Queueing mechanisms are major mechanisms used in variety of computer and communications systems. Their major objective is to control the system operation when it is under heavy traffic load in order to provide good and fair performance to the various users. Since these system are responsible for the resource allocation and taking care of the performance at times of high traffic load – it is natural that they become the target of attacks and their vulnerability should be examined. To demonstrate the vulnerability of the queuing mechanism is sensitive to attacks, if not designed properly, we consider a simple case study consisting of a single server (with single queue) system that operates under the First-Come-First-Served (FCFS) scheduling. The first step for measuring the vulnerability of system, concentrating only on measuring reduction of quality attacks. The paper suggests the measure of attack potency by evaluating the amount of performance degradation inflicted by an attacker with specific budget. The reader can observe that this measure is similar to our definition and in fact the difference between the Effectiveness and the Potency measures, is that we normalize the performance degradation the system suffers due to an attacker by the performance degradation the system suffers due to a regular user. Clearly, the potency parameter can be generalized to measure any sophisticated attack, however we think that the Effectiveness measure is preferable since it allows to derive meaningful information based on this number alone. Consider the case of a system with Vulnerability Factor of K