INTERNET-DRAFT                   Geoffrey Clemm, Rational Software
 draft-ietf-webdav-acl-04
draft-ietf-webdav-acl-05         Anne Hopkins, Microsoft Corporation
                                 Eric Sedlar, Oracle Corporation
                                 Jim Whitehead, U.C. Santa Cruz

Expires July 21, 2001            January 21,            April 23, 2001

                     WebDAV Access Control Protocol

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

This document specifies a set of methods, headers, and message bodies
that define the WebDAV Access Control extensions to the HTTP/1.1
protocol. This protocol permits a client to remotely read and modify
access control lists that instruct a server whether to grant or deny
operations upon a resource (such as HTTP method invocations) by a given
principal.

This document is a product of the Web Distributed Authoring and
Versioning (WebDAV) working group of the Internet Engineering Task
Force. Comments on this draft are welcomed, and should be addressed to
the acl@webdav.org mailing list. Other related documents can be found at
http://www.webdav.org/acl/, and http://www.ics.uci.edu/pub/ietf/webdav/.

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Table of Contents

1 INTRODUCTION......................................................3 INTRODUCTION......................................................4
1.1  Terms..........................................................4 Terms...........................................................5
1.2 Notational Conventions.........................................5 Conventions..........................................6

2 PRINCIPALS........................................................5 PRINCIPALS........................................................6

3 PRIVILEGES........................................................5 PRIVILEGES........................................................6
3.1 DAV:read Privilege.............................................6 Privilege..............................................7
3.2 DAV:write Privilege............................................6 Privilege.............................................7
3.3 DAV:read-acl Privilege.........................................7 Privilege..........................................8
3.4  DAV:write-acl Privilege........................................7 DAV:read-cuprivset Privilege....................................8
3.5 DAV:write-acl Privilege.........................................8
3.6 DAV:all Privilege..............................................7 Privilege...............................................8

4 PRINCIPAL PROPERTIES..............................................7 PROPERTIES..............................................8
4.1  DAV:is-principal...............................................7 DAV:is-principal................................................9
4.2  DAV:authentication-id..........................................7 DAV:alternate-URL...............................................9

5 ACCESS CONTROL PROPERTIES.........................................8 PROPERTIES.........................................9
5.1  DAV:owner......................................................8 DAV:owner.......................................................9
5.2  DAV:supported-privilege-set....................................8 DAV:supported-privilege-set....................................10
5.3  DAV:current-user-privilege-set.................................9 DAV:current-user-privilege-set.................................11
5.4  DAV:acl........................................................9 DAV:acl........................................................11
 5.4.1 ACE Principal................................................9 Principal...............................................11
 5.4.2 ACE Grant and Deny..........................................10 Deny..........................................13
 5.4.3 ACE Protection..............................................11 Protection..............................................13
 5.4.4 ACE Inheritance.............................................11 Inheritance.............................................13
5.5  DAV:acl-semantics.............................................11 DAV:acl-semantics..............................................13
5.6  DAV:principal-collection-set..................................11 DAV:principal-collection-set...................................14
5.7 Example: PROPFIND to retrieve access control properties.......12 properties........14

6 ACL SEMANTICS....................................................15 SEMANTICS....................................................17
6.1 ACE Combination...............................................15 Combination................................................17
 6.1.1 DAV:first-match ACE Combination.............................15 Combination.............................18
 6.1.2 DAV:all-grant-before-any-deny ACE Combination...............15 Combination...............18
 6.1.3 DAV:no-deny DAV:specific-deny-overrides-grant ACE Combination.................................15 Combination...........18
6.2 ACE Ordering..................................................16 Ordering...................................................18
 6.2.1 DAV:deny-before-grant ACE Ordering..........................16 Ordering..........................18
6.3 Required Principals...........................................16 Principals............................................18

7 ACCESS CONTROL AND EXISTING METHODS..............................16 METHODS..............................19
7.1  OPTIONS.......................................................16 OPTIONS........................................................19
 7.1.1 Example - OPTIONS...........................................16 OPTIONS...........................................19

8 ACCESS CONTROL METHODS...........................................17 METHODS...........................................19
8.1  ACL...........................................................17 ACL............................................................19
 8.1.1 ACL Preconditions...........................................17 Preconditions...........................................20
 8.1.2 Example: the ACL method.....................................17 method.....................................20
 8.1.3 Example: ACL method failure due to omission of protected ACE18 ACE21
 8.1.4 Example: ACL method failure due to inherited ACEs preceding
 non-inherited ACEs................................................19 ACEs................................................22

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 8.1.5 Example: ACL method failure due to an attempt to set grant and
 deny in a single ACE..............................................20

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 2] ACE..............................................23

9 INTERNATIONALIZATION CONSIDERATIONS..............................21 CONSIDERATIONS..............................24

10  SECURITY CONSIDERATIONS........................................22 CONSIDERATIONS........................................25
10.1 Increased Risk of Compromised Users...........................22 Users...........................25
10.2 Authentication-id Property and Dictionary Attacks.............22
 10.3 Risks of the read-acl Privilege...............................23 and cuprivset Privileges................25

11  AUTHENTICATION.................................................23  AUTHENTICATION.................................................26

12  IANA CONSIDERATIONS............................................23 CONSIDERATIONS............................................26

13  INTELLECTUAL PROPERTY..........................................23 PROPERTY..........................................26

14  ACKNOWLEDGEMENTS...............................................24  ACKNOWLEDGEMENTS...............................................26

15  REFERENCES.....................................................24  REFERENCES.....................................................27
15.1 Normative References..........................................24 References..........................................27
15.2 Informational References......................................25 References......................................28

16  AUTHORS' ADDRESSES.............................................25 ADDRESSES.............................................28

17  APPENDICIES....................................................25  APPENDICIES....................................................28
17.1 XML Document Type Definition..................................25 Definition..................................28

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1  INTRODUCTION

     The goal of the WebDAV access control extensions is to provide an
     interoperable mechanism for handling discretionary access control
     for content in 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 how you can access 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
     "how" is 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.

     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.

     In the interests of timeliness, the following set of security
     mechanisms are not addressed by this document:

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 3]

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

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

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

       * Specification of an ACL that applies globally to a method,
          rather than to a particular resource.

     This specification is organized as follows. Section 1.1 defines key
     concepts used throughout the specification, and is followed by 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 semantics of access control lists are
     described in Section 6, including sections on ACE combination
     (Section 6.1), ACE ordering (Section 6.2), and principals required
     to be present in an ACE (Section 6.3). Client discovery of access
     control capability using OPTIONS is described in Section 7.1, and
     the access control setting method, ACL, is specified in Section 8.

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     Internationalization considerations (Section 9) and security
     considerations (Section 10) round out the specification. An
     appendix (Section 17.1) provides an XML Document Type Definition
     (DTD) for the XML elements defined in the specification.

1.1 Terms

     This draft 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.

   principal collection

     A "principal collection" is a group of principals, and is
     represented in this protocol by a WebDAV collection containing HTTP
     resources that represent principals, and principal collections.

   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

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 4]

     The modifier "abstract", when applied to an atomic or aggregate
     privilege, means the privilege cannot be set in an access control
     element (ace).

   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 shared from the acl of another
     resource.

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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].

2  PRINCIPALS

     A principal is an HTTP a network resource that represents a distinct human
     or computational actor that initiates access to network resources.
     On many implementations, users and groups are represented as
     principals; other types of principals are also possible. A URL of
     any scheme MAY be used to identify a principal resource. However,
     servers implementing this specification SHOULD 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. Although an implementation MAY SHOULD support PROPFIND
     and PROPPATCH to access and modify information about a principal,
     it is not required to do so.

     A principal resource may or may not be a collection.  A collection
     principal may only contain other principals (not other types of
     resources).  Servers that support aggregation of principals (e.g.
     groups of users or other groups) MUST manifest them as collection
     principals.  The WebDAV methods for examining and maintaining
     collections (e.g. DELETE, PROPFIND) MAY be used to maintain
     collection principals.  Membership in a collection principal is
     recursive, so a principal in a collection principal GRPA contained
     by collection principal GRPB is a member of both GRPA and GRPB.
     Implementations not supporting recursive membership in principal
     collections can return an error if the client attempts to bind
     collection principals into other collection principals.

3  PRIVILEGES

     Ability to perform a given method on a resource SHOULD be
     controlled by one or more privileges.  Authors of protocol
     extensions that

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 5] 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 SHOULD be denied any HTTP access to that resource.

     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 an
     internal member of a collection.  Since these privileges control

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     the ability to update the state of a collection, these privileges
     would be aggregated by the DAV:write privilege on a collection, and
     granting the DAV:write privilege on a collection would also grant
     the add-member and remove-member privileges.

     Privileges may have the quality of being abstract, in which case
     they cannot be set in an ACE. Aggregate and atomic 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
     and classify non-abstract privileges.

     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, WebDAV defines a set of well-known privileges (e.g.
     DAV:read and DAV:write), which can at least be used to classify the
     other privileges defined on a particular resource. The access
     permissions on null and lock-null resources are solely those they
     inherit (if any), and they are not discoverable (i.e., the ACL
     properties specified in Section 5 are not defined on null and lock-
     null resources). On the transition from null or lock-null to a
     stateful resource, the initial access control list is set by the
     server's default ACL value policy (if any).

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.  Additionally, the read
     privilege MAY control the OPTIONS method.

     <!ELEMENT read EMPTY>

3.2 DAV:write Privilege

     The write privilege controls methods that modify the state of the
     resource, such as PUT and PROPPATCH.  Note that state modification

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 6]
     is also controlled via locking (see section 5.3 of [WEBDAV]), so
     effective write access requires that both write privileges and
     write locking requirements are satisfied.

     <!ELEMENT write EMPTY>

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3.3 DAV:read-acl Privilege

     The DAV:read-acl privilege controls the use of PROPFIND to retrieve
     the DAV:acl, and DAV:current-user-privilege-set properties DAV:acl property of the resource.

     <!ELEMENT read-acl EMPTY>

3.4 DAV:write-acl DAV:read-cuprivset Privilege

     The DAV:write-acl DAV:read-cuprivset privilege controls the use of the ACL method PROPFIND to modify
     retrieve the DAV:acl DAV:current-user-privilege-set property of the
     resource.

      <!ELEMENT write-acl EMPTY>

 3.5 DAV:all Privilege

   DAV:all is an aggregate privilege

     Clients are intended to use this property to visually indicate in
     their UI items that contains all privileges are dependent on the resource.

      <!ELEMENT all EMPTY>

 4  PRINCIPAL PROPERTIES

   Principals are manifested to clients as an HTTP permissions of a resource, identified
     for example, by a URL.  A principal MUST have a DAV:displayname property. graying out resources that are not writeable.

     This
   protocol defines the following additional properties for a
   principal. 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-cuprivset EMPTY>

3.5 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>

3.6 DAV:all Privilege

     DAV:all is an aggregate privilege that contains all privileges on
     the resource.

     <!ELEMENT all EMPTY>

4  PRINCIPAL PROPERTIES

     Principals are manifested to clients as an HTTP resource,
     identified by a URL.  A principal MUST have a DAV:displayname
     property.  This protocol defines the following additional
     properties for a principal. The name and value of these properties
     SHOULD NOT be returned by PROPFIND allprop request (as defined in
     Section 12.14.1 of [RFC2518]). In the descriptions below, a read-
     only property is defined as a property that MUST NOT be writeable
     using PROPPATCH.

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4.1 DAV:is-principal

     This is a read-only property that indicates whether this resource
     is a principal.  A resource MUST have a non-empty DAV:is-principal
     property if and only if it is a principal resource.   (Note: If we can just add a
   DAV:principal element to the DAV:resourcetype property, then we do
   not need a DAV:is-principal property.)

     <!ELEMENT is-principal (#PCDATA)>
     PCDATA value: any non-empty value ("T" "true" - resource is suggested)

 4.2 DAV:authentication-id

   A a principal, "false" - resource
     is not a principal (note that in cases where the "F" value might be
     used, this specification requires the property containing not be present at
     all).

4.2 DAV:alternate-URL

     This read-only property, if present, contains the name used URL of a network
     resource with additional descriptive information about the
     principal. This property identifies one or more additional network
     resources (i.e., it contains one or more URLs) that may be
     consulted by a client to authenticate this principal
   (typically typed into gain additional knowledge concerning a login prompt/dialog).
     principal. Two potential uses for this property are to store an
     ldap [RFC2255] or mailto [RFC2368] scheme URL. Support for this
     property is OPTIONAL.

     <!ELEMENT authentication-id (#PCDATA)>
      PCDATA value: any string

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 7] alternate-URL (href*)>

5  ACCESS CONTROL PROPERTIES

     This specification defines a number of new properties for WebDAV
     resources.  Access control properties may be retrieved just like
     other WebDAV properties, using the PROPFIND method.  Some access
   control properties (such as DAV:owner) MAY be updated with PROPFIND method.  Some access
     control properties (such as DAV:owner) MAY be updated with the
     PROPPATCH method.  In the descriptions below, a read-only property
     is defined as a property that MUST NOT be writeable using
     PROPPATCH. Since it is expensive, for many servers, to retrieve
     access control information, a PROPFIND allprop request (as defined
     in Section 12.14.1 of [RFC2518]) SHOULD NOT return the names and
     values of the
   PROPPATCH method. properties defined in this section.

     HTTP resources that support the WebDAV Access Control Protocol MUST
     contain the following properties. Null, and lock-null resources
     (described in Section 7.4 of [RFC2518]) MUST NOT contain the
     following properties:

5.1 DAV:owner

     This property identifies a particular principal as being the
     "owner" of the resource. Since the owner of a resource often has
     special access control capabilities (e.g., the owner frequently has
     permanent write-ACL privilege), clients might display the resource
     owner in their user interface.

     <!ELEMENT owner (href prop?)>
     <!ELEMENT prop (see [RFC2518], section 12.11)>

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     An implementation MAY include a list of selected properties of that
     principal resource.  Which properties (if any) are included is
     implementation defined. defined, but might reasonably include properties
     such as DAV:displayname, which is useful for the construction of
     access control user interfaces on the client. A server might
     support this capability if it wished to save the client the
     additional network round-trip delay required to retrieve this
     information using a PROPFIND request on the principal URL in the
     href element. Servers that do not directly support PROPFIND on
     principal resources might also support this feature, since it
     allows them to return a server-controlled subset of the properties
     on the principal resource.

     An implementation MAY allow the use of PROPPATCH to update the
     DAV:owner field. If the DAV:owner property is writeable, clients
     MUST NOT submit the prop element; only the href element can be
     modified by the client. The purpose of this restriction is to limit
     the scope of effect of a PROPPATCH to just the owner property's
     resource; setting the prop element would additionally require
     modification to properties of the principal resource identified by
     the href element.

5.2 DAV:supported-privilege-set

     This is a read-only property that identifies the privileges defined
     for the resource.

     <!ELEMENT supported-privilege-set (supported-privilege*)>

     Each privilege appears as an XML element, where aggregate
     privileges list as sub-elements all of the privileges that they
     aggregate.

     <!ELEMENT supported-privilege
      (privilege, abstract?, description, supported-privilege*)>
     <!ELEMENT privilege ANY>

     An abstract privilege of a resource MUST NOT be used in an ACE for
     that resource. Servers MUST fail an attempt to set an abstract
     privilege.

     <!ELEMENT abstract EMPTY>

     A description is a human-readable description of what this
     privilege controls access to.

     <!ELEMENT description #PCDATA>

     It is envisioned that a WebDAV ACL-aware administrative client
     would list the supported privileges in a dialog box, and allow the
     user to choose non-abstract privileges to apply in an ACE.  The
     privileges tree is useful programmatically to map well-known
     privileges (defined by WebDAV or other standards groups) into

Clemm, Hopkins, Sedlar, Whitehead                            [Page 10] 
     privileges that are supported by any particular server
     implementation.  The

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 8] privilege tree also serves to hide complexity
     in implementations allowing large number of privileges to be
     defined by displaying aggregates to the user.

5.3 DAV:current-user-privilege-set

     DAV:current-user-privilege-set is a read-only property containing
     the exact set of privileges (as computed by the server) granted to
     the currently authenticated HTTP user. Aggregate privileges and
     their contained privileges are listed. A user-agent can use the
     value of this property to adjust its user interface to make actions
     inaccessible (e.g, (e.g., by graying out a menu item or button) for which
     the current principal does not have permission. This is
     particularly useful for an access control user interface, which can
     be constructed without knowing the ACE combining semantics of the
     server. This property is also useful for determine determining what
     operations
   can be performed by the current principal, principal can perform, without having to
     actually execute an operation.

     <!ELEMENT current-user-privilege-set (privilege*)>
     <!ELEMENT privilege ANY>

     If the current user is granted a specific privilege, that privilege
     must belong to the set of privileges that may be set on this
     resource. Therefore, each element in the DAV:current-user-privilege-
   set DAV:current-user-
     privilege-set property MUST identify a non-abstract privilege from
     the DAV:supported-
   privilege-set DAV:supported-privilege-set property.

5.4 DAV:acl

     This is a read-only property that specifies the list of access
     control entries (ACEs), which define what principals are to get
     what privileges for this resource.

     <!ELEMENT acl (ace*)>

     Each DAV:ace element specifies the set of privileges to be either
     granted or denied to a single principal.  If the DAV:acl property
     is empty, no principal is granted any privilege.

     <!ELEMENT ace (principal, (grant|deny), protected?, inherited?)>

   An attempt to update the DAV:acl property with a PROPPATCH MUST
   fail.

5.4.1 ACE Principal

     The DAV:principal element identifies the principal to which this
     ACE applies.

     <!ELEMENT principal ((href, prop?)
      | all | authenticated | unauthenticated
      | property | self)>

Clemm, Hopkins, Sedlar, Whitehead                            [Page 11] 
     The current user matches DAV:href only if that user is
     authenticated as being (or being a member of) the principal
     identified by the URL

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 9] URL contained by that DAV:href. An implementation
     MAY include a DAV:prop element after the DAV:href element,
     containing a list of selected properties of that principal
     resource. Which properties (if any) are included in the DAV:prop
     element is implementation defined. The DAV:prop element is primarily intended for
   implementations can be used
     by servers  that do not support PROPFIND requests on principal
     resources to return principal-related information (such as the
     value of the DAV:displayname property) that a client would find
     useful in the creation of an access control user interface. A
     server might also support this capability if it wished to save the
     client the additional network round-trip delays required to
     retrieve this information via a series of PROPFIND requests on each
     principal URL. URL in the ACL. In the worst case, this is one additional
     PROPFIND per ACE.

     <!ELEMENT prop (see [RFC2518], section 12.11)>

     The current user always matches DAV:all.

     <!ELEMENT all EMPTY>

     The current user matches DAV:authenticated only if authenticated.

     <!ELEMENT authenticated EMPTY>

     The current user matches DAV:unauthenticated only if not
     authenticated.

     <!ELEMENT unauthenticated EMPTY>

     DAV:all is the union of DAV:authenticated, and DAV:unauthenticated.
     For a given request, the user matches either DAV:authenticated, or
     DAV:unauthenticated, but not both.

     The current user matches a DAV:property principal in a DAV:acl
     property of a resource only if the identified property of that
     resource contains a DAV:href that identifies a principal, and the
     current user is authenticated as being (or being a member of) that
     principal.  For example, if the DAV:property element contained
     <DAV:owner/>, the current user would match the DAV:property
     principal only if the current user is authenticated as matching the
     principal identified by the DAV:owner property of the resource.

     <!ELEMENT property ANY>

     The current user matches DAV:self in a DAV:acl property of the
     resource only if that resource is a principal object and the
     current user is authenticated as being that principal.

     <!ELEMENT self EMPTY>

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5.4.2 ACE Grant and Deny

     Each DAV:grant or DAV:deny element specifies the set of privileges
     to be either granted or denied to the specified principal.  A
     DAV:grant or DAV:deny element of the DAV:acl of a resource MUST
     only contain non-abstract elements specified in the DAV:supported-privilege-set DAV:supported-
     privilege-set of that resource.

     <!ELEMENT grant (privilege+)>
     <!ELEMENT deny (privilege+)>
     <!ELEMENT privilege ANY>

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 10]

5.4.3 ACE Protection

     If an ACE contains a DAV:protected element, an ACL request without
     that ACE MUST fail.

     <!ELEMENT protected EMPTY>

5.4.4 ACE Inheritance

     The presence of a DAV:inherited element indicates that this ACE is
     inherited from another resource that is identified by the URL
     contained in a DAV:href element.  An inherited ACE cannot be
     modified directly, but instead the ACL on the resource from which
     it is inherited must be modified.

     Note that ACE inheritance is not the same as ACL initialization.
     ACL initialization defines the ACL that a newly created resource
     will use (if not specified).  ACE inheritance refers to an ACE that
     is logically shared - where an update to the resource containing an
     ACE will affect the ACE of each resource that inherits that ACE.
     The method by which ACLs are initialized or by which ACEs are
     inherited is not defined by this document.

     <!ELEMENT inherited (href)>

5.5 DAV:acl-semantics

     This is a read-only property that defines the ACL semantics.  These
     semantics define how multiple ACEs that match the current user are
     combined, what are the constraints on how ACEs can be ordered, and
     which principals must have an ACE. A client user interface could
     use the value of this property to provide feedback to a human
     operator concerning the impact of proposed changes to an ACL.
     Alternately, a client could use this property to determine exactly,
     before submitting an ACL method invocation, what ACL changes it
     needs to make to accomplish a specific goal (or whether that goal
     is even achievable on this server).

     Since it is not practical to require all implementations to use the
     same ACL semantics, the DAV:acl-semantics property is used to
     identify the ACL semantics for a particular resource.  The DAV:acl-
     semantics element is defined in section 6.

Clemm, Hopkins, Sedlar, Whitehead                            [Page 13] 

5.6 DAV:principal-collection-set

     This read-only property contains zero, one, or more URLs that
     identify a collection principal. It is expected that
     implementations of this protocol will typically employ a relatively
     small number of locations in the URL namespace for principal, and
     collection principals. In cases where this assumption holds, the DAV:principal-
   collection-set
     DAV:principal-collection-set property will contain a small set of
     URLs identifying the top of collection hierarchy containing
     multiple principals and collection principals. An access control
     protocol user agent could use the contents of DAV:principal-collection-set to, for example, DAV:principal-
     collection-set to query the DAV:displayname property (specified in
     Section 13.2 of [RFC2518]) of all principals on that server,
     thereby yielding human-
   readable human-readable names for each principal that could
     be displayed in a user interface.

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 11]

     <!ELEMENT principal-collection-set (href*)>
     Since different servers can control different parts of the URL
     namespace, different resources on the same host MAY have different
     DAV:principal-collection-set values. The collections specified in
     the DAV:principal-collection-set MAY be located on different hosts
     from the resource. The URLs in DAV:principal-collection-set are not
   limited to http scheme URLs, and can, for example, SHOULD
     be ldap http or https scheme URLs. For security and scalability reasons,
     a server MAY report only a subset of the entire set of known
     collection principals, and therefore clients should not assume they
     have retrieved an exhaustive listing. Additionally, a server MAY
     elect to report none of the collection principals it knows about.

5.7 Example: PROPFIND to retrieve access control properties

     The following example shows how access control information can be
     retrieved by using the PROPFIND method to fetch the values of the
     DAV:owner, DAV:supported-privilege-set, DAV:current-user-privilege-
     set, and DAV:acl properties.

     >> Request <<

     PROPFIND /top/container/ HTTP/1.1
     Host: www.foo.org
     Content-type: text/xml; charset="utf-8"
     Content-Length: xxx
     Depth: 0
     Authorization: Digest username="ejw",
        realm="users@foo.org", nonce="...",
        uri="/top/container/", response="...", opaque="..."

     <?xml version="1.0" encoding="utf-8" ?>
     <D:propfind xmlns:D="DAV:">
       <D:owner/>
       <D:supported-privilege-set/>
       <D:current-user-privilege-set/>
       <D:acl/>
     </D:propfind>

Clemm, Hopkins, Sedlar, Whitehead                            [Page 14] 
     >> Response <<

     HTTP/1.1 207 Multi-Status
     Content-Type: text/xml; charset="utf-8"
     Content-Length: xxx

     <?xml version="1.0" encoding="utf-8" ?>
     <D:multistatus
        xmlns:D="DAV:"
         xmlns:A="http://www.acl.org/">
        xmlns:A="http://www.webdav.org/acl/"> <D:response> <D:propstat>
       <D:status>HTTP/1.1 200 OK</D:status>
       <D:prop>
         <D:owner>
           <D:href>http://www.foo.org/users/gclemm</D:href> </D:owner>

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 12]
         <D:supported-privilege-set>
           <D:supported-privilege>
             <D:privilege> <D:all/> </D:privilege>
             <D:abstract/>
             <D:description>Any operation</D:description>
             <D:supported-privilege>
               <D:privilege> <D:read/> </D:privilege>
               <D:description>Read any object</D:description>
             </D:supported-privilege>
             <D:supported-privilege>
               <D:privilege> <D:write/> </D:privilege>
               <D:abstract/>
               <D:description>Write any object</D:description>
               <D:supported-privilege>
                 <D:privilege> <A:create/> </D:privilege>
                 <D:description>Create an object</D:description>
               </D:supported-privilege>
               <D:supported-privilege>
                 <D:privilege> <A:update/> </D:privilege>
                 <D:description>Update an object</D:description>
               </D:supported-privilege>
               <D:supported-privilege>
                 <D:privilege> <A:delete/> </D:privilege>
                 <D:description>Delete an object</D:description>
               </D:supported-privilege>
             </D:supported-privilege>
             <D:supported-privilege>
               <D:privilege> <D:read-acl/> </D:privilege>
               <D:description>Read the ACL</D:description>
             </D:supported-privilege>
             <D:supported-privilege>
               <D:privilege> <D:write-acl/> </D:privilege>
               <D:description>Write the ACL</D:description>
             </D:supported-privilege>
           </D:supported-privilege>
         </D:supported-privilege-set>
         <D:current-user-privilege-set>
           <D:privilege> <D:read/> </D:privilege>
           <D:privilege> <D:read-acl/> </D:privilege>

Clemm, Hopkins, Sedlar, Whitehead                            [Page 15] 
         </D:current-user-privilege-set>
         <D:acl>
           <D:ace>
             <D:principal>
               <D:href>http://www.foo.org/users/esedlar</D:href>
               <D:prop>
                  <D:authentication-id>esedlar</D:authentication-id>
                 <D:displayname>Eric Sedlar</D:displayname>
               </D:prop> </D:principal>
             <D:grant>
               <D:privilege> <D:read/> </D:privilege>
               <D:privilege> <D:write/> </D:privilege>
               <D:privilege> <D:read-acl/> </D:privilege> </D:grant>
           </D:ace>
           <D:ace>
             <D:principal>

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 13]
               <D:href>http://www.foo.org/groups/marketing/</D:href>
             </D:principal>
             <D:deny>
               <D:privilege> <D:read/> </D:privilege> </D:deny>
           </D:ace>
           <D:ace>
             <D:principal>
               <D:property> <D:owner/> </D:property> </D:principal>
             <D:grant>
               <D:privilege> <D:read-acl/> </D:privilege>
               <D:privilege> <D:write-acl/> </D:privilege> </D:grant>
           </D:ace>
           <D:ace>
             <D:principal> <D:all/> </D:principal>
             <D:grant>
               <D:privilege> <D:read/> </D:privilege> </D:grant> </D:privilege></D:grant>
             <D:inherited>
               <D:href>http://www.foo.org/top/</D:href> </D:inherited>
           </D:ace> </D:acl>
         </D:prop>
       </D:propstat> </D:response> </D:multistatus>

     The value of the DAV:owner property is a single DAV:href XML
     element containing the URL of the principal that owns this
     resource.

     The value of the DAV:supported-privilege-set property is a tree of
     supported privileges:

        DAV:acl

       DAV:all (aggregate, abstract)
           |
         +-- DAV:read
         +-- DAV:write (aggregate, abstract)
              |
              +-- http://www.acl.org/create http://www.webdav.org/acl/create
              +-- http://www.acl.org/update http://www.webdav.org/acl/update
              +-- http://www.acl.org/delete http://www.webdav.org/acl/delete

Clemm, Hopkins, Sedlar, Whitehead                            [Page 16] 
           +-- DAV:read-acl
           +-- DAV:write-acl

     The DAV:current-user-privilege-set property contains two
     privileges, DAV:read, and DAV:read-acl. This indicates that the
     current authenticated user only has the ability to read the
     resource, and read the DAV:acl property on the resource.

     The DAV:acl property contains a set of four ACEs:

     ACE #1: The principal identified by the URL
     http://www.foo.org/users/esedlar is granted the DAV:read,
     DAV:write, and DAV:read-acl privileges.

     ACE #2: The principals identified by the URL
     http://www.foo.org/groups/marketing/ are denied the DAV:read
     privilege.  In this example, the principal URL identifies a group,
     which is represented by a collection principal.

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 14]

     ACE #3: In this ACE, the principal is a property principal,
     specifically the DAV:owner property. When evaluating this ACE, the
     value of the DAV:owner property is retrieved, and is examined to
     see if it contains a DAV:href XML element. If so, the URL within
     the DAV:href element is read, and identifies a principal. In this
     ACE, the owner is granted DAV:read-acl, and DAV:write-acl
     privileges.

     ACE #4: This ACE grants the DAV:all principal (all users) the
     DAV:read privilege. This ACE is inherited from the resource
     http://www.foo.org/top/, the parent collection of this resource.

6  ACL SEMANTICS

     The ACL semantics define how multiple ACEs that match the current
     user are combined, what are the constraints on how ACEs can be
     ordered, and which principals must have an ACE.

     <!ELEMENT acl-semantics acl-sem*>

     <!ELEMENT acl-sem (ace-combination, ace-ordering, required-
      principal)>
     principal*)>

6.1 ACE Combination

     The DAV:ace-combination element defines how privileges from
     multiple ACEs that match the current user will be combined to
     determine the access privileges for that user.  Multiple ACEs may
     match the same user because the same principal can appear in
     multiple ACEs, because multiple principals can identify the same
     user, and because one principal can be a member of another
     principal.

     <!ELEMENT ace-combination

Clemm, Hopkins, Sedlar, Whitehead                            [Page 17] 
      (first-match | all-grant-before-any-deny | no-deny)> specific-deny-
       overrides-grant)>

6.1.1 DAV:first-match ACE Combination

     The ACEs are evaluated in the order in which they appear in the
     ACL.  If the first ACE that matches the current user does not grant
     all the privileges needed for the request, the request MUST fail.

     <!ELEMENT first-match EMPTY>

6.1.2 DAV:all-grant-before-any-deny ACE Combination

     The ACEs are evaluated in the order in which they appear in the
     ACL.  If an evaluated ACE denies a privilege needed for the
     request, the request MUST fail.  If all ACEs have been evaluated
     without the user being granted all privileges needed for the
     request, the request MUST fail.

     <!ELEMENT all-grant-before-any-deny EMPTY>

6.1.3 DAV:no-deny DAV:specific-deny-overrides-grant ACE Combination

     All ACEs in the ACL are evaluated.  An "individual ACE" is one
     whose principal identifies the current user.  A "group ACE" is one
     whose

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 15] principal is a collection that contains a principal that
     identifies the current user.  A privilege is granted if it is
     granted by an individual ACE and not denied by an individual ACE,
     or if it is granted by a group ACE and not denied by an individual
     or group ACE.  A request MUST fail if any of its needed privileges
     are not granted.

     <!ELEMENT no-deny specific-deny-overrides-grant EMPTY>

6.2 ACE Ordering

     The DAV:ace-ordering element defines a constraint on how the ACEs
     can be ordered in the ACL.

     <!ELEMENT ace-ordering (deny-before-grant)? >

6.2.1 DAV:deny-before-grant ACE Ordering

     This element indicates that all deny ACEs must precede all grant
     ACEs.

     <!ELEMENT deny-before-grant EMPTY>

6.3 Required Principals

     The required principal elements identify which principals must have
     an ACE defined in the ACL.

     <!ELEMENT required-principal
       (href | all | authenticated | unauthenticated | property | self)>

Clemm, Hopkins, Sedlar, Whitehead                            [Page 18] 
     For example, the following element requires that the ACE ACL contain a
     DAV:owner property ACE:

     <D:required-principal xmlns:D="DAV:">
       <D:property> <D:owner/> </D:property>
     </D:required-principal>

7  ACCESS CONTROL AND EXISTING METHODS

     This section defines the impact of access control functionality on
     existing methods.

7.1 OPTIONS

     If the server supports access control, it MUST return "access-
     control" as a field in the DAV response header from an OPTIONS
     request on any resource implemented by that server.

 7.1.1Example

7.1.1 Example - OPTIONS

     >> REQUEST Request <<

       OPTIONS /foo.html HTTP/1.1
       Host: www.webdav.org
       Content-Length: 0

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 16]

     >> RESPONSE Response <<

       HTTP/1.1 200 OK
       DAV: 1, 2, access-control
       Allow: OPTIONS, GET, PUT, PROPFIND, PROPPATCH, ACL

     In this example, the OPTIONS response indicates that the server
     supports access control and that /foo.html can have its access
     control list modified by the ACL method.

8  ACCESS CONTROL METHODS

8.1 ACL

   A

     The ACL method modifies the DAV:acl property of a resource is modified by the ACL method. resource.  A new
     DAV:acl value must be written in its entirety, including any
     inherited ACEs.  Unless the DAV:acl property of the resource can be
     updated to be exactly the value specified in the ACL request, the
     ACL request MUST fail.  If a server restricts the set of ACEs
     visible to the current user via the DAV:acl property, then the ACL
     request would only replace the set of ACEs visible to the current
     user, and would not affect any ACE that was not visible.

     In order to avoid overwriting DAV:acl changes by another client, a
     client SHOULD acquire a WebDAV lock on the resource before
     retrieving the DAV:acl property of a resource that it intends on
     updating.

Clemm, Hopkins, Sedlar, Whitehead                            [Page 19] 
     When submitting ACEs, clients MUST NOT include the optional prop
     element (a child of the principal element). The purpose of this
     restriction is to limit the scope of effect of the ACL method to
     just the resource identified by the Request-URI; setting the prop
     element would additionally require property modification for one or
     more principal resources.

8.1.1 ACL Preconditions

     An implementation MAY enforce one or more of the following
     constraints on an ACL request.  If the constraint is violated, a
     403 (Forbidden) response MUST be returned and the indicated XML
     element MUST be returned in the response body.

     <DAV:protected/>: An implementation MAY protect an ACE from
     modification or deletion.  For example, some implementations
     implicitly grant the DAV:owner of a resource DAV:read-acl and
     DAV:write-acl privileges, and this cannot be changed by a client.

     <DAV:too-many-aces/>: An implementation MAY limit the number of
     ACEs in an ACL.  However, ACL-compliant servers MUST support at
     least one ACE granting privileges to a single principal, and one
     ACE granting privileges to a collection principal.

     <DAV:non-inherited-must-precede-inherited/>: All non-inherited ACEs
     MUST precede all inherited ACEs.

     <DAV:deny-must-precede-grant/>: All non-inherited deny ACEs MUST
     precede all non-inherited grant ACEs.

     If the following precondition is not met, the server MUST return a
     409 (Conflict) response, and the indicated XML element MUST be
     returned in the response body:

     <DAV:inhereted-exist-parent>: Inherited ACEs MUST exist on a parent
     resource.

8.1.2 Example: the ACL method

     In the following example, user "fielding", authenticated by
     information in the Authorization header, grants the principal

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 17]
     identified by the URL http://www.foo.org/users/esedlar  (i.e., the
     user "esedlar") read and write privileges, grants the owner of the
     resource read-acl and write-acl privileges, and grants everyone
     read privileges inherited from the parent collection
     http://www.foo.bar/top/.

     >> Request <<

     ACL /top/container/ HTTP/1.1
     Host: www.foo.org
     Content-Type: text/xml; charset="utf-8"
     Content-Length: xxxx
     Authorization: Digest username="fielding",
        realm="users@foo.org", nonce="...",

Clemm, Hopkins, Sedlar, Whitehead                            [Page 20] 
        uri="/top/container/", response="...", opaque="..."

     <?xml version="1.0" encoding="utf-8" ?>
     <D:acl xmlns:D="DAV:">
       <D:ace>
         <D:principal>
           <D:href>http://www.foo.org/users/esedlar</D:href>
         </D:principal>
         <D:grant>
           <D:privilege> <D:read/> </D:privilege>
           <D:privilege> <D:write/> </D:privilege> </D:grant>
       </D:ace>
       <D:ace>
         <D:principal>
           <D:property> <D:owner/> </D:property> </D:principal>
         <D:grant>
           <D:privilege> <D:read-acl/> </D:privilege>
           <D:privilege> <D:write-acl/> </D:privilege> </D:grant>
       </D:ace>
       <D:ace>
         <D:principal> <D:all/> </D:principal>
         <D:grant>
           <D:privilege> <D:read/> </D:privilege></D:grant>
         <D:inherited>
           <D:href>http://www.foo.org/top/</D:href> </D:inherited>
       </D:ace> </D:acl>

     >> Response <<

     HTTP/1.1 200 OK

8.1.3 Example: ACL method failure due to omission of protected ACE

     In the following request, user "fielding", authenticated by
     information in the Authorization header, attempts to grant the
     principal identified by the URL http://www.foo.org/users/esedlar
     (i.e., the user "esedlar") read privileges, but fails because an
   protected ACE has been omitted (e.g. privileges. Prior to the request,
     the DAV:acl property on the resource contained a protected ACE (see
     Section 5.4.3) granting the DAV:owner the DAV:read-acl and DAV:write-acl privileges must always be present
   since it is DAV:write-
     acl privileges. The ACL method invocation fails because this
     protected -- see Section 5.4.3).

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 18] ACE is omitted, thus violating the semantics of ACE
     protection..

     >> Request <<

     ACL /top/container/ HTTP/1.1
     Host: www.foo.org
     Content-Type: text/xml; charset="utf-8"
     Content-Length: xxxx
     Authorization: Digest username="fielding",
        realm="users@foo.org", nonce="...",
        uri="/top/container/", response="...", opaque="..."

     <?xml version="1.0" encoding="utf-8" ?>

Clemm, Hopkins, Sedlar, Whitehead                            [Page 21] 
     <D:acl xmlns:D="DAV:">
       <D:ace>
         <D:principal>
           <D:href>http://www.foo.org/users/esedlar</D:href>
         </D:principal>
         <D:grant>
           <D:privilege> <D:read/> </D:privilege> </D:grant>
       </D:ace>
     </D:acl>

     >> Response <<

     HTTP/1.1 403 Forbidden
     Content-Type: text/xml; charset="utf-8"
     Content-Length: xxx

     <?xml version="1.0" encoding="utf-8" ?>
     <DAV:protected/>

8.1.4 Example: ACL method failure due to inherited ACEs preceding non-
      inherited ACEs

     In the following request, user "ejw", authenticated by information
     in the Authorization header, tries to change the access control
     list on the resource http://www.foo.org/top/index.html. This
     resource has two inherited ACEs.

     Inherited ACE #1 grants the principal identified by URL
     http://www.foo.org/users/ejw (i.e., the user "ejw")
     http://www.foo.org/privs/write-all and DAV:read-acl privileges. On
     this server, http://www.foo.org/privs/write-all is an aggregate
     privilege containing DAV:write, and DAV:write-acl.

     Inherited ACE #2 grants principal DAV:all the DAV:read privilege.

     The request attempts to add a third ACE, granting the principal
     identified by the URL http://www.foo.org/users/gclemm (i.e., the
     user "gclemm") ôgclemmö) DAV:write permission, but in the request places the
     inherited ACEs before the non-inherited ACEs, causing an error on
     this specific server implementation. Note that on a different
     implementation, this request might be accepted.

     >> Request <<

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 19]

     ACL /top/index.html HTTP/1.1
     Host: www.foo.org
     Content-Type: text/xml; charset="utf-8"
     Content-Length: xxxx
     Authorization: Digest username="ejw",
        realm="users@foo.org", nonce="...",
        uri="/top/index.html", response="...", opaque="..."

     <?xml version="1.0" encoding="utf-8" ?>
     <D:acl xmlns:D="DAV:" xmlns:F="http://www.foo.org/privs/">

Clemm, Hopkins, Sedlar, Whitehead                            [Page 22] 
       <D:ace>
         <D:principal>
           <D:href>http://www.foo.org/users/ejw</D:href>
         </D:principal>
         <D:grant>
           <D:privilege><F:write-all/></D:privilege>
           <D:privilege><D:read-acl/></D:privilege>
         </D:grant>
         <D:inherited/>
       </D:ace>
       <D:ace>
         <D:principal><D:all/></D:principal>
         <D:grant><D:read/></D:grant>
         <D:inherited/>
       </D:ace>
       <D:ace>
         <D:principal>
           <D:href>http://www.foo.org/users/gclemm</D:href>
         </D:principal>
         <D:grant><D:write/></D:grant>
       </D:ace>
     </D:acl>

     >> Response <<

     HTTP/1.1 403 Forbidden
     Content-Type: text/xml; charset="utf-8"
     Content-Length: xxx

     <?xml version="1.0" encoding="utf-8" ?>
     <DAV:non-inherited-must-precede-inherited/>

8.1.5 Example: ACL method failure due to an attempt to set grant and deny
      in a single ACE.

     In this example, user "ygoland", authenticated by information in
     the Authorization header, tries to change the access control list
     on the resource http://www.foo.org/diamond/engagement-ring.gif. The
     ACL request includes a single, syntactically and semantically
     incorrect ACE, which attempts to grant the collection principal
     identified by the URL http://www.foo.org/users/friends/ DAV:read
     privilege and deny the principal identified by URL
     http://www.foo.org/users/ygoland-so (i.e., the user "ygoland-so")
     DAV:read privilege. However, it is illegal to have multiple

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 20]
     principal elements, as well as both a grant and deny element in the
     same ACE, so the request fails due to poor syntax.

     >> Request <<

     ACL /diamond/engagement-ring.gif HTTP/1.1
     Host: www.foo.org
     Content-Type: text/xml; charset="utf-8"
     Content-Length: xxxx
     Authorization: Digest username="ygoland",

Clemm, Hopkins, Sedlar, Whitehead                            [Page 23] 
        realm="users@foo.org", nonce="...",
        uri="/diamond/engagement-ring.gif", response="...", opaque="..."

     <?xml version="1.0" encoding="utf-8" ?>
     <D:acl xmlns:D="DAV:">
       <D:ace>
         <D:principal>
           <D:href>http://www.foo.org/users/friends/</D:href>
         </D:principal>
         <D:grant><D:read/></D:grant>
         <D:principal>
           <D:href>http://www.foo.org/users/ygoland-so</D:href>
         </D:principal>
         <D:deny><D:read/></D:deny>
       </D:ace>
     </D:acl>

     >> Response <<

     HTTP/1.1 400 Bad Request
     Content-Length: 0

     Note that if the request had been divided into two ACEs, one to
     grant, and one to deny, the request would have been syntactically
     well formed.

9  INTERNATIONALIZATION CONSIDERATIONS

     In this specification, the only human-readable content can be found
     in the DAV:authentication-id property, description XML element, found on principal resources. within the DAV:supported-
     privilege-set property.  This property element contains the name used to authenticate a principal,
   typically human-readable
     description of the capabilities controlled by a user entering this name into a password entry screen. privilege.  As a
     result, the authentication-id description element must be capable of representing
   names
     descriptions in multiple character sets.  Since DAV:authentication-id the description
     element is found within a WebDAV property, it is represented on-the-wire on-
     the-wire as XML [REC-XML], and hence can leverage XML's language
     tagging and character set encoding capabilities. Specifically, XML
     processors must, at minimum, be able to read XML elements encoded
     using the UTF-8 [UTF-8] encoding of the ISO 10646 multilingual
     plane. XML examples in this specification demonstrate use of the
     charset parameter of the Content-Type header, as defined in
     [RFC3023], as well as the XML "encoding" attribute, which together
     provide charset identification information for MIME and XML
     processors.

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 21]

     For properties XML elements other than DAV:authentication-id, the description element, it is expected
     that implementations will treat the property names names, privilege
     names, and values as tokens, and convert these tokens into human-readable human-
     readable text in the user's language and character set when
     displayed to a person.  Only a generic WebDAV property display
     utility would display these values in their raw form. form to a human
     user.

Clemm, Hopkins, Sedlar, Whitehead                            [Page 24] 
     For error reporting, we follow the convention of HTTP/1.1 status
     codes, including with each status code a short, English description
     of the code (e.g., 200 (OK)).  While the possibility exists that a
     poorly crafted user agent would display this message to a user,
     internationalized applications will ignore this message, and
     display an appropriate message in the user's language and character
     set.

     Further internationalization considerations for this protocol are
     described in the WebDAV Distributed Authoring protocol
     specification [RFC2518].

10 SECURITY CONSIDERATIONS

     Applications and users of this access control protocol should be
     aware of several security considerations, detailed below. In
     addition to the discussion in this document, the security
     considerations detailed in the HTTP/1.1 specification [RFC2616],
     the WebDAV Distributed Authoring Protocol specification [RFC2518],
     and the XML Media Types specification [RFC3023] should be
     considered in a security analysis of this protocol.

10.1 Increased Risk of Compromised Users

     In the absence of a mechanism for remotely manipulating access
     control specifications, lists, if a single user's authentication credentials are
     compromised, only those resources for which the user has access
     permission can be read, modified, moved, or deleted. With the
     introduction of this access control protocol, if a single
     compromised user has the ability to change ACLs for a broad range
     of other users (e.g., a super-user), the number of resources that
     could be altered by a single compromised user increases. This risk
     can be mitigated by limiting the number of people who have write-acl write-
     acl privileges across a broad range of resources.

10.2 Authentication-id Property and Dictionary Attacks

   Every principal has a DAV:authentication-id property defined on it,
   which provides the name used to authenticate this principal,
   typically the username portion of a username/password authentication
   scheme. An attacker can use the information in this property when
   attempting either a brute-force, or a dictionary attack to guess the
   principal's identifying password. By providing the username in
   DAV:authentication-id, the scope of an attack can be reduced to a
   single, valid username. Furthermore, it is possible that principals
   can potentially belong to a collection. In this case, it is possible
   to use the PROPFIND method to retrieve the DAV:authentication-id
   property from all of the principals in a collection, thus providing
   multiple usernames that can be the focus of attack.

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 22] 
   To reduce this risk, the DAV:authentication-id property should not
   be world-readable. Which principals are granted default read
   privilege for DAV:authentication-id should be carefully considered
   in any deployment of this protocol.

 10.3 Risks of the read-acl Privilege and cuprivset Privileges

     The ability to read the access privileges (stored in the DAV:acl
     property), or the privileges permitted the currently authenticated
     user (stored in the DAV:current-user-privilege-set property) on a
     resource may seem innocuous, since reading an ACL cannot possibly
     affect the resource's state. However, if all resources have world-
     readable ACLs, it is possible to perform an exhaustive search for
     those resources that have inadvertently left themselves in a
     vulnerable state, such as being world-writeable. In particular, the
     property retrieval method PROPFIND, executed with Depth infinity on
     an entire hierarchy, is a very efficient way to retrieve the
     DAV:acl or DAV:current-user-privilege-set properties. Once found,
     this vulnerability can be exploited by a denial of service attack
     in which the open resource is repeatedly overwritten. Alternately,
     writeable resources can be modified in undesirable ways.

     To reduce this risk, read-acl privileges should not be granted to
     unauthenticated principals, and restrictions on read-acl and

Clemm, Hopkins, Sedlar, Whitehead                            [Page 25] 
     cuprivset privileges for authenticated principals should be
     carefully analyzed when deploying this protocol. Access to the
     current-user-privilege-set property will involve a tradeoff of
     usability versus security. When the current-user-privilege-set is
     visible, user interfaces are expected to provide enhanced
     information concerning permitted and restricted operations, yet
     this information may also indicate a vulnerability that could be
     exploited. Deployment of this protocol will need to evaluate this
     tradeoff in light of the requirements of the deployment
     environment.

11 AUTHENTICATION

     Authentication mechanisms defined in WebDAV also apply to this
     WebDAV Access Control Protocol, in particular the Basic and Digest
     authentication mechanisms defined in [RFC2617].

12 IANA CONSIDERATIONS

     This document uses the namespace defined by [RFC2518] for XML
     elements.  All other IANA considerations mentioned in [RFC2518]
     also applicable to WebDAV ACL.

13 INTELLECTUAL PROPERTY

     The following notice is copied from RFC 2026, section 10.4, and
     describes the position of the IETF concerning intellectual property
     claims made against this document.

     The IETF takes no position regarding the validity or scope of any
     intellectual property or other rights that might be claimed to
     pertain to the implementation or use other technology described in
     this document or the extent to which any license under such rights
     might or might not be available; neither does it represent that it
     has made any effort to identify any such rights.  Information on
     the IETF's procedures with respect to rights in standards-track and
     standards-related documentation can be found in BCP-11.  Copies of
     claims of rights made available for publication and any assurances
     of licenses to be made available, or the result of an attempt made

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 23]
     to obtain a general license or permission for the use of such
     proprietary rights by implementers or users of this specification
     can be obtained from the IETF Secretariat.

     The IETF invites any interested party to bring to its attention any
     copyrights, patents or patent applications, or other proprietary
     rights that may cover technology that may be required to practice
     this standard.  Please address the information to the IETF
     Executive Director.

14 ACKNOWLEDGEMENTS

     This protocol is the collaborative product of the WebDAV ACL design
     team: Bernard Chester, Geoff Clemm (Rational), Anne Hopkins

Clemm, Hopkins, Sedlar, Whitehead                            [Page 26] 
     (Microsoft), Barry Lind (Xythos), Sean Lyndersay (Microsoft), Eric
     Sedlar (Oracle), Greg Stein (Apache.org), and Jim Whitehead (UC
     Santa Cruz). The authors are grateful for the detailed review and
     comments provided by Jim Amsden, Gino Basso, Murthy Chintalapati,
     Dennis Hamilton, Laurie Harper, Ron Jacobs, Chris Knight, and Remy
     Maucherat. Prior work on WebDAV access control protocols has been
     performed by Yaron Goland, Paul Leach, Lisa Dusseault, Howard
     Palmer, and Jon Radoff. We would like to acknowledge the foundation
     laid for us by the authors of the WebDAV and HTTP protocols upon
     which this protocol is layered, and the invaluable feedback from
     the WebDAV working group.

15 REFERENCES

15.1 Normative References

     [RFC2119] S.Bradner, "Key words for use in RFCs to Indicate
     Requirement Levels." RFC 2119, BCP 14, Harvard, March, 1997.

     [REC-XML] T. Bray, J. Paoli, C.M. Sperberg-McQueen, "Extensible
     Markup Language (XML)." World Wide Web Consortium Recommendation
     REC-xml-19980210. http://www.w3.org/TR/REC-xml-19980210.

     [RFC2616] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, L.
     Masinter, P. Leach, and T. Berners-Lee, "Hypertext Transfer
     Protocol -- HTTP/1.1." RFC 2616. U.C.Irvine, Compaq, Xerox,
     Microsoft, MIT/LCS, June, 1999.

     [RFC2617] J. Franks, P. Hallam-Baker, J. Hostetler, S. Lawrence, P.
     Leach, A. Luotonen, L. Stewart, "HTTP Authentication: Basic and
     Digest Access Authentication. " RFC 2617. Northwestern University,
     Verisign, AbiSource, Agranat, Microsoft, Netscape, Open Market,
     June, 1999.

     [RFC2518] Y. Goland, E. Whitehead, A. Faizi, S. R. Carter, D.
     Jensen, "HTTP Extensions for Distributed Authoring -- WEBDAV." RFC
     2518. Microsoft, U.C.Irvine, Netscape, Novell, February, 1999.

     [RFC2368] P. Hoffman, L. Masinter, J. Zawinski, "The mailto URL
     scheme." RFC 2368. Internet Mail Consortium, Xerox, Netscape, July,
     1998.

     [RFC2255] T. Howes, M. Smith, "The LDAP URL Format." RFC 2255.
     Netscape, December, 1997.

     [RFC3023] M. Murata, S. St.Laurent, D. Kohn, "XML Media Types." RFC
     3023. IBM Tokyo Research Laboratory, simonstl.com, Skymoon
     Ventures, January, 2001.

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 24]

     [UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode and
     ISO 10646." RFC 2279. Alis Technologies. January, 1998.

 15.2Informational

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15.2 Informational References

     [RFC2026] S.Bradner, "The Internet Standards Process û -- Revision 3."
     RFC 2026, BCP 9. Harvard, October, 1996.

16 AUTHORS' ADDRESSES

     Geoffrey Clemm
     Rational Software
     20 Maguire Road
     Lexington, MA 02421
     Email: geoffrey.clemm@rational.com

     Anne Hopkins
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     Email: annehop@microsoft.com

     Eric Sedlar
     Oracle Corporation
     500 Oracle Parkway
     Redwood Shores, CA 94065
     Email: esedlar@us.oracle.com

     Jim Whitehead
     U.C. Santa Cruz
     Dept. of Computer Science
     Baskin Engineering
     1156 High Street
     Santa Cruz, CA 95064
     Email: ejw@cse.ucsc.edu

17 APPENDICIES

 17.1XML

17.1 XML Document Type Definition

     <!-- Privileges -->

     <!ELEMENT read EMPTY>
     <!ELEMENT write EMPTY>
     <!ELEMENT read-acl EMPTY>
     <!ELEMENT read-cuprivset EMPTY>
     <!ELEMENT write-acl EMPTY>
     <!ELEMENT all EMPTY>

     <!-- Principal Properties (Section 4) -->

     <!ELEMENT is-principal (#PCDATA)>

     <!ELEMENT alternate-URL (href*)>

Clemm, Hopkins, Sedlar, Whitehead                            [Page 25] 
      <!ELEMENT is-principal (#PCDATA)>
      <!ELEMENT authentication-id (#PCDATA)> 28] 
     <!-- Access Control Properties (Section 5) -->

     <!-- DAV:owner Property (Section 5.1) -->

     <!ELEMENT owner (href prop?)>
     <!ELEMENT prop (see [RFC2518], section 12.11)>

     <!-- DAV:supported-privilege-set Property (Section 5.2) -->

     <!ELEMENT supported-privilege-set (supported-privilege*)>
     <!ELEMENT supported-privilege
      (privilege, abstract?, description, supported-privilege*)>

     <!ELEMENT privilege ANY>
     <!ELEMENT abstract EMPTY>
     <!ELEMENT description #PCDATA>
     <!ELEMENT privilege ANY>

     <!-- DAV:current-user-privilege-set Property (Section 5.3) -->

     <!ELEMENT current-user-privilege-set (privilege*)>

     <!-- DAV:acl Property (Section 5.4) -->

     <!ELEMENT acl (ace*)>

     <!ELEMENT ace (principal, (grant|deny), protected?, inherited?)>
     <!ELEMENT principal ((href, prop?)
      | all | authenticated | unauthenticated
      | property | self)>

     <!ELEMENT prop (see [RFC2518], section 12.11)>
     <!ELEMENT all EMPTY>
     <!ELEMENT authenticated EMPTY>
     <!ELEMENT unauthenticated EMPTY>
     <!ELEMENT property ANY>
     <!ELEMENT self EMPTY>

     <!ELEMENT grant (privilege+)>
     <!ELEMENT deny (privilege+)>
     <!ELEMENT privilege ANY>

     <!ELEMENT protected EMPTY>

     <!ELEMENT inherited (href)>

     <!-- DAV:principal-collection-set Property (Section 5.6) -->

     <!ELEMENT principal-collection-set (href*)>

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      <!ELEMENT principal-collection-set (href*)> 29] 
     <!-- DAV:acl-semantics Property (Section 6) -->

     <!ELEMENT acl-semantics acl-sem*>
     <!ELEMENT acl-sem (ace-combination, ace-ordering, required-
      principal)>
     principal*)>

     <!ELEMENT ace-combination
      (first-match | all-grant-before-any-deny | no-deny)> specific-deny-
     overrides-grant)>
     <!ELEMENT first-match EMPTY>
     <!ELEMENT all-grant-before-any-deny EMPTY>
     <!ELEMENT no-deny specific-deny-overrides-grant EMPTY>

     <!ELEMENT ace-ordering (deny-before-grant)? >
     <!ELEMENT deny-before-grant EMPTY>

     <!ELEMENT required-principal
       (href | all | authenticated | unauthenticated | property | self)>

     <!-- ACL method preconditions (Section 8.1.1) -->

     <!ELEMENT protected EMPTY>
     <!ELEMENT too-many-aces EMPTY>
     <!ELEMENT non-inherited-must-precede-inherited EMPTY>
     <!ELEMENT deny-must-precede-grant EMPTY>
     <!ELEMENT acl-requires-lock-token EMPTY>
     <!ELEMENT inherited-exist-parent EMPTY>

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