You can already do that with existing cars(pretty much anything that has power steering, so last 30 years of cars) and yet you don't see people hijacking vehicles left and right this way(or any other way, you can cut someone's brakes in 5 seconds and yet it's extremely rare). I think this is a made up problem.
It's not about usual hijacking or rigging. It's about telling car to speed and slam into pedestrians if it sees large enough group of them, for example, and until that time it can just behave normally. Think of terrorists getting access to programmable robot that weighs over a ton, that can accelerate to 60 miles/hr under 5 seconds and it can identify objects in its surrounding.
So.....a new Mercedes with driver assist package then? And Mercedes will not stop you from modding their cars to your heart's content, you don't have to ask them for permission to work on your car like you have to with a Tesla.
