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Usage control (UCON) proposed by R. Sandhu et al. [8, 9] is an attribute-based authorization model and its main novelties are mutability of attributes and continuity of control.

OASIS eXtensible Access Control Markup Language (XACML) [10] is a widely-used language to write authorization policies to protect resources in a distributed computing environment (e.g. Grid). The XACML policy specifies before-usage authorization process optionally complemented with obligation actions fulfillment. By now, XACML has insufficient facilities to express continuous usage control afterwards an access was granted and started.

In this paper, we introduce U-XACML, a new policy language, which enhances the original XACML with the UCON novelties. We extend a syntax and semantics of the XACML policy to define mutability of attributes and continuity of control. We introduce an architecture to enforce the U-XACML policy.