Is there any consequence to nuclear proliferation to countries? Let's say that there's a technology leak from Pakistan and it allows Iran and Syria to develop the nuclear weapons. Will Pakistan be held responsible for the proliferation of nuclear technology and what kind of punishment can we expect? What are the mechanisms to holding countries responsible for proliferating nuclear technology?
This has happened, see the AQ Khan network. It did have consequences for Pakistan, yet the overall political situation at the time influenced the severity.
Keep in mind that there are three different sorts of "consequences" for something like this:
- Actions of the world community, like the UNSC passing a sanctions resolution.
- Actions of individual states, like no longer trading with Pakistan.
- If the weapons are used, the victims might assume that it was a Pakistani attack, somewhat cloaked for deniability, and exercise their right to self-defense against Pakistan.
There is no right in international law to be trusted by other countries, and no right to be sold high technology. There is no right to have exports purchased by other countries, either. A naval blockade would be an act of war (remember the Cuban Missile Crisis). An embargo without blockade is no act of war.
So if another country dislikes what Pakistan is doing, they can order their citizens and companies to stop doing business with Pakistani citizens and companies. They can stop issuing visa to Pakistani individuals, or to Pakistani in general. They can encourage other countries to do the same. They can even threaten to stop doing business with entities which do business with Pakistan. If, say, Haiti were to take a dislike to Pakistan, Pakistan might not worry too much. If it is the United States or China, they have a problem. (Compare the Banco Delta Asia case.)
Just how serious the consequences would be would likely depend on the serverity of the proliferation, the degree of government involvement, the recipient, and the time between the action and the detection -- it is harder to get upset if it comes out decades later.
While the aforementioned Abdul Quadeer Khan (in the answer of o.m.) was working in a Dutch uranium enrichment plant he managed to get hold of various secret documents. These documents were later of key importance in the development of the Pakistan atomic bomb.
Although this espionage was made possible due to extreme laxity of the Dutch intelligence agencies, the Netherlands did not suffer any sanctions or other consequences by other countries because of it. The only person who was punished was Frits Veerman , a Dutch colleague of Abdul who suspicious about him and warned about him. He was ordered to shut up and later fired.
So based on historical precedent one would expect that a country will not suffer any consequences for accidentally leaking nuclear technology.