The earliest linkage that I know of between the vehicle data network and an entertainment system was to be able to automatically increase the radio/CD volume at

The earliest linkage that I know of between the vehicle data network and an entertainment system was to be able to automatically increase the radio/CD volume at higher speeds. This was long before phones were built in to the dashboard so there wasn't as much of a requirement for security, once you are inside the car the diagnostic connector will give you access to the whole network anyway. I don't know of a read-only CAN controller, plus not all data values will be periodically broadcast over the vehicle network so you may need to be able to send out a request for something as well as listen for a reply. The standard firewall tools ought to be easy enough to adapt to filter CAN messages. The first time I updated ECUs over the air would be over 15 years ago, I have not kept up with whether manufacturers were thinking of doing it routinely.