In the last few years we have witnessed a strong interest in the protection of Critical Infrastructures (CIs) such as power distribution networks, power plants, refineries, water distribution, transportation systems, hospitals and telecommunication networks. Despite their relevance for public safety and security, these infrastructures are highly exposed to a large number of threats, including natural hazards, component failures, criminal actions and terrorism. Several research projects address this topic. Many of them focus on building CI simulators for preventive analysis of system vulnerabilities, while others try to proactively strengthen partial sections of the CIs (such as fault tolerant components or secure control networks). Nevertheless, despite their positive results, those projects seldom provide mechanisms to assess, in real time, the risk level associated with each of the services provided by the addressed CI. Moreover, they do not take into account the high level of interdependency between heterogeneous CIs (power distribution failures, for instance, have a direct impact on telecommunication networks, which also affect other critical infrastructures and so on) or, when they do, they have to make compromises at the level of scalability, performance, or privacy of sensitive information. In this paper we present a CI alerting system that takes a step further, when compared to those approaches, by estimating in real time the risk level associated with each service provided by the CI (i.e. the current likelihood of service degradation or service shutdown induced on a given CI by "undesired" events occurred in that CI and/or in other interdependent CIs).