A method of preventing intrusion in communication traffic with a set of machines in a network is provided, the traffic including communication entities, including the steps of: providing a test system comprising test facilities replicating at least one of the machines in the set; directing at least part of the communication entities in the traffic toward the test system; running the communication entities directed toward the test system on the test facilities to detect possibly adverse effects on the test system, and i) in the presence of an adverse effect, blocking, by the test system, the communication entities leading to the adverse effect, and ii) in the absence of an adverse effect, allowing, by the test system, the communication entities not having the adverse effect to communicate with the set of machines.