After the Soviet Union fell apart, there was a mad scramble to try to inventory who had what in the way of nuke weapons and to account for each and every warhead. For a dirty bomb, you don't need a weapon, just the fissile materials from any nuclear process. This is where I think North Korea could be extremely dangerous. They could intentionally, or just through a profit motive, give nuclear materials to a variety of terrorist groups who would be more than happy to employ them in combination with conventional bomb-making materials/explosives.
Bill Cumming shared the link above.
