The main worry has been that anti air missile systems are not isolated, to be effective they have to be integrated in a wide network with other missile systems,

The main worry has been that anti air missile systems are not isolated, to be effective they have to be integrated in a wide network with other missile systems, observation radars, flying radars, sensors on fighters, ships, etc. This is a complex computer network. The systems operated by Greece and inherited by the Soviet Union in the cases of other Nato members are rather old with limited computational resources and limited software developed back in the 80s-90s. In the case of the S-400 we are probably talking about millions of lines of code developed only years ago and potentially with an update schedule, there is absolutely no way to be sure there is no cutting edge malware that could either infiltrate the rest of the air-defense network, or just ship radar data back to Russia about the signatures of friendly f-35s, that's the real reason the US does not want to sell f-35s to Turkey. It's one thing to pick up a return from an f-35 on your radar and another to be getting a return and also it's actual position along with IFF information. This data would be invaluable to the Russians and could actually undermine the f-35s stealth performance in a potential conflict in the future.