  Advanced electronic units inside modern vehicles have enhanced the driving
experience, but also introduced a myriad of security problems due to the
inherent limitations of the internal communication protocol. In the last two
decades, a number of security threats have been identified and accordingly,
security measures have been proposed. While a large body of research on the
vehicular security domain is focused on exposing vulnerabilities and proposing
counter measures, there is an apparent paucity of research aimed at reviewing
existing works on automotive security and at extracting insights. This paper
provides a systematic review of security threats and countermeasures for the
ubiquitous CAN bus communication protocol. It further exposes the limitations
of the existing security measures, and discusses a seemingly-overlooked,
simple, cost-effective and incrementally deployable solution which can provide
a reasonable defense against a major class of packet injection attacks and many
denial of service attacks.
