I read recently that they may have trained Zawahiri according to two former FSB agents. The guy in general is a ghost though. On the government side they often exerted influence in Syria, Egypt, probably others I've forgotten. Then you have the Soviet Union and their lot but they lost a lot of juice after they couldn't exert their former financial and material support. There was the theater attack, Beslan school massacre, numerous train bombings, Moscow apartment bombings(possible FSB false flag), everything that happens in the Caucasus post war could be described as terrorism. In the West I think we get a very limited view of what's actually happening there. There's still a low intensity conflict in Chechnya, Ingushetia, and Dagestan last I knew.
Is Chechnya facing wave of jihadist violence? - BBC News
Dagestan: Russia’s Most Troublesome Republic
Things will certainly ramp up in Russia if not at least the Caucasus again it's just a matter of when, and the historical grievances can't be understated. Supporting Assad just sweetens the deal for that lot.