This paper describes an approach for automated security analysis of network access controls in operational Infrastructure as a Service (IaaS) cloud computing environments. Our objective is to provide automated and experimental methods to analyze firewall access control mechanisms aiming at protecting cloud architectures. In order to determine the accessibilities in virtual infrastructure networks and detect unforeseen misconfigurations, we present an approach combining static and dynamic analyses, along with the analysis of discrepancies in the compared results. Our approach is sustained by experiments carried out on a VMware-based cloud platform.