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

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.