Due to the increasing complexity of distributed systems, security testing is becoming increasingly critical in insuring reliability of such systems in relation to their security requirements. . To challenge this issue, we rely in this paper1 on model based active testing. In this paper we propose a framework to specify security policies and test their implementation. Our framework makes it possible to automatically generate test sequences, in order to validate the conformance of a security policy. This framework contains several new methods to ease the test case generation. To demonstrate the reliability of our framework, we present a Vehicular Networks System as an ongoing case study.