Recently, flexible networks contain various services designed and implemented by different vendors to accomplish specific tasks. Trust to security systems of such services is a significant challenge. In this paper, we propose a framework to assess trust to security systems of web services based on security policy specifications and security mechanisms running on flexible networks. The framework may be used to implement an automated system to assess trust of a specific security system protecting web services. The trust assessment process is explained with a scenario to show the applicability of the framework.