Access control policies for XML typically use regular path expressions such as XPath for specifying the objects for access control policies. However such access control policies are burdens to the engines for XML query languages. To relieve this burden, we introduce static analysis for XML access control. Given an access control policy, query expression, and an optional schema, static analysis determines if this query expression is guaranteed not to access elements or attributes that are permitted by the schema but hidden by the access control policy. Static analysis can be performed without evaluating any query expression against an actual database. Run-time checking is required only when static analysis is unable to determine whether to grant or deny access requests. A nice side-effect of static analysis is query optimization: access-denied expressions in queries can be evaluated to empty lists at compile time. We have built a prototype of static analysis for XQuery, and shown the effectivenessand scalability through experiments.
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