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RFC 3744

Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol

Pages: 72
Proposed Standard
Errata
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Network Working Group                                           G. Clemm
Request for Comments: 3744                                           IBM
Category: Standards Track                                     J. Reschke
                                                              greenbytes
                                                               E. Sedlar
                                                      Oracle Corporation
                                                            J. Whitehead
                                                         U.C. Santa Cruz
                                                                May 2004


           Web Distributed Authoring and Versioning (WebDAV)
                        Access Control Protocol

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

This document specifies a set of methods, headers, message bodies, properties, and reports that define Access Control extensions to the WebDAV Distributed Authoring Protocol. This protocol permits a client to read and modify access control lists that instruct a server whether to allow or deny operations upon a resource (such as HyperText Transfer Protocol (HTTP) method invocations) by a given principal. A lightweight representation of principals as Web resources supports integration of a wide range of user management repositories. Search operations allow discovery and manipulation of principals using human names.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Terms. . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Notational Conventions . . . . . . . . . . . . . . . . . 8 2. Principals . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3. Privileges . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. DAV:read Privilege . . . . . . . . . . . . . . . . . . . 10 3.2. DAV:write Privilege. . . . . . . . . . . . . . . . . . . 10 3.3. DAV:write-properties Privilege . . . . . . . . . . . . . 10 3.4. DAV:write-content Privilege. . . . . . . . . . . . . . . 11 3.5. DAV:unlock Privilege . . . . . . . . . . . . . . . . . . 11 3.6. DAV:read-acl Privilege . . . . . . . . . . . . . . . . . 11 3.7. DAV:read-current-user-privilege-set Privilege. . . . . . 12 3.8. DAV:write-acl Privilege. . . . . . . . . . . . . . . . . 12 3.9. DAV:bind Privilege . . . . . . . . . . . . . . . . . . . 12 3.10. DAV:unbind Privilege . . . . . . . . . . . . . . . . . . 12 3.11. DAV:all Privilege. . . . . . . . . . . . . . . . . . . . 13 3.12. Aggregation of Predefined Privileges . . . . . . . . . . 13 4. Principal Properties . . . . . . . . . . . . . . . . . . . . . 13 4.1. DAV:alternate-URI-set. . . . . . . . . . . . . . . . . . 14 4.2. DAV:principal-URL. . . . . . . . . . . . . . . . . . . . 14 4.3. DAV:group-member-set . . . . . . . . . . . . . . . . . . 14 4.4. DAV:group-membership . . . . . . . . . . . . . . . . . . 14 5. Access Control Properties. . . . . . . . . . . . . . . . . . . 15 5.1. DAV:owner. . . . . . . . . . . . . . . . . . . . . . . . 15 5.1.1. Example: Retrieving DAV:owner . . . . . . . . . . 15 5.1.2. Example: An Attempt to Set DAV:owner. . . . . . . 16 5.2. DAV:group. . . . . . . . . . . . . . . . . . . . . . . . 18 5.3. DAV:supported-privilege-set. . . . . . . . . . . . . . . 18 5.3.1. Example: Retrieving a List of Privileges Supported on a Resource . . . . . . . . . . . . . 19 5.4. DAV:current-user-privilege-set . . . . . . . . . . . . . 21 5.4.1. Example: Retrieving the User's Current Set of Assigned Privileges . . . . . . . . . . . . . . . 22 5.5. DAV:acl. . . . . . . . . . . . . . . . . . . . . . . . . 23 5.5.1. ACE Principal . . . . . . . . . . . . . . . . . . 23 5.5.2. ACE Grant and Deny. . . . . . . . . . . . . . . . 25 5.5.3. ACE Protection. . . . . . . . . . . . . . . . . . 25 5.5.4. ACE Inheritance . . . . . . . . . . . . . . . . . 25 5.5.5. Example: Retrieving a Resource's Access Control List. . . . . . . . . . . . . . . . . . . . . . . 25 5.6. DAV:acl-restrictions . . . . . . . . . . . . . . . . . . 27 5.6.1. DAV:grant-only. . . . . . . . . . . . . . . . . . 27 5.6.2. DAV:no-invert ACE Constraint. . . . . . . . . . . 28 5.6.3. DAV:deny-before-grant . . . . . . . . . . . . . . 28 5.6.4. Required Principals . . . . . . . . . . . . . . . 28 5.6.5. Example: Retrieving DAV:acl-restrictions. . . . . 28
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       5.7.  DAV:inherited-acl-set. . . . . . . . . . . . . . . . . . 29
       5.8.  DAV:principal-collection-set . . . . . . . . . . . . . . 30
             5.8.1. Example: Retrieving DAV:principal-collection-set. 30
       5.9.  Example: PROPFIND to retrieve access control properties. 32
   6.  ACL Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 36
   7.  Access Control and existing methods. . . . . . . . . . . . . . 37
       7.1.  Any HTTP method. . . . . . . . . . . . . . . . . . . . . 37
             7.1.1. Error Handling. . . . . . . . . . . . . . . . . . 37
       7.2.  OPTIONS. . . . . . . . . . . . . . . . . . . . . . . . . 38
             7.2.1. Example - OPTIONS . . . . . . . . . . . . . . . . 39
       7.3.  MOVE . . . . . . . . . . . . . . . . . . . . . . . . . . 39
       7.4.  COPY . . . . . . . . . . . . . . . . . . . . . . . . . . 39
       7.5.  LOCK . . . . . . . . . . . . . . . . . . . . . . . . . . 39
   8.  Access Control Methods . . . . . . . . . . . . . . . . . . . . 40
       8.1.  ACL. . . . . . . . . . . . . . . . . . . . . . . . . . . 40
             8.1.1. ACL Preconditions . . . . . . . . . . . . . . . . 40
             8.1.2. Example: the ACL method . . . . . . . . . . . . . 42
             8.1.3. Example: ACL method failure due to protected
                    ACE conflict. . . . . . . . . . . . . . . . . . . 43
             8.1.4. Example: ACL method failure due to an
                    inherited ACE conflict. . . . . . . . . . . . . . 44
             8.1.5. Example: ACL method failure due to an attempt
                    to set grant and deny in a single ACE . . . . . . 45
   9.  Access Control Reports . . . . . . . . . . . . . . . . . . . . 46
       9.1.  REPORT Method. . . . . . . . . . . . . . . . . . . . . . 46
       9.2.  DAV:acl-principal-prop-set Report. . . . . . . . . . . . 47
             9.2.1. Example: DAV:acl-principal-prop-set Report. . . . 48
       9.3.  DAV:principal-match REPORT . . . . . . . . . . . . . . . 49
             9.3.1. Example: DAV:principal-match REPORT . . . . . . . 50
       9.4.  DAV:principal-property-search REPORT . . . . . . . . . . 51
             9.4.1. Matching. . . . . . . . . . . . . . . . . . . . . 53
             9.4.2. Example: successful DAV:principal-property-search
                    REPORT. . . . . . . . . . . . . . . . . . . . . . 54
       9.5.  DAV:principal-search-property-set REPORT . . . . . . . . 56
             9.5.1. Example: DAV:principal-search-property-set
                    REPORT. . . . . . . . . . . . . . . . . . . . . . 58
   10. XML Processing . . . . . . . . . . . . . . . . . . . . . . . . 59
   11. Internationalization Considerations. . . . . . . . . . . . . . 59
   12. Security Considerations. . . . . . . . . . . . . . . . . . . . 60
       12.1. Increased Risk of Compromised Users. . . . . . . . . . . 60
       12.2. Risks of the DAV:read-acl and
             DAV:current-user-privilege-set Privileges. . . . . . . . 60
       12.3. No Foreknowledge of Initial ACL. . . . . . . . . . . . . 61
   13. Authentication . . . . . . . . . . . . . . . . . . . . . . . . 61
   14. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 62
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 62
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   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 62
       16.1. Normative References . . . . . . . . . . . . . . . . . . 62
       16.2. Informative References . . . . . . . . . . . . . . . . . 63
   Appendices
   A.  WebDAV XML Document Type Definition Addendum . . . . . . . . . 64
   B.  WebDAV Method Privilege Table (Normative). . . . . . . . . . . 67
   Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 71
   Full Copyright Statement. . . . . . . . . . . . .  . . . . . . . . 72

1. Introduction

The goal of the WebDAV access control extensions is to provide an interoperable mechanism for handling discretionary access control for content and metadata managed by WebDAV servers. WebDAV access control can be implemented on content repositories with security as simple as that of a UNIX file system, as well as more sophisticated models. The underlying principle of access control is that who you are determines what operations you can perform on a resource. The "who you are" is defined by a "principal" identifier; users, client software, servers, and groups of the previous have principal identifiers. The "operations you can perform" are determined by a single "access control list" (ACL) associated with a resource. An ACL contains a set of "access control entries" (ACEs), where each ACE specifies a principal and a set of privileges that are either granted or denied to that principal. When a principal submits an operation (such as an HTTP or WebDAV method) to a resource for execution, the server evaluates the ACEs in the ACL to determine if the principal has permission for that operation. Since every ACE contains the identifier of a principal, client software operated by a human must provide a mechanism for selecting this principal. This specification uses http(s) scheme URLs to identify principals, which are represented as WebDAV-capable resources. There is no guarantee that the URLs identifying principals will be meaningful to a human. For example, http://www.example.com/u/256432 and http://www.example.com/people/Greg.Stein are both valid URLs that could be used to identify the same principal. To remedy this, every principal resource has the DAV:displayname property containing a human-readable name for the principal. Since a principal can be identified by multiple URLs, it raises the problem of determining exactly which principal is being referenced in a given ACE. It is impossible for a client to determine that an ACE granting the read privilege to http://www.example.com/people/ Greg.Stein also affects the principal at http://www.example.com/u/ 256432. That is, a client has no mechanism for determining that two
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   URLs identify the same principal resource.  As a result, this
   specification requires clients to use just one of the many possible
   URLs for a principal when creating ACEs.  A client can discover which
   URL to use by retrieving the DAV:principal-URL property (Section 4.2)
   from a principal resource.  No matter which of the principal's URLs
   is used with PROPFIND, the property always returns the same URL.

   With a system having hundreds to thousands of principals, the problem
   arises of how to allow a human operator of client software to select
   just one of these principals.  One approach is to use broad
   collection hierarchies to spread the principals over a large number
   of collections, yielding few principals per collection.  An example
   of this is a two level hierarchy with the first level containing 36
   collections (a-z, 0-9), and the second level being another 36,
   creating collections /a/a/, /a/b/, ..., /a/z/, such that a principal
   with last name "Stein" would appear at /s/t/Stein.  In effect, this
   pre-computes a common query, search on last name, and encodes it into
   a hierarchy.  The drawback with this scheme is that it handles only a
   small set of predefined queries, and drilling down through the
   collection hierarchy adds unnecessary steps (navigate down/up) when
   the user already knows the principal's name.  While organizing
   principal URLs into a hierarchy is a valid namespace organization,
   users should not be forced to navigate this hierarchy to select a
   principal.

   This specification provides the capability to perform substring
   searches over a small set of properties on the resources representing
   principals.  This permits searches based on last name, first name,
   user name, job title, etc.  Two separate searches are supported, both
   via the REPORT method, one to search principal resources
   (DAV:principal-property-search, Section 9.4), the other to determine
   which properties may be searched at all (DAV:principal-search-
   property-set, Section 9.5).

   Once a principal has been identified in an ACE, a server evaluating
   that ACE must know the identity of the principal making a protocol
   request, and must validate that that principal is who they claim to
   be, a process known as authentication.  This specification
   intentionally omits discussion of authentication, as the HTTP
   protocol already has a number of authentication mechanisms [RFC2617].
   Some authentication mechanism (such as HTTP Digest Authentication,
   which all WebDAV compliant implementations are required to support)
   must be available to validate the identity of a principal.
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   The following issues are out of scope for this document:

   o  Access control that applies only to a particular property on a
      resource (excepting the access control properties DAV:acl and
      DAV:current-user-privilege-set), rather than the entire resource,

   o  Role-based security (where a role can be seen as a dynamically
      defined group of principals),

   o  Specification of the ways an ACL on a resource is initialized,

   o  Specification of an ACL that applies globally to all resources,
      rather than to a particular resource.

   o  Creation and maintenance of resources representing people or
      computational agents (principals), and groups of these.

   This specification is organized as follows.  Section 1.1 defines key
   concepts used throughout the specification, and is followed by a more
   in-depth discussion of principals (Section 2), and privileges
   (Section 3).  Properties defined on principals are specified in
   Section 4, and access control properties for content resources are
   specified in Section 5.  The ways ACLs are to be evaluated is
   described in Section 6.  Client discovery of access control
   capability using OPTIONS is described in Section 7.2.  Interactions
   between access control functionality and existing HTTP and WebDAV
   methods are described in the remainder of Section 7.  The access
   control setting method, ACL, is specified in Section 8.  Four reports
   that provide limited server-side searching capabilities are described
   in Section 9.  Sections on XML processing (Section 10),
   Internationalization considerations (Section 11), security
   considerations (Section 12), and authentication (Section 13) round
   out the specification.  An appendix (Appendix A) provides an XML
   Document Type Definition (DTD) for the XML elements defined in the
   specification.

1.1. Terms

This document uses the terms defined in HTTP [RFC2616] and WebDAV [RFC2518]. In addition, the following terms are defined: principal A "principal" is a distinct human or computational actor that initiates access to network resources. In this protocol, a principal is an HTTP resource that represents such an actor.
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   group

      A "group" is a principal that represents a set of other
      principals.

   privilege

      A "privilege" controls access to a particular set of HTTP
      operations on a resource.

   aggregate privilege

      An "aggregate privilege" is a privilege that contains a set of
      other privileges.

   abstract privilege

      The modifier "abstract", when applied to a privilege on a
      resource, means the privilege cannot be set in an access control
      element (ACE) on that resource.

   access control list (ACL)

      An "ACL" is a list of access control elements that define access
      control to a particular resource.

   access control element (ACE)

      An "ACE" either grants or denies a particular set of (non-
      abstract) privileges for a particular principal.

   inherited ACE

      An "inherited ACE" is an ACE that is dynamically shared from the
      ACL of another resource.  When a shared ACE changes on the primary
      resource, it is also changed on inheriting resources.

   protected property

      A "protected property" is one whose value cannot be updated except
      by a method explicitly defined as updating that specific property.
      In particular, a protected property cannot be updated with a
      PROPPATCH request.
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1.2. Notational Conventions

The augmented BNF used by this document to describe protocol elements is described in Section 2.1 of [RFC2616]. Because this augmented BNF uses the basic production rules provided in Section 2.2 of [RFC2616], those rules apply to this document as well. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Definitions of XML elements in this document use XML element type declarations (as found in XML Document Type Declarations), described in Section 3.2 of [REC-XML]. When an XML element type in the "DAV:" namespace is referenced in this document outside of the context of an XML fragment, the string "DAV:" will be prefixed to the element name.

2. Principals

A principal is a network resource that represents a distinct human or computational actor that initiates access to network resources. Users and groups are represented as principals in many implementations; other types of principals are also possible. A URI of any scheme MAY be used to identify a principal resource. However, servers implementing this specification MUST expose principal resources at an http(s) URL, which is a privileged scheme that points to resources that have additional properties, as described in Section 4. So, a principal resource can have multiple URIs, one of which has to be an http(s) scheme URL. Although an implementation SHOULD support PROPFIND and MAY support PROPPATCH to access and modify information about a principal, it is not required to do so. A principal resource may be a group, where a group is a principal that represents a set of other principals, called the members of the group. If a person or computational agent matches a principal resource that is a member of a group, they also match the group. Membership in a group is recursive, so if a principal is a member of group GRPA, and GRPA is a member of group GRPB, then the principal is also a member of GRPB.

3. Privileges

Ability to perform a given method on a resource MUST be controlled by one or more privileges. Authors of protocol extensions that define new HTTP methods SHOULD specify which privileges (by defining new privileges, or mapping to ones below) are required to perform the method. A principal with no privileges to a resource MUST be denied any HTTP access to that resource, unless the principal matches an ACE
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   constructed using the DAV:all, DAV:authenticated, or
   DAV:unauthenticated pseudo-principals (see Section 5.5.1).  Servers
   MUST report a 403 "Forbidden" error if access is denied, except in
   the case where the privilege restricts the ability to know the
   resource exists, in which case 404 "Not Found" may be returned.

   Privileges may be containers of other privileges, in which case they
   are termed "aggregate privileges".  If a principal is granted or
   denied an aggregate privilege, it is semantically equivalent to
   granting or denying each of the aggregated privileges individually.
   For example, an implementation may define add-member and remove-
   member privileges that control the ability to add and remove a member
   of a group.  Since these privileges control the ability to update the
   state of a group, these privileges would be aggregated by the
   DAV:write privilege on a group, and granting the DAV:write privilege
   on a group would also grant the add-member and remove-member
   privileges.

   Privileges may be declared to be "abstract" for a given resource, in
   which case they cannot be set in an ACE on that resource.  Aggregate
   and non-aggregate privileges are both capable of being abstract.
   Abstract privileges are useful for modeling privileges that otherwise
   would not be exposed via the protocol.  Abstract privileges also
   provide server implementations with flexibility in implementing the
   privileges defined in this specification.  For example, if a server
   is incapable of separating the read resource capability from the read
   ACL capability, it can still model the DAV:read and DAV:read-acl
   privileges defined in this specification by declaring them abstract,
   and containing them within a non-abstract aggregate privilege (say,
   read-all) that holds DAV:read, and DAV:read-acl.  In this way, it is
   possible to set the aggregate privilege, read-all, thus coupling the
   setting of DAV:read and DAV:read-acl, but it is not possible to set
   DAV:read, or DAV:read-acl individually.  Since aggregate privileges
   can be abstract, it is also possible to use abstract privileges to
   group or organize non-abstract privileges.  Privilege containment
   loops are not allowed; therefore, a privilege MUST NOT contain
   itself.  For example, DAV:read cannot contain DAV:read.

   The set of privileges that apply to a particular resource may vary
   with the DAV:resourcetype of the resource, as well as between
   different server implementations.  To promote interoperability,
   however, this specification defines a set of well-known privileges
   (e.g., DAV:read, DAV:write, DAV:read-acl, DAV:write-acl, DAV:read-
   current-user-privilege-set, and DAV:all), which can at least be used
   to classify the other privileges defined on a particular resource.
   The access permissions on null resources (defined in [RFC2518],
   Section 3) are solely those they inherit (if any), and they are not
   discoverable (i.e., the access control properties specified in
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   Section 5 are not defined on null resources).  On the transition from
   null to stateful resource, the initial access control list is set by
   the server's default ACL value policy (if any).

   Server implementations MAY define new privileges beyond those defined
   in this specification.  Privileges defined by individual
   implementations MUST NOT use the DAV: namespace, and instead should
   use a namespace that they control, such as an http scheme URL.

3.1. DAV:read Privilege

The read privilege controls methods that return information about the state of the resource, including the resource's properties. Affected methods include GET and PROPFIND. Any implementation-defined privilege that also controls access to GET and PROPFIND must be aggregated under DAV:read - if an ACL grants access to DAV:read, the client may expect that no other privilege needs to be granted to have access to GET and PROPFIND. Additionally, the read privilege MUST control the OPTIONS method. <!ELEMENT read EMPTY>

3.2. DAV:write Privilege

The write privilege controls methods that lock a resource or modify the content, dead properties, or (in the case of a collection) membership of the resource, such as PUT and PROPPATCH. Note that state modification is also controlled via locking (see section 5.3 of [RFC2518]), so effective write access requires that both write privileges and write locking requirements are satisfied. Any implementation-defined privilege that also controls access to methods modifying content, dead properties or collection membership must be aggregated under DAV:write, e.g., if an ACL grants access to DAV:write, the client may expect that no other privilege needs to be granted to have access to PUT and PROPPATCH. <!ELEMENT write EMPTY>

3.3. DAV:write-properties Privilege

The DAV:write-properties privilege controls methods that modify the dead properties of the resource, such as PROPPATCH. Whether this privilege may be used to control access to any live properties is determined by the implementation. Any implementation-defined privilege that also controls access to methods modifying dead properties must be aggregated under DAV:write-properties - e.g., if
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   an ACL grants access to DAV:write-properties, the client can safely
   expect that no other privilege needs to be granted to have access to
   PROPPATCH.

   <!ELEMENT write-properties EMPTY>

3.4. DAV:write-content Privilege

The DAV:write-content privilege controls methods that modify the content of an existing resource, such as PUT. Any implementation- defined privilege that also controls access to content must be aggregated under DAV:write-content - e.g., if an ACL grants access to DAV:write-content, the client can safely expect that no other privilege needs to be granted to have access to PUT. Note that PUT - when applied to an unmapped URI - creates a new resource and therefore is controlled by the DAV:bind privilege on the parent collection. <!ELEMENT write-content EMPTY>

3.5. DAV:unlock Privilege

The DAV:unlock privilege controls the use of the UNLOCK method by a principal other than the lock owner (the principal that created a lock can always perform an UNLOCK). While the set of users who may lock a resource is most commonly the same set of users who may modify a resource, servers may allow various kinds of administrators to unlock resources locked by others. Any privilege controlling access by non-lock owners to UNLOCK MUST be aggregated under DAV:unlock. A lock owner can always remove a lock by issuing an UNLOCK with the correct lock token and authentication credentials. That is, even if a principal does not have DAV:unlock privilege, they can still remove locks they own. Principals other than the lock owner can remove a lock only if they have DAV:unlock privilege and they issue an UNLOCK with the correct lock token. Lock timeout is not affected by the DAV:unlock privilege. <!ELEMENT unlock EMPTY>

3.6. DAV:read-acl Privilege

The DAV:read-acl privilege controls the use of PROPFIND to retrieve the DAV:acl property of the resource. <!ELEMENT read-acl EMPTY>
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3.7. DAV:read-current-user-privilege-set Privilege

The DAV:read-current-user-privilege-set privilege controls the use of PROPFIND to retrieve the DAV:current-user-privilege-set property of the resource. Clients are intended to use this property to visually indicate in their UI items that are dependent on the permissions of a resource, for example, by graying out resources that are not writable. This privilege is separate from DAV:read-acl because there is a need to allow most users access to the privileges permitted the current user (due to its use in creating the UI), while the full ACL contains information that may not be appropriate for the current authenticated user. As a result, the set of users who can view the full ACL is expected to be much smaller than those who can read the current user privilege set, and hence distinct privileges are needed for each. <!ELEMENT read-current-user-privilege-set EMPTY>

3.8. DAV:write-acl Privilege

The DAV:write-acl privilege controls use of the ACL method to modify the DAV:acl property of the resource. <!ELEMENT write-acl EMPTY>

3.9. DAV:bind Privilege

The DAV:bind privilege allows a method to add a new member URL to the specified collection (for example via PUT or MKCOL). It is ignored for resources that are not collections. <!ELEMENT bind EMPTY>

3.10. DAV:unbind Privilege

The DAV:unbind privilege allows a method to remove a member URL from the specified collection (for example via DELETE or MOVE). It is ignored for resources that are not collections. <!ELEMENT unbind EMPTY>
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3.11. DAV:all Privilege

DAV:all is an aggregate privilege that contains the entire set of privileges that can be applied to the resource. <!ELEMENT all EMPTY>

3.12. Aggregation of Predefined Privileges

Server implementations are free to aggregate the predefined privileges (defined above in Sections 3.1-3.10) subject to the following limitations: DAV:read-acl MUST NOT contain DAV:read, DAV:write, DAV:write-acl, DAV:write-properties, DAV:write-content, or DAV:read-current-user- privilege-set. DAV:write-acl MUST NOT contain DAV:write, DAV:read, DAV:read-acl, or DAV:read-current-user-privilege-set. DAV:read-current-user-privilege-set MUST NOT contain DAV:write, DAV:read, DAV:read-acl, or DAV:write-acl. DAV:write MUST NOT contain DAV:read, DAV:read-acl, or DAV:read- current-user-privilege-set. DAV:read MUST NOT contain DAV:write, DAV:write-acl, DAV:write- properties, or DAV:write-content. DAV:write MUST contain DAV:bind, DAV:unbind, DAV:write-properties and DAV:write-content.

4. Principal Properties

Principals are manifested to clients as a WebDAV resource, identified by a URL. A principal MUST have a non-empty DAV:displayname property (defined in Section 13.2 of [RFC2518]), and a DAV:resourcetype property (defined in Section 13.9 of [RFC2518]). Additionally, a principal MUST report the DAV:principal XML element in the value of the DAV:resourcetype property. The element type declaration for DAV:principal is: <!ELEMENT principal EMPTY>
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   This protocol defines the following additional properties for a
   principal.  Since it can be expensive for a server to retrieve access
   control information, the name and value of these properties SHOULD
   NOT be returned by a PROPFIND allprop request (as defined in Section
   12.14.1 of [RFC2518]).

4.1. DAV:alternate-URI-set

This protected property, if non-empty, contains the URIs of network resources with additional descriptive information about the principal. This property identifies additional network resources (i.e., it contains one or more URIs) that may be consulted by a client to gain additional knowledge concerning a principal. One expected use for this property is the storage of an LDAP [RFC2255] scheme URL. A user-agent encountering an LDAP URL could use LDAP [RFC2251] to retrieve additional machine-readable directory information about the principal, and display that information in its user interface. Support for this property is REQUIRED, and the value is empty if no alternate URI exists for the principal. <!ELEMENT alternate-URI-set (href*)>

4.2. DAV:principal-URL

A principal may have many URLs, but there must be one "principal URL" that clients can use to uniquely identify a principal. This protected property contains the URL that MUST be used to identify this principal in an ACL request. Support for this property is REQUIRED. <!ELEMENT principal-URL (href)>

4.3. DAV:group-member-set

This property of a group principal identifies the principals that are direct members of this group. Since a group may be a member of another group, a group may also have indirect members (i.e., the members of its direct members). A URL in the DAV:group-member-set for a principal MUST be the DAV:principal-URL of that principal. <!ELEMENT group-member-set (href*)>

4.4. DAV:group-membership

This protected property identifies the groups in which the principal is directly a member. Note that a server may allow a group to be a member of another group, in which case the DAV:group-membership of
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   those other groups would need to be queried in order to determine the
   groups in which the principal is indirectly a member.  Support for
   this property is REQUIRED.

   <!ELEMENT group-membership (href*)>



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