Does OBD-II have any direct control capability? I had thought it was a diagnostic (read-only) interface.
I'm pretty sure diesel performance kits (small PDA-like device) that let you alter engine and transmission performance connects via the OBD-II port.
It depends on the manufacturer as to what custom protocols/capabilities they have on top of OBD-II.
OBD-II diagnostics are mandated by law, but manufactures often connect almost all microcontrollers to that connector. This allows updating all of your firmware from one port. That can be a security problem when connected to the internet.
I was not aware of this; I assumed it was read-only except for reset flags, thank you for pointing this out.
There are numerous holes in various firmware that allow cross-talk between the various CAN busses anyway. I don't think anyone who set out to research automotive security has ever had to move on to another vehicle due to lack of exploitable flaws.