As a result of thinking about the DSD question, it occurred to me to wonder if there is anything in normative or non-normative XACML pubs that says that an unrecognized resource attribute must result in a DENY decision.
Let's say that "unrecognized" means that there is no rule in the set being used by the PDP that references the attribute.
It seems to me that this should be a requirement, as the purpose of resource attributes is to specify (via the applied rule set) what subject attributes are required for access to the resource. If there's no rule that references the resource attribute then there is a definite possibility that the subject does not have all the attributes intended to be required by whatever policy led to the inclusion of the unrecognized resource attribute in the protected resource's metadata.
The same does not apply to unrecognized subject attributes, which can be ignored as they are simply "extra" from the perspective of the PDP (and are presumably provisioned for access to other resources not protected by the current PDP/PEP.)
Martin
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Martin F Smith, PrincipalBFC Consulting, LLC
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