In this paper, we introduce a formal property characterizing access control policies for which the interpretations of access control as mechanism over objects and as mechanism over information contained into objects are similar. This leads us to define both a flow based interpretation of access control policies and the information flows generated during the executions of a system implementing an access control mechanism. When these two interpretations are not equivalent, we propose to add a mechanism dedicated to illegal information flow detection to the mechanism of access control over objects. Such a mechanism is parameterized by the access control policy and is proved sound and complete. Finally, we briefly describe two real implementations, at two levels of granularity, of our illegal flow detection mechanism: one for the Linux operating system and one for the Java Virtual Machine. We show that the whole approach is effective in detecting real life computer attacks.