Message-based communication system monitor, comprising an inspector (16) capable of inspecting a message so as to determine whether it satisfies a specific pattern of a source, characterized in that it comprises an analyser (12) capable of inspecting a communication stream, so as to derive therefrom a structured representation of messages, in correspondence with a plurality of rules, and in that the inspector (16) is devised so as to: *apply a stored pattern to a structured representation of the inspected message, so as to establish a signature, and *compare the signature with reference data, so as to classify the inspected message in the case of correspondence.