If you’re a state actor it probably wouldn’t be too hard to create US citizen IDs anyone could fall into.
Hack a hospital to give a birth certificate for a new born baby (easily within NK’s reach), and then mail the documents for a SS card. In eighteen years have an agent take the GED and SAT, and score well enough to get into state school. Then graduate and apply to the list of target companies you want to infiltrate.
You’d have to go pretty deep into someone’s identity to unearth this sort of approach. Probably not enough for a military clearance, but enough for anything not directly defense related.