We consider the problem of extending XML databases with fine-grained,high-level access control policies specified using XPath expressions. Mostprior work checks individual updates dynamically, which is expensive (requiringworst-case execution time proportional to the size of the database). On theother hand, static enforcement can be performed without accessing the databasebut may be incomplete, in the sense that it may forbid accesses that dynamicenforcement would allow. We introduce topological characterizations of XPathfragments in order to study the problem of determining when an access controlpolicy can be enforced statically without loss of precision. We introduce thenotion of fair policies that are statically enforceable, and study thecomplexity of determining fairness and of static enforcement itself.
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