  Third-party dongles for cars, e.g. from insurance companies, can extract
sensitive data and even send commands to the car via the standardized OBD-II
interface. Due to the lack of message authentication mechanisms, this leads to
major security vulnerabilities for example regarding the connection with
malicious devices. Therefore, we apply a modular, protocol-independent firewall
approach by placing a man-in-the-middle between the third-party dongle and the
car's OBD-II interface. With this privileged network position, we demonstrate
how the data flow accessible through the OBD-II interface can be modified or
restricted. We can modify the messages contents or delay the arrival of
messages by using our fine-granular configurable rewriting rules, specifically
designed to work protocol agnostic. We have implemented our modular approach
for a configurable firewall at the OBD-II interface and successfully tested it
against third-party dongles available on the market. Thus, our approach enables
a security layer to enhance automotive privacy and security of dongle users,
which is of high relevance due to missing message authentications on the level
of the electronic control units.
