This paper describes a next-generation security information and event management (SIEM) platform that performs real-time impact assessment of cyber attacks that target monitoring and control systems in interdependent critical infrastructures. To assess the effects of cyber attacks on the services provided by critical infrastructures, the platform combines security analysis with simulations produced by the Infrastructure Interdependencies Simulator (i2Sim). The approach is based on the mixed holistic reductionist (MHR) methodology that models the relationships between functional components of critical infrastructures and the provided services. The effectiveness of the approach is demonstrated using a scenario involving a dam that feeds a hydroelectric power plant. The scenario considers an attack on a legacy SCADA system and wireless sensor network that reduces electricity production and degrades the services provided by the interdependent systems. The results demonstrate that the attack is detected in a timely manner, risk assessment is performed effectively and service level variations can be predicted. The paper also shows how the impact of attacks on services can be estimated when limits are imposed on information sharing.