Extensibility for security monitoring in 5G vehicular networks remains largely unexplored despite strong requirements for interoperability, to support multiple properties (e.g., security, privacy, trust, sustainability) and to reach trade-offs. We discuss ITS security monitoring challenges and propose an extensible monitoring architecture to meet them. We design and implement a sample security monitoring probe for CAM and DENM and demonstrate on simulations the probe capabilities on a cooperative collision detection use case.