Using Expert Systems to Statically Detect "Dynamic" Conflicts in XACML

Policy specification languages such as XACML often provide mechanisms to resolve dynamic conflicts that occur when trying to determine if a request should be permitted or denied access by a policy. Examples include "deny-overrides" or "first-applicable." Such algorithms are primitive and potentially a risk for corporate computer security. While they can be useful for resolving dynamic conflicts, they are not justified for conflicts that can be easily detected statically. It is better to find those at compile time and remove them before run time. Many different approaches have been used for static conflict detection. However, most of them do not scale well because they rely on pair-wise comparison of the access control logic of policies and rules. We propose an extension of a Prolog-based expert system approach due to Eronen and Zitting. This approach uses constraint logic programming techniques (CLP), which are well-adapted to hierarchical XACML policy logic and avoid pair-wise comparisons altogether by taking advantage of Prolog's built-in powerful indexing system. We demonstrate that expert systems can indeed detect conflicts statically, even those that are generally believed to only be detectable at run time, by inferring the values of attributes that would cause a conflict. As a result, relying on the XACML policy combining algorithms can be avoided in most cases except in federated systems. Finally we provide performance measurements for two different architectures represented in Prolog and give some analysis.