The NSA is hacking everyone, all the time, but especially North Korea. The latest reports are that the NSA hacked the nation en masse for years, and had installed a “early warning radar” that gave them the ability to monitor the nation’s hacking activities.
It was this system of malware that officials are at least implying was used as the source of the secret “proof” they have that North Korea was behind the Sony Pictures hack, and certainly such a system would give them access to such evidence, if it in fact exists.
Absent in this, however, is any credible explanation for why the “early warning” system didn’t give them any early warning about the hack, if they are so sure North Korea did it.
That the private investigators into the hack saw such strong evidence for it being a hack by a disgruntled former employee and nothing to do with North Korea is also a problem, since the FBI, NSA, etc. never really discredited this theory, and just dismissed it out of hand.