The eXtensible Access Control Markup Language (XACML) is an XML-based standard markup language for specifying access control policies. The standard, published by OASIS, defines a declarative fine-grained, attribute-based access control policy language, an architecture, and a processing model describing how to evaluate access requests according to the rules defined in policies.[2]
XACML is primarily an attribute-based access control system. In XACML, attributes – information about the subject accessing a resource, the resource to be addressed, and the environment – act as inputs for the decision of whether access is granted or not.[3] XACML can also be used to implement role-based access control.[4]
In XACML, access control decisions to be taken are expressed as Rules. Each Rule comprises a series of conditions which decide whether a given request is approved or not. If a Rule is applicable to a request but the conditions within the Rule fail to evaluate, the result is Indeterminate. Rules are grouped together in Policies, and a PolicySet contains Policies and possibly other PolicySets. Each of these also includes a Target, a simple condition that determines whether it should be evaluated for a given request. Combining algorithms can be used to combine Rules and Policies with potentially differing results in various ways. XACML also supports obligations and advice expressions. Obligations specify actions which must be executed during the processing of a request, for example for logging. Advice expressions are similar, but may be ignored.[3]
XACML separates access control functionality into several components. Each operating environment in which access control is used has a Policy Enforcement Point (PEP) which implements the functionality to demand authorization and to grant or deny access to resources. These refer to an environment-independent and central Policy Decision Point (PDP) which actually makes the decision on whether access is granted. The PDP refers to policies stored in the Policy Retrieval Point (PRP). Policies are managed through a Policy Administration Point (PAP).[3]
Version 3.0 was ratified by OASIS in January 2013.[5]
Version 2.0 was ratified by OASIS standards organization on February 1, 2005.[citation needed]
Version 3.0 was ratified by OASIS in January 2013.
Architecture
Terminology
Non-normative terminology (following RFC 2904, except for PAP)
Abbr.
Term
Description
PAP
Policy Administration Point
Point which manages access authorization policies
PDP
Policy Decision Point
Point which evaluates access requests against authorization policies before issuing access decisions
PEP
Policy Enforcement Point
Point which intercepts user's access request to a resource, makes a decision request to the PDP to obtain the access decision
(i.e. access to the resource is approved or rejected), and acts on the received decision
PIP
Policy Information Point
The system entity that acts as a source of attribute values (i.e. a resource, subject, environment)
PRP
Policy Retrieval Point
Point where the XACML access authorization policies are stored, typically a database or the filesystem.
Flow
This image shows the XACML architecture and a sample authorization flow.
A user sends a request which is intercepted by the Policy Enforcement Point (PEP)
The PEP converts the request into a XACML authorization request
The PEP forwards the authorization request to the Policy Decision Point (PDP)
The PDP evaluates the authorization request against the policies it is configured with. The policies are acquired via the Policy Retrieval Point (PRP) and managed by the Policy Administration Point (PAP). If needed it also retrieves attribute values from underlying Policy Information Points (PIP).
The PDP reaches a decision (Permit / Deny / NotApplicable / Indeterminate) and returns it to the PEP
Policy elements
Structural elements
XACML is structured into 3 levels of elements:
PolicySet,
Policy,
Rule.
A policy set can contain any number of policy elements and policy set elements. A policy can contain any number of rule elements.
Attributes and categories
Policies, policy sets, rules and requests all use subjects, resources, environments, 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 one or more attributes.
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.
Targets
XACML provides a target, which is basically a set of simplified conditions for the subject, resource, and action that must be met for a policy set, policy, or rule to apply to a given request. Once a policy or policy set is found to apply to a given request, its rules are evaluated to determine the access decision and response.
In addition to being a way to check applicability, target information also provides a way to index policies, which is useful if you need to store many policies and then quickly sift through them to find which ones apply. When a request to access that service arrives, the PDP will know where to look for policies that might apply to this request because the policies are indexed based on their target constraints. Note that a target may also specify that it applies to any request.
Policy set, policy and rule can all contain target elements.
Conditions
Conditions only exist in rules. Conditions are essentially an advanced form of a target which can use a broader range of functions and more importantly can be used to compare two or more attributes together, e.g. subject-id==doctor-id. With conditions, it is possible to implement segregation of duty checks or relationship-based access control.
Obligations & Advice
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 approved. If the PEP is unable to comply with the directive, the approved 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.
Obligations can be used for "break-the-glass" scenarios or trust elevation ("you cannot transfer $1,000 without two-factor authentication - here is the link to the 2FA page").
In addition to obligations, XACML supports advice which are identical to obligations with the difference that a PEP is not obligated to enforce the advice (hence its name).
Combining algorithms
What happens in XACML if there are two rules (or policies) that contradict each other? Imagine for instance a first rule that would say managers can view documents and a second rule that would say no one can work before 9am. What if the request is about Alice trying to view a document at 8am? Which rule wins? This is what combining algorithms tell us. They help resolve conflicts.
XACML defines a number of combining algorithms that can be identified by a RuleCombiningAlgId or PolicyCombiningAlgId attribute of the <Policy> or <PolicySet> elements, respectively. The rule-combining algorithm defines a procedure for arriving at an access decision given the individual results of evaluation of a set of rules. Similarly, the policy-combining algorithm defines a procedure for arriving at an access decision given the individual results of evaluation of a set of policies.
Functions
XACML defines a long list of functions (close to 300) to manipulate and compare attributes to other attributes and values:
Equality, inequality and other matching functions
Arithmetic functions
String functions
Logical functions (and, or, not)
Set and bag functions
Higher order functions
Regular expression functions
XPath functions
The functions and their identifiers are fully described in the standard. Functions are type-specific i.e. there is a function for string equality and a different one for integer equality.
Equality, inequality and other matching functions
Arithmetic functions
Refer to the standard for a formal definition of these function.
add (double and integer)
subtract (double and integer)
multiply (double and integer)
divide (double and integer)
mod (double and integer)
abs (double and integer)
round
floor
String functions
Refer to the standard for a formal definition of these function.
string-concatenate
string-starts-with
string-ends-with
string-contains
string-substring
Logical functions (and, or, not)
Set and bag functions
Regular expression functions
XPath functions
Higher order functions
The list of higher order functions is as listed below. For a formal definition, refer to the XACML standard.
Description: this function takes in a boolean function and 2 or more attribute values or bags. The higher-order function applies the boolean function to the remaining parameters.
Example: anyOf(function[stringEqual], allowedRoles, stringOneAndOnly(role)) will return true if (a) role is single-valued, (b) there is at least one value in the attribute bag allowedRoles equal to the value inside the single-valued attribute bag role.
XACML 3.0 introduces administrative delegation, the JSON Profile of XACML (request/response), the REST Profile of XACML, the Multiple Decision Profile of XACML, and many more.
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. 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, authorizing 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
Sample policies
Date and time-based authorization in XACML
This rule implements the use-it-lose-it access control paradigm. If a user does not log in for 30 days, then they lose access.
In pseudo-code: deny if currentDateTime > lastLogin + 30 days
<Rule RuleId="f6637b3f-3690-4cce-989c-2ce9c053d6fa" Effect="Deny"><Description>Use it or lose it: this policy denies access if lastLogin is more than 30 days away from today's date</Description><Target/><Condition><Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:any-of"><Function FunctionId="urn:oasis:names:tc:xacml:1.0:function:dateTime-greater-than"/><Apply FunctionId="urn:oasis:names:tc:xacml:3.0:function:dateTime-add-dayTimeDuration"><Apply FunctionId="urn:oasis:names:tc:xacml:1.0:function:dateTime-one-and-only"><AttributeDesignator
Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject"
AttributeId="com.acme.user.lastLogin"
DataType="http://www.w3.org/2001/XMLSchema#dateTime" MustBePresent="false"/></Apply><AttributeValue DataType="http://www.w3.org/2001/XMLSchema#dayTimeDuration">P30D</AttributeValue></Apply><AttributeDesignator
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:environment"
AttributeId="urn:oasis:names:tc:xacml:1.0:environment:current-dateTime"
DataType="http://www.w3.org/2001/XMLSchema#dateTime" MustBePresent="false"/></Apply></Condition></Rule>
Time-based authorization in XACML
This rule grants access if the current time is greater than 9am and less than 5pm.
<Rule RuleId="c01d7519-be21-4985-88d8-10941f44590a" Effect="Permit"><Description>Allow if time between 9 and 5</Description><Target><AnyOf><AllOf><Match MatchId="urn:oasis:names:tc:xacml:1.0:function:time-greater-than"><AttributeValue DataType="http://www.w3.org/2001/XMLSchema#time">09:00:00</AttributeValue><AttributeDesignator
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:environment"
AttributeId="urn:oasis:names:tc:xacml:1.0:environment:current-time"
MustBePresent="false"
DataType="http://www.w3.org/2001/XMLSchema#time"/></Match></AllOf></AnyOf><AnyOf><AllOf><Match MatchId="urn:oasis:names:tc:xacml:1.0:function:time-less-than"><AttributeValue DataType="http://www.w3.org/2001/XMLSchema#time">17:00:00</AttributeValue><AttributeDesignator
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:environment"
AttributeId="urn:oasis:names:tc:xacml:1.0:environment:current-time"
MustBePresent="false"
DataType="http://www.w3.org/2001/XMLSchema#time"/></Match></AllOf></AnyOf></Target></Rule>
The following contains an Obligation block. Obligations are statements that can be returned along with a decision to enrich the decision flow. In this example, the PEP must log that access was granted.
By default a PDP processes a single request at a time e.g. "Can Alice view item #1?". The PDP then replies with a single decision. At times, though, it is necessary to send multiple requests in one go e.g. "Can Alice view / edit / delete items #1, #2, #3?". The Multiple Decision Profile of XACML allows for this use case. The PDP will typically do the product of all combinations i.e. in the example aforementioned there will be 1 x 3 x 3 = 9 decisions returned in a single response.
Sample JSON Request for the Multiple Decision Profile
The way to enable the MDP is to send an array of objects for any of the categories rather than an array of one object (or simply an object). For instance, AccessSubject is an object but Resource is an array of objects. The latter will trigger the MDP process in PDPs that support the profile. Note as well the use of the IncludeInResult attribute which tells the PDP to return the XACML attribute and its value in the response so that decisions can be correlated to the relevant attribute values.
In 2013 and 2014, the XACML Technical Committee focused on designing new profiles to facilitate developer integration. These include:
The REST profile of XACML written by Remon Sinnema of EMC
The JSON profile of XACML written by David Brossard of Axiomatics
The ALFA profile of XACML written by Pablo Giambiagi, Srijith Nair, and David Brossard of Axiomatics
All three profiles were showcased at the Cloud Identity Summit 2014 in Monterey, California. Using these profiles, integrating fine-grained authorization into applications becomes much easier.
The ALFA Profile of XACML
ALFA stands for Abbreviated Language for Authorization. It is a lightweight syntax used to implement policy-based access control policies. For examples refer to the main article.
The JSON Profile of XACML
The JSON profile of XACML simplifies the integration between the PEP and the PDP.
XACML is almost entirely a policy definition language based on XML and XSLT, defined by an open OASIS specification. The XACML specification does not cover the design or implementation of Policy Decision Point (PDP), only the policy language they consume. Many proprietary and open-source PDPs use XACML as their policy definition language.
Open Policy Agent (OPA) is an open-source Policy Decision Point (PDP) implementation, capable of interpreting policy language to render policy decisions. OPA is a general-purpose PDP implementation which can be used for any scenario where a policy decision is required, much like PDP implementations that support the XACML specification.
OPA's policy definition language is (Rego), which is a JSON-based, Turing-incomplete language based on Datalog.
Policies written in XACML can be translated to Rego, and vice-versa.
XACML and SAML
Example of cross-enterprise federation where two enterprises federate using SAML and two Security Token Services (STS), establishing a circle of trust. The SAML in this picture is used for identity exchange / virtualization. The XACML is used server-side to determine whether access should be granted to the application functionality (functional access control) and the data behind it (data access control).
SAML is an identity SSO and federation standard used for authentication. SAML is used as a common identity token format between different applications. SAML and XACML are both defined by OASIS. SAML and XACML were designed to interoperate where SAML is used to carry identity information / virtual identities and XACML is used to drive the access control logic through policies.
XACML and OAuth
OAuth 2.0 is considered to be an authorization standard. It differs from XACML though in its origin, its purpose, and its applications. OAuth is about:
delegated access control: I, the user, delegate another user or service access to the resource I own. For instance via OAuth, I grant Twitter (the service) the ability to post on my Facebook wall (the resource).
handling the password anti-pattern. Whenever you want to integrate 2 services together, in a traditional, legacy model you have to provide service B with your user credentials on service A so that service B can pretend to be you with Service A. This has many risks of course. Using OAuth eliminates the issues with these patterns and lets the user control what service B can do on behalf of the user with service A.
HTTP-based services / resources
managing owner (user) approval
XACML does not handle user approval or delegated access or password management. XACML simply provides:
An access control architecture with the notion of a Policy Decision Point (PDP) as previously discussed and a Policy Enforcement Point (PEP).
a policy language with which to express a wide range of access control policies including policies that can use consents handled / defined via OAuth.
XACML and OAuth can be combined together to deliver a more comprehensive approach to authorization.
^See for example De la Rosa Algarín, Alberto; Ziminski, Timoteus B.; Demurjian 1, Steven A.; Kuykendall, Robert; Rivera Sánchez, Yaira K. (2013). Defining and Enforcing XACML Role-based Security Policies within an XML Security Framework. Proceedings of the 9th International Conference on Web Information Systems and Technologies. doi:10.5220/0004366200160025.((cite conference)): CS1 maint: numeric names: authors list (link)
