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XACML stands for eXtensible Access Control Markup Language. The standard defines a declarative access control policy language implemented in XML and a processing model describing how to evaluate authorization requests according to the rules defined in policies.

As a published standard specification, one of the goals of XACML is to promote common terminology and interoperability between authorization implementations by multiple vendors. XACML is primarily an Attribute Based Access Control system (ABAC), where attributes (bits of data) associated with a user or action or resource are inputs into the decision of whether a given user may access a given resource in a particular way. Role-based access control (RBAC) can also be implemented in XACML as a specialization of ABAC.

The XACML model supports and encourages the separation of the authorization decision from the point of use. When authorization decisions are baked into client applications (or based on local machine userids and Access Control Lists (ACLs)), it is very difficult to update the decision criteria when the governing policy changes. When the client is decoupled from the authorization decision, authorization policies can be updated on the fly and affect all clients immediately.

The latest version 3.0 was ratified by OASIS standards organization in August, 2010.

The first committee draft of XACML 3.0 was released April 16, 2009. ^[1]

The first version of administrative policy profile working draft was publicly released on April 1, 2009.^[2]

Terminology

Non normative terminology (following RFC 2904, except for PAP)

Policy Elements

XACML is structured into 3 levels of elements:

• PolicySet,
• Policy, and
• Rule.

All 3 elements can contain Target elements. PolicySet elements can contain PolicySet and Policy. Policy can contain Rule. See http://docs.oasis-open.org/xacml/3.0/xacml-3.0-core-spec-cs-01-en.pdf for more details.

Policies are defined with a collection of Rules. Both Rules and Requests use Subjects, Resources and Actions.

• A Subject element is the entity requesting access. A Subject has one or more Attributes.
• The Resource element is a data, service or system component. A Resource has a single Attribute.
• An Action element defines the type of access requested on the Resource. Actions have one or more Attributes.
• An Environment element can optionally provide additional information.

Obligations

Within XACML, a concept called obligations can be used. An obligation is a directive from the Policy Decision Point (PDP) to the Policy Enforcement Point (PEP) on what must be carried out before or after an access is granted. If the PEP is unable to comply with the directive, the granted access may or must not be realized. The augmentation of obligations eliminates a gap between formal requirements and policy enforcement. An example of an obligation could look like this:

Access control rule:

         Allow access to resource MedicalJournal with attribute patientID=x 
               if Subject match DesignatedDoctorOfPatient
               and action is read
         with obligation
              on Permit: doLog_Inform(patientID, Subject, time)
              on Deny  : doLog_UnauthorizedLogin(patientID, Subject, time)

The XACML's obligation can be an effective way to meet formal requirements (non-repudiation for example) that can be hard to implement as access control rules. Furthermore, any formal requirements will be part of the access control policy as obligations and not as separate functions, which makes policies consistent and centralization of the IT environment easier to achieve.

New in XACML 3.0

Delegation

The implementation of delegation is new in XACML 3.0. The delegation mechanism is used to support decentralized administration of access policies. It allows an authority (delegator) to delegate all or parts of its own authority or someone else's authority to another user (delegate) without any need to involve modification of the root policy.

This is because, in this delegation model, the delegation rights are separated from the access rights. These are instead referred to as administrative control policies^[3]. Access control and administrative policies work together as in the following scenario:

A partnership of companies' many services are protected by an access control system. The system implements the following central rules to protect its resources and to allow delegation:

Access control rules:

             Allow access
                   to resource with attribute WebService 
                   if subject is Employee and action is read or write. 
             

Administration control rules:

             Allow delegation of access control rule #1
                     to subjects with attribute Consultant.
             Conditions: 
                        delegation must expire within 6 months,
                        resource must not have attribute StrictlyInternal.
               

(Attributes can be fetched from an external source, e.g. a LDAP catalog.)

When a consultant enters the corporation, a delegation can be issued locally by the consultant's supervisor, granting the consultant access to systems directly.

The delegator (the supervisor in this scenario) may only have the right to delegate a limited set of access rights to consultants.

Other features

Other new features of XACML 3.0 are listed at http://www.webfarmr.eu/2010/07/enhancements-and-new-features-in-xacml-3-axiomatics/

The XACML TC is also publishing a list of changes here: http://wiki.oasis-open.org/xacml/DifferencesBetweenXACML2.0AndXACML3.0

See also

• Role-based access control
• Mandatory access control
• Discretionary access control
• PERMIS
• GeoXACML
• HERAS-AF
• TAS3 - Specifies 4-point authorization usng XACML

Notes

External links

• eXtensible Access Control Markup Language
• HERAS-AF: An Open Source Project providing an XACML-based Security Framework
• JBoss XACML - Open Source LGPL licensed library
• OASIS XACML committee website
• OASIS declaration of issues with two software patents of IBM
• XACML 2.0 PDP and PAP implemented as Axis2 web services
• XACML authorization for many PAM enabled applications
• SICSACML XACML
• SunXACML
• ZXID http://zxid.org