I've been wondering this for a long time - what's to stop a terrorist from flying and attaching a drone with an explosive payload to a large commercial flight? Especially at night while the plane is taxiing?
As to how to stop it, it’s just like any other terrorist attack. SIGINT is the main tool.
At lower altitude, less impact, though it would still be problematic.
Keeping the charge on the target aircraft whilst it climbed to cruising height is likely the hard part.