The issue is the stupid media center. People want that to have Internet. Then at the same time they want it to display all of the climate details and other system info that comes off the CAN.
Yeah, no. Auto mfr.'s want to reduce the display count to make cars cheaper to make and the interiors simpler to build. I've never ever spoken to or heard form anyone who 'wants' or even kinda likes having their climate controls on the same screen as their maps, pandora, etc. It's confusing, usually cluttered, and complicates things unnecessarily.
There are typically at least two CAN buses in many recent cars, one for critical stuff like engine, transmission, etc, and another for less critical things like environmental controls, there is also LIN, FlexRay, MOST, and maybe another one or two that are less popular.
One problem is that there is no limitation in the CAN protocol to prevent a node from impersonating the master node. Another is the mutability of a node's firmware.
Two systems that accept input from the same touchscreen can't be isolated in hardware. From the software side, any portion of each system's interface that overlaps at the touchscreen cannot be isolated. On top of that, if the touchscreen is reprogrammable, then it can be attacked directly and touchscreens are commodity devices not normally considered part of the security infrastructure.
I don't think anybody is upset that their climate control system is exposed to hackers. There are no accelerator/steering/braking controls on the touchscreen (I assume) so those could be isolated but it would require having to separate systems for critical and non-critical operations. The only reason not to do that is because it's more complicated which translates to more expensive.
The climate control isn't mechanically independent of the engine and other mechanical systems. Even in the primitive cars before digital contols, the heater is dependent on the engine's thermostat (well except for old Bugs and 911's, but then that heat never really worked). In modern cars, the AC may be connected to the engine control system so that power can be redeployed for fuel efficiency or for power in emergency situations or to keep emissions within specification.
"Climate control system" is an abstraction over belt driven moving parts.
