Janus Vehicles: Entity-Centric Threat Modelling of Protocol-Conformant Vehicles under a Malicious-Manufacturer Assumption

Automotive cybersecurity commonly assumes benign end-product manufacturers and design authorities, focusing on supply-chain threats, external attacks, and compromised network participants observable through misbehaviour. This assumption overlooks malicious-by-design vehicles that remain authentic and protocol-conformant while performing undeclared sensing, communication, or context-triggered actuation. We formulate an Entity-Centric Threat Model spanning three subcomponents. The first, Malign Entity Archetypes, describes four types of malign entity: peer-benign, purpose-malicious vehicles; compromised vehicles; reappropriated genuine components; and imitated vehicles or components. The model exposes two assurance gaps: authenticity does not establish declared-purpose conformance, while malicious functionality can evade detection based on behavioural history or peer-visible evidence. Reappropriated components can remain genuine and retain usable identities or capabilities outside authorised contexts, reducing the effectiveness of signal fingerprinting and certificate-based authentication. To structure this threat space, we introduce a Behaviour-Locus Matrix that maps covert, overt, and concealment actions across hardware, software, and network loci. For peer-benign, purpose-malicious vehicles, we further present a Behavioural Interaction Model that represents how hidden capabilities are contextually activated, realised through actions, manifested within the vehicle and its environment, and potentially observed or concealed.