INTERNET-DRAFT                   Geoffrey Clemm, Rational Software
     draft-ietf-webdav-acl-03
 draft-ietf-webdav-acl-04         Anne Hopkins, Microsoft Corporation
                                  Eric Sedlar, Oracle Corporation
                                  Jim Whitehead, U.C. Santa Cruz

 Expires May 24, July 21, 2001            January 21, 2001            November 24, 2000

                     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.

 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......................................................3
 1.1 Terms .................................................3  Terms..........................................................4
 1.2  Notational Conventions ................................4 Conventions.........................................5

 2 PRINCIPALS ..............................................4 PRINCIPALS........................................................5

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

 4 PRINCIPAL PROPERTIES ....................................6 PROPERTIES..............................................7
 4.1 DAV:is-principal ......................................6  DAV:is-principal...............................................7
 4.2 DAV:authentication-id .................................6  DAV:authentication-id..........................................7

 5 ACCESS CONTROL PROPERTIES ...............................7 PROPERTIES.........................................8
 5.1 DAV:owner .............................................7  DAV:owner......................................................8
 5.2 DAV:supported-privilege-set ...........................7  DAV:supported-privilege-set....................................8
 5.3 DAV:current-user-privilege-set ........................8  DAV:current-user-privilege-set.................................9
 5.4 DAV:acl ...............................................8  DAV:acl........................................................9
  5.4.1 ACE Principal .....................................8 Principal................................................9
  5.4.2 ACE Grant and Deny ................................9 Deny..........................................10
  5.4.3 ACE Protection ...................................10 Protection..............................................11
  5.4.4 ACE Inheritance ..................................10 Inheritance.............................................11
 5.5 DAV:acl-semantics ....................................10
      5.5.1  first-match Semantics ............................14
      5.5.2  all-grant-before-any-deny Semantics ..............14
      5.5.3  no-deny Semantics ................................14  DAV:acl-semantics.............................................11
 5.6 DAV:principal-collection-set .........................10  DAV:principal-collection-set..................................11
 5.7  Example: PROPFIND to retrieve access control properties11 properties.......12

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

 7 ACCESS CONTROL AND EXISTING METHODS ....................14
     6.1 OPTIONS ..............................................15
      6.1.1 METHODS..............................16
 7.1  OPTIONS.......................................................16
  7.1.1 Example - OPTIONS ................................15

     7 OPTIONS...........................................16

 8 ACCESS CONTROL METHODS .................................16
     7.1 ACL ..................................................16
      7.1.1 METHODS...........................................17
 8.1  ACL...........................................................17
  8.1.1 ACL Preconditions ................................16
      7.1.2 Preconditions...........................................17
  8.1.2 Example: the ACL method.....................................17
  8.1.3 Example: ACL method ..........................17
      7.1.3 failure due to omission of protected ACE18
  8.1.4 Example: ACL method failure ......................17

     8 INTERNATIONALIZATION CONSIDERATIONS ....................18

     9 SECURITY CONSIDERATIONS ................................19

     10  AUTHENTICATION .......................................20

     11  IANA CONSIDERATIONS ..................................20

     12  INTELLECTUAL PROPERTY ................................20

     13  ACKNOWLEDGEMENTS .....................................21 due to inherited ACEs preceding
  non-inherited ACEs................................................19
  8.1.5 Example: ACL method failure due to an attempt to set grant and
  deny in a single ACE..............................................20

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

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

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

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

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

 14  REFERENCES ...........................................21  ACKNOWLEDGEMENTS...............................................24

 15  AUTHORS' ADDRESSES ...................................22  REFERENCES.....................................................24
 15.1 Normative References..........................................24
 15.2 Informational References......................................25

 16  STILL TO DO ..........................................22  AUTHORS' ADDRESSES.............................................25

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

 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
          rights 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 has intentionally omits discussion of authentication,
   as the HTTP protocol already has a number of authentication mechanisms[RFC2617] .
   mechanisms [RFC2617].  Some authentication mechanism (such as HTTP
   Digest Authentication, which all WebDAV compliant implementations
   are required to support) must be availableto available to validate the identity
   of a principal.

   In the interests of timeliness, the following set of security
   mechanisms is currently viewed as out of scope of are not addressed by this document:

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        * Access control that applies only to a particular property on
          a resource, rather than the entire resource. resource,

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

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

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

     1.1 Terms resource.

   This draft uses specification is organized as follows. Section 1.1 defines key
   concepts used throughout the terms specification, and is followed by more
   in-depth discussion of principals (Section 2), and privileges
   (Section 3). Properties defined on principals are specified in HTTP [RFC2616]
   Section 4, and WebDAV
          [RFC2518].  In addition, the following terms access control properties for content resources are defined:

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 3] 
        principal

          A "principal" is a distinct human or computational actor that
          initiates
   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 network resources.  In this protocol, a
          principal 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.
   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 " privilege" is a privilege that comprises contains a set of other
   privileges.

   abstract privilege

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

   access control element (ace)

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

   inherited ace

   An "inherited ace " ace" is an ace that is shared from the acl of another
   resource.

 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 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.   Although
   an implementation MAY support PROPFIND and PROPPATCH to access and
   modify information about a principal, it is not required to do so.

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 4]

   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

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   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 aggregates 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 remove-
   member privileges that control the ability to add and remove an
   internal member of a collection.  Since these privileges control 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.

          The set of privileges that apply to a particular resource

   Privileges may
          vary with have the DAV:resourcetype quality of the resource, as well as
          between different server implementations.  To promote
          interoperability, however, WebDAV defines a being abstract, in which case
   they cannot be set in an ACE. Aggregate and atomic privileges are
   both capable of well-known being abstract. Abstract privileges (e.g. DAV:read and DAV:write), which can at least are useful for
   modeling privileges that otherwise would not be used to classify exposed via the
   protocol. Abstract privileges also provide server implementations
   with flexibility in implementing the other privileges defined on in this
   specification.  For example, if a
          particular resource.

     3.1 DAV:read Privilege

          The server is incapable of separating
   the read privilege controls 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.

 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.  The  Additionally, the read
   privilege does not MAY control the OPTIONS method since the
          OPTIONS method returns capabilities rather than state.

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 5] 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

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   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>

 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 of the
   resource.

      <!ELEMENT read-acl EMPTY>

 3.4 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>

 3.5 DAV:all Privilege

          The

   DAV:all is an aggregate privilege controls 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.

 4.1 DAV:is-principal

   This property 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" is suggested)

 4.2 DAV:authentication-id

   A property containing the name used to authenticate this principal
   (typically typed into a login prompt/dialog).

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

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 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 the
   PROPPATCH method.

   HTTP resources that support the WebDAV Access Control Protocol MUST
   contain the following properties:

 5.1 DAV:owner

   This property identifies a particular principal as being the "owner"
   of the resource.

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

   An implementation MAY include a list of selected properties of that
   principal resource.  Which properties (if any) are included is
   implementation defined.  An implementation MAY allow the use of
   PROPPATCH to update the DAV:owner field.

 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 is used to classify the non-abstract
          privilege elements.  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-

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 7] 
          known well-known privileges
   (defined by WebDAV or other standards groups) into privileges that
   are supported by any particular server implementation.  The

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   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

          This

   DAV:current-user-privilege-set is a read-only property containing a list
   the exact set of privileges (as computed by the server) granted to
   the currently authenticated HTTP user.  The current  A user-agent can use the
   value of this property to adjust its user has no access privileges interface to an object protected make actions
   inaccessible (e.g, by an ACL
          unless that user matches one graying out a menu item or more of button) for which
   the principals
          specified in current principal does not have permission. This is particularly
   useful for an access control user interface, which can be
   constructed without knowing the ACEs. ACE combining semantics of the
   server. This property is also useful for determine what operations
   can be performed by the current principal, without having to
   actually execute an operation.

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

          Each

   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 privilege from the DAV:supported-privilege-set DAV:supported-
   privilege-set property.

 5.4 DAV:acl

   This property 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)>

   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

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   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

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 8] included in the DAV:prop element is implementation
   defined.  The DAV:prop element is primarily intended for
   implementations that do not support PROPFIND requests on the
   principal URL.

      <!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>

 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 elements specified in the DAV:supported-privilege-set of
   that resource.

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

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 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.

   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 DAV:acl-
   semantics element is defined in section 6.

 5.6 DAV:principal-collection-set

          Often

   This read-only property contains zero, one, or more URLs that
   identify a versioning implementation constrains where collection principal. It is expected that implementations
   of this protocol will typically employ a principal
          can be located relatively small number of
   locations in the URL space.  The namespace for principal, and collection
   principals. In cases where this assumption holds, the DAV:principal-
   collection-set enumerates which collections may 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,
   query the DAV:displayname property (specified in Section 13.2 of
   [RFC2518]) of all principals on that server, thereby yielding human-
   readable names for each principal that could be displayed in a user
   interface.

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      <!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 . values. The collections specified in
   the DAV:principal-collection-set MAY be located on different hosts
   from the resource.

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 10] The URLs in DAV:principal-collection-set are not
   limited to http scheme URLs, and can, for example, be ldap 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, 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"> encoding="utf-8" ?>
      <D:propfind xmlns:D="DAV:">
        <D:owner/>
        <D:supported-privilege-set/>
        <D:current-user-privilege-set/>
        <D:acl/>
      </D:propfind>

      >> 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:D="DAV:"
         xmlns:A="http://www.acl.org/"> <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>
            <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: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>

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 11]
                  <D:description>Create an object</D:description>
                </D:supported-privilege>
                <D:supported-privilege>
                  <D:privilege> <A:update> <A:update/> </D:privilege>
                  <D:description>Update an object</D:description>
                </D:supported-privilege>
                <D:supported-privilege>
                  <D:privilege> <A:delete> <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:privilege> ACL</D:description>
              </D:supported-privilege>
              <D:supported-privilege>
                <D:privilege> <D:write-acl/> </D:privilege>
                <D:description>Write the ACL</D:privilege> 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>
          </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:privilege> </D:grant>
            </D:ace>
            <D:ace>
              <D:principal>
                    <D:href>http://www.foo.org/groups/marketing/</d:href>

 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: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:privilege> </D:grant>
            </D:ace>
            <D:ace>

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 12]
              <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:inheritied>
                <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 (abstract) (aggregate, abstract)
            |
          +-- DAV:read
          +-- DAV:write (abstract) (aggregate, abstract)
               |
               +-- http://www.acl.org/create
               +-- http://www.acl.org/update
               +-- http://www.acl.org/delete
            +-- 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

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 13]
   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 ANY>
           ANY value: zero or more of the ACL semantic elements acl-sem*>

      <!ELEMENT acl-sem (ace-combination, ace-ordering, required-
      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 rights 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
       (first-match | all-grant-before-any-deny | no-deny)>

 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 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 EMPTY>

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 14]

 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)>

   For example, the following element requires that the ACE 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.

     7.1.1 Example

 7.1.1Example - OPTIONS

          >>REQUEST

      >> REQUEST <<

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

          >>RESPONSE

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 16] 
      >> RESPONSE <<

        HTTP/1.1 200 OK
        DAV: 1, 2, access-control

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 15]
        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 DAV:acl property of a resource is modified by the ACL method.  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.

 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.

          <DAV:acl-requires-lock-token/>: If a resource is locked, the
          lock token MUST be specified in the ACL request.

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 16]

 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 /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" ?>
      <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: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

     8.1.3Example:

 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 implicit
   protected ACE has been

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 17] omitted (e.g. the ACE granting the DAV:owner
   DAV:read-acl and DAV:write-acl privileges). privileges must always be present
   since it is protected -- see Section 5.4.3).

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 18] 
      >> Request <<

      ACL /top/container /top/container/ HTTP/1.1
      Host: www.foo.bar 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" ?>
      <D:acl xmlns:D="DAV:">
        <D:ace>
          <D:principal>
                <D:href>http://www.foo.bar/users/esedlar</D:href>
            <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:cannot-change-implicit-ace/>

     9  INTERNATIONALIZATION CONSIDERATIONS
      <DAV:protected/>

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

   In this specification, the only human-readable content can be
          found following request, user "ejw", authenticated by information
   in the DAV:authentication-id property, found Authorization header, tries to change the access control list
   on
          principal resources. the resource http://www.foo.org/top/index.html. This property contains resource has
   two inherited ACEs.

   Inherited ACE #1 grants the name used to
          authenticate a principal, typically principal identified by a URL
   http://www.foo.org/users/ejw (i.e., the user entering "ejw")
   http://www.foo.org/privs/write-all and DAV:read-acl privileges. On
   this
          name into a password entry screen.  As 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 result, third ACE, granting the
          authentication-id must be capable of representing names principal
   identified by the URL http://www.foo.org/users/gclemm (i.e., the
   user "gclemm") 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/">
        <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
          multiple character sets.  Since DAV:authentication-id is 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",
         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, found on principal resources.
   This property contains the name used to authenticate a principal,
   typically by a user entering this name into a password entry screen.
   As a result, the authentication-id must be capable of representing
   names in multiple character sets.  Since DAV:authentication-id is a
   WebDAV property, it is represented on-the-wire as XML [REC-
          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 [RFC2376], [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 other than DAV:authentication-id, it is expected that
   implementations will treat the property names and values as tokens,
   and convert these tokens into human-

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 18] 
          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.

   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 (discussed
          in [RFC2376]) [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, 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), 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
   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

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 19] 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 permission
   privilege for DAV:authentication-id should be carefully considered
   in any deployment of this protocol.

 10.3 Risks of the read-acl Privilege

   The ability to read the access permissions privileges (stored in the DAV:acl
   property), or the privileges permitted the currently authenticated
   user (stored in the DAV:current-user-privilege-
          set 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 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 privileges
   for authenticated principals should be carefully
          analysed analyzed when
   deploying this protocol.

 11 AUTHENTICATION

   Authentication mechanisms defined in WebDAV will also apply to this
   WebDAV ACL. 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;

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 20] 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
   (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, 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] 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.

     Clemm, Hopkins, Sedlar, Whitehead                        [Page 21]

   [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.

   [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.

     15.2 Informational

 15.2Informational References

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

          [RFC2396] E. Whitehead, M. Murata, "XML Media Types." RFC
          2376. U.C. Irvine, Fuji Xerox Info. Systems. July, 1998. (This
          RFC will soon be superseded by <draft-murata-xml-09.txt>,
          which has been approved by the IESG as a Proposed Standard,
          but not yet issued as an RFC.)

 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 STILL TO DO

          * If we can add more elements to DAV:resourcetype, we can
            eliminate DAV:is-principal. APPENDICIES

 17.1XML Document Type Definition

      <!-- Privileges -->

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

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

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 22] 
          * Add back the XML schema if provides info not in the DTD's.

          * Consider adding a DAV:matching-principals, which identifies 25] 
      <!ELEMENT is-principal (#PCDATA)>
      <!ELEMENT authentication-id (#PCDATA)>

      <!-- 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 ACL principals that match the current user.

          * Add DAV:ordering-constraints, DAV:required-principals, and
            DAV:ace-combination-semantics as sub-elements of DAV:acl-
            semantics. | 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) -->

 Clemm, Hopkins, Sedlar, Whitehead                            [Page 23] 26] 
      <!ELEMENT principal-collection-set (href*)>

      <!-- DAV:acl-semantics Property (Section 6) -->

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

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