[potatochup]

Usually there is a whitelisted set of messages available through the OBD port, it doesn't give you unfettered access to the CAN bus.

[tjoff]

Enough for https://comma.ai/ to work so can't be that restricted?

Edit: thanks to responders, I misremembered how it worked.

[bri3d]

Comma.ai plugs into the vehicle's "backing" CAN busses (Powertrain, Sensors, infotainment, etc.) behind the Gateway, not the OBD port. This requires the removal of at least a few trim pieces and connectors. For example, the Giraffe module taps in at the high-mounted camera module on many cars, requiring the removal of ceiling trim and the installation of a custom connector / tap onto the CAN bus.

[jaywalk]

Look at the installation procedure for that device. It requires removing the rearview mirror and plugging the device into the existing camera's wiring harness. It's not controlling anything via the OBD-C port.