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The direction OEMs are going is to try and have their cake and eat it too. Data connection is not optional, so security is achieved through an isolated network interface boxe that talks to the outside and is assumed to be pwned/pwnable. The rest of the architecture is hardened accordingly, with a single internal interface to the rest of the vehicle on a secured bus and attestation/secured computing platform, etc.

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That's great until an unknowing mechanic connects a pwned device to the secure system that infects it and enables it to receive commands from the "assumed pwned" network box.

They wouldn't have a direct connection regardless, but this is also part of the threat model on every vehicle I've been involved with.

I have layman knowledge of this and do not understand. How does this compare to the level of security described in the GP comment (no connection at all)?

It might help to imagine there's a data diode between the interface box and the rest of the system. In an ideal world, there should be few differences beyond the actual data being sent. In practice, the analysis is a lot more complicated. I've also seen cases where there should be no connection hardware at all, but someone forgot to disable Bluetooth on a dev board or something and it shows up in a red team exercise.

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