point, collectively, where we don't do much more than roll our eyes and shrug it off. It's a bit like when the Gemans decided to test tank warfare by getting involved in the Spanish Civil War.