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Signature-based intrusion detection systems use a set of attack descriptions to analyze event streams, looking for evidence of malicious behavior. If the signatures are expressed in a well-defined language, it is possible to analyze the attack signatures and automatically generate events or series of events that conform to the attack descriptions. This approach has been used in tools whose goal is to force intrusion detection systems to generate a large number of detection alerts. The resulting "alert storm" is used to desensitize intrusion detection system administrators and hide attacks in the event stream. We apply a similar technique to perform testing of intrusion detection systems. Signatures from one intrusion detection system are used as input to an event stream generator that produces randomized synthetic events that match the input signatures. The resulting event stream is then fed to a number of different intrusion detection systems and the results are analyzed. This paper presents the general testing approach and describes the first prototype of a tool, called Mucus, that automatically generates network traffic using the signatures of the Snort network-based intrusion detection system. The paper describes preliminary cross-testing experiments with both an open-source and a commercial tool and reports the results. An evasion attack that was discovered as a result of analyzing the test results is also presented.