They have agents operating in Japan in significant numbers, too. In the past, North Korea gave veiled references and threats to unleashing a WMD attack on specific US military bases in Japan, if not in downtown Tokyo towards Japanese civilians. This could in fact take the form of a suitcase nuke, which for all intents and purposes could a) already be in Japan, and b) would have the necessary delivery system/infrastructure (agents of Chongryon or other infiltrated agents), to do the deed. In fact I have noticed a considerably high presence of Japanese police and security personnel at various Japanese train stations and bullet train stops these days. Who knows what for.
But what will they accomplish except self destruction or takeover by either those that they attack or allies of same?
I doubt that a country without the ability to either feed its populace nor electrify its country can attack and hold even a few miles of South Korea.
Granted, there might be an attempt and destruction, but the repercussions would be disastrous for those in North Korea who would order such actions.
Such logic can be used on paper and not in real life however.