Depends what you want to do. They don’t require a network connection to operate as a vehicle. So if you don’t care about the remote app features (local ones such as lock/unlock still work over BLE), live traffic, streaming music or updates, then a network connection isn’t necessary.

If you do want any of those features, then you would need to either get root access to the gateway and infotainment systems to modify the endpoints or take over the C&C server (formerly named “mothership”) domains and certificates.

I set the VPN tunnel from the VPS to deny everything to the internal network by default, then put the services that need to be accessed on the allow list in the firewall. So the VPN endpoint from the VPS can only hit the very specific IPs/ports/protocols that were explicitly allowed. There is still the possibility of a compromise chain of VPS->service->container/VM->hypervisor->internal network access, but I feel comfortable with those layers.

You could also setup an IDS such as Snort to pick up on that exploit traffic between the services and internal VPN endpoint if extra security is necessary on top of fail2ban and log alerts on the VPS.