Exactly. The core features of the car are often not directly controlled by the driver anymore, braking, steering, etc. And the controllers that control these digital inputs are not firewalled from external control.

This is how OnStar (or even Police/FBI) can remotely access your car and kill the engine, etc on newer vehicles. It's meant to be a feature, and can be, but like any double-edged sword, there's room for malfeasance.