There often are multiple CAN buses (though there is a push to a single high speed bus) and the ECU (or BCM and so on) are supposed to make sure the commands are

There often are multiple CAN buses (though there is a push to a single high speed bus) and the ECU (or BCM and so on) are supposed to make sure the commands are safe. In practice they often communicate with one another (manufactures want to sell features like remote start or use cheaper system such as electric parking brake) and the module does not do as much verification as it should.