I'm assuming the firmware update requires A Magical Dance Of Keys and Buttons to access firmware update mode (if not, shoot them) and if the private keys were compromised, you still need physical access to the vehicle (And it's keys, or keyfob if keyless) to update it.
So that's less of an issue than you might think. Im quite sure you don't just turn the car on with the USB drive plugged in, that would be stupendously dense.
Note - I don't know what the procedure is, and I genuinely don't care, to be blunt, as I specifically avoid cars that have nannying controls for everything. And penis extensions like Jeeps.