The design of computer and communication systems has been based, for decades, on the fundamental assumption that the objective of all users is to improve their own performance. In recent years we have experienced a wave of DDoS attacks threatening the welfare of the internet. These are launched by malicious users whose pure incentive is to degrade the performance of other, innocent, users. The traditional systems turn out to be quite vulnerable to these attacks. The objective of this work is to take a first step to close this fundamental gap, aiming at laying a foundation that can be used in future computer/network designs taking into account the malicious users. Our approach is based on proposing a metric that evaluates the vulnerability of a system. We then evaluate the commonly used data structure in network mechanisms, the hash data structure, using our vulnerability metric. We show that a Closed Hash is much more vulnerable than an Open Hash to DDoS attacks, even though the two systems are considered to be equivalent via traditional performance evaluation. We also apply the metric to queueing mechanisms common to computer and communications systems. Lastly we apply it to the practical case of a hash table whose requests are controlled by a queue, showing that even after the attack has ended, the regular users still suffer from performance degradation or even a total denial of service.