It isn't hard, and for most cars the isolation should be in place: The [CAN] Bus for the modem (and that for OBD2) is not the same one as the one that is used f

It isn't hard, and for most cars the isolation should be in place: The [CAN] Bus for the modem (and that for OBD2) is not the same one as the one that is used for stuff like brakes. They are interconnected through some Gateway device which filters signals, so that you can't manipulate all vehicle state from all positions within the vehicle. Of course it's only some amount of protection (if you are able to compromise the gateway software), but far better than nothing. However some car manufacturers don't have separated networks (due to cost reasons).