A flow sensitive security model is presented to analyse information flow in federated cloud systems. Each cloud and the entities of the cloud system are classified into different security levels which form a security lattice. Opacity --- a general technique for unifying
security properties --- turns out to be a promising analytical technique in the context of cloud computing systems. The proposed approach can help to track and control the secure information flow in federated cloud systems. It can also be used to analyze the impact of different resources allocation strategies
