In the complex world of information services, we are realizing that system dependencies upon one another have not only operational implications but also security implications. These security implications are multifold. Beyond allowing an attacker to propagate over an information system by leveraging stepping stones vulnerabilities, it also allows a defender to select the most interesting enforcement points for its policies, overall reducing the cost of managing the security of these complex systems. In this paper, we present a dependency model that has been designed for the purpose of providing security operators with a quantitative decision support system for deploying and managing security policies.