Multi-facet dependency of human life on COMPUTER networks and its widespread vulnerability has made network robustness a necessity. With cost as a limiting factor, network robustness is considered as a great challenge for network administrators. This goal would be achievable by prioritizing the vulnerabilities based on their risk and choosing the most hazardous ones for elimination. Nowadays, CVSS is being used as the most widely used vulnerability scoring system. In CVSS, vulnerability ranking is based on its intrinsic features while temporal features such as the probability of developing exploitation tools, are ignored. So, DYNAMIC risk evaluation is not possible with CVSS and it is incapable of performing effective vulnerability discretion. This is because, only limited number of vulnerabilities are available for prioritization of infinite number of vulnerabilities. In addition, CVSS only ranks single step attacks whilst a wide variety of attacks are multi-step attacks. In this paper, a security system is proposed that is an improvement over CVSS and some other existing vulnerability scoring systems. It performs DYNAMIC risk evaluation of multi-step attacks by considering vulnerabilities' temporal features. As the introduced MODEL is developed based on security metrics of the security MODEL, security evaluation of multi-step attacks is now possible by CVSS. Also, the capability of risk evaluation of zero-day attacks is one unique feature of the proposed system which cannot be accomplished by the present vulnerability scoring systems. In CVSS, the impact of exploiting 35. 5% of vulnerabilities on confidentiality, integrity and availability are scored the same. But, in the proposed system, by considering the relative priority of the three mentioned security parameters, vulnerability discrimination of risk score of the mentioned percentage of vulnerabilities may be possible. On the other hand, the continuity of the probability assessment function of the proposed method in comparison to the discrete one in CVSS, improves the score diversity.