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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 3744

  INTERNET-DRAFT                   Geoffrey Clemm, IBM
  draft-ietf-webdav-acl-10         Anne Hopkins, Microsoft Corporation
                                   Eric Sedlar, Oracle Corporation
                                   Jim Whitehead, U.C. Santa Cruz
  
  Expires September 15, 2003       March 15, 2003
  
  
                           WebDAV Access Control Protocol
  
  
  Status of this Memo
  This document is an Internet-Draft and is subject to 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, message bodies,
  properties, and reports that define Access Control extensions to the
  WebDAV Distributed Authoring Protocol. This protocol permits a client to
  read and modify access control lists that instruct a server whether to
  allow or deny operations upon a resource (such as HyperText Transfer
  Protocol (HTTP) method invocations) by a given principal. A lightweight
  representation of principals as Web resources supports integration of a
  wide range of user management repositories. Search operations allow
  discovery and manipulation of principals using human names.
  
  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.example.com/acl/, and
  http://www.ics.uci.edu/pub/ietf/webdav/.
  
  
  
  
  
  
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  Table of Contents
  
  WEBDAV ACCESS CONTROL PROTOCOL............................1
  
  STATUS OF THIS MEMO.......................................1
  
  ABSTRACT..................................................1
  
  TABLE OF CONTENTS.........................................2
  
  1 INTRODUCTION...........................................4
  1.1 Terms.................................................6
  1.2 Notational Conventions................................7
  
  2 PRINCIPALS.............................................7
  
  3 PRIVILEGES.............................................8
  3.1 DAV:read Privilege....................................9
  3.2 DAV:write Privilege...................................9
  3.3 DAV:write-properties.................................10
  3.4 DAV:write-content....................................10
  3.5 DAV:unlock...........................................10
  3.6 DAV:read-acl Privilege...............................11
  3.7 DAV:read-current-user-privilege-set Privilege........11
  3.8 DAV:write-acl Privilege..............................11
  3.9 DAV:delete Privilege.................................11
  3.10 DAV:all Privilege..................................11
  3.11 Aggregation of Predefined Privileges...............12
  
  4 PRINCIPAL PROPERTIES..................................12
  4.1 DAV:alternate-URI-set................................12
  4.2 DAV:principal-URL....................................13
  4.3 DAV:group-member-set.................................13
  4.4 DAV:group-membership.................................13
  
  5 ACCESS CONTROL PROPERTIES.............................13
  5.1 DAV:owner............................................14
   5.1.1 Example: Retrieving DAV:owner....................14
   5.1.2 Example: An Attempt to Set DAV:owner.............15
  5.2 DAV:supported-privilege-set..........................16
   5.2.1 Example: Retrieving a List of Privileges Supported on a Resource
       16
  5.3 DAV:current-user-privilege-set.......................19
   5.3.1 Example: Retrieving the User's Current Set of Assigned
   Privileges..............................................19
  5.4 DAV:acl..............................................20
   5.4.1 ACE Principal....................................20
   5.4.2 ACE Grant and Deny...............................21
   5.4.3 ACE Protection...................................22
   5.4.4 ACE Inheritance..................................22
   5.4.5 Example: Retrieving a Resource's Access Control List    22
  5.5 DAV: acl-restrictions................................24
   5.5.1 DAV:grant-only...................................24
  
  
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   5.5.2 DAV:no-invert ACE Constraint.....................24
   5.5.3 DAV:deny-before-grant............................24
   5.5.4 Required Principals..............................24
   Example: Retrieving DAV:acl-restrictions................25
  5.6 DAV:inherited-acl-set................................26
  5.7 DAV:principal-collection-set.........................26
   5.7.1 Example: Retrieving DAV:principal-collection-set.27
  5.8 Example: PROPFIND to retrieve access control properties28
  
  6 ACL EVALUATION........................................31
  
  7 ACCESS CONTROL AND EXISTING METHODS...................32
  7.1 OPTIONS..............................................32
   7.1.1 Example - OPTIONS................................32
  7.2 MOVE.................................................33
  7.3 COPY.................................................33
  7.4 LOCK.................................................33
  
  8 ACCESS CONTROL METHODS................................33
  8.1 ACL..................................................33
   8.1.1 ACL Preconditions................................34
   8.1.2 Example: the ACL method..........................35
   8.1.3 Example: ACL method failure due to protected ACE conflict    36
   8.1.4 Example: ACL method failure due to an inherited ACE conflict 37
   8.1.5 Example: ACL method failure due to an attempt to set grant and
   deny in a single ACE....................................38
  
  9 ACCESS CONTROL REPORTS................................39
  9.1 REPORT Method........................................39
  9.2 DAV:acl-principal-prop-set Report....................40
   9.2.1 Example: DAV:acl-principal-prop-set Report.......41
  9.3 DAV:principal-match REPORT...........................42
   9.3.1 Example: DAV:principal-match REPORT..............43
  9.4 DAV:principal-property-search REPORT.................44
   9.4.1 Matching.........................................46
   9.4.2 Example: successful DAV:principal-property-search REPORT 46
   9.4.3 Example: Unsuccessful DAV:principal-property-search REPORT   48
  9.5 DAV:principal-search-property-set REPORT.............49
   9.5.1 Example: DAV:principal-search-property-set REPORT50
  
  10  XML PROCESSING.......................................51
  
  11  INTERNATIONALIZATION CONSIDERATIONS..................51
  
  12  SECURITY CONSIDERATIONS..............................52
  12.1 Increased Risk of Compromised Users................52
  12.2 Risks of the DAV:read-acl and DAV:current-user-privilege-set
  Privileges...............................................53
  12.3 No Foreknowledge of Initial ACL....................53
  
  13  AUTHENTICATION.......................................54
  
  14  IANA CONSIDERATIONS..................................54
  
  
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  15  INTELLECTUAL PROPERTY................................54
  
  16  ACKNOWLEDGEMENTS.....................................55
  
  17  REFERENCES...........................................55
  17.1 Normative References...............................55
  17.2 Informational References...........................56
  
  18  AUTHORS' ADDRESSES...................................57
  
  19  APPENDICES...........................................58
  19.1 WebDAV XML Document Type Definition Addendum.......58
  19.2 WebDAV Method Privilege Table (Normative)..........60
  
  
  1  INTRODUCTION
  
       The goal of the WebDAV access control extensions is to provide an
       interoperable mechanism for handling discretionary access control
       for content and metadata managed by WebDAV servers.  WebDAV access
       control can be implemented on content repositories with security as
       simple as that of a UNIX file system, as well as more sophisticated
       models.  The underlying principle of access control is that who you
       are determines what operations you can perform on a resource. The
       "who you are" is defined by a "principal" identifier; users, client
       software, servers, and groups of the previous have principal
       identifiers. The "operations you can perform" are determined by a
       single "access control list" (ACL) associated with a resource.  An
       ACL contains a set of "access control entries" (ACEs), where each
       ACE specifies a principal and a set of privileges that are either
       granted or denied to that principal. When a principal submits an
       operation (such as an HTTP or WebDAV method) to a resource for
       execution, the server evaluates the ACEs in the ACL to determine if
       the principal has permission for that operation.
  
       Since every ACE contains the identifier of a principal, client
       software operated by a human must provide a mechanism for selecting
       this principal. This specification uses http(s) scheme URLs to
       identify principals, which are represented as WebDAV-capable
       resources. There is no guarantee that the URLs identifying
       principals will be meaningful to a human. For example,
       http://www.example.com/u/256432 and
       http://www.example.com/people/Greg.Stein are both valid URLs that
       could be used to identify the same principal. To remedy this, every
       principal resource has the DAV:displayname property containing a
       human-readable name for the principal.
  
       Since a principal can be identified by multiple URLs, it raises the
       problem of determining exactly which principal is being referenced
       in a given ACE. It is impossible for a client to determine that an
       ACE granting the read privilege to
       http://www.example.com/people/Greg.Stein also affects the principal
       at http://www.example.com/u/256432. That is, a client has no
       mechanism for determining that two URLs identify the same principal
  
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       resource.  As a result, this specification requires clients to use
       just one of the many possible URLs for a principal when creating
       ACEs. A client can discover which URL to use by retrieving the
       DAV:principal-URL property (Section 4.2) from a principal resource.
       No matter which of the principal's URLs is used with PROPFIND, the
       property always returns the same URL.
  
       With a system having hundreds to thousands of principals, the
       problem arises of how to allow a human operator of client software
       to select just one of these principals. One approach is to use
       broad collection hierarchies to spread the principals over a large
       number of collections, yielding few principals per collection. An
       example of this is a two level hierarchy with the first level
       containing 36 collections (a-z, 0-9), and the second level being
       another 36, creating collections /a/a/, /a/b/, ..., /a/z/, such
       that a principal with last name "Stein" would appear at /s/t/Stein.
       In effect, this pre-computes a common query, search on last name,
       and encodes it into a hierarchy. The drawback with this scheme is
       that it handles only a small set of predefined queries, and
       drilling down through the collection hierarchy adds unnecessary
       steps (navigate down/up) when the user already knows the
       principal's name. While organizing principal URLs into a hierarchy
       is a valid namespace organization, users should not be forced to
       navigate this hierarchy to select a principal.
  
       This specification provides the capability to perform substring
       searches over a small set of properties on the resources
       representing principals. This permits searches based on last name,
       first name, user name, job title, etc. Two separate searches are
       supported, both via the REPORT method, one to search principal
       resources (DAV:principal-property-search, Section 9.4), the other
       to determine which properties may be searched at all
       (DAV:principal-search-property-set, Section 9.5).
  
       Once a principal has been identified in an ACE, a server evaluating
       that ACE must know the identity of the principal making a protocol
       request, and must validate that that principal is who they claim to
       be, a process known as authentication. This specification
       intentionally omits discussion of authentication, as the HTTP
       protocol already has a number of authentication mechanisms
       [RFC2617].  Some authentication mechanism (such as HTTP Digest
       Authentication, which all WebDAV compliant implementations are
       required to support) must be available to validate the identity of
       a principal.
  
       The following issues are out of scope for this document:
  
       Access control that applies only to a particular property on a
       resource (excepting the access control properties DAV:acl and
       DAV:current-user-privilege-set), rather than the entire resource,
  
       Role-based security (where a role can be seen as a dynamically
       defined group of principals),
  
  
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       Specification of the ways an ACL on a resource is initialized,
  
       Specification of an ACL that applies globally to all resources,
       rather than to a particular resource.
  
       Creation and maintenance of resources representing people or
       computational agents (principals), and groups of these.
  
       This specification is organized as follows. Section 1.1 defines key
       concepts used throughout the specification, and is followed by a
       more in-depth discussion of principals (Section 2), and privileges
       (Section 3). Properties defined on principals are specified in
       Section 4, and access control properties for content resources are
       specified in Section 5. The ways ACLs are to be evaluated is
       described in section 6. Client discovery of access control
       capability using OPTIONS is described in Section 7.1. Interactions
       between access control functionality and existing HTTP and WebDAV
       methods are described in the remainder of Section 7. The access
       control setting method, ACL, is specified in Section 8. Four
       reports that provide limited server-side searching capabilities are
       described in Section 9. Sections on XML processing (Section 10),
       Internationalization considerations (Section 11), security
       considerations (Section 12), and authentication (Section 13) round
       out the specification. An appendix (Section 19.1) 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.
  
     group
  
       A "group" is a principal that represents a set of other principals.
  
     privilege
  
       A "privilege" controls access to a particular set of HTTP
       operations on a resource.
  
     aggregate privilege
  
       An "aggregate privilege" is a privilege that contains a set of
       other privileges.
  
     abstract privilege
  
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       The modifier "abstract", when applied to a privilege on a resource,
       means the privilege cannot be set in an access control element
       (ACE) on that resource .
  
     access control list (ACL)
  
       An "ACL" is a list of access control elements that define access
       control to a particular resource.
  
     access control element (ACE)
  
       An "ACE" either grants or denies a particular set of (non-abstract)
       privileges for a particular principal.
  
     inherited ACE
  
       An "inherited ACE" is an ACE that is dynamically shared from the
       ACL of another resource. When a shared ACE changes on the primary
       resource, it is also changed on inheriting resources.
  
     protected property
  
       A "protected property" is one whose value cannot be updated except
       by a method explicitly defined as updating that specific property.
       In particular, a protected property cannot be updated with a
       PROPPATCH request.
  
  
  1.2 Notational Conventions
  
       The augmented BNF used by this document to describe protocol
       elements is described in Section 2.1 of [RFC2616]. Because this
       augmented BNF uses the basic production rules provided in Section
       2.2 of [RFC2616], those rules apply to this document as well.
  
       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
       this document are to be interpreted as described in [RFC2119].
  
       Definitions of XML elements in this document use XML element type
       declarations (as found in XML Document Type Declarations),
       described in Section 3.2 of [REC-XML]. When an XML element type in
       the "DAV:" namespace is referenced in this document outside of the
       context of an XML fragment, the string "DAV:" will be prefixed to
       the element name.
  
  
  2  PRINCIPALS
  
       A principal is a network resource that represents a distinct human
       or computational actor that initiates access to network resources.
       Users and groups are represented as principals in many
       implementations; other types of principals are also possible. A URI
       of any scheme MAY be used to identify a principal resource.
  
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       However, servers implementing this specification MUST expose
       principal resources at an http(s) URL, which is a privileged scheme
       that points to resources that have additional properties, as
       described in Section 4. So, a principal resource can have multiple
       URIs, one of which has to be an http(s) scheme URL. Although an
       implementation SHOULD support PROPFIND and MAY support PROPPATCH to
       access and modify information about a principal, it is not required
       to do so.
  
       A principal resource may be a group, where a group is a principal
       that represents a set of other principals, called the members of
       the group.  If a person or computational agent matches a principal
       resource that is a member of a group, they also match the group.
       Membership in a group is recursive, so if a principal is a member
       of group GRPA, and GRPA is a member of group GRPB, then the
       principal is also a member of GRPB.
  
  
  3  PRIVILEGES
  
       Ability to perform a given method on a resource MUST be controlled
       by one or more privileges.  Authors of protocol extensions that
       define new HTTP methods SHOULD specify which privileges (by
       defining new privileges, or mapping to ones below) are required to
       perform the method.  A principal with no privileges to a resource
       MUST be denied any HTTP access to that resource, unless the
       principal matches an ACE constructed using the DAV:all,
       DAV:authenticated, or DAV:unauthenticated pseudo-principals (see
       Section 5.4.1).  Servers MUST report a 403 "Forbidden" error if
       access is denied, except in the case where the privilege restricts
       the ability to know the resource exists, in which case 404 "Not
       Found" may be returned.
  
       Privileges may be containers of other privileges, in which case
       they are termed "aggregate privileges".  If a principal is granted
       or denied an aggregate privilege, it is semantically equivalent to
       granting or denying each of the aggregated privileges individually.
       For example, an implementation may define add-member and remove-
       member privileges that control the ability to add and remove a
       member of a group.  Since these privileges control the ability to
       update the state of a group, these privileges would be aggregated
       by the DAV:write privilege on a group, and granting the DAV:write
       privilege on a group would also grant the add-member and remove-
       member privileges.
  
       Privileges may be declared to be "abstract" for a given resource,
       in which case they cannot be set in an ACE on that resource.
       Aggregate and non-aggregate privileges are both capable of being
       abstract. Abstract privileges are useful for modeling privileges
       that otherwise would not be exposed via the protocol. Abstract
       privileges also provide server implementations with flexibility in
       implementing the privileges defined in this specification.  For
       example, if a server is incapable of separating the read resource
       capability from the read ACL capability, it can still model the
  
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       DAV:read and DAV:read-acl privileges defined in this specification
       by declaring them abstract, and containing them within a non-
       abstract aggregate privilege (say, read-all) that holds DAV:read,
       and DAV:read-acl. In this way, it is possible to set the aggregate
       privilege, read-all, thus coupling the setting of DAV:read and
       DAV:read-acl, but it is not possible to set DAV:read, or DAV:read-
       acl individually. Since aggregate privileges can be abstract, it is
       also possible to use abstract privileges to group or organize non-
       abstract privileges. Privilege containment loops are not allowed;
       therefore, a privilege MUST NOT contain itself. For example,
       DAV:read cannot contain DAV:read.
  
       The set of privileges that apply to a particular resource may vary
       with the DAV:resourcetype of the resource, as well as between
       different server implementations.  To promote interoperability,
       however, this specification defines a set of well-known privileges
       (e.g. DAV:read, DAV:write, DAV:read-acl, DAV:write-acl, DAV:read-
       current-user-privilege-set, and DAV:all), which can at least be
       used to classify the other privileges defined on a particular
       resource. The access permissions on null resources (defined in
       [RFC2518], Section 3) are solely those they inherit (if any), and
       they are not discoverable (i.e., the access control properties
       specified in Section 5 are not defined on null resources). On the
       transition from null to stateful resource, the initial access
       control list is set by the server's default ACL value policy (if
       any).
  
       Server implementations MAY define new privileges beyond those
       defined in this specification. Privileges defined by individual
       implementations MUST NOT use the DAV: namespace, and instead should
       use a namespace that they control, such as an http scheme URL.
  
  
  3.1 DAV:read Privilege
  
       The read privilege controls methods that return information about
       the state of the resource, including the resource's properties.
       Affected methods include GET and PROPFIND.  Any implementation-
       defined privilege that also controls access to GET and PROPFIND
       must be aggregated under DAV:read—if an ACL grants access to
       DAV:read, the client may expect that no other privilege needs to be
       granted to have access to GET and PROPFIND.  Additionally, the read
       privilege MUST control the OPTIONS method.
  
       <!ELEMENT read EMPTY>
  
  3.2 DAV:write Privilege
  
       The write privilege controls methods that lock a resource or modify
       the content, dead properties, or (in the case of a collection)
       membership of the resource, such as PUT and PROPPATCH.  Note that
       state modification is also controlled via locking (see section 5.3
       of [WEBDAV]), so effective write access requires that both write
       privileges and write locking requirements are satisfied.  Any
  
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       implementation-defined privilege that also controls access to
       methods modifying content, dead properties or collection membership
       must be aggregated under DAV:write, e.g. if an ACL grants access to
       DAV:write, the client may expect that no other privilege needs to
       be granted to have access to PUT and PROPPATCH.
  
       <!ELEMENT write EMPTY>
  
  3.3 DAV:write-properties
  
       The DAV:write-properties privilege controls methods that modify the
       dead properties of the resource, such as PROPPATCH.  Whether this
       privilege may be used to control access to any live properties is
       determined by the implementation.  Any implementation-defined
       privilege that also controls access to methods modifying dead
       properties must be aggregated under DAV:write-properties—e.g. if an
       ACL grants access to DAV:write-properties, the client can safely
       expect that no other privilege needs to be granted to have access
       to PROPPATCH.
  
       <!ELEMENT write-properties EMPTY>
  
  3.4 DAV:write-content
  
       The DAV:write-content privilege controls methods that modify the
       content or (in the case of a collection) membership of the
       resource, such as PUT and DELETE.  Any implementation-defined
       privilege that also controls access to content or alteration of
       collection membership must be aggregated under DAV:write-content—
       e.g. if an ACL grants access to DAV:write-content, the client can
       safely expect that no other privilege needs to be granted to have
       access to PUT or DELETE.
  
       <!ELEMENT write-content EMPTY>
  
  3.5 DAV:unlock
  
       The DAV:unlock privilege controls the use of the UNLOCK method by a
       principal other than the lock owner (the principal that created a
       lock can always perform an UNLOCK).  While the set of users who may
       lock a resource is most commonly the same set of users who may
       modify a resource, servers may allow various kinds of
       administrators to unlock resources locked by others. Any privilege
       controlling access by non-lock owners to UNLOCK MUST be aggregated
       under DAV:unlock.
  
       A lock owner can always remove a lock by issuing an UNLOCK with the
       correct lock token and authentication credentials. That is, even if
       a principal does not have DAV:unlock privilege, they can still
       remove locks they own. Principals other than the lock owner can
       remove a lock only if they have DAV:unlock privilege and they issue
       an UNLOCK with the correct lock token. Lock timeout is not affected
       by the DAV:unlock privilege.
  
  
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       <!ELEMENT unlock EMPTY>
  
  3.6 DAV:read-acl Privilege
  
       The DAV:read-acl privilege controls the use of PROPFIND to retrieve
       the DAV:acl property of the resource.
  
       <!ELEMENT read-acl EMPTY>
  
  3.7 DAV:read-current-user-privilege-set Privilege
  
       The DAV:read-current-user-privilege-set privilege controls the use
       of PROPFIND to retrieve the DAV:current-user-privilege-set property
       of the resource.
  
       Clients are intended to use this property to visually indicate in
       their UI items that are dependent on the permissions of a resource,
       for example, by graying out resources that are not writeable.
  
       This privilege is separate from DAV:read-acl because there is a
       need to allow most users access to the privileges permitted the
       current user (due to its use in creating the UI), while the full
       ACL contains information that may not be appropriate for the
       current authenticated user. As a result, the set of users who can
       view the full ACL is expected to be much smaller than those who can
       read the current user privilege set, and hence distinct privileges
       are needed for each.
  
       <!ELEMENT read-current-user-privilege-set EMPTY>
  
  3.8 DAV:write-acl Privilege
  
       The DAV:write-acl privilege controls use of the ACL method to
       modify the DAV:acl property of the resource.
  
       <!ELEMENT write-acl EMPTY>
  
  3.9 DAV:delete Privilege
  
       The DAV:delete privilege controls use of the DELETE method on the
       specified resource.  You must also have DAV:write-content on the
       collection containing the resource for the DELETE to succeed.
  
       <!ELEMENT delete EMPTY>
  
  3.10DAV:all Privilege
  
       DAV:all is an aggregate privilege that contains the entire set of
       privileges that can be applied to the resource.
  
       <!ELEMENT all EMPTY>
  
  
  
  
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  3.11Aggregation of Predefined Privileges
  
       Server implementations are free to aggregate the predefined
       privileges (defined above in Sections 3.1-3.9) subject to the
       following limitations:
  
       DAV:read-acl MUST NOT contain DAV:read, DAV:write, DAV:write-acl,
       DAV:write-properties, DAV:write-content, or DAV:read-current-user-
       privilege-set.
  
       DAV:write-acl MUST NOT contain DAV:write, DAV:read, DAV:read-acl,
       or DAV:read-current-user-privilege-set.
  
       DAV:read-current-user-privilege-set MUST NOT contain DAV:write,
       DAV:read, DAV:read-acl, or DAV:write-acl.
  
       DAV:write MUST NOT contain DAV:read, DAV:read-acl, or DAV:read-
       current-user-privilege-set.
  
       DAV:read MUST NOT contain DAV:write, DAV:write-acl, DAV:write-
       properties, or DAV:write-content.
  
       DAV:write MUST contain DAV:write-properties and DAV:write-content.
  
  
  4  PRINCIPAL PROPERTIES
  
       Principals are manifested to clients as a WebDAV resource,
       identified by a URL.  A principal MUST have a non-empty
       DAV:displayname property (defined in Section 13.2 of [RFC2518]),
       and a DAV:resourcetype property (defined in Section 13.9 of
       [RFC2518]).  Additionally, a principal MUST report the
       DAV:principal XML element in the value of the DAV:resourcetype
       property.  The element type declaration for DAV:principal is:
  
       <!ELEMENT principal EMPTY>
  
       This protocol defines the following additional properties for a
       principal. Since it can be expensive for a server to retrieve
       access control information, the name and value of these properties
       SHOULD NOT be returned by a PROPFIND allprop request (as defined in
       Section 12.14.1 of [RFC2518]).
  
  
  4.1 DAV:alternate-URI-set
  
       This protected property, if non-empty, contains the URIs of network
       resources with additional descriptive information about the
       principal. This property identifies additional network resources
       (i.e., it contains one or more URIs) that may be consulted by a
       client to gain additional knowledge concerning a principal. One
       expected use for this property is the storage of an LDAP [RFC2255]
       scheme URL. A user-agent encountering an LDAP URL could use LDAP
       [RFC2589] to retrieve additional machine-readable directory
  
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       information about the principal, and display that information in
       its user interface. Support for this property is REQUIRED, and the
       value is empty if no alternate URI exists for the principal.
  
       <!ELEMENT alternate-URI-set (href*)>
  
  4.2 DAV:principal-URL
  
        A principal may have many URLs, but there must be one "principal
       URL" that clients can use to uniquely identify a principal.  This
       protected property contains the URL that MUST be used to identify
       this principal in an ACL request. Support for this property is
       REQUIRED.
  
       <!ELEMENT principal-URL (href)>
  
  4.3 DAV:group-member-set
  
       This property of a group principal identifies the principals that
       are direct members of this group. Since a group may be a member of
       another group, a group may also have indirect members (i.e. the
       members of its direct members).  A URL in the DAV:group-member-set
       for a principal MUST be the DAV:principal-URL of that principal.
  
       <!ELEMENT group-member-set (href*)>
  
  
  4.4 DAV:group-membership
  
        This protected property identifies the groups in which the
       principal is directly a member.  Note that a server may allow a
       group to be a member of another group, in which case the DAV:group-
       membership of those other groups would need to be queried in order
       to determine the groups in which the principal is indirectly a
       member. Support for this property is REQUIRED.
  
       <!ELEMENT group-membership (href*)>
  
  
  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.  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
       properties defined in this section.
  
       Access control properties (especially DAV:acl and DAV:inherited-
       acl-set) are defined on the resource identified by the Request-URI
       of a PROPFIND request. A direct consequence is that if the resource
       is accessible via multiple URI, the value of access control
       properties is the same across these URI.
  
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       HTTP resources that support the WebDAV Access Control Protocol MUST
       contain the following properties. Null resources (described in
       Section 3 of [RFC2518]) MUST NOT contain the following properties.
  
  
  5.1 DAV:owner
  
       This protected 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 DAV:write-acl privilege), clients might display the
       resource owner in their user interface.
  
       <!ELEMENT owner (href)>
  
  5.1.1Example: Retrieving DAV:owner
  
       This example shows a client request for the value of the DAV:owner
       property from a collection resource with URL
       http://www.example.com/papers/. The principal making the request is
       authenticated using Digest authentication. The value of DAV:owner
       is the URL http://www.example.com/acl/users/gstein, wrapped in the
       DAV:href XML element.
  
       >> Request <<
  
       PROPFIND /papers/ HTTP/1.1
       Host: www.example.com
       Content-type: text/xml; charset="utf-8"
       Content-Length: xxx
       Depth: 0
       Authorization: Digest username="jim",
          realm="jim@webdav.org", nonce="...",
          uri="/papers/", response="...", opaque="..."
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:propfind xmlns:D="DAV:">
         <D:prop>
           <D:owner/>
         </D:prop>
       </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:">
          <D:response>
             <D:href>http://www.example.com/papers/</D:href>
             <D:propstat>
                <D:prop>
  
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                   <D:owner>
                      <D:href>http://www.example.com/acl/users/gstein</D:href>
                   </D:owner>
                </D:prop>
                <D:status>HTTP/1.1 200 OK</D:status>
            </D:propstat>
          </D:response>
       </D:multistatus>
  
  5.1.2Example: An Attempt to Set DAV:owner
  
       The following example shows a client request to modify the value of
       the DAV:owner property on the resource with URL
       <http://www.example.com/papers>. Since DAV:owner is a protected
       property, the server responds with a 207 (Multi-Status) response
       that contains a 403 (Forbidden) status code for the act of setting
       DAV:owner. Section 8.2.1 of [RFC2518] describes PROPPATCH status
       code information, and Section 11 of [RFC2518] describes the Multi-
       Status response.
  
       >> Request <<
  
       PROPPATCH /papers/ HTTP/1.1
       Host: www.example.com
       Content-type: text/xml; charset="utf-8"
       Content-Length: xxx
       Depth: 0
       Authorization: Digest username="jim",
          realm="jim@webdav.org", nonce="...",
          uri="/papers/", response="...", opaque="..."
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:propertyupdate xmlns:D="DAV:">
          <D:set>
             <D:prop>
                <D:owner>
                   <D:href>http://www.example.com/acl/users/jim</D:href>
                </D:owner>
             </D:prop>
          </D:set>
       </D:propertyupdate>
  
  
       >> 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:">
          <D:response>
             <D:href>http://www.example.com/papers/</D:href>
             <D:propstat>
  
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                <D:prop><D:owner/></D:prop>
                <D:status>HTTP/1.1 403 Forbidden</D:status>
                <D:responsedescription>
                  Failure to set protected property (DAV:owner)
                </D:responsedescription>
             </D:propstat>
          </D:response>
       </D:multistatus>
  
  
  
  5.2 DAV:supported-privilege-set
  
       This is a protected 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 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. Servers MUST indicate the human
       language of the description using the xml:lang attribute and SHOULD
       consider the HTTP Accept-Language request header when selecting one
       of multiple available languages.
  
       <!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
       privileges that are supported by any particular server
       implementation.  The privilege tree also serves to hide complexity
       in implementations allowing large number of privileges to be
       defined by displaying aggregates to the user.
  
  
  5.2.1Example: Retrieving a List of Privileges Supported on a Resource
  
       This example shows a client request for the DAV:supported-
       privilege-set property on the resource
  
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       http://www.example.com/papers/. The value of the DAV:supported-
       privilege-set property is a tree of supported privileges (using
       "[XML Namespace , localname]" to identify each privilege):
  
          [DAV:, all] (aggregate, abstract)
             |
             +-- [DAV:, read] (aggregate)
                    |
                    +-- [DAV:, read-acl] (abstract)
                    +-- [DAV:, read-current-user-privilege-set] (abstract)
             |
             +-- [DAV:, write] (aggregate)
                    |
                    +-- [DAV:, write-acl] (abstract)
                    +-- [DAV:, write-properties]
                    +-- [DAV:, write-content]
             |
             +-- [DAV:, unlock]
  
  
       This privilege tree is not normative (except that it reflects the
       normative aggregation rules given in Section 3.11), and many
       possible privilege trees are possible.
  
  
  
       >> Request <<
  
       PROPFIND /papers/ HTTP/1.1
       Host: www.example.com
       Content-type: text/xml; charset="utf-8"
       Content-Length: xxx
       Depth: 0
       Authorization: Digest username="gclemm",
          realm="gclemm@webdav.org", nonce="...",
          uri="/papers/", response="...", opaque="..."
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:propfind xmlns:D="DAV:">
         <D:prop>
           <D:supported-privilege-set/>
         </D:prop>
       </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:">
         <D:response>
           <D:href>http://www.example.com/papers/</D:href>
  
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           <D:propstat>
             <D:prop>
               <D:supported-privilege-set>
                 <D:supported-privilege>
                   <D:privilege> <D:all/> </D:privilege>
                   <D:abstract/>
                   <D:description xml:lang="en">Any operation</D:description>
                   <D:supported-privilege>
                     <D:privilege> <D:read/> </D:privilege>
                     <D:description xml:lang="en">Read any object</D:description>
                     <D:supported-privilege>
                       <D:privilege> <D:read-acl/> </D:privilege>
                       <D:abstract/>
                       <D:description xml:lang="en">Read ACL</D:description>
                     </D:supported-privilege>
                     <D:supported-privilege>
                       <D:privilege>
                         <D:read-current-user-privilege-set/>
                       </D:privilege>
                       <D:abstract/>
                       <D:description xml:lang="en">Read current user privilege
       set property</D:description>
                     </D:supported-privilege>
                     </D:supported-privilege>
                     <D:supported-privilege>
                     <D:privilege> <D:write/> </D:privilege>
                     <D:description xml:lang="en">Write any object</D:description>
                     <D:supported-privilege>
                       <D:privilege> <D:write-acl/> </D:privilege>
                       <D:description xml:lang="en">Write ACL</D:description>
                       <D:abstract/>
                     </D:supported-privilege>
                     <D:supported-privilege>
                       <D:privilege> <D:write-properties/> </D:privilege>
                       <D:description xml:lang="en">Write
       properties</D:description>
                     </D:supported-privilege>
                     <D:supported-privilege>
                       <D:privilege> <D:write-content/> </D:privilege>
                       <D:description xml:lang="en">Write resource
       content</D:description>
                     </D:supported-privilege>
                     </D:supported-privilege>
                     <D:supported-privilege>
                       <D:privilege> <D:unlock/> </D:privilege>
                       <D:description xml:lang="en">Unlock
       resource</D:description>
                     </D:supported-privilege>
                 </D:supported-privilege>
               </D:supported-privilege-set>
             </D:prop>
             <D:status>HTTP/1.1 200 OK</D:status>
           </D:propstat>
         </D:response>
  
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       </D:multistatus>
  
  5.3 DAV:current-user-privilege-set
  
       DAV:current-user-privilege-set is a protected 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., by graying out a menu item or button) for which
       the current principal does not have permission. This property is
       also useful for determining what operations the current 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 property MUST identify a non-abstract privilege from
       the DAV:supported-privilege-set property.
  
  
  5.3.1Example: Retrieving the User's Current Set of Assigned Privileges
  
       Continuing the example from Section 5.2.1, this example shows a
       client requesting the DAV:current-user-privilege-set property from
       the resource with URL http://www.example.com/papers/. The username
       of the principal making the request is "khare", and Digest
       authentication is used in the request. The principal with username
       "khare" has been granted the DAV:read privilege. Since the DAV:read
       privilege contains the DAV:read-acl and DAV:read-current-user-
       privilege-set privileges (see Section 5.2.1), the principal with
       username "khare" can read the ACL property, and the DAV:current-
       user-privilege-set property. However, the DAV:all, DAV:read-acl,
       DAV:write-acl and DAV:read-current-user-privilege-set privileges
       are not listed in the value of DAV:current-user-privilege-set,
       since (for this example) they are abstract privileges. DAV:write is
       not listed since the principal with username "khare" is not listed
       in an ACE granting that principal write permission.
  
       >> Request <<
  
       PROPFIND /papers/ HTTP/1.1
       Host: www.example.com
       Content-type: text/xml; charset="utf-8"
       Content-Length: xxx
       Depth: 0
       Authorization: Digest username="khare",
          realm="khare@webdav.org", nonce="...",
          uri="/papers/", response="...", opaque="..."
  
       <?xml version="1.0" encoding="utf-8" ?>
  
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       <D:propfind xmlns:D="DAV:">
         <D:prop>
           <D:current-user-privilege-set/>
         </D:prop>
       </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:">
         <D:response>
           <D:href>http://www.example.com/papers/</D:href>
           <D:propstat>
             <D:prop>
               <D:current-user-privilege-set>
                 <D:privilege> <D:read/> </D:privilege>
               </D:current-user-privilege-set>
             </D:prop>
             <D:status>HTTP/1.1 200 OK</D:status>
           </D:propstat>
         </D:response>
       </D:multistatus>
  
  5.4 DAV:acl
  
       This is a protected 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 (invert | principal, (grant|deny), protected?, inherited?)>
  
  5.4.1ACE Principal
  
       The DAV:principal element identifies the principal to which this
       ACE applies.
  
       <!ELEMENT principal ((href)
        | 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 contained by that DAV:href.
  
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       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 (that is, DAV:authenticated and
       DAV:unauthenticated are disjoint sets).
  
       The current user matches a DAV:property principal in a DAV:acl
       property of a resource only if the value of the identified property
       of that resource contains at most one DAV:href XML element, the URI
       value of DAV:href 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>
  
       Alternately, some servers may support ACEs applying to those users
       NOT matching the current principal, e.g. all users not in a
       particular group.  This can be done by wrapping the DAV:principal
       element with DAV:invert.
  
       <!ELEMENT invert principal>
  
  
       The current user matches DAV:self in a DAV:acl property of the
       resource only if that resource is a principal and that principal
       matches the current user or, if the principal is a group, a member
       of that group matches the current user.
  
       <!ELEMENT self EMPTY>
  
  5.4.2ACE 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 of that resource.
  
       <!ELEMENT grant (privilege+)>
  
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       <!ELEMENT deny (privilege+)>
       <!ELEMENT privilege ANY>
  
  5.4.3ACE Protection
  
       A server indicates an ACE is protected by including the
       DAV:protected element in the ACE. If the ACL of a resource contains
       an ACE with a DAV:protected element, an attempt to remove that ACE
       from the ACL MUST fail.
  
       <!ELEMENT protected EMPTY>
  
  5.4.4ACE 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)>
  
  5.4.5Example: Retrieving a Resource's Access Control List
  
       Continuing the example from Sections 5.2.1 and 5.3.1, this example
       shows a client requesting the DAV:acl property from the resource
       with URL http://www.example.com/papers/. There are two ACEs defined
       in this ACL:
  
       ACE #1: The group identified by URL
       http://www.example.com/acl/groups/maintainers (the group of site
       maintainers) is granted DAV:write privilege. Since (for this
       example) DAV:write contains the DAV:write-acl privilege (see
       Section 5.2.1), this means the "maintainers" group can also modify
       the access control list.
  
       ACE #2: All principals (DAV:all) are granted the DAV:read
       privilege. Since (for this example) DAV:read contains DAV:read-acl
       and DAV:read-current-user-privilege-set, this means all users
       (including all members of the "maintainers" group) can read the
       DAV:acl property and the DAV:current-user-privilege-set property.
  
  
       >> Request <<
  
       PROPFIND /papers/ HTTP/1.1
  
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       Host: www.example.com
       Content-type: text/xml; charset="utf-8"
       Content-Length: xxx
       Depth: 0
       Authorization: Digest username="masinter",
          realm="webdav.org", nonce="...",
          uri="/papers/", response="...", opaque="..."
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:propfind xmlns:D="DAV:">
         <D:prop>
           <D:acl/>
         </D:prop>
       </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:">
         <D:response>
           <D:href>http://www.example.com/papers/</D:href>
           <D:propstat>
              <D:prop>
               <D:acl>
                 <D:ace>
                   <D:principal>
       <D:href>http://www.example.com/acl/groups/maintainers</D:href>
                   </D:principal>
                   <D:grant>
                     <D:privilege> <D:write/> </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:ace>
               </D:acl>
             </D:prop>
             <D:status>HTTP/1.1 200 OK</D:status>
           </D:propstat>
         </D:response>
       </D:multistatus>
  
  
  
  
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  5.5 DAV: acl-restrictions
  
       This protected property defines the types of ACLs supported by this
       server, to avoid clients needlessly getting errors.  When a client
       tries to set an ACL via the ACL method, the server may reject the
       attempt to set the ACL as specified.  The following properties
       indicate the restrictions the client must observe before setting an
       ACL:
  
            <grant-only>        Deny ACEs are not supported
  
            <no-invert>         Inverted ACEs are not supported
  
            <deny-before-grant> All deny ACEs must occur before any grant
       ACEs
  
            <required-principal>     Indicates which principals are
       required to be present
  
  
  
       <!ELEMENT acl-restrictions (grant-only?, no-invert?, deny-before-grant?,
       required-principal?)>
  
  
  
  5.5.1DAV:grant-only
  
       This element indicates that ACEs with deny clauses are not allowed.
  
       <!ELEMENT grant-only EMPTY>
  
  5.5.2DAV:no-invert ACE Constraint
  
       This element indicates that ACEs with the <invert> element are not
       allowed.
  
       <!ELEMENT no-invert EMPTY>
  
  5.5.3DAV:deny-before-grant
  
       This element indicates that all deny ACEs must precede all grant
       ACEs.
  
       <!ELEMENT deny-before-grant EMPTY>
  
  5.5.4Required Principals
  
       The required principal elements identify which principals must have
       an ACE defined in the ACL.
  
       <!ELEMENT required-principal
         (all? | authenticated? | unauthenticated? | self? | href* | property*)>
  
  
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       For example, the following element requires that the ACL contain a
       DAV:owner property ACE:
  
       <D:required-principal xmlns:D="DAV:">
         <D:property> <D:owner/> </D:property>
       </D:required-principal>
  
  
  
        Example: Retrieving DAV:acl-restrictions
  
       In this example, the client requests the value of the DAV:acl-
       restrictions property. Digest authentication provides credentials
       for the principal operating the client.
  
  
  
       >> Request <<
  
       PROPFIND /papers/ HTTP/1.1
       Host: www.example.com
       Content-type: text/xml; charset="utf-8"
       Content-Length: xxx
       Depth: 0
       Authorization: Digest username="srcarter",
          realm="srcarter@webdav.org", nonce="...",
          uri="/papers/", response="...", opaque="..."
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:propfind xmlns:D="DAV:">
         <D:prop>
           <D:acl-restrictions/>
         </D:prop>
       </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:">
         <D:response>
           <D:href>http://www.example.com/papers/</D:href>
           <D:propstat>
               <D:prop>
               <D:acl-restrictions>
                   <D:principal-only-one-ace/>
                 <D:required-principal>
                   <D:all/>
                 </D:required-principal>
               </D:acl-restrictions>
  
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             </D:prop>
             <D:status>HTTP/1.1 200 OK</D:status>
           </D:propstat>
         <D:response>
       </D:multistatus>
  
  
  
  5.6 DAV:inherited-acl-set
  
       This protected property contains a set of URLs that identify other
       resources that also control the access to this resource.  To have a
       privilege on a resource, not only must the ACL on that resource
       (specified in the DAV:acl property of that resource) grant the
       privilege, but so must the ACL of each resource identified in the
       DAV:inherited-acl-set property of that resource.  Effectively, the
       privileges granted by the current ACL are ANDed with the privileges
       granted by each inherited ACL.
  
       <!ELEMENT inherited-acl-set (href*)>
  
  
  
  5.7 DAV:principal-collection-set
  
       This protected property of a resource contains a set of URLs that
       identify the root collections that contain the principals that are
       available on the server that implements this resource.  A WebDAV
       Access Control Protocol user agent could use the contents of
       DAV:principal-collection-set to retrieve 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.
  
       <!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 SHOULD
       be http or https scheme URLs. For security and scalability reasons,
       a server MAY report only a subset of the entire set of known
       principal collections, and therefore clients should not assume they
       have retrieved an exhaustive listing. Additionally, a server MAY
       elect to report none of the principal collections it knows about,
       in which case the property value would be empty.
  
       The value of DAV:principal-collection-set gives the scope of the
       DAV:principal-property-search REPORT (defined in Section 9.4).
       Clients use the DAV:principal-property-search REPORT to populate
       their user interface with a list of principals. Therefore, servers
       that limit a client's ability to obtain principal information will
       interfere with the client's ability to manipulate access control
  
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       lists, due to the difficulty of getting the URL of a principal for
       use in an ACE.
  
  
  5.7.1Example: Retrieving DAV:principal-collection-set
  
       In this example, the client requests the value of the
       DAV:principal-collection-set property on the collection resource
       identified by URL http://www.example.com/papers/. The property
       contains the two URLs, http://www.example.com/acl/users/ and
       http://www.example.com/acl/groups/, both wrapped in DAV:href XML
       elements. Digest authentication provides credentials for the
       principal operating the client.
  
       The client might reasonably follow this request with two separate
       PROPFIND requests to retrieve the DAV:displayname property of the
       members of the two collections (/acl/users and /acl/groups). This
       information could be used when displaying a user interface for
       creating access control entries.
  
  
       >> Request <<
  
       PROPFIND /papers/ HTTP/1.1
       Host: www.example.com
       Content-type: text/xml; charset="utf-8"
       Content-Length: xxx
       Depth: 0
       Authorization: Digest username="yarong",
          realm="yarong@webdav.org", nonce="...",
          uri="/papers/", response="...", opaque="..."
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:propfind xmlns:D="DAV:">
         <D:prop>
           <D:principal-collection-set/>
         </D:prop>
       </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:">
        <D:response>
         <D:href>http://www.example.com/papers/</D:href>
         <D:propstat>
          <D:prop>
            <D:principal-collection-set>
             <D:href>http://www.example.com/acl/users/</D:href>
  
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             <D:href>http://www.example.com/acl/groups/</D:href>
            </D:principal-collection-set>
          </D:prop>
          <D:status>HTTP/1.1 200 OK</D:status>
         </D:propstat>
        </D:response>
       </D:multistatus>
  
  5.8 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.example.com
       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:prop>
           <D:owner/>
           <D:supported-privilege-set/>
           <D:current-user-privilege-set/>
           <D:acl/>
         </D:prop>
       </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:A="http://www.example.com/acl/"> <D:response>
         <D:href>http://www.example.com/top/container/</D:href>
         <D:propstat>
         <D:prop>
           <D:owner>
             <D:href>http://www.example.com/users/gclemm</D:href> </D:owner>
           <D:supported-privilege-set>
             <D:supported-privilege>
               <D:privilege> <D:all/> </D:privilege>
  
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               <D:abstract/>
               <D:description xml:lang="en">Any operation</D:description>
               <D:supported-privilege>
                 <D:privilege> <D:read/> </D:privilege>
                 <D:description xml:lang="en">Read any object</D:description>
               </D:supported-privilege>
               <D:supported-privilege>
                 <D:privilege> <D:write/> </D:privilege>
                 <D:abstract/>
                 <D:description xml:lang="en">Write any object</D:description>
                 <D:supported-privilege>
                   <D:privilege> <A:create/> </D:privilege>
                   <D:description xml:lang="en">Create an object</D:description>
                 </D:supported-privilege>
                 <D:supported-privilege>
                   <D:privilege> <A:update/> </D:privilege>
                   <D:description xml:lang="en">Update an object</D:description>
                 </D:supported-privilege>
                 <D:supported-privilege>
                   <D:privilege> <A:delete/> </D:privilege>
                   <D:description xml:lang="en">Delete an object</D:description>
                 </D:supported-privilege>
               </D:supported-privilege>
               <D:supported-privilege>
                 <D:privilege> <D:read-acl/> </D:privilege>
                 <D:description xml:lang="en">Read the ACL</D:description>
               </D:supported-privilege>
               <D:supported-privilege>
                 <D:privilege> <D:write-acl/> </D:privilege>
                 <D:description xml:lang="en">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>
           </D:current-user-privilege-set>
           <D:acl>
             <D:ace>
               <D:principal>
                 <D:href>http://www.example.com/users/esedlar</D:href>
                 </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>
                 <D:href>http://www.example.com/groups/marketing</D:href>
               </D:principal>
               <D:deny>
                 <D:privilege> <D:read/> </D:privilege> </D:deny>
             </D:ace>
  
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             <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.example.com/top</D:href> </D:inherited>
             </D:ace> </D:acl>
           </D:prop>
           <D:status>HTTP/1.1 200 OK</D:status>
         </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 (using "[XML Namespace , localname]" to
       identify each privilege):
  
          [DAV:, all] (aggregate, abstract)
             |
             +-- [DAV:, read]
             +-- [DAV:, write] (aggregate, abstract)
                    |
                    +-- [http://www.example.com/acl, create]
                    +-- [http://www.example.com/acl, update]
                    +-- [http://www.example.com/acl, 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.example.com/users/esedlar is granted the DAV:read,
       DAV:write, and DAV:read-acl privileges.
  
       ACE #2: The principals identified by the URL
       http://www.example.com/groups/marketing are denied the DAV:read
       privilege.  In this example, the principal URL identifies a group.
  
  
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       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.example.com/top, the parent collection of this resource.
  
  
  6  ACL EVALUATION
  
       WebDAV ACLs are evaluated in similar manner as ACLs on Windows NT
       and in NFSv4 [NFSV4]).  An ACL is evaluated to determine whether or
       not access will be granted for a WebDAV request.  ACEs are
       maintained in a particular order, and are evaluated until all of
       the permissions required by the current request have been granted,
       at which point the ACL evaluation is terminated and access is
       granted.  If, during ACL evaluation, a <deny> ACE (matching the
       current user) is encountered for a privilege which has not yet been
       granted, the ACL evaluation is terminated and access is denied.
       Failure to have all required privileges granted results in access
       being denied.
  
  
  
       Note that the semantics of many other existing ACL systems may be
       represented via this mechanism, by mixing deny and grant ACEs.  For
       example, consider the standard "rwx" privilege scheme used by UNIX.
       In this scheme, if the current user is the owner of the file,
       access is granted if the corresponding privilege bit is set and
       denied if not set, regardless of the permissions set on the file’s
       group and for the world.  An ACL for UNIX permissions of "r--rw-r--
       "might be constructed like:
  
             <D:acl>
             <D:ace>
               <D:principal><D:property><D:owner/></D:property></D:principal>
               <D:grant><D:privilege><D:read/></D:privilege></D:grant>
             </D:ace>
             <D:ace>
               <D:principal><D:property><D:owner/></D:property></D:principal>
               <D:deny><D:privilege><D:all/></D:privilege></D:deny>
             </D:ace>
             <D:ace>
               <D:principal><D:property><D:group/></D:property></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:group/></D:property></D:principal>
  
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               <D:deny><D:privilege><D:all/></D:privilege></D:deny>
             </D:ace>
             <D:ace>
               <D:principal><D:all></D:principal>
               <D:grant><D:privilege><D:read/></D:privilege></D:grant>
             </D:ace>
             </D:acl>
       and the <acl-restrictions> would be defined as:
  
       <D:no-invert/><D:principal-only-one-ace/>
       <D:required-principal>
         <D:all/>
         <D:property><D:owner/></D:property>
         <D:property><D:group/><D:group/>
       </D:required-principal>
  
       Note that the client can still get errors from a UNIX server in
       spite of obeying the <acl-restrictions>, including <D:allowed-
       principal> (adding an ACE specifying a principal other than the
       ones in the ACL above) or <D:ace-conflict> (by trying to reorder
       the ACEs in the example above), as these particular implementation
       semantics are too complex to be captured with the simple (but
       general) declarative restrictions.
  
  
  
  
  
  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. A value of
       "access-control" in the DAV header MUST indicate that the server
       supports all MUST level requirements and REQUIRED features
       specified in this document.
  
  
  7.1.1Example - OPTIONS
  
       >> Request <<
  
         OPTIONS /foo.html HTTP/1.1
         Host: www.example.com
         Content-Length: 0
  
       >> Response <<
  
  
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         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.
  
  
  7.2 MOVE
  
       When a resource is moved from one location to another due to a MOVE
       request, the non-inherited and non-protected ACEs in the DAV:acl
       property of the resource MUST NOT be modified, or the MOVE request
       fails. Handling of inherited and protected ACEs is intentionally
       undefined to give server implementations flexibility in how they
       implement ACE inheritance and protection.
  
  
  7.3 COPY
  
       The DAV:acl property on the resource at the destination of a COPY
       MUST be the same as if the resource was created by an individual
       resource creation request (e.g. MKCOL, PUT). Clients wishing to
       preserve the DAV:acl property across a copy need to read the
       DAV:acl property prior to the COPY, then perform an ACL operation
       on the new resource at the destination to restore, insofar as this
       is possible, the original access control list.
  
  
  7.4 LOCK
  
       A lock on a resource ensures that only the lock owner can modify
       ACEs that are not inherited and not protected  (these are the only
       ACEs that a client can modify with an ACL request). A lock does not
       protect inherited or protected ACEs, since a client cannot modify
       them with an ACL request on that resource.
  
  
  8  ACCESS CONTROL METHODS
  
  
  8.1 ACL
  
       The ACL method modifies the access control list (which can be read
       via the DAV:acl property) of a resource.  Specifically, the ACL
       method only permits modification to ACEs that are not inherited,
       and are not protected. An ACL method invocation modifies all non-
       inherited and non-protected ACEs in a resource's access control
       list to exactly match the ACEs contained within in the DAV:acl XML
       element (specified in Section 5.4) of the request body. An ACL
       request body MUST contain only one DAV:acl XML element. Unless the
       non-inherited and non-protected ACEs of the DAV:acl property of the
  
  
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       resource can be updated to be exactly the value specified in the
       ACL request, the ACL request MUST fail.
  
       It is possible that the ACEs visible to the current user in the
       DAV:acl property may only be a portion of the complete set of ACEs
       on that resource. If this is the case, an ACL request only modifies
       the set of ACEs visible to the current user, and does not affect
       any non-visible ACE.
  
       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.
  
       Implementation Note: Two common operations are to add or remove an
       ACE from an existing access control list. To accomplish this, a
       client uses the PROPFIND method to retrieve the value of the
       DAV:acl property, then parses the returned access control list to
       remove all inherited and protected ACEs (these ACEs are tagged with
       the DAV:inherited and DAV:protected XML elements). In the remaining
       set of non-inherited, non-protected ACEs, the client can add or
       remove one or more ACEs before submitting the final ACE set in the
       request body of the ACL method.
  
  
  8.1.1ACL Preconditions
  
       An implementation MUST enforce the following constraints on an ACL
       request.  If the constraint is violated, a 403 (Forbidden) or 409
       (Conflict) response MUST be returned and the indicated XML element
       MUST be returned as a child of a top level DAV:error element in an
       XML response body.
  
       Though these status elements are generally expressed as empty XML
       elements (and are defined as EMPTY in the DTD), implementations MAY
       return additional descriptive XML elements as children of the
       status element. Clients MUST be able to accept children of these
       status elements. Clients that do not understand the additional XML
       elements should ignore them.
  
       (DAV:no-ace-conflict): The ACEs submitted in the ACL request MUST
       NOT conflict with each other.  This is a catchall error code
       indicating that an implementation-specific ACL restriction has been
       violated.
  
       (DAV:no-protected-ace-conflict): The ACEs submitted in the ACL
       request MUST NOT conflict with the protected ACEs on the resource.
       For example, if the resource has a protected ACE granting DAV:write
       to a given principal, then it would not be consistent if the ACL
       request submitted an ACE denying DAV:write to the same principal.
  
       (DAV:no-inherited-ace-conflict): The ACEs submitted in the ACL
       request MUST NOT conflict with the inherited ACEs on the resource.
       For example, if the resource inherits an ACE from its parent
  
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       collection granting DAV:write to a given principal, then it would
       not be consistent if the ACL request submitted an ACE denying
       DAV:write to the same principal. Note that reporting of this error
       will be implementation-dependent. Implementations MUST either
       report this error or allow the ACE to be set, and then let normal
       ACE evaluation rules determine whether the new ACE has any impact
       on the privileges available to a specific principal.
  
       (DAV:limited-number-of-aces): The number of ACEs submitted in the
       ACL request MUST NOT exceed the number of ACEs allowed on that
       resource.  However, ACL-compliant servers MUST support at least one
       ACE granting privileges to a single principal, and one ACE granting
       privileges to a group.
  
       (DAV:deny-before-grant): All non-inherited deny ACEs MUST precede
       all non-inherited grant ACEs.
  
        (DAV:grant-only): The ACEs submitted in the ACL request MUST NOT
       include a deny ACE.  This precondition applies only when the ACL
       restrictions of the resource include the DAV:grant-only constraint
       (defined in Section 5.5.1).
  
       (DAV:no-invert):  The ACL request MUST NOT include a DAV:invert
       element.   This precondition applies only when the ACL semantics of
       the resource includes the DAV:no-invert constraint (defined in
       Section 6.3.4).
  
       (DAV:no-abstract): The ACL request MUST NOT attempt to grant or
       deny an abstract privilege (see Section 5.2).
  
       (DAV:not-supported-privilege): The ACEs submitted in the ACL
       request MUST be supported by the resource.
  
       (DAV:missing-required-principal): The result of the ACL request
       MUST have at least one ACE for each principal identified in a
       DAV:required-principal XML element in the ACL semantics of that
       resource (see Section 5.5.4).
  
       (DAV:recognized-principal): Every principal URL in the ACL request
       MUST identify a principal resource.
  
       (DAV:allowed-principal): The principals specified in the ACEs
       submitted in the ACL request MUST be allowed as principals for the
       resource. For example, a server where only authenticated principals
       can access resources would not allow the DAV:all or
       DAV:unauthenticated principals to be used in an ACE, since these
       would allow unauthenticated access to resources.
  
  
  8.1.2Example: the ACL method
  
       In the following example, user "fielding", authenticated by
       information in the Authorization header, grants the principal
       identified by the URL http://www.example.com/users/esedlar  (i.e.,
  
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       the user "esedlar") read and write privileges, grants the owner of
       the resource read-acl and write-acl privileges, and grants everyone
       read privileges.
  
       >> Request <<
  
       ACL /top/container/ HTTP/1.1
       Host: www.example.com
       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.example.com/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:ace> </D:acl>
  
       >> Response <<
  
       HTTP/1.1 200 OK
  
  8.1.3Example: ACL method failure due to protected ACE conflict
  
       In the following request, user "fielding", authenticated by
       information in the Authorization header, attempts to deny the
       principal identified by the URL
       http://www.example.com/users/esedlar  (i.e., the user "esedlar")
       write privileges. Prior to the request, the DAV:acl property on the
       resource contained a protected ACE (see Section 5.4.3) granting
       DAV:owner the DAV:read and DAV:write privileges. The principal
  
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       identified by URL http://www.example.com/users/esedlar is the owner
       of the resource. The ACL method invocation fails because the
       submitted ACE conflicts with the protected ACE, thus violating the
       semantics of ACE protection.
  
       >> Request <<
  
       ACL /top/container/ HTTP/1.1
       Host: www.example.com
       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.example.com/users/esedlar</D:href>
           </D:principal>
           <D:deny>
             <D:privilege> <D:write/> </D:privilege>
           </D:deny>
         </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" ?>
       <D:error xmlns:D="DAV:">
         <D:no-protected-ace-conflict/>
       </D:error>
  
  8.1.4Example: ACL method failure due to an inherited ACE conflict
  
       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.example.com/top/index.html. This
       resource has two inherited ACEs.
  
       Inherited ACE #1 grants the principal identified by URL
       http://www.example.com/users/ejw (i.e., the user "ejw")
       http://www.example.com/privs/write-all and DAV:read-acl privileges.
       On this server, http://www.example.com/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.
  
  
  
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       The request attempts to set a (non-inherited) ACE, denying the
       principal identified by the URL http://www.example.com/users/ejw
       (i.e., the user "ejw") DAV:write permission. This conflicts with
       inherited ACE #1. Note that the decision to report an inherited ACE
       conflict is specific to this server implementation. Another server
       implementation could have allowed the new ACE to be set, and then
       used normal ACE evaluation rules to determine whether the new ACE
       has any impact on the privileges available to a principal.
  
       >> Request <<
  
       ACL /top/index.html HTTP/1.1
       Host: www.example.com
       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.example.com/privs/">
         <D:ace>
             <D:principal>
               <D:href>http://www.example.com/users/ejw</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" ?>
       <D:error xmlns:D="DAV:">
         <D:no-inherited-ace-conflict xmlns:D="DAV:"/>
       </D:error>
  
  8.1.5Example: 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.example.com/diamond/engagement-ring.gif.
       The ACL request includes a single, syntactically and semantically
       incorrect ACE, which attempts to grant the group identified by the
       URL http://www.example.com/users/friends DAV:read privilege and
       deny the principal identified by URL
       http://www.example.com/users/ygoland-so (i.e., the user "ygoland-
       so") DAV:read privilege. However, it is illegal to have multiple
       principal elements, as well as both a grant and deny element in the
       same ACE, so the request fails due to poor syntax.
  
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       >> Request <<
  
       ACL /diamond/engagement-ring.gif HTTP/1.1
       Host: www.example.com
       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.example.com/users/friends</D:href>
             </D:principal>
             <D:grant><D:read/></D:grant>
             <D:principal>
               <D:href>http://www.example.com/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  ACCESS CONTROL REPORTS
  
  
  9.1 REPORT Method
  
       The REPORT method (defined in Section 3.6 of [RFC3253]) provides an
       extensible mechanism for obtaining information about a resource.
       Unlike the PROPFIND method, which returns the value of one or more
       named properties, the REPORT method can involve more complex
       processing. REPORT is valuable in cases where the server has access
       to all of the information needed to perform the complex request
       (such as a query), and where it would require multiple requests for
       the client to retrieve the information needed to perform the same
       request.
  
       A server that supports the WebDAV Access Control Protocol MUST
       support the DAV:expand-property report (defined in Section 3.8 of
       [RFC3253]).
  
  
  
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  9.2 DAV:acl-principal-prop-set Report
  
       The DAV:acl-principal-prop-set report returns, for all principals
       in the DAV:acl property (of the Request-URI) that are identified by
       http(s) URLs or by a DAV:property principal, the value of the
       properties specified in the REPORT request body. In the case where
       a principal URL appears multiple times, the DAV:acl-principal-prop-
       set report MUST return the properties for that principal only once.
       Support for this report is REQUIRED.
  
       One expected use of this report is to retrieve the human readable
       name (found in the DAV:displayname property) of each principal
       found in an ACL. This is useful for constructing user interfaces
       that show each ACE in a human readable form.
  
     Marshalling
  
       The request body MUST be a DAV:acl-principal-prop-set XML element.
  
       <!ELEMENT acl-principal-prop-set ANY>
       ANY value: a sequence of one or more elements, with at most one DAV:prop
       element.
       prop: see RFC 2518, Section 12.11
  
  
       This report is only defined when the Depth header has value "0";
       other values result in a 400 (Bad Request) error response. Note
       that [RFC3253], Section 3.6, states that if the Depth header is not
       present, it defaults to a value of "0".
  
       The response body for a successful request MUST be a
       DAV:multistatus XML element (i.e., the response uses the same
       format as the response for PROPFIND). In the case where there are
       no response elements, the returned multistatus XML element is
       empty.
  
       multistatus: see RFC 2518, Section 12.9
  
       The response body for a successful DAV:acl-principal-prop-set
       REPORT request MUST contain a DAV:response element for each
       principal identified by an http(s) URL listed in a DAV:principal
       XML element of an ACE within the DAV:acl property of the resource
       identified by the Request-URI.
  
     Postconditions:
  
       (DAV:number-of-matches-within-limits): The number of matching
       principals must fall within server-specific, predefined limits. For
       example, this condition might be triggered if a search
       specification would cause the return of an extremely large number
       of responses.
  
  
  
  
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  9.2.1Example: DAV:acl-principal-prop-set Report
  
       Resource http://www.example.com/index.html has an ACL with three
       ACEs:
  
       ACE #1: All principals (DAV:all) have DAV:read and DAV:read-
       current-user-privilege-set access.
  
       ACE #2: The principal identified by
       http://www.example.com/people/gstein (the user "gstein") is granted
       DAV:write,  DAV:write-acl, DAV:read-acl privileges.
  
       ACE #3: The group identified by
       http://www.example.com/groups/authors (the "authors" group) is
       granted DAV:write and DAV:read-acl privileges.
  
       The following example shows a DAV:acl-principal-prop-set report
       requesting the DAV:displayname property. It returns the value of
       DAV:displayname for resources http://www.example.com/people/gstein
       and http://www.example.com/groups/authors , but not for DAV:all,
       since this is not an http(s) URL.
  
  
       >> Request <<
  
       REPORT /index.html HTTP/1.1
       Host: www.example.com
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxxx
       Depth: 0
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:acl-principal-prop-set xmlns:D="DAV:">
         <D:prop>
           <D:displayname/>
         </D:prop>
       </D:acl-principal-prop-set>
  
       >> Response <<
  
       HTTP/1.1 207 Multi-Status
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxxx
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:multistatus xmlns:D="DAV:">
         <D:response>
           <D:href>http://www.example.com/people/gstein</D:href>
           <D:propstat>
             <D:prop>
               <D:displayname>Greg Stein</D:displayname>
             </D:prop>
             <D:status>HTTP/1.1 200 OK</D:status>
           </D:propstat>
  
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         </D:response>
         <D:response>
           <D:href>http://www.example.com/groups/authors</D:href>
           <D:propstat>
             <D:prop>
               <D:displayname>Site authors</D:displayname>
             </D:prop>
             <D:status>HTTP/1.1 200 OK</D:status>
           </D:propstat>
         </D:response>
       </D:multistatus>
  
  9.3 DAV:principal-match REPORT
  
       The DAV:principal-match REPORT is used to identify all members (at
       any depth) of the collection identified by the Request-URI that
       match the current user. In particular, if the collection contains
       principals, the report can be used to identify all members of the
       collection that match the current user. Alternatively, if the
       collection contains resources that have a property that identifies
       a principal (e.g. DAV:owner), the report can be used to identify
       all members of the collection whose property identifies a principal
       that matches the current user. For example, this report can return
       all of the resources in a collection hierarchy that are owned by
       the current user. Support for this report is REQUIRED.
  
     Marshalling:
  
       The request body MUST be a DAV:principal-match XML element.
  
       <!ELEMENT principal-match ((principal-property | self), prop?)>
       <!ELEMENT principal-property ANY>
       ANY value: an element whose value identifies a property. The expectation is
       the value of the named property typically contains an href element that
       contains the URI of a principal
       <!ELEMENT self EMPTY>
       prop: see RFC 2518, Section 12.11
  
  
       This report is only defined when the Depth header has value "0";
       other values result in a 400 (Bad Request) error response. Note
       that [RFC3253], Section 3.6, states that if the Depth header is not
       present, it defaults to a value of "0".
  
       The response body for a successful request MUST be a
       DAV:multistatus XML element. In the case where there are no
       response elements, the returned multistatus XML element is empty.
  
       multistatus: see RFC 2518, Section 12.9
  
  
       The response body for a successful DAV:principal-match REPORT
       request MUST contain a DAV:response element for each member of the
       collection that matches the current user. When the DAV:principal-
  
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       property element is used, a match occurs if the current user is
       matched by the principal identified by the URI found in the
       DAV:href element of the property identified by the DAV:principal-
       property element. When the DAV:self element is used in a
       DAV:principal-match report issued against a group, it matches the
       group if a member identifies the same principal as the current
       user.
  
       If DAV:prop is specified in the request body, the properties
       specified in the DAV:prop element MUST be reported in the
       DAV:response elements.
  
  
  9.3.1Example: DAV:principal-match REPORT
  
       The following example identifies the members of the collection
       identified by the URL http://www.example.com/doc that are owned by
       the current user. The current user ("gclemm") is authenticated
       using Digest authentication.
  
       >> Request <<
  
       REPORT /doc/ HTTP/1.1
       Host: www.example.com
       Authorization: Digest username="gclemm",
          realm="gclemm@webdav.org", nonce="...",
          uri="/papers/", response="...", opaque="..."
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxxx
       Depth: 0
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:principal-match xmlns:D="DAV:">
         <D:principal-property>
           <D:owner/>
         </D:principal-property>
       </D:principal-match>
  
       >> Response <<
  
       HTTP/1.1 207 Multi-Status
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxxx
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:multistatus xmlns:D="DAV:">
         <D:response>
           <D:href>http://www.example.com/doc/foo.html</D:href>
           <D:status>HTTP/1.1 200 OK</D:status>
         </D:response>
         <D:response>
           <D:href>http://www.example.com/doc/img/bar.gif</D:href>
           <D:status>HTTP/1.1 200 OK</D:status>
         </D:response>
  
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       </D:multistatus>
  
  9.4 DAV:principal-property-search REPORT
  
       The DAV:principal-property-search REPORT performs a search for all
       principals whose properties contain character data that matches the
       search criteria specified in the request. One expected use of this
       report is to discover the URL of a principal associated with a
       given person or group by searching for them by name. This is done
       by searching over DAV:displayname, which is defined on all
       principals.
  
       The actual search method (exact matching vs. substring matching vs,
       prefix-matching, case-sensitivity) deliberately is left to the
       server implementation to allow implementation on a wide set of
       possible user management systems. In cases where the implementation
       of DAV:principal-property-search is not constrained by the
       semantics of an underlying user management repository, preferred
       default semantics are caseless substring matches.
  
       For implementation efficiency, servers do not typically support
       searching on all properties. A client can discover the set of
       searchable properties by using the DAV:principal-search-property-
       set REPORT, defined in Section 9.5.
  
       Support for the DAV:principal-property-search report is REQUIRED.
  
       Implementation Note: The value of a WebDAV property is a sequence
       of well-formed XML, and hence can include any character in the
       Unicode/ISO-10646 standard, that is, most known characters in human
       languages. Due to the idiosyncrasies of case mapping across human
       languages, implementation of case-insensitive matching is non-
       trivial. Implementors of servers that do perform substring matching
       are strongly encouraged to consult [CaseMap], especially Section
       2.3 ("Caseless Matching"), for guidance when implementing their
       case-insensitive matching algorithms.
  
       Implementation Note: Some implementations of this protocol will use
       an LDAP repository for storage of principal metadata. The schema
       describing each attribute (akin to a WebDAV property) in an LDAP
       repository specifies whether it supports case-sensitive or caseless
       searching. One of the benefits of leaving the search method to the
       discretion of the server implementation is the default LDAP
       attribute search behavior can be used when implementing the
       DAV:principal-property-search report.
  
     Marshalling:
  
       The request body MUST be a DAV:principal-property-search XML
       element containing a search specification and an optional list of
       properties. For every principal that matches the search
       specification, the response will contain the value of the requested
       properties on that principal.
  
  
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       <!ELEMENT principal-property-search
        ((property-search+), prop?, apply-to-principal-collection-set?) >
  
       By default, the report searches all members (at any depth) of the
       collection identified by the Request-URI.  If DAV:apply-to-
       principal-collection-set is specified in the request body, the
       request is applied instead to each collection identified by the
       DAV:prinicipal-collection-set property of the resource identified
       by the Request-URI.
  
       The DAV:property-search element contains a prop element enumerating
       the properties to be searched and a match element, containing the
       search string.
  
       <!ELEMENT property-search (prop, match) >
       prop: see RFC 2518, Section 12.11
  
       <!ELEMENT match #PCDATA >
  
       Multiple property-search elements or multiple elements within a
       DAV:prop element will be interpreted with a logical AND.
  
       This report is only defined when the Depth header has value "0";
       other values result in a 400 (Bad Request) error response. Note
       that [RFC3253], Section 3.6, states that if the Depth header is not
       present, it defaults to a value of "0".
  
       The response body for a successful request MUST be a
       DAV:multistatus XML element. In the case where there are no
       response elements, the returned multistatus XML element is empty.
  
       multistatus: see RFC 2518, Section 12.9
  
       The response body for a successful DAV:principal-property-search
       REPORT request MUST contain  a DAV:response element for each
       principal whose property values satisfy the search specification
       given in DAV:principal-property-search.
  
       The response body for an unsuccessful DAV:principal-property-search
       REPORT request MUST contain, after the XML element indicating the
       failed precondition or postcondition, a DAV:prop element containing
       the property that caused the pre/postcondition to fail.
  
       If DAV:prop is specified in the request body, the properties
       specified in the DAV:prop element MUST be reported in the
       DAV:response elements.
  
     Preconditions:
  
       (DAV:property-must-be-searchable): All properties specified in the
       DAV:principal-property-search REPORT must be searchable.
  
     Postconditions:
  
  
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       (DAV:number-of-matches-within-limits): The number of matching
       principals must fall within server-specific, predefined limits. For
       example, this condition might be triggered if a search
       specification would cause the return of an extremely large number
       of responses.
  
  
  9.4.1Matching
  
       There are several cases to consider when matching strings. The
       easiest case is when a property value is "simple" and has only
       character information item content (see [REC-XML-INFOSET]). For
       example, the search string "julian" would match the DAV:displayname
       property with value "Julian Reschke". Note that the on-the-wire
       marshalling of DAV:displayname in this case is:
  
       <D:displayname xmlns:D="DAV:">Julian Reschke</D:displayname>
  
  
       The name of the property is encoded into the XML element
       information item, and the character information item content of the
       property is "Julian Reschke".
  
       A more complicated case occurs when properties have mixed content
       (that is, compound values consisting of multiple child element
       items, other types of information items, and character information
       item content). Consider the property "aprop" in the namespace
       "http://www.example.com/props/", marshalled as:
  
       <W:aprop xmlns:W="http://www.example.com/props/">
       {cdata 0}<W:elem1>{cdata 1}</W:elem1>
         <W:elem2>{cdata 2}</W:elem2>{cdata 3}
       </W:aprop>
  
  
       In this case, matching is performed on each individual contiguous
       sequence of character information items. In the example above, a
       search string would be compared to the four following strings:
  
       {cdata 0}
       {cdata 1}
       {cdata 2}
       {cdata 3}
  
  
       That is, four individual matches would be performed, one each for
       {cdata 0}, {cdata 1}, {cdata 2}, and {cdata 3}.
  
  
  9.4.2Example: successful DAV:principal-property-search REPORT
  
       In this example, the client requests the principal URLs of all
       users whose DAV:displayname property contains the substring "doE"
       and whose "title" property in the namespace
  
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       "http://BigCorp.com/ns/" (that is, their professional title)
       contains "Sales".  In addition, the client requests five properties
       to be returned with the matching principals:
  
       In the DAV: namespace: displayname
       In the http://www.example.com/ns/ namespace: department, phone,
       office, salary
  
       The response shows that two principal resources meet the search
       specification, "John Doe" and "Zygdoebert Smith". The property
       "salary" in namespace "http://www.example.com/ns/" is not returned,
       since the principal making the request does not have sufficient
       access permissions to read this property.
  
       >> Request <<
  
       REPORT /users/ HTTP/1.1
       Host: www.example.com
       Content-Type: text/xml; charset=utf-8
       Content-Length: xxxx
       Depth: 0
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:principal-property-search xmlns:D="DAV:">
         <D:property-search>
           <D:prop>
             <D:displayname/>
           </D:prop>
           <D:match>doE</D:match>
         </D:property-search>
         <D:property-search>
           <D:prop xmlns:B="http://www.example.com/ns/">
             <B:title/>
           </D:prop>
           <D:match>Sales</D:match>
         </D:property-search>
         <D:prop xmlns:B="http://www.example.com/ns/">
           <D:displayname/>
           <B:department/>
           <B:phone/>
           <B:office/>
           <B:salary/>
         </D:prop>
       </D:principal-property-search>
  
  
       >> Response <<
  
  
  
       HTTP/1.1 207 Multi-Status
       Content-Type: text/xml; charset=utf-8
       Content-Length: xxxx
  
  
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       <?xml version="1.0" encoding="utf-8" ?>
       <D:multistatus xmlns:D="DAV:" xmlns:B="http://BigCorp.com/ns/">
         <D:response>
           <D:href>http://www.example.com/users/jdoe</D:href>
           <D:propstat>
             <D:prop>
               <D:displayname>John Doe</D:displayname>
               <B:department>Widget Sales</B:department>
               <B:phone>234-4567</B:phone>
               <B:office>209</B:office>
             </D:prop>
             <D:status>HTTP/1.1 200 OK</D:status>
           </D:propstat>
           <D:propstat>
             <D:prop>
               <B:salary/>
             </D:prop>
             <D:status>HTTP/1.1 403 Forbidden</D:status>
           </D:propstat>
         </D:response>
         <D:response>
           <D:href>http://www.example.com/users/zsmith</D:href>
           <D:propstat>
             <D:prop>
               <D:displayname>Zygdoebert Smith</D:displayname>
               <B:department>Gadget Sales</B:department>
               <B:phone>234-7654</B:phone>
               <B:office>114</B:office>
             </D:prop>
             <D:status>HTTP/1.1 200 OK</D:status>
           </D:propstat>
           <D:propstat>
             <D:prop>
               <B:salary/>
             </D:prop>
             <D:status>HTTP/1.1 403 Forbidden</D:status>
           </D:propstat>
         </D:response>
       </D:multistatus>
  
  
  9.4.3Example: Unsuccessful DAV:principal-property-search REPORT
  
       In this example, the client requests a search on the non-searchable
       property "phone" in the namespace "http://www.example.com/ns/".
       The response is a 403 (Forbidden), with a response body containing
       a DAV:property-must-be-searchable XML element as the value of a
       DAV:error XML element.
  
  
  
       >> Request <<
  
       REPORT /users/ HTTP/1.1
  
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       Host: www.example.com
       Content-Type: text/xml; charset=utf-8
       Content-Length: xxxx
       Depth: 0
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:principal-property-search xmlns:D="DAV:">
         <D:property-search>
           <D:prop xmlns:B="http://www.example.com/ns/">
             <B:phone/>
           </D:prop>
           <D:match>232</D:match>
         </D:property-search>
       </D:principal-property-search>
  
  
       >> Response <<
  
  
       HTTP/1.1 403 Forbidden
       Content-Type: text/xml; charset=utf-8
       Content-Length: xxxx
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:error xmlns:D="DAV:">
         <D:property-must-be-searchable>
           <D:prop xmlns:B="http://www.example.com/ns/">
             <B:phone/>
           </D:prop>
         </D:property-must-be-searchable>
       </D:error>
  
  
  9.5 DAV:principal-search-property-set REPORT
  
       The DAV:principal-search-property-set REPORT identifies those
       properties that may be searched using the DAV:principal-property-
       search REPORT (defined in Section 9.4).
  
       Servers MUST support the DAV:principal-search-property-set REPORT
       on all collections identified in the value of a DAV:principal-
       collection-set property.
  
       An access control protocol user agent could use the results of the
       DAV:principal-search-property-set REPORT to present a query
       interface to the user for retrieving principals.
  
       Support for this report is REQUIRED.
  
       Implementation Note: Some clients will have only limited screen
       real estate for the display of lists of searchable properties. In
       this case, a user might appreciate having the most frequently
       searched properties be displayed on-screen, rather than having to
       scroll through a long list of searchable properties. One mechanism
  
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       for signaling the most frequently searched properties is to return
       them towards the start of a list of properties. A client can then
       preferentially display the list of properties in order, increasing
       the likelihood that the most frequently searched properties will
       appear on-screen, and will not require scrolling for their
       selection.
  
     Marshalling:
  
       The request body MUST be an empty DAV:principal-search-property-set
       XML element.
  
       This report is only defined when the Depth header has value "0";
       other values result in a 400 (Bad Request) error response. Note
       that [RFC3253], Section 3.6, states that if the Depth header is not
       present, it defaults to a value of "0".
  
       The response body MUST be  a DAV:principal-search-property-set XML
       element, containing a DAV:principal-search-property XML element for
       each property that may be searched with the DAV:principal-property-
       search REPORT. A server MAY limit its response to just a subset of
       the searchable properties, such as those likely to be useful to an
       interactive access control client.
  
       <!ELEMENT principal-search-property-set (principal-search-property*) >
  
  
       Each DAV:principal-search-property XML element contains exactly one
       searchable property, and a description of the property.
  
       <!ELEMENT principal-search-property (prop, description) >
  
       The DAV:prop element contains one principal property on which the
       server is able to perform a DAV:principal-property-search REPORT.
  
       prop: see RFC 2518, Section 12.11
  
       The description element is a human-readable description of what
       information this property represents. Servers MUST indicate the
       human language of the description using the xml:lang attribute and
       SHOULD consider the HTTP Accept-Language request header when
       selecting one of multiple available languages.
  
       <!ELEMENT description #PCDATA >
  
  9.5.1Example: DAV:principal-search-property-set REPORT
  
       In this example, the client determines the set of searchable
       principal properties by requesting the DAV:principal-search-
       property-set REPORT on the root of the server's principal URL
       collection set, identified by http://www.example.com/users/.
  
       >> Request <<
  
  
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       REPORT /users/ HTTP/1.1
       Host: www.example.com
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxx
       Accept-Language: en, de
       Authorization: BASIC d2FubmFtYWs6cGFzc3dvcmQ=
       Depth: 0
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:principal-search-property-set xmlns:D="DAV:"/>
  
  
       >> Response <<
  
       HTTP/1.1 200 OK
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxx
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:principal-search-property-set xmlns:D="DAV:">
         <D:principal-search-property>
           <D:prop>
             <D:displayname/>
           </D:prop>
           <D:description xml:lang="en">Full name</D:description>
         </D:principal-search-property>
         <D:principal-search-property>
           <D:prop xmlns:B="http://BigCorp.com/ns/">
             <B:title/>
           </D:prop>
           <D:description xml:lang="en">Job title</D:description>
         </D:principal-search-property>
       </D:principal-search-property-set>
  
  10 XML PROCESSING
  
       Implementations of this specification MUST support the XML element
       ignore rule, as specified in Section 23.3.2 of [RFC2518], and the
       XML Namespace recommendation [REC-XML-NAMES].
  
       Note that use of the DAV namespace is reserved for XML elements and
       property names defined in a standards-track or Experimental IETF
       RFC.
  
  
  
  
  11 INTERNATIONALIZATION CONSIDERATIONS
  
       In this specification, the only human-readable content can be found
       in the description XML element, found within the DAV:supported-
       privilege-set property.  This element contains a human-readable
       description of the capabilities controlled by a privilege.  As a
       result, the description element must be capable of representing
  
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       descriptions in multiple character sets.  Since the description
       element is found within a WebDAV property, it is represented on the
       wire as XML [REC-XML], and hence can leverage XML's language
       tagging and character set encoding capabilities. Specifically, XML
       processors at minimum must 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. Futhermore, this specification requires server
       implementations to tag description fields with the xml:lang
       attribute (see Section 2.12 of [REC-XML]), which specifies the
       human language of the description. Additionally, server
       implementations should take into account the value of the Accept-
       Language HTTP header to determine which description string to
       return.
  
       For XML elements other than the description element, it is expected
       that implementations will treat the property names, privilege
       names, and values as tokens, and convert these tokens into 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 to a human
       user.
  
       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].
  
  
  12 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.
  
  
  12.1Increased Risk of Compromised Users
  
       In the absence of a mechanism for remotely manipulating access
       control lists, if a single user's authentication credentials are
  
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       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 privileges across a broad range of resources.
  
  
  12.2Risks of the DAV:read-acl and DAV:current-user-privilege-set 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 read-
       current-user-privilege-set 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.
  
  
  12.3No Foreknowledge of Initial ACL
  
       In an effort to reduce protocol complexity, this protocol
       specification intentionally does not address the issue of how to
       manage or discover the initial ACL that is placed upon a resource
       when it is created. The only way to discover the initial ACL is to
       create a new resource, then retrieve the value of the DAV:acl
       property. This assumes the principal creating the resource also has
       been granted the DAV:read-acl privilege.
  
       As a result, it is possible that a principal could create a
       resource, and then discover that its ACL grants privileges that are
       undesirable. Furthermore, this protocol makes it possible (though
  
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       unlikely) that the creating principal could be unable to modify the
       ACL, or even delete the resource. Even when the ACL can be
       modified, there will be a short period of time when the resource
       exists with the initial ACL before its new ACL can be set.
  
       Several factors mitigate this risk. Human principals are often
       aware of the default access permissions in their editing
       environments and take this into account when writing information.
       Furthermore, default privilege policies are usually very
       conservative, limiting the privileges granted by the initial ACL.
  
  
  13 AUTHENTICATION
  
       Authentication mechanisms defined for use with HTTP and WebDAV also
       apply to this WebDAV Access Control Protocol, in particular the
       Basic and Digest authentication mechanisms defined in [RFC2617].
       Implementation of the ACL spec requires that Basic authentication,
       if used, MUST only be supported over secure transport such as TLS.
  
  
  14 IANA CONSIDERATIONS
  
       This document uses the namespace defined by [RFC2518] for XML
       elements. That is, this specification uses the "DAV:" URI
       namespace, previously registered in the URI schemes registry. All
       other IANA considerations mentioned in [RFC2518] are also
       applicable to this specification.
  
  
  15 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
       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
  
  
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       this standard.  Please address the information to the IETF
       Executive Director.
  
  
  16 ACKNOWLEDGEMENTS
  
       This protocol is the collaborative product of the WebDAV ACL design
       team: Bernard Chester, Geoff Clemm, Anne Hopkins, Barry Lind, Sean
       Lyndersay, Eric Sedlar, Greg Stein, and Jim Whitehead. The authors
       are grateful for the detailed review and comments provided by Jim
       Amsden, Dylan Barrell, Gino Basso, Murthy Chintalapati, Lisa
       Dusseault, Stefan Eissing, Tim Ellison, Yaron Goland, Dennis
       Hamilton, Laurie Harper, Eckehard Hermann, Ron Jacobs, Chris
       Knight, Remy Maucherat, Larry Masinter, Joe Orton, Peter Raymond,
       Julian Reschke, and Keith Wannamaker. We thank Keith Wannamaker for
       the initial text of the principal property search sections. 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 DeltaV, WebDAV and HTTP protocols upon which this
       protocol is layered, and the invaluable feedback from the WebDAV
       working group.
  
  
  17 REFERENCES
  
  
  17.1Normative References
  
       [RFC2119] S.Bradner, "Key words for use in RFCs to Indicate
       Requirement Levels." RFC 2119, BCP 14, March, 1997.
  
       [REC-XML] T. Bray, J. Paoli, C.M. Sperberg-McQueen, "Extensible
       Markup Language (XML)." World Wide Web Consortium Recommendation
       REC-xml.http://www.w3.org/TR/REC-xml
  
       [REC-XML-NAMES] T. Bray, D. Hollander, A. Layman, "Name Spaces in
       XML" World Wide Web Consortium Recommendation REC-xml-names.
       http://www.w3.org/TR/REC-xml-names/
  
       [RFC3253] G. Clemm, J. Amsden, T. Ellison, C. Kaler, J. Whitehead,
       "Versioning Extensions to WebDAV." RFC 3253, March 2002.
  
       [REC-XML-INFOSET] J. Cowan, R. Tobin, "XML Information Set." World
       Wide Web Consortium Recommendation REC-xml-infoset.
       http://www.w3.org/TR/xml-infoset/
  
       [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, 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, June, 1999.
  
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       [RFC2518] Y. Goland, E. Whitehead, A. Faizi, S. R. Carter, D.
       Jensen, "HTTP Extensions for Distributed Authoring -- WEBDAV." RFC
       2518, February, 1999.
  
       [RFC2368] P. Hoffman, L. Masinter, J. Zawinski, "The mailto URL
       scheme." RFC 2368, July, 1998.
  
       [RFC3023] M. Murata, S. St.Laurent, D. Kohn, "XML Media Types." RFC
       3023, January, 2001.
  
       [RFC3010] S. Shepler, B. Callaghan, D. Robinson, R. Thurlow, C.
       Beame, M. Eisler, D.Noveck "NFS version 4 Protocol." RFC 3010,
       December 2000.
  
        [UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode and
       ISO 10646." RFC 2279, January, 1998.
  
  
  17.2Informational References
  
       [RFC2026] S.Bradner, "The Internet Standards Process - Revision 3."
       RFC 2026, BCP 9. Harvard, October, 1996.
  
       [RFC2255] T. Howes, M. Smith, "The LDAP URL Format." RFC 2255.
       Netscape, December, 1997.
  
       [RFC2251] M. Wahl, T. Howes, S. Kille, "Lightweight Directory
       Access Protocol (v3)." RFC 2251. Critical Angle, Netscape, Isode,
       December, 1997.
  
       [CaseMap] M. Davis, "Case Mappings", Unicode Standard Annex #21,
       March 26, 2001.  http://www.unicode.org/unicode/reports/tr21
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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  18 AUTHORS' ADDRESSES
  
       Geoffrey Clemm
  
       IBM
  
       20 Maguire Road
  
       Lexington, MA 02421
  
       Email: geoffrey.clemm@us.ibm.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: eric.sedlar@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
  
  
  
  
  
  
  
  
  
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  19 APPENDICES
  
  
  19.1WebDAV XML Document Type Definition Addendum
  
       All XML elements defined in this Document Type Definition (DTD)
       belong to the DAV namespace. This DTD should be viewed as an
       addendum to the DTD provided in [RFC2518], section 23.1.
  
       <!-- Privileges -->
  
       <!ELEMENT read EMPTY>
       <!ELEMENT write EMPTY>
       <!ELEMENT write-properties EMPTY>
       <!ELEMENT write-content EMPTY>
       <!ELEMENT unlock EMPTY>
       <!ELEMENT read-acl EMPTY>
       <!ELEMENT read-current-user-privilege-set EMPTY>
       <!ELEMENT write-acl EMPTY>
       <!ELEMENT all EMPTY>
  
  
       <!-- Principal Properties (Section 4) -->
  
       <!ELEMENT principal EMPTY>
  
       <!ELEMENT alternate-URI-set (href*)>
       <!ELEMENT principal-URL (href)>
       <!ELEMENT group-member-set (href*)>
       <!ELEMENT group-membership (href*)>
  
       <!-- 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) -->
  
  
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       <!ELEMENT current-user-privilege-set (privilege*)>
  
  
       <!-- DAV:acl Property (Section 5.4) -->
  
       <!ELEMENT acl (ace)* >
       <!ELEMENT ace (invert | principal, (grant|deny), protected?, inherited?)>
       <!ELEMENT invert principal>
  
       <!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:inherited-acl-set Property (Section 5.6) -->
  
       <!ELEMENT inherited-acl-set (href*)>
  
  
       <!-- DAV:principal-collection-set Property (Section 5.6) -->
  
       <!ELEMENT principal-collection-set (href*)>
  
  
       <!-- DAV:acl-semantics Property (Section 6) -->
  
       <!ELEMENT acl-semantics (ace-combination?, ace-ordering?, allowed-ace?,
       required-principal?)>
  
       <!ELEMENT ace-combination
        (first-match | all-grant-before-any-deny | specific-deny-overrides-grant)>
       <!ELEMENT first-match EMPTY>
       <!ELEMENT all-grant-before-any-deny EMPTY>
       <!ELEMENT specific-deny-overrides-grant EMPTY>
  
       <!ELEMENT ace-ordering (deny-before-grant)? >
       <!ELEMENT deny-before-grant EMPTY>
  
       <!ELEMENT allowed-ace (grant-only |
  
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                       no-invert)*>
       <!ELEMENT grant-only EMPTY>
       <!ELEMENT no-invert EMPTY>
  
  
  
       <!ELEMENT required-principal
         (all? | authenticated? | unauthenticated? | self? | href* |property*)>
  
  
       <!-- ACL method preconditions (Section 8.1.1) -->
  
       <!ELEMENT no-ace-conflict EMPTY>
       <!ELEMENT no-protected-ace-conflict EMPTY>
       <!ELEMENT no-inherited-ace-conflict EMPTY>
       <!ELEMENT limited-number-of-aces EMPTY>
       <!ELEMENT no-abstract EMPTY>
       <!ELEMENT not-supported-privilege EMPTY>
       <!ELEMENT missing-required-principal EMPTY>
       <!ELEMENT recognized-principal EMPTY>
       <!ELEMENT allowed-principal EMPTY>
  
  
       <!-- REPORTs (Section 9) -->
  
       <!ELEMENT acl-principal-prop-set ANY>
       ANY value: a sequence of one or more elements, with at most one DAV:prop
       element.
  
       <!ELEMENT principal-match ((principal-property | self), prop?)>
       <!ELEMENT principal-property ANY>
       ANY value: an element whose value identifies a property. The expectation is
       the value of the named property typically contains an href element that
       contains the URI of a principal
       <!ELEMENT self EMPTY>
  
       <!ELEMENT principal-property-search ((property-search+), prop?) >
       <!ELEMENT property-search (prop, match) >
       <!ELEMENT match #PCDATA >
  
       <!ELEMENT principal-search-property-set (principal-search-property*) >
       <!ELEMENT principal-search-property (prop, description) >
  
  19.2WebDAV Method Privilege Table (Normative)
  
  The following table of WebDAV methods (as defined in RFC 2518, 2616, and
  3253) clarifies which privileges are required for access for each
  method.  Note that the privileges listed, if denied, MUST cause access
  to be denied.  However, given that a specific implementation MAY define
  an additional custom privilege to control access to existing methods,
  having all of the indicated privileges does not mean that access will be
  granted.  Note that lack of the indicated privileges does not imply that
  access will be denied, since a particular implementation may use a sub-
  privilege aggregated under the indicated privilege to control access.
  
  Clemm, et al.                                      [Page 60]

INTERNET-DRAFT        WebDAV Bindings         June 27, 2003
  
  
  Privileges required refer to the current resource being processed unless
  otherwise specified.
  
  
  
  METHOD    PRIVILEGES
   GET      <D:read>
   HEAD     <D:read>
   OPTIONS  <D:read>
   PUT      <D:write-content> (on parent coll if resource
       doesn't already exist, or on existing resource
       otherwise)
   PROPPATCH     <D:write-properties>
   ACL      <D:write-acl>
   PROPFIND      <D:read> (plus <read-acl> and
       <read-current-user-privilege-set> as needed)
   COPY     <D:read>, <D:write-content> on target collection
   MOVE (no target exists) <D:write-content> on source&target coll, plus
  <dav:read>
       on the resource being moved MAY be required
   MOVE (target exists)    As above, plus <D:delete> on the resource to be
  overwritten
   DELETE   <D:delete>, <D:write-content> on parent collection
   LOCK     <D:write-content>
   MKCOL    <D:write-content> (on parent coll)
   UNLOCK   <D:unlock>
   CHECKOUT      <D:write>
   CHECKIN  <D:write>
   REPORT   <D:read> (on all referenced resources)
   VERSION-CONTROL    <D:write>
   MERGE    <D:write-content>
   MKWORKSPACE   <D:write-content> on parent collection
   BASELINE-CONTROL   <D:write>
   MKACTIVITY    <D:write-content> on parent collection
  

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