Multi-tenancy authorization models for collaborative cloud services

The cloud service model intrinsically caters to multiple tenants, most obviously in public clouds but also in private clouds for large organizations. Currently most cloud service providers (CSPs) isolate user activities and data within a single tenant boundary with no or minimum cross-tenant interaction. It is anticipated that this situation will evolve soon to foster cross-tenant collaboration supported by Authorization as a Service (AaaS). At present there is no widely accepted model for cross-tenant authorization. Recently, Calero et al [12] informally presented a multi-tenancy authorization system (MTAS) which extends the well-known role-based access control (RBAC) model by building trust relations among collaborating tenants. In this paper we formalize this MTAS model and propose extensions for finer-grained cross-tenant trust. We also develop an administration model for MTAS (AMTAS). We demonstrate the utility and practical feasibility of MTAS by means of an example policy specification in XACML. We anticipate researchers will develop additional multi-tenant authorization models before eventual consolidation and unification.