How does a single network 100% of vehicles are tied into avoid the problem of an attacker only having to subvert one node on that network to compromise everybod

How does a single network 100% of vehicles are tied into avoid the problem of an attacker only having to subvert one node on that network to compromise everybody? (Not to mention the difficulty/impossibility of getting to 100% in a reasonable timeframe.) Seems like that's been shown time and again to be practically impossible.