OpenPilot doesn't require you to flash anything to your car, everything is done using the same CAN messages that the stock ADAS system sends. Further, the microcontroller that communicates between OpenPilot and the vehicle is hardcoded to block any CAN messages OpenPilot sends that are deemed to be too fast to react to:
https://github.com/commaai/openpilot/blob/a2ae18d1dbd1e59c38...