Network Working Group G. Clemm
Request for Comments: 3744 IBM
Category: Standards Track J. Reschke
greenbytes
E. Sedlar
Oracle Corporation
J. Whitehead
U.C. Santa Cruz
May 2004
Web Distributed Authoring and Versioning (WebDAV)
Access Control Protocol
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies a set of methods, headers, message bodies,
properties, and reports that define Access Control extensions to the
WebDAV Distributed Authoring Protocol. This protocol permits a
client to read and modify access control lists that instruct a server
whether to allow or deny operations upon a resource (such as
HyperText Transfer Protocol (HTTP) method invocations) by a given
principal. A lightweight representation of principals as Web
resources supports integration of a wide range of user management
repositories. Search operations allow discovery and manipulation of
principals using human names.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terms. . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 8
2. Principals . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Privileges . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. DAV:read Privilege . . . . . . . . . . . . . . . . . . . 10
3.2. DAV:write Privilege. . . . . . . . . . . . . . . . . . . 10
3.3. DAV:write-properties Privilege . . . . . . . . . . . . . 10
3.4. DAV:write-content Privilege. . . . . . . . . . . . . . . 11
3.5. DAV:unlock Privilege . . . . . . . . . . . . . . . . . . 11
3.6. DAV:read-acl Privilege . . . . . . . . . . . . . . . . . 11
3.7. DAV:read-current-user-privilege-set Privilege. . . . . . 12
3.8. DAV:write-acl Privilege. . . . . . . . . . . . . . . . . 12
3.9. DAV:bind Privilege . . . . . . . . . . . . . . . . . . . 12
3.10. DAV:unbind Privilege . . . . . . . . . . . . . . . . . . 12
3.11. DAV:all Privilege. . . . . . . . . . . . . . . . . . . . 13
3.12. Aggregation of Predefined Privileges . . . . . . . . . . 13
4. Principal Properties . . . . . . . . . . . . . . . . . . . . . 13
4.1. DAV:alternate-URI-set. . . . . . . . . . . . . . . . . . 14
4.2. DAV:principal-URL. . . . . . . . . . . . . . . . . . . . 14
4.3. DAV:group-member-set . . . . . . . . . . . . . . . . . . 14
4.4. DAV:group-membership . . . . . . . . . . . . . . . . . . 14
5. Access Control Properties. . . . . . . . . . . . . . . . . . . 15
5.1. DAV:owner. . . . . . . . . . . . . . . . . . . . . . . . 15
5.1.1. Example: Retrieving DAV:owner . . . . . . . . . . 15
5.1.2. Example: An Attempt to Set DAV:owner. . . . . . . 16
5.2. DAV:group. . . . . . . . . . . . . . . . . . . . . . . . 18
5.3. DAV:supported-privilege-set. . . . . . . . . . . . . . . 18
5.3.1. Example: Retrieving a List of Privileges
Supported on a Resource . . . . . . . . . . . . . 19
5.4. DAV:current-user-privilege-set . . . . . . . . . . . . . 21
5.4.1. Example: Retrieving the User's Current Set of
Assigned Privileges . . . . . . . . . . . . . . . 22
5.5. DAV:acl. . . . . . . . . . . . . . . . . . . . . . . . . 23
5.5.1. ACE Principal . . . . . . . . . . . . . . . . . . 23
5.5.2. ACE Grant and Deny. . . . . . . . . . . . . . . . 25
5.5.3. ACE Protection. . . . . . . . . . . . . . . . . . 25
5.5.4. ACE Inheritance . . . . . . . . . . . . . . . . . 25
5.5.5. Example: Retrieving a Resource's Access Control
List. . . . . . . . . . . . . . . . . . . . . . . 25
5.6. DAV:acl-restrictions . . . . . . . . . . . . . . . . . . 27
5.6.1. DAV:grant-only. . . . . . . . . . . . . . . . . . 27
5.6.2. DAV:no-invert ACE Constraint. . . . . . . . . . . 28
5.6.3. DAV:deny-before-grant . . . . . . . . . . . . . . 28
5.6.4. Required Principals . . . . . . . . . . . . . . . 28
5.6.5. Example: Retrieving DAV:acl-restrictions. . . . . 28
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5.7. DAV:inherited-acl-set. . . . . . . . . . . . . . . . . . 29
5.8. DAV:principal-collection-set . . . . . . . . . . . . . . 30
5.8.1. Example: Retrieving DAV:principal-collection-set. 30
5.9. Example: PROPFIND to retrieve access control properties. 32
6. ACL Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 36
7. Access Control and existing methods. . . . . . . . . . . . . . 37
7.1. Any HTTP method. . . . . . . . . . . . . . . . . . . . . 37
7.1.1. Error Handling. . . . . . . . . . . . . . . . . . 37
7.2. OPTIONS. . . . . . . . . . . . . . . . . . . . . . . . . 38
7.2.1. Example - OPTIONS . . . . . . . . . . . . . . . . 39
7.3. MOVE . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7.4. COPY . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7.5. LOCK . . . . . . . . . . . . . . . . . . . . . . . . . . 39
8. Access Control Methods . . . . . . . . . . . . . . . . . . . . 40
8.1. ACL. . . . . . . . . . . . . . . . . . . . . . . . . . . 40
8.1.1. ACL Preconditions . . . . . . . . . . . . . . . . 40
8.1.2. Example: the ACL method . . . . . . . . . . . . . 42
8.1.3. Example: ACL method failure due to protected
ACE conflict. . . . . . . . . . . . . . . . . . . 43
8.1.4. Example: ACL method failure due to an
inherited ACE conflict. . . . . . . . . . . . . . 44
8.1.5. Example: ACL method failure due to an attempt
to set grant and deny in a single ACE . . . . . . 45
9. Access Control Reports . . . . . . . . . . . . . . . . . . . . 46
9.1. REPORT Method. . . . . . . . . . . . . . . . . . . . . . 46
9.2. DAV:acl-principal-prop-set Report. . . . . . . . . . . . 47
9.2.1. Example: DAV:acl-principal-prop-set Report. . . . 48
9.3. DAV:principal-match REPORT . . . . . . . . . . . . . . . 49
9.3.1. Example: DAV:principal-match REPORT . . . . . . . 50
9.4. DAV:principal-property-search REPORT . . . . . . . . . . 51
9.4.1. Matching. . . . . . . . . . . . . . . . . . . . . 53
9.4.2. Example: successful DAV:principal-property-search
REPORT. . . . . . . . . . . . . . . . . . . . . . 54
9.5. DAV:principal-search-property-set REPORT . . . . . . . . 56
9.5.1. Example: DAV:principal-search-property-set
REPORT. . . . . . . . . . . . . . . . . . . . . . 58
10. XML Processing . . . . . . . . . . . . . . . . . . . . . . . . 59
11. Internationalization Considerations. . . . . . . . . . . . . . 59
12. Security Considerations. . . . . . . . . . . . . . . . . . . . 60
12.1. Increased Risk of Compromised Users. . . . . . . . . . . 60
12.2. Risks of the DAV:read-acl and
DAV:current-user-privilege-set Privileges. . . . . . . . 60
12.3. No Foreknowledge of Initial ACL. . . . . . . . . . . . . 61
13. Authentication . . . . . . . . . . . . . . . . . . . . . . . . 61
14. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 62
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 62
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16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 62
16.1. Normative References . . . . . . . . . . . . . . . . . . 62
16.2. Informative References . . . . . . . . . . . . . . . . . 63
Appendices
A. WebDAV XML Document Type Definition Addendum . . . . . . . . . 64
B. WebDAV Method Privilege Table (Normative). . . . . . . . . . . 67
Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 71
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 72
1. Introduction
The goal of the WebDAV access control extensions is to provide an
interoperable mechanism for handling discretionary access control for
content and metadata managed by WebDAV servers. WebDAV access
control can be implemented on content repositories with security as
simple as that of a UNIX file system, as well as more sophisticated
models. The underlying principle of access control is that who you
are determines what operations you can perform on a resource. The
"who you are" is defined by a "principal" identifier; users, client
software, servers, and groups of the previous have principal
identifiers. The "operations you can perform" are determined by a
single "access control list" (ACL) associated with a resource. An
ACL contains a set of "access control entries" (ACEs), where each ACE
specifies a principal and a set of privileges that are either granted
or denied to that principal. When a principal submits an operation
(such as an HTTP or WebDAV method) to a resource for execution, the
server evaluates the ACEs in the ACL to determine if the principal
has permission for that operation.
Since every ACE contains the identifier of a principal, client
software operated by a human must provide a mechanism for selecting
this principal. This specification uses http(s) scheme URLs to
identify principals, which are represented as WebDAV-capable
resources. There is no guarantee that the URLs identifying
principals will be meaningful to a human. For example,
http://www.example.com/u/256432 and
http://www.example.com/people/Greg.Stein are both valid URLs that
could be used to identify the same principal. To remedy this, every
principal resource has the DAV:displayname property containing a
human-readable name for the principal.
Since a principal can be identified by multiple URLs, it raises the
problem of determining exactly which principal is being referenced in
a given ACE. It is impossible for a client to determine that an ACE
granting the read privilege to http://www.example.com/people/
Greg.Stein also affects the principal at http://www.example.com/u/
256432. That is, a client has no mechanism for determining that two
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URLs identify the same principal resource. As a result, this
specification requires clients to use just one of the many possible
URLs for a principal when creating ACEs. A client can discover which
URL to use by retrieving the DAV:principal-URL property (Section 4.2)
from a principal resource. No matter which of the principal's URLs
is used with PROPFIND, the property always returns the same URL.
With a system having hundreds to thousands of principals, the problem
arises of how to allow a human operator of client software to select
just one of these principals. One approach is to use broad
collection hierarchies to spread the principals over a large number
of collections, yielding few principals per collection. An example
of this is a two level hierarchy with the first level containing 36
collections (a-z, 0-9), and the second level being another 36,
creating collections /a/a/, /a/b/, ..., /a/z/, such that a principal
with last name "Stein" would appear at /s/t/Stein. In effect, this
pre-computes a common query, search on last name, and encodes it into
a hierarchy. The drawback with this scheme is that it handles only a
small set of predefined queries, and drilling down through the
collection hierarchy adds unnecessary steps (navigate down/up) when
the user already knows the principal's name. While organizing
principal URLs into a hierarchy is a valid namespace organization,
users should not be forced to navigate this hierarchy to select a
principal.
This specification provides the capability to perform substring
searches over a small set of properties on the resources representing
principals. This permits searches based on last name, first name,
user name, job title, etc. Two separate searches are supported, both
via the REPORT method, one to search principal resources
(DAV:principal-property-search, Section 9.4), the other to determine
which properties may be searched at all (DAV:principal-search-
property-set, Section 9.5).
Once a principal has been identified in an ACE, a server evaluating
that ACE must know the identity of the principal making a protocol
request, and must validate that that principal is who they claim to
be, a process known as authentication. This specification
intentionally omits discussion of authentication, as the HTTP
protocol already has a number of authentication mechanisms [RFC2617].
Some authentication mechanism (such as HTTP Digest Authentication,
which all WebDAV compliant implementations are required to support)
must be available to validate the identity of a principal.
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The following issues are out of scope for this document:
o Access control that applies only to a particular property on a
resource (excepting the access control properties DAV:acl and
DAV:current-user-privilege-set), rather than the entire resource,
o Role-based security (where a role can be seen as a dynamically
defined group of principals),
o Specification of the ways an ACL on a resource is initialized,
o Specification of an ACL that applies globally to all resources,
rather than to a particular resource.
o Creation and maintenance of resources representing people or
computational agents (principals), and groups of these.
This specification is organized as follows. Section 1.1 defines key
concepts used throughout the specification, and is followed by a more
in-depth discussion of principals (Section 2), and privileges
(Section 3). Properties defined on principals are specified in
Section 4, and access control properties for content resources are
specified in Section 5. The ways ACLs are to be evaluated is
described in Section 6. Client discovery of access control
capability using OPTIONS is described in Section 7.2. Interactions
between access control functionality and existing HTTP and WebDAV
methods are described in the remainder of Section 7. The access
control setting method, ACL, is specified in Section 8. Four reports
that provide limited server-side searching capabilities are described
in Section 9. Sections on XML processing (Section 10),
Internationalization considerations (Section 11), security
considerations (Section 12), and authentication (Section 13) round
out the specification. An appendix (Appendix A) provides an XML
Document Type Definition (DTD) for the XML elements defined in the
specification.
1.1. Terms
This document uses the terms defined in HTTP [RFC2616] and WebDAV
[RFC2518]. In addition, the following terms are defined:
principal
A "principal" is a distinct human or computational actor that
initiates access to network resources. In this protocol, a
principal is an HTTP resource that represents such an actor.
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group
A "group" is a principal that represents a set of other
principals.
privilege
A "privilege" controls access to a particular set of HTTP
operations on a resource.
aggregate privilege
An "aggregate privilege" is a privilege that contains a set of
other privileges.
abstract privilege
The modifier "abstract", when applied to a privilege on a
resource, means the privilege cannot be set in an access control
element (ACE) on that resource.
access control list (ACL)
An "ACL" is a list of access control elements that define access
control to a particular resource.
access control element (ACE)
An "ACE" either grants or denies a particular set of (non-
abstract) privileges for a particular principal.
inherited ACE
An "inherited ACE" is an ACE that is dynamically shared from the
ACL of another resource. When a shared ACE changes on the primary
resource, it is also changed on inheriting resources.
protected property
A "protected property" is one whose value cannot be updated except
by a method explicitly defined as updating that specific property.
In particular, a protected property cannot be updated with a
PROPPATCH request.
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1.2. Notational Conventions
The augmented BNF used by this document to describe protocol elements
is described in Section 2.1 of [RFC2616]. Because this augmented BNF
uses the basic production rules provided in Section 2.2 of [RFC2616],
those rules apply to this document as well.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Definitions of XML elements in this document use XML element type
declarations (as found in XML Document Type Declarations), described
in Section 3.2 of [REC-XML]. When an XML element type in the "DAV:"
namespace is referenced in this document outside of the context of an
XML fragment, the string "DAV:" will be prefixed to the element name.
2. Principals
A principal is a network resource that represents a distinct human or
computational actor that initiates access to network resources.
Users and groups are represented as principals in many
implementations; other types of principals are also possible. A URI
of any scheme MAY be used to identify a principal resource. However,
servers implementing this specification MUST expose principal
resources at an http(s) URL, which is a privileged scheme that points
to resources that have additional properties, as described in Section
4. So, a principal resource can have multiple URIs, one of which has
to be an http(s) scheme URL. Although an implementation SHOULD
support PROPFIND and MAY support PROPPATCH to access and modify
information about a principal, it is not required to do so.
A principal resource may be a group, where a group is a principal
that represents a set of other principals, called the members of the
group. If a person or computational agent matches a principal
resource that is a member of a group, they also match the group.
Membership in a group is recursive, so if a principal is a member of
group GRPA, and GRPA is a member of group GRPB, then the principal is
also a member of GRPB.
3. Privileges
Ability to perform a given method on a resource MUST be controlled by
one or more privileges. Authors of protocol extensions that define
new HTTP methods SHOULD specify which privileges (by defining new
privileges, or mapping to ones below) are required to perform the
method. A principal with no privileges to a resource MUST be denied
any HTTP access to that resource, unless the principal matches an ACE
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constructed using the DAV:all, DAV:authenticated, or
DAV:unauthenticated pseudo-principals (see Section 5.5.1). Servers
MUST report a 403 "Forbidden" error if access is denied, except in
the case where the privilege restricts the ability to know the
resource exists, in which case 404 "Not Found" may be returned.
Privileges may be containers of other privileges, in which case they
are termed "aggregate privileges". If a principal is granted or
denied an aggregate privilege, it is semantically equivalent to
granting or denying each of the aggregated privileges individually.
For example, an implementation may define add-member and remove-
member privileges that control the ability to add and remove a member
of a group. Since these privileges control the ability to update the
state of a group, these privileges would be aggregated by the
DAV:write privilege on a group, and granting the DAV:write privilege
on a group would also grant the add-member and remove-member
privileges.
Privileges may be declared to be "abstract" for a given resource, in
which case they cannot be set in an ACE on that resource. Aggregate
and non-aggregate privileges are both capable of being abstract.
Abstract privileges are useful for modeling privileges that otherwise
would not be exposed via the protocol. Abstract privileges also
provide server implementations with flexibility in implementing the
privileges defined in this specification. For example, if a server
is incapable of separating the read resource capability from the read
ACL capability, it can still model the DAV:read and DAV:read-acl
privileges defined in this specification by declaring them abstract,
and containing them within a non-abstract aggregate privilege (say,
read-all) that holds DAV:read, and DAV:read-acl. In this way, it is
possible to set the aggregate privilege, read-all, thus coupling the
setting of DAV:read and DAV:read-acl, but it is not possible to set
DAV:read, or DAV:read-acl individually. Since aggregate privileges
can be abstract, it is also possible to use abstract privileges to
group or organize non-abstract privileges. Privilege containment
loops are not allowed; therefore, a privilege MUST NOT contain
itself. For example, DAV:read cannot contain DAV:read.
The set of privileges that apply to a particular resource may vary
with the DAV:resourcetype of the resource, as well as between
different server implementations. To promote interoperability,
however, this specification defines a set of well-known privileges
(e.g., DAV:read, DAV:write, DAV:read-acl, DAV:write-acl, DAV:read-
current-user-privilege-set, and DAV:all), which can at least be used
to classify the other privileges defined on a particular resource.
The access permissions on null resources (defined in [RFC2518],
Section 3) are solely those they inherit (if any), and they are not
discoverable (i.e., the access control properties specified in
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Section 5 are not defined on null resources). On the transition from
null to stateful resource, the initial access control list is set by
the server's default ACL value policy (if any).
Server implementations MAY define new privileges beyond those defined
in this specification. Privileges defined by individual
implementations MUST NOT use the DAV: namespace, and instead should
use a namespace that they control, such as an http scheme URL.
3.1. DAV:read Privilege
The read privilege controls methods that return information about the
state of the resource, including the resource's properties. Affected
methods include GET and PROPFIND. Any implementation-defined
privilege that also controls access to GET and PROPFIND must be
aggregated under DAV:read - if an ACL grants access to DAV:read, the
client may expect that no other privilege needs to be granted to have
access to GET and PROPFIND. Additionally, the read privilege MUST
control the OPTIONS method.
3.2. DAV:write Privilege
The write privilege controls methods that lock a resource or modify
the content, dead properties, or (in the case of a collection)
membership of the resource, such as PUT and PROPPATCH. Note that
state modification is also controlled via locking (see section 5.3 of
[RFC2518]), so effective write access requires that both write
privileges and write locking requirements are satisfied. Any
implementation-defined privilege that also controls access to methods
modifying content, dead properties or collection membership must be
aggregated under DAV:write, e.g., if an ACL grants access to
DAV:write, the client may expect that no other privilege needs to be
granted to have access to PUT and PROPPATCH.
3.3. DAV:write-properties Privilege
The DAV:write-properties privilege controls methods that modify the
dead properties of the resource, such as PROPPATCH. Whether this
privilege may be used to control access to any live properties is
determined by the implementation. Any implementation-defined
privilege that also controls access to methods modifying dead
properties must be aggregated under DAV:write-properties - e.g., if
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an ACL grants access to DAV:write-properties, the client can safely
expect that no other privilege needs to be granted to have access to
PROPPATCH.
3.4. DAV:write-content Privilege
The DAV:write-content privilege controls methods that modify the
content of an existing resource, such as PUT. Any implementation-
defined privilege that also controls access to content must be
aggregated under DAV:write-content - e.g., if an ACL grants access to
DAV:write-content, the client can safely expect that no other
privilege needs to be granted to have access to PUT. Note that PUT -
when applied to an unmapped URI - creates a new resource and
therefore is controlled by the DAV:bind privilege on the parent
collection.
3.5. DAV:unlock Privilege
The DAV:unlock privilege controls the use of the UNLOCK method by a
principal other than the lock owner (the principal that created a
lock can always perform an UNLOCK). While the set of users who may
lock a resource is most commonly the same set of users who may modify
a resource, servers may allow various kinds of administrators to
unlock resources locked by others. Any privilege controlling access
by non-lock owners to UNLOCK MUST be aggregated under DAV:unlock.
A lock owner can always remove a lock by issuing an UNLOCK with the
correct lock token and authentication credentials. That is, even if
a principal does not have DAV:unlock privilege, they can still remove
locks they own. Principals other than the lock owner can remove a
lock only if they have DAV:unlock privilege and they issue an UNLOCK
with the correct lock token. Lock timeout is not affected by the
DAV:unlock privilege.
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.
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3.7. DAV:read-current-user-privilege-set Privilege
The DAV:read-current-user-privilege-set privilege controls the use of
PROPFIND to retrieve the DAV:current-user-privilege-set property of
the resource.
Clients are intended to use this property to visually indicate in
their UI items that are dependent on the permissions of a resource,
for example, by graying out resources that are not writable.
This privilege is separate from DAV:read-acl because there is a need
to allow most users access to the privileges permitted the current
user (due to its use in creating the UI), while the full ACL contains
information that may not be appropriate for the current authenticated
user. As a result, the set of users who can view the full ACL is
expected to be much smaller than those who can read the current user
privilege set, and hence distinct privileges are needed for each.
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.
3.9. DAV:bind Privilege
The DAV:bind privilege allows a method to add a new member URL to the
specified collection (for example via PUT or MKCOL). It is ignored
for resources that are not collections.
3.10. DAV:unbind Privilege
The DAV:unbind privilege allows a method to remove a member URL from
the specified collection (for example via DELETE or MOVE). It is
ignored for resources that are not collections.
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3.11. DAV:all Privilege
DAV:all is an aggregate privilege that contains the entire set of
privileges that can be applied to the resource.
3.12. Aggregation of Predefined Privileges
Server implementations are free to aggregate the predefined
privileges (defined above in Sections 3.1-3.10) subject to the
following limitations:
DAV:read-acl MUST NOT contain DAV:read, DAV:write, DAV:write-acl,
DAV:write-properties, DAV:write-content, or DAV:read-current-user-
privilege-set.
DAV:write-acl MUST NOT contain DAV:write, DAV:read, DAV:read-acl, or
DAV:read-current-user-privilege-set.
DAV:read-current-user-privilege-set MUST NOT contain DAV:write,
DAV:read, DAV:read-acl, or DAV:write-acl.
DAV:write MUST NOT contain DAV:read, DAV:read-acl, or DAV:read-
current-user-privilege-set.
DAV:read MUST NOT contain DAV:write, DAV:write-acl, DAV:write-
properties, or DAV:write-content.
DAV:write MUST contain DAV:bind, DAV:unbind, DAV:write-properties and
DAV:write-content.
4. Principal Properties
Principals are manifested to clients as a WebDAV resource, identified
by a URL. A principal MUST have a non-empty DAV:displayname property
(defined in Section 13.2 of [RFC2518]), and a DAV:resourcetype
property (defined in Section 13.9 of [RFC2518]). Additionally, a
principal MUST report the DAV:principal XML element in the value of
the DAV:resourcetype property. The element type declaration for
DAV:principal is:
Clemm, et al. Standards Track [Page 13]
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This protocol defines the following additional properties for a
principal. Since it can be expensive for a server to retrieve access
control information, the name and value of these properties SHOULD
NOT be returned by a PROPFIND allprop request (as defined in Section
12.14.1 of [RFC2518]).
4.1. DAV:alternate-URI-set
This protected property, if non-empty, contains the URIs of network
resources with additional descriptive information about the
principal. This property identifies additional network resources
(i.e., it contains one or more URIs) that may be consulted by a
client to gain additional knowledge concerning a principal. One
expected use for this property is the storage of an LDAP [RFC2255]
scheme URL. A user-agent encountering an LDAP URL could use LDAP
[RFC2251] to retrieve additional machine-readable directory
information about the principal, and display that information in its
user interface. Support for this property is REQUIRED, and the value
is empty if no alternate URI exists for the principal.
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.
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.
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
Clemm, et al. Standards Track [Page 14]
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those other groups would need to be queried in order to determine the
groups in which the principal is indirectly a member. Support for
this property is REQUIRED.
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.
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 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.
Servers MAY implement DAV:owner as protected property and MAY return
an empty DAV:owner element as property value in case no owner
information is available.
5.1.1. Example: 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.
Clemm, et al. Standards Track [Page 15]
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>> 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="users@example.com", nonce="...",
uri="/papers/", response="...", opaque="..."
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
http://www.example.com/papers/
http://www.example.com/acl/users/gstein
HTTP/1.1 200 OK
5.1.2. Example: 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 . Since DAV:owner is a protected property on
this particular server, it 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, Section 11 of [RFC2518] describes the
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Multi-Status response and Sections 1.6 and 3.12 of [RFC3253] describe
additional error marshaling for PROPPATCH attempts on protected
properties.
>> 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="users@example.com", nonce="...",
uri="/papers/", response="...", opaque="..."
http://www.example.com/acl/users/jim
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
http://www.example.com/papers/
HTTP/1.1 403 Forbidden
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5.2. DAV:group
This property identifies a particular principal as being the "group"
of the resource. This property is commonly found on repositories
that implement the Unix privileges model.
Servers MAY implement DAV:group as protected property and MAY return
an empty DAV:group element as property value in case no group
information is available.
5.3. DAV:supported-privilege-set
This is a protected property that identifies the privileges defined
for the resource.
Each privilege appears as an XML element, where aggregate privileges
list as sub-elements all of the privileges that they aggregate.
An abstract privilege MUST NOT be used in an ACE for that resource.
Servers MUST fail an attempt to set an abstract privilege.
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.
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.
Clemm, et al. Standards Track [Page 18]
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5.3.1. Example: 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 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 3.12), 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="users@example.com", nonce="...",
uri="/papers/", response="...", opaque="..."
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>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
http://www.example.com/papers/
Any operation
Read any object
Read ACL
Read current user privilege set property
Write any object
Clemm, et al. Standards Track [Page 20]
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Write ACL
Write properties
Write resource content
Unlock resource
HTTP/1.1 200 OK
5.4. 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.
Clemm, et al. Standards Track [Page 21]
RFC 3744 WebDAV Access Control Protocol May 2004
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.
5.4.1. Example: Retrieving the User's Current Set of Assigned
Privileges
Continuing the example from Section 5.3.1, 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 5.3.1), 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="users@example.com", nonce="...",
uri="/papers/", response="...", opaque="..."
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RFC 3744 WebDAV Access Control Protocol May 2004
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
http://www.example.com/papers/
HTTP/1.1 200 OK
5.5. 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.
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.
5.5.1. ACE Principal
The DAV:principal element identifies the principal to which this ACE
applies.
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.
Clemm, et al. Standards Track [Page 23]
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The current user always matches DAV:all.
The current user matches DAV:authenticated only if authenticated.
The current user matches DAV:unauthenticated only if not
authenticated.
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
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
http://www.example.com/papers/
http://www.example.com/acl/groups/maintainers
HTTP/1.1 200 OK
5.6. 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:
Deny ACEs are not supported
Inverted ACEs are not supported
All deny ACEs must occur before any grant ACEs
Indicates which principals are required to be
present
5.6.1. DAV:grant-only
This element indicates that ACEs with deny clauses are not allowed.
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5.6.2. DAV:no-invert ACE Constraint
This element indicates that ACEs with the element are not
allowed.
5.6.3. DAV:deny-before-grant
This element indicates that all deny ACEs must precede all grant
ACEs.
5.6.4. Required Principals
The required principal elements identify which principals must have
an ACE defined in the ACL.
For example, the following element requires that the ACL contain a
DAV:owner property ACE:
5.6.5. 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="users@example.com", nonce="...",
uri="/papers/", response="...", opaque="..."
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RFC 3744 WebDAV Access Control Protocol May 2004
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
http://www.example.com/papers/
HTTP/1.1 200 OK
5.7. 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.
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RFC 3744 WebDAV Access Control Protocol May 2004
5.8. 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.
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
lists, due to the difficulty of getting the URL of a principal for
use in an ACE.
5.8.1. Example: 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.
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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="users@example.com", nonce="...",
uri="/papers/", response="...", opaque="..."
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
http://www.example.com/papers/
http://www.example.com/acl/users/
http://www.example.com/acl/groups/
HTTP/1.1 200 OK
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5.9. 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@example.com", nonce="...",
uri="/top/container/", response="...", opaque="..."
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
http://www.example.com/top/container/
http://www.example.com/users/gclemm
Any operation
Read any object
Write any object
Create an object
Update an object
Delete an object
Read the ACL
Write the ACL
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http://www.example.com/users/esedlar
http://www.example.com/groups/mrktng
http://www.example.com/top
HTTP/1.1 200 OK
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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/mrktng are denied the DAV:read privilege. In this example,
the principal URL identifies a group.
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.
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6. ACL Evaluation
WebDAV ACLs are evaluated in similar manner as ACLs on Windows NT and
in NFSv4 [RFC3530]). 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 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:
Clemm, et al. Standards Track [Page 36]
RFC 3744 WebDAV Access Control Protocol May 2004
and the would be defined as:
Note that the client can still get errors from a UNIX server in spite
of obeying the , including
(adding an ACE specifying a principal other than the ones in the ACL
above) or (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. Any HTTP method
7.1.1. Error Handling
The WebDAV ACL mechanism requires the usage of HTTP method
"preconditions" as described in section 1.6 of RFC3253 for ALL HTTP
methods. All HTTP methods have an additional precondition called
DAV:need-privileges. If an HTTP method fails due to insufficient
privileges, the response body to the "403 Forbidden" error MUST
contain the element, which in turn contains the
Clemm, et al. Standards Track [Page 37]
RFC 3744 WebDAV Access Control Protocol May 2004
element, which contains one or more
elements indicating which resource had insufficient
privileges, and what the lacking privileges were:
Since some methods require multiple permissions on multiple
resources, this information is needed to resolve any ambiguity.
There is no requirement that all privilege violations be reported -
for implementation reasons, some servers may only report the first
privilege violation. For example:
>> Request <<
MOVE /a/b/ HTTP/1.1
Host: www.example.com
Destination: http://www.example.com/c/d
>> Response <<
HTTP/1.1 403 Forbidden
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
/a
/c
7.2. 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.
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RFC 3744 WebDAV Access Control Protocol May 2004
7.2.1. Example - OPTIONS
>> Request <<
OPTIONS /foo.html HTTP/1.1
Host: www.example.com
Content-Length: 0
>> Response <<
HTTP/1.1 200 OK
DAV: 1, 2, access-control
Allow: OPTIONS, GET, PUT, PROPFIND, PROPPATCH, ACL
In this example, the OPTIONS response indicates that the server
supports access control and that /foo.html can have its access
control list modified by the ACL method.
7.3. 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.
7.4. 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.
7.5. 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.
Clemm, et al. Standards Track [Page 39]
RFC 3744 WebDAV Access Control Protocol May 2004
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 5.5) 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 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.
8.1.1. ACL 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
Clemm, et al. Standards Track [Page 40]
RFC 3744 WebDAV Access Control Protocol May 2004
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
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 5.6.1).
(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 5.6.2).
(DAV:no-abstract): The ACL request MUST NOT attempt to grant or deny
an abstract privilege (see Section 5.3).
Clemm, et al. Standards Track [Page 41]
RFC 3744 WebDAV Access Control Protocol May 2004
(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).
(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.
8.1.2. Example: 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., 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@example.com", nonce="...",
uri="/top/container/", response="...", opaque="..."
http://www.example.com/users/esedlar
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>> Response <<
HTTP/1.1 200 OK
8.1.3. Example: 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 5.5.3) granting DAV:owner the DAV:read and DAV:write
privileges. The principal 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@example.com", nonce="...",
uri="/top/container/", response="...", opaque="..."
Clemm, et al. Standards Track [Page 43]
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http://www.example.com/users/esedlar
>> Response <<
HTTP/1.1 403 Forbidden
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
8.1.4. Example: 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.
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.
Clemm, et al. Standards Track [Page 44]
RFC 3744 WebDAV Access Control Protocol May 2004
>> 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@example.com", nonce="...",
uri="/top/index.html", response="...", opaque="..."
http://www.example.com/users/ejw
>> Response <<
HTTP/1.1 403 Forbidden
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
8.1.5. Example: ACL method failure due to an attempt to set grant and
deny in a single ACE
In this example, user "ygoland", authenticated by information in the
Authorization header, tries to change the access control list on the
resource http://www.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.
Clemm, et al. Standards Track [Page 45]
RFC 3744 WebDAV Access Control Protocol May 2004
>> 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@example.com", nonce="...",
uri="/diamond/engagement-ring.gif", response="...",
opaque="..."
http://www.example.com/users/friends
http://www.example.com/users/ygoland-so
>> 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.
Clemm, et al. Standards Track [Page 46]
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A server that supports the WebDAV Access Control Protocol MUST
support the DAV:expand-property report (defined in Section 3.8 of
[RFC3253]).
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.
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.
Clemm, et al. Standards Track [Page 47]
RFC 3744 WebDAV Access Control Protocol May 2004
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.
9.2.1. Example: 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
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RFC 3744 WebDAV Access Control Protocol May 2004
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
http://www.example.com/people/gstein
Greg Stein
HTTP/1.1 200 OK
http://www.example.com/groups/authors
Site authors
HTTP/1.1 200 OK
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 are
principals and 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.
Clemm, et al. Standards Track [Page 49]
RFC 3744 WebDAV Access Control Protocol May 2004
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
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-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.
9.3.1. Example: 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="users@example.com", nonce="...",
uri="/papers/", response="...", opaque="..."
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
Depth: 0
Clemm, et al. Standards Track [Page 50]
RFC 3744 WebDAV Access Control Protocol May 2004
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
http://www.example.com/doc/foo.html
HTTP/1.1 200 OK
http://www.example.com/doc/img/bar.gif
HTTP/1.1 200 OK
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 search requesting properties that are
not searchable for a particular principal will not match that
principal.
Support for the DAV:principal-property-search report is REQUIRED.
Clemm, et al. Standards Track [Page 51]
RFC 3744 WebDAV Access Control Protocol May 2004
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 "The Unicode Standard"
[UNICODE4], especially Section 5.18, Subsection "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.
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:principal-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.
prop: see RFC 2518, Section 12.11
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RFC 3744 WebDAV Access Control Protocol May 2004
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.
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:
None
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.
9.4.1. Matching
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
marshaling of DAV:displayname in this case is:
Julian Reschke
Clemm, et al. Standards Track [Page 53]
RFC 3744 WebDAV Access Control Protocol May 2004
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/", marshaled as:
{cdata 0}{cdata 1}
{cdata 2} {cdata 3}
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}.
9.4.2. Example: 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 "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
Clemm, et al. Standards Track [Page 54]
RFC 3744 WebDAV Access Control Protocol May 2004
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
doE
Sales
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset=utf-8
Content-Length: xxxx
http://www.example.com/users/jdoe
Clemm, et al. Standards Track [Page 55]
RFC 3744 WebDAV Access Control Protocol May 2004
John Doe
Widget Sales
234-4567
209
HTTP/1.1 200 OK
HTTP/1.1 403 Forbidden
http://www.example.com/users/zsmith
Zygdoebert Smith
Gadget Sales
234-7654
114
HTTP/1.1 200 OK
HTTP/1.1 403 Forbidden
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.
Clemm, et al. Standards Track [Page 56]
RFC 3744 WebDAV Access Control Protocol May 2004
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 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.
Each DAV:principal-search-property XML element contains exactly
one searchable property, and a description of the property.
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
Clemm, et al. Standards Track [Page 57]
RFC 3744 WebDAV Access Control Protocol May 2004
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.
9.5.1. Example: 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 <<
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
Full name
Job title
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
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 [RFC3629] 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. Furthermore,
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
Clemm, et al. Standards Track [Page 59]
RFC 3744 WebDAV Access Control Protocol May 2004
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.
12.1. Increased Risk of Compromised Users
In the absence of a mechanism for remotely manipulating access
control lists, if a single user's authentication credentials are
compromised, only those resources for which the user has access
permission can be read, modified, moved, or deleted. With the
introduction of this access control protocol, if a single compromised
user has the ability to change ACLs for a broad range of other users
(e.g., a super-user), the number of resources that could be altered
by a single compromised user increases. This risk can be mitigated
by limiting the number of people who have write-acl privileges across
a broad range of resources.
12.2. Risks 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-writable. 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, writable
resources can be modified in undesirable ways.
Clemm, et al. Standards Track [Page 60]
RFC 3744 WebDAV Access Control Protocol May 2004
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.
12.3. No 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
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.
Clemm, et al. Standards Track [Page 61]
RFC 3744 WebDAV Access Control Protocol May 2004
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. 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, 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.
16. References
16.1. Normative References
[REC-XML] Bray, T., Paoli, J., Sperberg-McQueen, C. and E.
Maler, "Extensible Markup Language (XML) 1.0
((Third ed)", W3C REC REC-xml-20040204, February
2004, .
[REC-XML-INFOSET] Cowan, J. and R. Tobin, "XML Information Set
(Second Edition)", W3C REC REC-xml-infoset-
20040204, February 2004,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Clemm, et al. Standards Track [Page 62]
RFC 3744 WebDAV Access Control Protocol May 2004
[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S.
and D. Jensen, "HTTP Extensions for Distributed
Authoring -- WEBDAV", RFC 2518, February 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P. and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC
2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J.,
Lawrence, S., Leach, P., Luotonen, A. and L.
Stewart, "HTTP Authentication: Basic and Digest
Access Authentication", RFC 2617, June 1999.
[RFC3023] Murata, M., St.Laurent, S. and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[RFC3253] Clemm, G., Amsden, J., Ellison, T., Kaler, C. and
J. Whitehead, "Versioning Extensions to WebDAV",
RFC 3253, March 2002.
[RFC3530] Shepler, S., Ed., Callaghan, B., Robinson, D.,
Thurlow, R., Beame, C., Eisler, M. and D. Noveck,
"Network File System (NFS) version 4 Protocol", RFC
3530, April 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629 November 2003.
16.2. Informative References
[RFC2251] Wahl, M., Howes, T. and S. Kille, "Lightweight
Directory Access Protocol (v3)", RFC 2251, December
1997.
[RFC2255] Howes, T. and M. Smith, "The LDAP URL Format", RFC
2255, December 1997.
[UNICODE4] The Unicode Consortium, "The Unicode Standard -
Version 4.0", Addison-Wesley , August 2003,
|
| HEAD | |
| OPTIONS | |
| PUT (target exists) | on target |
| | resource |
| PUT (no target exists) | on parent collection |
| | of target |
| PROPPATCH | |
| ACL | |
| PROPFIND | (plus and |
| | as needed) |
| COPY (target exists) | , and |
| | on target |
| | resource |
| COPY (no target exists) | , on target |
| | collection |
| MOVE (no target exists) | on source collection |
| | and on target |
| | collection |
| MOVE (target exists) | As above, plus on |
| | the target collection |
| DELETE | on parent collection |
| LOCK (target exists) | |
| LOCK (no target exists) | on parent collection |
| MKCOL | on parent collection |
| UNLOCK | |
| CHECKOUT | |
| CHECKIN | |
| REPORT | (on all referenced |
| | resources) |
| VERSION-CONTROL | |
| MERGE | |
| MKWORKSPACE | on parent |
| | collection |
| BASELINE-CONTROL | and |
| | |
| MKACTIVITY | on parent |
| | collection |
+---------------------------------+---------------------------------+
Clemm, et al. Standards Track [Page 68]
RFC 3744 WebDAV Access Control Protocol May 2004
Index
A
ACL method 40
C
Condition Names
DAV:allowed-principal (pre) 42
DAV:deny-before-grant (pre) 41
DAV:grant-only (pre) 41
DAV:limited-number-of-aces (pre) 41
DAV:missing-required-principal (pre) 42
DAV:no-abstract (pre) 41
DAV:no-ace-conflict (pre) 41
DAV:no-inherited-ace-conflict (pre) 41
DAV:no-invert (pre) 41
DAV:no-protected-ace-conflict (pre) 41
DAV:not-supported-privilege (pre) 42
DAV:number-of-matches-within-limits (post) 48, 53
DAV:recognized-principal (pre) 42
D
DAV header
compliance class 'access-control' 38
DAV:acl property 23
DAV:acl-principal-prop-set report 48
DAV:acl-restrictions property 27
DAV:all privilege 13
DAV:allowed-principal precondition 42
DAV:alternate-URI-set property 14
DAV:bind privilege 12
DAV:current-user-privilege-set property 21
DAV:deny-before-grant precondition 41
DAV:grant-only precondition 41
DAV:group property 18
DAV:group-member-set property 14
DAV:group-membership property 14
DAV:inherited-acl-set property 29
DAV:limited-number-of-aces precondition 41
DAV:missing-required-principal precondition 42
DAV:no-abstract precondition 41
DAV:no-ace-conflict precondition 41
DAV:no-inherited-ace-conflict precondition 41
DAV:no-invert precondition 41
DAV:no-protected-ace-conflict precondition 41
DAV:not-supported-privilege precondition 42
DAV:number-of-matches-within-limits postcondition 48, 53
DAV:owner property 15
Clemm, et al. Standards Track [Page 69]
RFC 3744 WebDAV Access Control Protocol May 2004
DAV:principal resource type 13
DAV:principal-collection-set property 30
DAV:principal-match report 50
DAV:principal-property-search 51
DAV:principal-search-property-set 56
DAV:principal-URL property 14
DAV:read privilege 10
DAV:read-acl privilege 11
DAV:read-current-user-privilege-set privilege 12
DAV:recognized-principal precondition 42
DAV:supported-privilege-set property 18
DAV:unbind privilege 12
DAV:unlock privilege 11
DAV:write privilege 10
DAV:write-acl privilege 12
DAV:write-content privilege 10
DAV:write-properties privilege 10
M
Methods
ACL 40
P
Privileges
DAV:all 13
DAV:bind 12
DAV:read 10
DAV:read-acl 11
DAV:read-current-user-privilege-set 12
DAV:unbind 12
DAV:unlock 11
DAV:write 10
DAV:write-acl 12
DAV:write-content 11
DAV:write-properties 10
Properties
DAV:acl 23
DAV:acl-restrictions 27
DAV:alternate-URI-set 14
DAV:current-user-privilege-set 21
DAV:group 18
DAV:group-member-set 14
DAV:group-membership 14
DAV:inherited-acl-set 29
DAV:owner 15
DAV:principal-collection-set 30
DAV:principal-URL 14
DAV:supported-privilege-set 18
Clemm, et al. Standards Track [Page 70]
RFC 3744 WebDAV Access Control Protocol May 2004
R
Reports
DAV:acl-principal-prop-set 47
DAV:principal-match 49
DAV:principal-property-search 51
DAV:principal-search-property-set 56
Resource Types
DAV:principal 13
Authors' Addresses
Geoffrey Clemm
IBM
20 Maguire Road
Lexington, MA 02421
EMail: geoffrey.clemm@us.ibm.com
Julian F. Reschke
greenbytes GmbH
Salzmannstrasse 152
Muenster, NW 48159
Germany
EMail: julian.reschke@greenbytes.de
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
1156 High Street
Santa Cruz, CA 95064
EMail: ejw@cse.ucsc.edu
Clemm, et al. Standards Track [Page 71]
RFC 3744 WebDAV Access Control Protocol May 2004
Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
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except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
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Acknowledgement
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Clemm, et al. Standards Track [Page 72]
RFC, FYI, BCP