An Operator-driven Approach for Modeling Interdependencies in Critical Infrastructures based on Critical Services and Sectors Elisa Canzani 1 , Helmut Kaufmann 2 , Ulrike Lechner 1 , 1 Universität der Bundeswehr München, Department of Computer Science, Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany {Elisa.Canzani, Ulrike.Lechner}@unibw.de 2 Airbus Aerospace and Defense, Cybersecurity Research Lab, Willy-Messerschmidt-Straße 1, 85521 Ottobrunn, Germany {Helmut.Kaufmann}@airbus.com Abstract. To trigger disruptive cascading effects among Critical Infrastructures (CIs), advanced cyber attacks take advantage of dependences among organizations. Consequently, proper countermeasures in case of threats to CIs must consider interdependencies between them. The strategic use of information systems to coordinate response efforts of CI operators is a major objective towards more resilient societies. As relevant contribution to the development of a cyber incident early warning system for CI operators, this paper presents a system dynamics interdependency model based on critical services that different operators must provide to guarantee correct functioning of CIs. We explain model requirements and demonstrate how it can be used to gain situational awareness in the context of European CIs. Keywords: Critical Infrastructures · Interdependency Modeling · System Dynamics · Early Warning System · Incident Response Coordination 1 Introduction Understanding complex dynamics of interdependencies to coordinate response efforts would help to prevent networks of Critical Infrastructures (CIs) from potential catastrophic effects [1]. However, existing security measures to protect a CI from threats and cyber attacks do not usually cross the organization’s boundaries [2]. As a first step towards unifying CI protection programs among the European member states, the European Union Agency for Network and Information Security (ENISA) has recently issued guidelines to identify CI assets and services [3]. On the basis of the work of ENISA, we move one step further by proposing an operator- driven approach to model and analyse CI dependencies and related critical services. To develop the interdependency model, we apply the block building methodology based on System Dynamics (SD) of Canzani [4]. We extend the three building blocks developed in [4] to highlight the relevance of critical sectors, CIs, and services.