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XACML policy language structure and syntax

XACML policy language structure and syntax¶

In order to render an authorization decision, it is possible to combine the two separate policies to form the single policy applicable to the request.

XACML defines three top-level policy elements:

Element that contains a boolean expression that can be evaluated in isolation, but that is not intended to be accessed in isolation by a PDP. So, it is not intended to form the basis of an authorization decision by itself. It is intended to exist in isolation only within a XACML PAP, where it may form the basic unit of management.
Element that contains a set of elements and a specified procedure for combining the results of their evaluation. It is the basic unit of policy used by the PDP, and so it is intended to form the basis of an authorization decision.
Element that contains a set of or other elements and a specified procedure for combining the results of their evaluation. It is the standard means for combining separate policies into a single combined policy.

As XACML is used in Attribute-based Access Controlling, in XACML all the attributes are categorized into the following four main categories: But from XACML 3.0, custom categories are also supported.

Subject
Resource
Action
Environment

A Rule is the most elementary unit of policy. It may exist in isolation only within one of the major actors of the XACML domain. The main components of a Rule are as follows:

<Target>- This defines the set of requests to which the rule is intended to apply in the form of a logical expression on attributes in the request.
<Effect>- The effect of the rule indicates the rule-writer's intended consequence of a "True" evaluation of the rule. Two values are allowed: "Permit" and "Deny".
<Condition>- A Boolean expression that refines the applicability of the rule beyond the predicates implied by its target. Therefore, it may be absent.
<ObligationExpressions>- Obligation expressions may be added by the writer of the policy. When a PDP evaluates a policy containing obligation expressions, it evaluates the obligation expressions into obligations and returns certain of those obligations to the PEP in the response context.
<AttributeSelector>- This allows the policy to specify an attribute with given identifier, category and data type. The AttributeSelector on the other hand, provides a mean to lookup the value of attributes using a XPath query by specifying the data type and XPath expression. Attribute selectors are then executed against the XML content that may have been sent along in the initial XACML request.

A sample XACML 3.0 policy is as follows:

<Policy xmlns="urn:oasis:names:tc:xacml:3.0:core:schema:wd-17" PolicyId="samplePolicy" RuleCombiningAlgId="urn:oasis:names:tc:xacml:3.0:rule-combining-algorithm:deny-overrides" Version="1.0">
    <Target>
        <AnyOf>
            <AllOf>
                <Match MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
                    <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">read</AttributeValue>
                    <AttributeDesignator AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id" Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action" DataType="http://www.w3.org/2001/XMLSchema#string" MustBePresent="true"/>
                </Match>
            </AllOf>
        </AnyOf>
    </Target>
    <Rule Effect="Permit" RuleId="permit"/>
</Policy>

Improvements in XACML 3.0¶

The XACML 3.0 core specification highlights the following main changes in comparison with XACML 2.0.

Custom attribute categories

Custom attribute categories can be defined with XACML 3.0. However, in XACML 2.0, attributes have been organized into subject, resource, environment or action. For instance, lets's say that you want to create an attribute category called “foo” in your policy and request. You can do it with XACML 3.0 easily. According to the XACML 3.0 policy schema, the category of XACML element is identified by a XML attribute called “ Category ”.

In XACML 2.0 Policy, you can define the attribute designator element as follows, However, it must be a pre-defined category such as subject, resource, environment or action.

<ResourceAttributeDesignator AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id" DataType="http://www.w3.org/2001/XMLSchema#string"/>

In a XACML 3.0 Policy, you can define it as follows. Category can be anything as it is defined as an attribute of the AttributeDesignator element.

<AttributeDesignator AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id" Category="resource" DataType="http://www.w3.org/2001/XMLSchema#string"/>

Improvements in `Obligation`

In general, obligations can have the following:

An Obligation has an identifier, which is used to distinguish different types of obligations
An Obligation can have arguments
Obligations apply to Permit (or Deny) decisions only

A PDP will return, as part of a Permit or Deny response, a (possibly empty) subset of the obligations that appear in the policy.

There are several improvements with Obligations in XACML 3.0 when compared to 2.0.

One of the main improvements is the introduction of Obligation Expressions. This adds dynamic expressions into the obligation statements. For a more in-depth understanding, see the following example:

Let's assume that you want to do following with the Obligation: “On deny, inform the PEP to send an email to the user”.

In XACML 2.0, you need to define the obligation element with the user email statically.

<Obligation ObligationId="send-email" FulfillOn="Deny">
    <AttributeAssignment AttributeId="email" DataType="http://www.w3.org/2001/XMLSchema#string">[email protected]</AttributeAssignment>
</Obligation>

However, the user may not be same for each XACML request that is evaluated. Therefore it is not possible to configure the email statically in the Obligation element. Obligation can only inform PEP to send an email to user (it lets the PEP figure out the value of user’s email).

<Obligation ObligationId="send-email" FulfillOn="Deny">
    <AttributeAssignment AttributeId="text" DataType="http://www.w3.org/2001/XMLSchema#string">please send email to user</AttributeAssignment>
</Obligation>

However, in XACML 3.0, the email of each user can be retrieved using PIP in dynamically as we can define an expression element inside the ObligationExpression . Therefore, obligation can inform PEP to send an email to user@foo.com address.

<ObligationExpression ObligationId="send-email" FulfillOn="Deny">
    <AttributeAssignmentExpression AttributeId="email" DataType="http://www.w3.org/2001/XMLSchema#string">
        <AttributeDesignator AttributeId="email" Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject" DataType="http://www.w3.org/2001/XMLSchema#string" MustBePresent="false"/>
    </AttributeAssignmentExpression>
</ObligationExpression>

In XACML 2.0, obligations can only be added to policies and policy sets. However, with XACML 3.0, rules can also contain obligations. At the root of all XACML policies, is a policy or a policy set . A policy represents a single access control policy, expressed through a set of rules. A policy set is a container that can hold other policies or policy sets, as well as references to policies found in remote locations.

Introducing `Advice`

Advice is a newly introduced feature with XACML 3.0. Advice is similar to obligations and it shares much of its syntax. The difference is contractual: the PEP can disregard any advice it receives. PEPs do not have to comply with advice statements; PEPs can consider or discard the statement. A common scenario is to explain why something was denied: “User, Alex is denied because Alex does not have a valid email”.

The XACML specification says that any advice returned with a decision can be safely ignored by compliant PEPs. This means that PEPs should work as described in the previous section, regardless of what the PEP does with the advice it may receive. For example, a PEP must allow access if it receives a Permit decision with no obligations, regardless of any advice in the decision.

Improvements in `Target`

Since it is possible to define custom attribute categories, there are improvements in some related elements in the XACML 3.0 policy, when compared to 2.0. With XACML 3.0, the Target element can be used to define more complex authorization logic within itself when compared with 2.0.

In XACML 2.0, the Target element contains the OR and AND relationship between the same category. However, in XACML 3.0, AllOf and AnyOf elements have been introduced to Target element. That clearly helps to define the OR and AND relationship between different categories.

As an example, let's look at a Target element. In XACML 2.0, we have an AND relationship between foo1 and foo2 resources and an OR relationship between bar1 and bar2 actions. However, we cannot create an OR relationship between a foo1 resource and bar1 action. so we cannot define something such as “Target would be matched when Alex can access the foo resource or do a bar action” by using the Target element.

<Target>
     <Resources>
         <Resource>
             <ResourceMatch MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
                 <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">foo1</AttributeValue>
                 <ResourceAttributeDesignator AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id" DataType="http://www.w3.org/2001/XMLSchema#string"/>
             </ResourceMatch>
             <ResourceMatch MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
                 <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">foo2</AttributeValue>
                 <ResourceAttributeDesignator AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id" DataType="http://www.w3.org/2001/XMLSchema#string"/>
             </ResourceMatch>
         </Resource>
     </Resources>
     <Actions>
         <Action>
             <ActionMatch MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
                 <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">bar1</AttributeValue>
                 <ActionAttributeDesignator AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id" DataType="http://www.w3.org/2001/XMLSchema#string"/>
             </ActionMatch>
         </Action>
         <Action>
             <ActionMatch MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
                 <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">bar2</AttributeValue>
                 <ActionAttributeDesignator AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id" DataType="http://www.w3.org/2001/XMLSchema#string"/>
             </ActionMatch>
         </Action>
     </Actions>
</Target>

XACML 3.0 has an AND relationship between “ foo ” resource and bar1 role and an OR relationship between bar2 action. So we cannot define something as “Target would be matched, when Alex can access foo resource and do bar1 action or do bar2 action”.

<Target>
     <AnyOf>
         <AllOf>
             <Match MatchId="urn:oasis:names:tc:xacml:1.0:function:string-regexp-match">
                 <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">foo</AttributeValue>
                 <AttributeDesignator MustBePresent="false" Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource" AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id" DataType="http://www.w3.org/2001/XMLSchema#string"/>
             </Match>
             <Match MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
                 <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">bar1</AttributeValue>
                 <AttributeDesignator MustBePresent="false" Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action"
 AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id" DataType="http://www.w3.org/2001/XMLSchema#string"/>
             </Match>
         </AllOf>
         <AllOf>
             <Match MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
                 <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">bar2</AttributeValue>
                 <AttributeDesignator MustBePresent="false" Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action"
 AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id" DataType="http://www.w3.org/2001/XMLSchema#string"/>
             </Match>
         </AllOf>
     </AnyOf>
</Target>

More Functions and Algorithms

XACML3 has introduced new String functions such as:

urn:oasis:names:tc:xacml:3.0:function:string-starts-with
urn:oasis:names:tc:xacml:3.0:function:string-ends-with
urn:oasis:names:tc:xacml:3.0:function:string-contains
urn:oasis:names:tc:xacml:3.0:function:string-substring

Some improvements to other functions such as:

urn:oasis:names:tc:xacml:3.0:function:dayTimeDuration-equal
urn:oasis:names:tc:xacml:3.0:function:yearMonthDuration-equal
urn:oasis:names:tc:xacml:3.0:function:dateTime-add-dayTimeDuration

Also improvements to existing combine algorithms (deny-overrides, permit-overrides, ordered-deny-overrides and ordered-permit-overrides) and new two combine algorithms for policy and rule combining.

urn:oasis:names:tc:xacml:3.0:rule-combining-algorithm:deny-unless-permit
urn:oasis:names:tc:xacml:3.0:policy-combining-algorithm:deny-unless-permit
urn:oasis:names:tc:xacml:3.0:rule-combining-algorithm:permit-unless-deny
urn:oasis:names:tc:xacml:3.0:policy-combining-algorithm:permit-unless-deny

Improvements in XPath

New XPath data type is introduced with XACML 3.0. In XACML 2.0, XPath is defined as a String and cannot define the context that the namespace prefix is going to resolve. Also XPath based multiple decisions scheme is introduced with XACML 3.0.

Improvement in XACML Request and Response

As it is possible to define custom attribute categories, many types of attribute categories can be in the XACML 3.0 request. XACML 2.0 request can contain only subject, resource, environment or action categories.

The XACML Response can contain additional data such as:

Request attributes that are defined in the XACML response.
Applicable policy ids for a given XACML request are defined in the XACML response.

XACML 3.0 Multiple Decision Profile

This is a useful profile that allows you to request more than one access control decision in a single XACML request context, and also allows you to request a single combined decision based on multiple individual decisions.

The XACML 3.0 Multiple Decision Profile (MDP) is particularly useful in scenarios where the PEP needs to request decisions for multiple requests in one XACML request, and provides a considerable improvement in performance between the PEP and the PDP in such scenarios.

For a set of tutorials that demonstrate how to work with XACML MDP requests in WSO2 Identity Server, see Working with XACML Multiple Decision Profile Requests .

XACML 3.0 JSON Profile

This is a new profile that provides a standardized interface between the PEP and the PDP using JSON. The decision request and response structure is specified in the core XACML specification.

With the introduction of the XACML 3.0 JSON profile , WSO2 Identity Server supports the JSON format in addition to the default XML format with regard to XACML 3.0 requests and responses.

Tip

WSO2 Identity Server also supports working with XACML MDP requests and responses in JSON format .

Following are some of the key points to keep in mind when you work with XACML 3.0 JSON requests and responses via WSO2 Identity Server:

Certain parts of JSON requests and responses have default values to avoid bloating. For example, the default value of the data-type of an attribute should be string.
The name of the XACML XML attribute element has changed to the category object in JSON so that it is possible to call the parent element.
The <AttributeValue> element in the XML representation no longer exists. Instead a value property is introduced to the attribute object.
The AdviceId and the ObligationId attributes of <Advice/> and <Obligation/> XML elements are renamed to Id in JSON.
The order of objects and values in the serialized form (JSON) does not matter in XACML.

You can use the short name of identifiers instead of the URI. Following are the supported identifier URIs and the short name for each:

Identifier URI	Short name
urn:oasis:names:tc:xacml:3.0:attribute-category:resource	Resource
urn:oasis:names:tc:xacml:3.0:attribute-category:action	Action
urn:oasis:names:tc:xacml:3.0:attribute-category:environment	Environment
urn:oasis:names:tc:xacml:3.0:attribute-category:access-subject	AccessSubject
urn:oasis:names:tc:xacml:3.0:attribute-category:recipient-subject	RecipientSubject
urn:oasis:names:tc:xacml:3.0:attribute-category:intermediary-subject	IntermediarySubject
urn:oasis:names:tc:xacml:3.0:attribute-category:codebase	Codebase
urn:oasis:names:tc:xacml:3.0:attribute-category:requesting-machine	RequestingMachine

The JSON format supports the fully qualified XACML data-type URI, and also supports the short name of the data-type.

XACML data type identifier	JSON shorthand type code	Mapping/inference rule
`http://www.w3.org/2001/XMLSchema#string`	string	JSON "String"
`http://www.w3.org/2001/XMLSchema#boolean`	boolean	JSON "Boolean"
`http://www.w3.org/2001/XMLSchema#integer`	integer	JSON "Number" without a fractional portion, and within the integer range defined by the XML schema in [XMLDatatypes]
`http://www.w3.org/2001/XMLSchema#double`	double	JSON "Number" with a fractional portion, or out of the integer range defined by the XML schema in [XMLDatatypes]
`http://www.w3.org/2001/XMLSchema#time`	time	None. Inference must fail.
`http://www.w3.org/2001/XMLSchema#date`	date	None. Inference must fail.
`http://www.w3.org/2001/XMLSchema#dateTime`	dateTime	None. Inference must fail.
`http://www.w3.org/2001/XMLSchema\#dayTimeDuration`	dayTimeDuration	None. Inference must fail.
`http://www.w3.org/2001/XMLSchema\#yearMonthDuration`	yearMonthDuration	None. Inference must fail.
`http://www.w3.org/2001/XMLSchema\#anyURI`	anyURI	None. Inference must fail.
`http://www.w3.org/2001/XMLSchema\#hexBinary`	hexBinary	None. Inference must fail.
`http://www.w3.org/2001/XMLSchema\#base64Binary`	base64Binary	None. Inference must fail.
urn:oasis:names:tc:xacml:1.0:data-type:rfc822Name	rfc822Name	None. Inference must fail.
urn:oasis:names:tc:xacml:1.0:data-type:x500Name	x500Name	None. Inference must fail.
urn:oasis:names:tc:xacml:1.0:data-type:ipAddress	ipAddress	None. Inference must fail.
urn:oasis:names:tc:xacml:1.0:data-type:dnsName	dnsName	None. Inference must fail.
urn:oasis:names:tc:xacml:1.0:data-type:xpathExpression	xpathExpression	None. Inference must fail.

xpathExpression data-type values are represented as JSON objects, and each object contains the following properties:

Attribute	Type	Required	Default value
XPathCategory	URI	Yes. You can use the short names defined for identifier URIs as values here.	None
Namespaces	Array of namespace declarations	No	None
XPath	String	Yes	None

The namespaces property should contain the following properties:

Attribute	Type	Required	Default value
Prefix	String	No	None
Namespace	URI	Yes	None

Following is a sample JSON attribute format that contains the fully qualified XACML data-type URI:

{
    "Attribute": {
       "AttributeId": "urn:oasis:names:tc:xacml:3.0:content-selector",
       "DataType": "xpathExpression",
       "Value": {
               "XPathCategory": "urn:oasis:names:tc:xacml:3.0:attribute-category:resource",
               "Namespaces": [{
                       "Namespace":   "urn:oasis:names:tc:xacml:3.0:core:schema:wd-17"
                       }, {
                       "Prefix": "md",
                       "Namespace":   "urn:example:med:schemas:record"
                           }],
               "XPath":"md:record/md:patient/md:patientDoB"
        }
   }
}

The MultiRequests object is optional in the JSON representation of XACML. The purpose of the MultiRequests object is to support the XACML multiple decision profile.

The JSON attribute object contains an array of attribute objects. The attribute object contains the following properties:

Property name	Type	Required	Default value
AttributeId	URI	Yes	None. The identifier used in the XML representation of a XACML attribute will be used in its JSON representation
Value	Either string, boolean, number (this maps to either a XACML integer or double as defined in supported data types), object, array of strings, array of boolean, Array of number, array of object, or a mixed array of string and number where the string values represents a numerical value.	Yes	None.
Issuer	String	No	Null
Data Type	URI	No	The data type value can be omitted in the JSON representation. The default value is `http://www.w3.org/2001/XMLSchema#string` .
IncludeInResult	Boolean	No	False

The results of the JSON request is represented by the decision object in the form of a JSON Object. This can have following properties:

Property name Type Required Default value

Decision String Yes. Possible values are: Permit, Deny, NotApplicable, Indeterminate None.
The results can have the status, obligations, associatedAdvice, category, and policyIdentifierList, which are optional.

Following is a sample JSON request that you can try out with WSO2 Identity Server:

{
   "Request": {
      "AccessSubject": {
         "Attribute": [
            {
               "AttributeId": "subject-id",
               "Value": "sam",
               "DataType": "string",
               "IncludeInResult": true
            }
         ]
      },

      "Resource": {
         "Attribute": [
            {
               "AttributeId": "resource-id",
               "Value": "index.jsp",
               "DataType": "string",
               "IncludeInResult": true
            }
         ]
      },
      "Action": {
            "Attribute": [{
                    "AttributeId": "action-id",
                    "Value": "modify-welcome",
                    "DataType": "string",
                    "IncludeInResult": true
                }
            ]
        }

   }
}

Following is a sample XACML JSON response that you will get for the above request:

{
    "Response": [
        {
            "Decision": "Deny",
            "Status": {
                "StatusCode": {
                    "Value": "urn:oasis:names:tc:xacml:1.0:status:ok"
                }
            },
            "AccessSubject": {
                "Attribute": [
                    {
                        "AttributeId": "subject-id",
                        "Value": "dinali",
                        "IncludeInResult": "true",
                        "DataType": "string"
                    }
                ]
            },
            "Resource": {
                "Attribute": [
                    {
                        "AttributeId": "resource-id",
                        "Value": "index.jsp",
                        "IncludeInResult": "true",
                        "DataType": "string"
                    }
                ]
            },
            "Action": {
                "Attribute": [
                    {
                        "AttributeId": "action-id",
                        "Value": "modify-welcome",
                        "IncludeInResult": "true",
                        "DataType": "string"
                    }
                ]
            }
        }
    ]
}

For a tutorial that demonstrate how WSO2 IS supports fine-grained authorization using XACML requests in JSON format, see Fine-grained Authorization using XACML Requests in JSON Format.

Administrative Delegation Profile

This is also a new profile that comes with XACML 3.0. This allows you to define policies about who can write policies about what. For example, "Alex may issue a policy but only about resources in department X”.

XACML 2.0 and XACML 3.0 samples¶

The biggest difference between XACML 2.0 and XACML 3.0 for your client app is that the structure of the attributes in the authentication request has changed significantly in XACML 3.0.

In XACML 2.0, attributes were organized into subject, resource, environment, or action categories using XML element tags:

<Request  xmlns="urn:oasis:names:tc:xacml:2.0:context:schema:os"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="urn:oasis:names:tc:xacml:2.0:context:schema:os  access_control-xacml-2.0-context-schema-os.xsd">
        <Subject>
            <Attribute
                  AttributeId="urn:oasis:names:tc:xacml:1.0:subject:subject-id"
                  DataType="http://www.w3.org/2001/XMLSchema#string">
                <AttributeValue>Julius Hibbert</AttributeValue>
            </Attribute>
        </Subject>
        <Resource>
            <Attribute
                  AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id"
                  DataType="http://www.w3.org/2001/XMLSchema#anyURI">
                <AttributeValue>http://medico.com/record/patient/BartSimpson</AttributeValue>
            </Attribute>
        </Resource>
        <Action>
            <Attribute
                  AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id"
                  DataType="http://www.w3.org/2001/XMLSchema#string">
                <AttributeValue>read</AttributeValue>
            </Attribute>
        </Action>
        <Environment/>
</Request>

In XACML 3.0, these categories are indicated using XML attributes instead of XML element tags:

<?xml version="1.0" encoding="utf-8"?>
<Request xsi:schemaLocation="urn:oasis:names:tc:xacml:3.0:core:schema:wd-17 http://docs.oasis-open.org/xacml/3.0/xacml-core-v3-schema-wd-17.xsd" ReturnPolicyIdList="false" CombinedDecision="false" xmlns="urn:oasis:names:tc:xacml:3.0:core:schema:wd-17" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Attributes Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject">
    <Attribute IncludeInResult="false" AttributeId="urn:oasis:names:tc:xacml:1.0:subject:subject-id">
      <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">Julius Hibbert</AttributeValue>
    </Attribute>
  </Attributes>
  <Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource">
    <Attribute IncludeInResult="false" AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id">
      <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#anyURI">http://medico.com/record/patient/BartSimpson</AttributeValue>
    </Attribute>
  </Attributes>
  <Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action">
    <Attribute IncludeInResult="false" AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id">
      <AttributeValue DataType="http://www.w3.org/2001/XMLSchema#string">read</AttributeValue>
    </Attribute>
  </Attributes>
  <Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:environment" />
</Request>

The <Subject> element in XACML 2.0 becomes <Attributes Category="urn:oasis:names:tc:xacml:1.0:subject-category:access-subject"> in XACML 3.0, for example. This is the same for the resource, environment, and action categories.

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XACML policy language structure and syntax