I believe it's a major threat to international security, which is why I've devoted a good chunk of my career over the last 20 years to working on this problem.
I do think, however, that the probability is lower than it was, say, at the time of the September 11 attacks. Since then the capabilities of the core of al Qaeda, the part of al Qaeda that had the greatest nuclear ambitions, have been greatly reduced since the death of Bin Laden and the capture and killing of many others. There's a large quantity of nuclear material that is now much more secure than it used to be.
What is the probability? No one can really know, but I would argue that given the huge consequences, even a very small probability is enough to say the risk is too high and we need to take action to reduce it.
One analogy I often use is that no one in their right mind would operate a nuclear power plant upwind of a major city if it had one chance in a hundred every year of blowing sky-high. Everybody would understand that it was too big a risk. My view is that we may be taking a bigger risk than that in the way that the world manages nuclear materials around the world today.