There were no treaties governing the placement of nuclear weapon systems at the time (now there are a few that do impinge on that/*, though none would prevent Russia from putting nukes in Cuba, I don't think). So there was no international law that governed this issue, no.

Note that they did not "give" missiles to Cuba. Cuba hosted nuclear weapons that were kept under Soviet control. So this is considered "basing" or "hosting" and not "giving." The United States had extensive nuclear basing arrangements at exactly the same period in the Cold War, putting thousands of nuclear weapons in sites in Europe, North Africa, and Asia, to "contain" the Soviet Union.

Here is a map showing just the nuclear-armed bomber deployments alone (if we included missiles, it would be even more) from the period of 1945-1958. So this gives a sense of the context; the USSR was actually doing exactly the same thing the US had been doing for some time, and one of the triggers of the Cuban crisis was the US deployment of intermediate-range missiles to Turkey.

(*The Nuclear Non-Proliferation Treaty, for example, would imply that the Russians could not "give" the Cubans nuclear weapons, but could still host them there. Had the now defunct Intermediate Nuclear Forces Treaty been in effect, it would have banned the specific missiles they put there. There are other treaties that govern the placement of weapons today — including nuclear free zones, the Seabed Arms Control Treaty, etc. — but these are to my knowledge the only ones that would have impinged on the specifics of the Cuban Missile Crisis at the time, had they been in place.)