Security assessment is an important task for operation of modern computer networks. The paper suggests the security assessment technique based on attack graphs which can be implemented in contemporary SIEM systems. It is based on the security metrics taxonomy and different techniques for calculation of security metrics according to the data about current events. Proposed metrics form the basis for security awareness and reflect current security situation, including development of attacks, attacks sources and targets, attackers’ characteristics. The technique suggested is demonstrated on a case study.