Critical infrastructures and systems are today exposed not only to traditional safety and availability problems, but also to new kinds of security threats. These are mainly due to the large number of new vulnerabilities and architectural weaknesses introduced by the extensive use of information and communication technologies (ICT) into such complex systems. In this paper we present the outcomes of an exhaustive ICT security assessment, targeting an operational power plant, which consisted also of the simulation of potential cyber attacks. The assessment shows that the plant is considerably vulnerable to malicious attacks. This situation cannot be ignored, because the potential outcomes of an induced plant malfunction can be severe.