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Cloud system peculiarities, such as enormous resources and long-lasting accesses, introduce new security and management challenges. This paper presents an advanced authorization framework based on the Usage Control (UCON) model and the OASIS XACML standard to regulate the usage of Cloud resources. Our framework addresses the issue of long lasting accesses and it is able to interrupt accesses that are in progress when the corresponding access rights do not hold any more. We provide the implementation of our framework and its integration with the OpenNebula toolkit.