[evck]

Modern vehicles report OBD-II signals over Controller Area Network, which is used to link most the controllers in your vehicle. All cars after 2008 in the states use CAN for OBD-II.

Most manufacturers have the OBD-II CAN bus connected to the engine controller for diagnostic purposes. If you can gain control of the CAN bus, you can do some real damage to the car. Furthermore, the hardware must write to the bus to get OBD-II data (it's a request/response system), so it's not a read only device.

[ghostfish]

This, a million times this. My understanding also is that the ODB port on many/most vehicles goes straight into the CAN bus controlling everything in the car. It's entirely possible you could gain control over any microcontroller in the car, send false messages, drown the bus, etc. through a device like this. It's a TERRIBLE idea from a security perspective.