Data access security in federated information systems with loose coupling among local data sources is hard to achieve mainly for two reasons: local data information source heterogeneity (data models, access security models, semantics), local autonomy which do not allow to create a global integrated consistent security schema. To solve some of such problems we propose a role-based object model to describe the local data access security schemas (discretionary and non-discretionary models). Interoperability among the various local data sources is achieved by a rich descriptive layer at the federated level. The global security policy allows to define the choices concerning information flow control both for importation (from the federation to a local system) and exportation (from a local system to the federation).