One document matched: draft-ietf-webdav-acl-04.txt
Differences from draft-ietf-webdav-acl-03.txt
INTERNET-DRAFT Geoffrey Clemm, Rational Software
draft-ietf-webdav-acl-04 Anne Hopkins, Microsoft Corporation
Eric Sedlar, Oracle Corporation
Jim Whitehead, U.C. Santa Cruz
Expires July 21, 2001 January 21, 2001
WebDAV Access Control Protocol
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document specifies a set of methods, headers, and message bodies
that define the WebDAV Access Control extensions to the HTTP/1.1
protocol. This protocol permits a client to remotely read and modify
access control lists that instruct a server whether to grant or deny
operations upon a resource (such as HTTP method invocations) by a given
principal.
This document is a product of the Web Distributed Authoring and
Versioning (WebDAV) working group of the Internet Engineering Task
Force. Comments on this draft are welcomed, and should be addressed to
the acl@webdav.org mailing list. Other related documents can be found
at http://www.webdav.org/acl/, and
http://www.ics.uci.edu/pub/ietf/webdav/.
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Table of Contents
1 INTRODUCTION......................................................3
1.1 Terms..........................................................4
1.2 Notational Conventions.........................................5
2 PRINCIPALS........................................................5
3 PRIVILEGES........................................................5
3.1 DAV:read Privilege.............................................6
3.2 DAV:write Privilege............................................6
3.3 DAV:read-acl Privilege.........................................7
3.4 DAV:write-acl Privilege........................................7
3.5 DAV:all Privilege..............................................7
4 PRINCIPAL PROPERTIES..............................................7
4.1 DAV:is-principal...............................................7
4.2 DAV:authentication-id..........................................7
5 ACCESS CONTROL PROPERTIES.........................................8
5.1 DAV:owner......................................................8
5.2 DAV:supported-privilege-set....................................8
5.3 DAV:current-user-privilege-set.................................9
5.4 DAV:acl........................................................9
5.4.1 ACE Principal................................................9
5.4.2 ACE Grant and Deny..........................................10
5.4.3 ACE Protection..............................................11
5.4.4 ACE Inheritance.............................................11
5.5 DAV:acl-semantics.............................................11
5.6 DAV:principal-collection-set..................................11
5.7 Example: PROPFIND to retrieve access control properties.......12
6 ACL SEMANTICS....................................................15
6.1 ACE Combination...............................................15
6.1.1 DAV:first-match ACE Combination.............................15
6.1.2 DAV:all-grant-before-any-deny ACE Combination...............15
6.1.3 DAV:no-deny ACE Combination.................................15
6.2 ACE Ordering..................................................16
6.2.1 DAV:deny-before-grant ACE Ordering..........................16
6.3 Required Principals...........................................16
7 ACCESS CONTROL AND EXISTING METHODS..............................16
7.1 OPTIONS.......................................................16
7.1.1 Example - OPTIONS...........................................16
8 ACCESS CONTROL METHODS...........................................17
8.1 ACL...........................................................17
8.1.1 ACL Preconditions...........................................17
8.1.2 Example: the ACL method.....................................17
8.1.3 Example: ACL method failure due to omission of protected ACE18
8.1.4 Example: ACL method failure due to inherited ACEs preceding
non-inherited ACEs................................................19
8.1.5 Example: ACL method failure due to an attempt to set grant and
deny in a single ACE..............................................20
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9 INTERNATIONALIZATION CONSIDERATIONS..............................21
10 SECURITY CONSIDERATIONS........................................22
10.1 Increased Risk of Compromised Users...........................22
10.2 Authentication-id Property and Dictionary Attacks.............22
10.3 Risks of the read-acl Privilege...............................23
11 AUTHENTICATION.................................................23
12 IANA CONSIDERATIONS............................................23
13 INTELLECTUAL PROPERTY..........................................23
14 ACKNOWLEDGEMENTS...............................................24
15 REFERENCES.....................................................24
15.1 Normative References..........................................24
15.2 Informational References......................................25
16 AUTHORS' ADDRESSES.............................................25
17 APPENDICIES....................................................25
17.1 XML Document Type Definition..................................25
1 INTRODUCTION
The goal of the WebDAV access control extensions is to provide an
interoperable mechanism for handling discretionary access control
for content in WebDAV servers. WebDAV access control can be
implemented on content repositories with security as simple as that
of a UNIX file system, as well as more sophisticated models. The
underlying principle of access control is that who you are
determines how you can access a resource. The "who you are" is
defined by a "principal" identifier; users, client software,
servers, and groups of the previous have principal identifiers. The
"how" is determined by a single "access control list" (ACL)
associated with a resource. An ACL contains a set of "access
control entries" (ACEs), where each ACE specifies a principal and a
set of privileges that are either granted or denied to that
principal. When a principal submits an operation (such as an HTTP or
WebDAV method) to a resource for execution, the server evaluates the
ACEs in the ACL to determine if the principal has permission for
that operation.
This specification intentionally omits discussion of authentication,
as the HTTP protocol already has a number of authentication
mechanisms [RFC2617]. Some authentication mechanism (such as HTTP
Digest Authentication, which all WebDAV compliant implementations
are required to support) must be available to validate the identity
of a principal.
In the interests of timeliness, the following set of security
mechanisms are not addressed by this document:
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* Access control that applies only to a particular property on
a resource, rather than the entire resource,
* Role-based security (where a role can be seen as a
dynamically defined collection of principals),
* Specification of the ways an ACL on a resource is
initialized,
* Specification of an ACL that applies globally to a method,
rather than to a particular resource.
This specification is organized as follows. Section 1.1 defines key
concepts used throughout the specification, and is followed by more
in-depth discussion of principals (Section 2), and privileges
(Section 3). Properties defined on principals are specified in
Section 4, and access control properties for content resources are
specified in Section 5. The semantics of access control lists are
described in Section 6, including sections on ACE combination
(Section 6.1), ACE ordering (Section 6.2), and principals required
to be present in an ACE (Section 6.3). Client discovery of access
control capability using OPTIONS is described in Section 7.1, and
the access control setting method, ACL, is specified in Section 8.
Internationalization considerations (Section 9) and security
considerations (Section 10) round out the specification. An appendix
(Section 17.1) provides an XML Document Type Definition (DTD) for
the XML elements defined in the specification.
1.1 Terms
This draft uses the terms defined in HTTP [RFC2616] and WebDAV
[RFC2518]. In addition, the following terms are defined:
principal
A "principal" is a distinct human or computational actor that
initiates access to network resources. In this protocol, a
principal is an HTTP resource that represents such an actor.
principal collection
A "principal collection" is a group of principals, and is
represented in this protocol by a WebDAV collection containing HTTP
resources that represent principals, and principal collections.
privilege
A "privilege" controls access to a particular set of HTTP operations
on a resource.
aggregate privilege
An "aggregate privilege" is a privilege that contains a set of other
privileges.
abstract privilege
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The modifier "abstract", when applied to an atomic or aggregate
privilege, means the privilege cannot be set in an access control
element (ace).
access control list (acl)
An "acl" is a list of access control elements that define access
control to a particular resource.
access control element (ace)
An "ace" either grants or denies a particular set of (non-abstract)
privileges for a particular principal.
inherited ace
An "inherited ace" is an ace that is shared from the acl of another
resource.
1.2 Notational Conventions
The augmented BNF used by this document to describe protocol
elements is described in Section 2.1 of [RFC2616]. Because this
augmented BNF uses the basic production rules provided in Section
2.2 of [RFC2616], those rules apply to this document as well.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2 PRINCIPALS
A principal is an HTTP resource that represents a distinct human or
computational actor that initiates access to network resources. On
many implementations, users and groups are represented as
principals; other types of principals are also possible. Although
an implementation MAY support PROPFIND and PROPPATCH to access and
modify information about a principal, it is not required to do so.
A principal resource may or may not be a collection. A collection
principal may only contain other principals (not other types of
resources). Servers that support aggregation of principals (e.g.
groups of users or other groups) MUST manifest them as collection
principals. The WebDAV methods for examining and maintaining
collections (e.g. DELETE, PROPFIND) MAY be used to maintain
collection principals. Membership in a collection principal is
recursive, so a principal in a collection principal GRPA contained
by collection principal GRPB is a member of both GRPA and GRPB.
Implementations not supporting recursive membership in principal
collections can return an error if the client attempts to bind
collection principals into other collection principals.
3 PRIVILEGES
Ability to perform a given method on a resource SHOULD be controlled
by one or more privileges. Authors of protocol extensions that
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define new HTTP methods SHOULD specify which privileges (by defining
new privileges, or mapping to ones below) are required to perform
the method. A principal with no privileges to a resource SHOULD be
denied any HTTP access to that resource.
Privileges may be containers of other privileges, in which case they
are termed aggregate privileges. If a principal is granted or
denied an aggregate privilege, it is semantically equivalent to
granting or denying each of the aggregated privileges individually.
For example, an implementation may define add-member and remove-
member privileges that control the ability to add and remove an
internal member of a collection. Since these privileges control the
ability to update the state of a collection, these privileges would
be aggregated by the DAV:write privilege on a collection, and
granting the DAV:write privilege on a collection would also grant
the add-member and remove-member privileges.
Privileges may have the quality of being abstract, in which case
they cannot be set in an ACE. Aggregate and atomic privileges are
both capable of being abstract. Abstract privileges are useful for
modeling privileges that otherwise would not be exposed via the
protocol. Abstract privileges also provide server implementations
with flexibility in implementing the privileges defined in this
specification. For example, if a server is incapable of separating
the read resource capability from the read ACL capability, it can
still model the DAV:read and DAV:read-acl privileges defined in this
specification by declaring them abstract, and containing them within
a non-abstract aggregate privilege (say, read-all) that holds
DAV:read, and DAV:read-acl. In this way, it is possible to set the
aggregate privilege, read-all, thus coupling the setting of DAV:read
and DAV:read-acl, but it is not possible to set DAV:read, or
DAV:read-acl individually. Since aggregate privileges can be
abstract, it is also possible to use abstract privileges to group
and classify non-abstract privileges.
The set of privileges that apply to a particular resource may vary
with the DAV:resourcetype of the resource, as well as between
different server implementations. To promote interoperability,
however, WebDAV defines a set of well-known privileges (e.g.
DAV:read and DAV:write), which can at least be used to classify the
other privileges defined on a particular resource.
3.1 DAV:read Privilege
The read privilege controls methods that return information about
the state of the resource, including the resource's properties.
Affected methods include GET and PROPFIND. Additionally, the read
privilege MAY control the OPTIONS method.
<!ELEMENT read EMPTY>
3.2 DAV:write Privilege
The write privilege controls methods that modify the state of the
resource, such as PUT and PROPPATCH. Note that state modification
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is also controlled via locking (see section 5.3 of [WEBDAV]), so
effective write access requires that both write privileges and write
locking requirements are satisfied.
<!ELEMENT write EMPTY>
3.3 DAV:read-acl Privilege
The DAV:read-acl privilege controls the use of PROPFIND to retrieve
the DAV:acl, and DAV:current-user-privilege-set properties of the
resource.
<!ELEMENT read-acl EMPTY>
3.4 DAV:write-acl Privilege
The DAV:write-acl privilege controls use of the ACL method to modify
the DAV:acl property of the resource.
<!ELEMENT write-acl EMPTY>
3.5 DAV:all Privilege
DAV:all is an aggregate privilege that contains all privileges on
the resource.
<!ELEMENT all EMPTY>
4 PRINCIPAL PROPERTIES
Principals are manifested to clients as an HTTP resource, identified
by a URL. A principal MUST have a DAV:displayname property. This
protocol defines the following additional properties for a
principal.
4.1 DAV:is-principal
This property indicates whether this resource is a principal. A
resource MUST have a non-empty DAV:is-principal property if and only
if it is a principal resource. (Note: If we can just add a
DAV:principal element to the DAV:resourcetype property, then we do
not need a DAV:is-principal property.)
<!ELEMENT is-principal (#PCDATA)>
PCDATA value: any non-empty value ("T" is suggested)
4.2 DAV:authentication-id
A property containing the name used to authenticate this principal
(typically typed into a login prompt/dialog).
<!ELEMENT authentication-id (#PCDATA)>
PCDATA value: any string
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5 ACCESS CONTROL PROPERTIES
This specification defines a number of new properties for WebDAV
resources. Access control properties may be retrieved just like
other WebDAV properties, using the PROPFIND method. Some access
control properties (such as DAV:owner) MAY be updated with the
PROPPATCH method.
HTTP resources that support the WebDAV Access Control Protocol MUST
contain the following properties:
5.1 DAV:owner
This property identifies a particular principal as being the "owner"
of the resource.
<!ELEMENT owner (href prop?)>
<!ELEMENT prop (see [RFC2518], section 12.11)>
An implementation MAY include a list of selected properties of that
principal resource. Which properties (if any) are included is
implementation defined. An implementation MAY allow the use of
PROPPATCH to update the DAV:owner field.
5.2 DAV:supported-privilege-set
This is a read-only property that identifies the privileges defined
for the resource.
<!ELEMENT supported-privilege-set (supported-privilege*)>
Each privilege appears as an XML element, where aggregate privileges
list as sub-elements all of the privileges that they aggregate.
<!ELEMENT supported-privilege
(privilege, abstract?, description, supported-privilege*)>
<!ELEMENT privilege ANY>
An abstract privilege of a resource MUST NOT be used in an ACE for
that resource. Servers MUST fail an attempt to set an abstract
privilege.
<!ELEMENT abstract EMPTY>
A description is a human-readable description of what this privilege
controls access to.
<!ELEMENT description #PCDATA>
It is envisioned that a WebDAV ACL-aware administrative client would
list the supported privileges in a dialog box, and allow the user to
choose non-abstract privileges to apply in an ACE. The privileges
tree is useful programmatically to map well-known privileges
(defined by WebDAV or other standards groups) into privileges that
are supported by any particular server implementation. The
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privilege tree also serves to hide complexity in implementations
allowing large number of privileges to be defined by displaying
aggregates to the user.
5.3 DAV:current-user-privilege-set
DAV:current-user-privilege-set is a read-only property containing
the exact set of privileges (as computed by the server) granted to
the currently authenticated HTTP user. 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 is particularly
useful for an access control user interface, which can be
constructed without knowing the ACE combining semantics of the
server. This property is also useful for determine what operations
can be performed by the current principal, without having to
actually execute an operation.
<!ELEMENT current-user-privilege-set (privilege*)>
<!ELEMENT privilege ANY>
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 privilege from the DAV:supported-
privilege-set property.
5.4 DAV:acl
This property specifies the list of access control entries (ACEs),
which define what principals are to get what privileges for this
resource.
<!ELEMENT acl (ace*)>
Each DAV:ace element specifies the set of privileges to be either
granted or denied to a single principal. If the DAV:acl property is
empty, no principal is granted any privilege.
<!ELEMENT ace (principal, (grant|deny), protected?, inherited?)>
An attempt to update the DAV:acl property with a PROPPATCH MUST
fail.
5.4.1 ACE Principal
The DAV:principal element identifies the principal to which this ACE
applies.
<!ELEMENT principal ((href, prop?)
| all | authenticated | unauthenticated
| property | self)>
The current user matches DAV:href only if that user is authenticated
as being (or being a member of) the principal identified by the URL
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contained by that DAV:href. An implementation MAY include a
DAV:prop element after the DAV:href element, containing a list of
selected properties of that principal resource. Which properties
(if any) are included in the DAV:prop element is implementation
defined. The DAV:prop element is primarily intended for
implementations that do not support PROPFIND requests on the
principal URL.
<!ELEMENT prop (see [RFC2518], section 12.11)>
The current user always matches DAV:all.
<!ELEMENT all EMPTY>
The current user matches DAV:authenticated only if authenticated.
<!ELEMENT authenticated EMPTY>
The current user matches DAV:unauthenticated only if not
authenticated.
<!ELEMENT unauthenticated EMPTY>
DAV:all is the union of DAV:authenticated, and DAV:unauthenticated.
For a given request, the user matches either DAV:authenticated, or
DAV:unauthenticated, but not both.
The current user matches a DAV:property principal in a DAV:acl
property of a resource only if the identified property of that
resource contains a DAV:href that identifies a principal, and the
current user is authenticated as being (or being a member of) that
principal. For example, if the DAV:property element contained
<DAV:owner/>, the current user would match the DAV:property
principal only if the current user is authenticated as matching the
principal identified by the DAV:owner property of the resource.
<!ELEMENT property ANY>
The current user matches DAV:self in a DAV:acl property of the
resource only if that resource is a principal object and the current
user is authenticated as being that principal.
<!ELEMENT self EMPTY>
5.4.2 ACE Grant and Deny
Each DAV:grant or DAV:deny element specifies the set of privileges
to be either granted or denied to the specified principal. A
DAV:grant or DAV:deny element of the DAV:acl of a resource MUST only
contain elements specified in the DAV:supported-privilege-set of
that resource.
<!ELEMENT grant (privilege+)>
<!ELEMENT deny (privilege+)>
<!ELEMENT privilege ANY>
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5.4.3 ACE Protection
If an ACE contains a DAV:protected element, an ACL request without
that ACE MUST fail.
<!ELEMENT protected EMPTY>
5.4.4 ACE Inheritance
The presence of a DAV:inherited element indicates that this ACE is
inherited from another resource that is identified by the URL
contained in a DAV:href element. An inherited ACE cannot be
modified directly, but instead the ACL on the resource from which it
is inherited must be modified.
Note that ACE inheritance is not the same as ACL initialization.
ACL initialization defines the ACL that a newly created resource
will use (if not specified). ACE inheritance refers to an ACE that
is logically shared - where an update to the resource containing an
ACE will affect the ACE of each resource that inherits that ACE.
The method by which ACLs are initialized or by which ACEs are
inherited is not defined by this document.
<!ELEMENT inherited (href)>
5.5 DAV:acl-semantics
This is a read-only property that defines the ACL semantics. These
semantics define how multiple ACEs that match the current user are
combined, what are the constraints on how ACEs can be ordered, and
which principals must have an ACE.
Since it is not practical to require all implementations to use the
same ACL semantics, the DAV:acl-semantics property is used to
identify the ACL semantics for a particular resource. The DAV:acl-
semantics element is defined in section 6.
5.6 DAV:principal-collection-set
This read-only property contains zero, one, or more URLs that
identify a collection principal. It is expected that implementations
of this protocol will typically employ a relatively small number of
locations in the URL namespace for principal, and collection
principals. In cases where this assumption holds, the DAV:principal-
collection-set property will contain a small set of URLs identifying
the top of collection hierarchy containing multiple principals and
collection principals. An access control protocol user agent could
use the contents of DAV:principal-collection-set to, for example,
query the DAV:displayname property (specified in Section 13.2 of
[RFC2518]) of all principals on that server, thereby yielding human-
readable names for each principal that could be displayed in a user
interface.
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<!ELEMENT principal-collection-set (href*)>
Since different servers can control different parts of the URL
namespace, different resources on the same host MAY have different
DAV:principal-collection-set values. The collections specified in
the DAV:principal-collection-set MAY be located on different hosts
from the resource. The URLs in DAV:principal-collection-set are not
limited to http scheme URLs, and can, for example, be ldap scheme
URLs. For security and scalability reasons, a server MAY report only
a subset of the entire set of known collection principals, and
therefore clients should not assume they have retrieved an
exhaustive listing. Additionally, a server MAY elect to report none
of the collection principals it knows about.
5.7 Example: PROPFIND to retrieve access control properties
The following example shows how access control information can be
retrieved by using the PROPFIND method to fetch the values of the
DAV:owner, DAV:supported-privilege-set, DAV:current-user-privilege-
set, and DAV:acl properties.
>> Request <<
PROPFIND /top/container/ HTTP/1.1
Host: www.foo.org
Content-type: text/xml; charset="utf-8"
Content-Length: xxx
Depth: 0
Authorization: Digest username="ejw",
realm="users@foo.org", nonce="...",
uri="/top/container/", response="...", opaque="..."
<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
<D:owner/>
<D:supported-privilege-set/>
<D:current-user-privilege-set/>
<D:acl/>
</D:propfind>
>> Response <<
HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus
xmlns:D="DAV:"
xmlns:A="http://www.acl.org/"> <D:response> <D:propstat>
<D:status>HTTP/1.1 200 OK</D:status>
<D:prop>
<D:owner>
<D:href>http://www.foo.org/users/gclemm</D:href> </D:owner>
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<D:supported-privilege-set>
<D:supported-privilege>
<D:privilege> <D:all/> </D:privilege>
<D:abstract/>
<D:description>Any operation</D:description>
<D:supported-privilege>
<D:privilege> <D:read/> </D:privilege>
<D:description>Read any object</D:description>
</D:supported-privilege>
<D:supported-privilege>
<D:privilege> <D:write/> </D:privilege>
<D:abstract/>
<D:description>Write any object</D:description>
<D:supported-privilege>
<D:privilege> <A:create/> </D:privilege>
<D:description>Create an object</D:description>
</D:supported-privilege>
<D:supported-privilege>
<D:privilege> <A:update/> </D:privilege>
<D:description>Update an object</D:description>
</D:supported-privilege>
<D:supported-privilege>
<D:privilege> <A:delete/> </D:privilege>
<D:description>Delete an object</D:description>
</D:supported-privilege>
</D:supported-privilege>
<D:supported-privilege>
<D:privilege> <D:read-acl/> </D:privilege>
<D:description>Read the ACL</D:description>
</D:supported-privilege>
<D:supported-privilege>
<D:privilege> <D:write-acl/> </D:privilege>
<D:description>Write the ACL</D:description>
</D:supported-privilege>
</D:supported-privilege>
</D:supported-privilege-set>
<D:current-user-privilege-set>
<D:privilege> <D:read/> </D:privilege>
<D:privilege> <D:read-acl/> </D:privilege>
</D:current-user-privilege-set>
<D:acl>
<D:ace>
<D:principal>
<D:href>http://www.foo.org/users/esedlar</D:href>
<D:prop>
<D:authentication-id>esedlar</D:authentication-id>
<D:displayname>Eric Sedlar</D:displayname>
</D:prop> </D:principal>
<D:grant>
<D:privilege> <D:read/> </D:privilege>
<D:privilege> <D:write/> </D:privilege>
<D:privilege> <D:read-acl/> </D:privilege> </D:grant>
</D:ace>
<D:ace>
<D:principal>
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<D:href>http://www.foo.org/groups/marketing/</D:href>
</D:principal>
<D:deny>
<D:privilege> <D:read/> </D:privilege> </D:deny>
</D:ace>
<D:ace>
<D:principal>
<D:property> <D:owner/> </D:property> </D:principal>
<D:grant>
<D:privilege> <D:read-acl/> </D:privilege>
<D:privilege> <D:write-acl/> </D:privilege> </D:grant>
</D:ace>
<D:ace>
<D:principal> <D:all/> </D:principal>
<D:grant>
<D:privilege> <D:read/> </D:privilege> </D:grant>
<D:inherited>
<D:href>http://www.foo.org/top/</D:href> </D:inherited>
</D:ace> </D:acl>
</D:prop>
</D:propstat> </D:response> </D:multistatus>
The value of the DAV:owner property is a single DAV:href XML element
containing the URL of the principal that owns this resource.
The value of the DAV:supported-privilege-set property is a tree of
supported privileges:
DAV:acl (aggregate, abstract)
|
+-- DAV:read
+-- DAV:write (aggregate, abstract)
|
+-- http://www.acl.org/create
+-- http://www.acl.org/update
+-- http://www.acl.org/delete
+-- DAV:read-acl
+-- DAV:write-acl
The DAV:current-user-privilege-set property contains two privileges,
DAV:read, and DAV:read-acl. This indicates that the current
authenticated user only has the ability to read the resource, and
read the DAV:acl property on the resource.
The DAV:acl property contains a set of four ACEs:
ACE #1: The principal identified by the URL
http://www.foo.org/users/esedlar is granted the DAV:read, DAV:write,
and DAV:read-acl privileges.
ACE #2: The principals identified by the URL
http://www.foo.org/groups/marketing/ are denied the DAV:read
privilege. In this example, the principal URL identifies a group,
which is represented by a collection principal.
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ACE #3: In this ACE, the principal is a property principal,
specifically the DAV:owner property. When evaluating this ACE, the
value of the DAV:owner property is retrieved, and is examined to see
if it contains a DAV:href XML element. If so, the URL within the
DAV:href element is read, and identifies a principal. In this ACE,
the owner is granted DAV:read-acl, and DAV:write-acl privileges.
ACE #4: This ACE grants the DAV:all principal (all users) the
DAV:read privilege. This ACE is inherited from the resource
http://www.foo.org/top/, the parent collection of this resource.
6 ACL SEMANTICS
The ACL semantics define how multiple ACEs that match the current
user are combined, what are the constraints on how ACEs can be
ordered, and which principals must have an ACE.
<!ELEMENT acl-semantics acl-sem*>
<!ELEMENT acl-sem (ace-combination, ace-ordering, required-
principal)>
6.1 ACE Combination
The DAV:ace-combination element defines how privileges from multiple
ACEs that match the current user will be combined to determine the
access privileges for that user. Multiple ACEs may match the same
user because the same principal can appear in multiple ACEs, because
multiple principals can identify the same user, and because one
principal can be a member of another principal.
<!ELEMENT ace-combination
(first-match | all-grant-before-any-deny | no-deny)>
6.1.1 DAV:first-match ACE Combination
The ACEs are evaluated in the order in which they appear in the ACL.
If the first ACE that matches the current user does not grant all
the privileges needed for the request, the request MUST fail.
<!ELEMENT first-match EMPTY>
6.1.2 DAV:all-grant-before-any-deny ACE Combination
The ACEs are evaluated in the order in which they appear in the ACL.
If an evaluated ACE denies a privilege needed for the request, the
request MUST fail. If all ACEs have been evaluated without the user
being granted all privileges needed for the request, the request
MUST fail.
<!ELEMENT all-grant-before-any-deny EMPTY>
6.1.3 DAV:no-deny ACE Combination
All ACEs in the ACL are evaluated. An "individual ACE" is one whose
principal identifies the current user. A "group ACE" is one whose
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principal is a collection that contains a principal that identifies
the current user. A privilege is granted if it is granted by an
individual ACE and not denied by an individual ACE, or if it is
granted by a group ACE and not denied by an individual or group ACE.
A request MUST fail if any of its needed privileges are not granted.
<!ELEMENT no-deny EMPTY>
6.2 ACE Ordering
The DAV:ace-ordering element defines a constraint on how the ACEs
can be ordered in the ACL.
<!ELEMENT ace-ordering (deny-before-grant)? >
6.2.1 DAV:deny-before-grant ACE Ordering
This element indicates that all deny ACEs must precede all grant
ACEs.
<!ELEMENT deny-before-grant EMPTY>
6.3 Required Principals
The required principal elements identify which principals must have
an ACE defined in the ACL.
<!ELEMENT required-principal
(href | all | authenticated | unauthenticated | property |
self)>
For example, the following element requires that the ACE contain a
DAV:owner property ACE:
<D:required-principal xmlns:D="DAV:">
<D:property> <D:owner/> </D:property>
</D:required-principal>
7 ACCESS CONTROL AND EXISTING METHODS
This section defines the impact of access control functionality on
existing methods.
7.1 OPTIONS
If the server supports access control, it MUST return "access-
control" as a field in the DAV response header from an OPTIONS
request on any resource implemented by that server.
7.1.1Example - OPTIONS
>> REQUEST <<
OPTIONS /foo.html HTTP/1.1
Host: www.webdav.org
Content-Length: 0
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>> RESPONSE <<
HTTP/1.1 200 OK
DAV: 1, 2, access-control
Allow: OPTIONS, GET, PUT, PROPFIND, PROPPATCH, ACL
In this example, the OPTIONS response indicates that the server
supports access control and that /foo.html can have its access
control list modified by the ACL method.
8 ACCESS CONTROL METHODS
8.1 ACL
A DAV:acl property of a resource is modified by the ACL method. A
new DAV:acl value must be written in its entirety, including any
inherited ACEs. Unless the DAV:acl property of the resource can be
updated to be exactly the value specified in the ACL request, the
ACL request MUST fail. If a server restricts the set of ACEs
visible to the current user via the DAV:acl property, then the ACL
request would only replace the set of ACEs visible to the current
user, and would not affect any ACE that was not visible.
In order to avoid overwriting DAV:acl changes by another client, a
client SHOULD acquire a WebDAV lock on the resource before
retrieving the DAV:acl property of a resource that it intends on
updating.
8.1.1 ACL Preconditions
An implementation MAY enforce one or more of the following
constraints on an ACL request. If the constraint is violated, a 403
(Forbidden) response MUST be returned and the indicated XML element
MUST be returned in the response body.
<DAV:protected/>: An implementation MAY protect an ACE from
modification or deletion. For example, some implementations
implicitly grant the DAV:owner of a resource DAV:read-acl and
DAV:write-acl privileges, and this cannot be changed by a client.
<DAV:too-many-aces/>: An implementation MAY limit the number of ACEs
in an ACL. However, ACL-compliant servers MUST support at least one
ACE granting privileges to a single principal, and one ACE granting
privileges to a collection principal.
<DAV:non-inherited-must-precede-inherited/>: All non-inherited ACEs
MUST precede all inherited ACEs.
<DAV:deny-must-precede-grant/>: All non-inherited deny ACEs MUST
precede all non-inherited grant ACEs.
8.1.2 Example: the ACL method
In the following example, user "fielding", authenticated by
information in the Authorization header, grants the principal
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identified by the URL http://www.foo.org/users/esedlar (i.e., the
user "esedlar") read and write privileges, grants the owner of the
resource read-acl and write-acl privileges, and grants everyone read
privileges inherited from the parent collection
http://www.foo.bar/top/.
>> Request <<
ACL /top/container/ HTTP/1.1
Host: www.foo.org
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
Authorization: Digest username="fielding",
realm="users@foo.org", nonce="...",
uri="/top/container/", response="...", opaque="..."
<?xml version="1.0" encoding="utf-8" ?>
<D:acl xmlns:D="DAV:">
<D:ace>
<D:principal>
<D:href>http://www.foo.org/users/esedlar</D:href>
</D:principal>
<D:grant>
<D:privilege> <D:read/> </D:privilege>
<D:privilege> <D:write/> </D:privilege> </D:grant>
</D:ace>
<D:ace>
<D:principal>
<D:property> <D:owner/> </D:property> </D:principal>
<D:grant>
<D:privilege> <D:read-acl/> </D:privilege>
<D:privilege> <D:write-acl/> </D:privilege> </D:grant>
</D:ace>
<D:ace>
<D:principal> <D:all/> </D:principal>
<D:grant>
<D:privilege> <D:read/> </D:privilege></D:grant>
<D:inherited>
<D:href>http://www.foo.org/top/</D:href> </D:inherited>
</D:ace> </D:acl>
>> Response <<
HTTP/1.1 200 OK
8.1.3 Example: ACL method failure due to omission of protected ACE
In the following request, user "fielding", authenticated by
information in the Authorization header, attempts to grant the
principal identified by the URL http://www.foo.org/users/esedlar
(i.e., the user "esedlar") read privileges, but fails because an
protected ACE has been omitted (e.g. the ACE granting the DAV:owner
DAV:read-acl and DAV:write-acl privileges must always be present
since it is protected -- see Section 5.4.3).
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>> Request <<
ACL /top/container/ HTTP/1.1
Host: www.foo.org
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
Authorization: Digest username="fielding",
realm="users@foo.org", nonce="...",
uri="/top/container/", response="...", opaque="..."
<?xml version="1.0" encoding="utf-8" ?>
<D:acl xmlns:D="DAV:">
<D:ace>
<D:principal>
<D:href>http://www.foo.org/users/esedlar</D:href>
</D:principal>
<D:grant>
<D:privilege> <D:read/> </D:privilege> </D:grant>
</D:ace>
</D:acl>
>> Response <<
HTTP/1.1 403 Forbidden
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<DAV:protected/>
8.1.4 Example: ACL method failure due to inherited ACEs preceding non-
inherited ACEs
In the following request, user "ejw", authenticated by information
in the Authorization header, tries to change the access control list
on the resource http://www.foo.org/top/index.html. This resource has
two inherited ACEs.
Inherited ACE #1 grants the principal identified by URL
http://www.foo.org/users/ejw (i.e., the user "ejw")
http://www.foo.org/privs/write-all and DAV:read-acl privileges. On
this server, http://www.foo.org/privs/write-all is an aggregate
privilege containing DAV:write, and DAV:write-acl.
Inherited ACE #2 grants principal DAV:all the DAV:read privilege.
The request attempts to add a third ACE, granting the principal
identified by the URL http://www.foo.org/users/gclemm (i.e., the
user "gclemm") DAV:write permission, but in the request places the
inherited ACEs before the non-inherited ACEs, causing an error on
this specific server implementation. Note that on a different
implementation, this request might be accepted.
>> Request <<
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ACL /top/index.html HTTP/1.1
Host: www.foo.org
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
Authorization: Digest username="ejw",
realm="users@foo.org", nonce="...",
uri="/top/index.html", response="...", opaque="..."
<?xml version="1.0" encoding="utf-8" ?>
<D:acl xmlns:D="DAV:" xmlns:F="http://www.foo.org/privs/">
<D:ace>
<D:principal>
<D:href>http://www.foo.org/users/ejw</D:href>
</D:principal>
<D:grant>
<D:privilege><F:write-all/></D:privilege>
<D:privilege><D:read-acl/></D:privilege>
</D:grant>
<D:inherited/>
</D:ace>
<D:ace>
<D:principal><D:all/></D:principal>
<D:grant><D:read/></D:grant>
<D:inherited/>
</D:ace>
<D:ace>
<D:principal>
<D:href>http://www.foo.org/users/gclemm</D:href>
</D:principal>
<D:grant><D:write/></D:grant>
</D:ace>
</D:acl>
>> Response <<
HTTP/1.1 403 Forbidden
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<DAV:non-inherited-must-precede-inherited/>
8.1.5 Example: ACL method failure due to an attempt to set grant and
deny in a single ACE.
In this example, user "ygoland", authenticated by information in the
Authorization header, tries to change the access control list on the
resource http://www.foo.org/diamond/engagement-ring.gif. The ACL
request includes a single, syntactically and semantically incorrect
ACE, which attempts to grant the collection principal identified by
the URL http://www.foo.org/users/friends/ DAV:read privilege and
deny the principal identified by URL
http://www.foo.org/users/ygoland-so (i.e., the user "ygoland-so")
DAV:read privilege. However, it is illegal to have multiple
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principal elements, as well as both a grant and deny element in the
same ACE, so the request fails due to poor syntax.
>> Request <<
ACL /diamond/engagement-ring.gif HTTP/1.1
Host: www.foo.org
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx
Authorization: Digest username="ygoland",
realm="users@foo.org", nonce="...",
uri="/diamond/engagement-ring.gif", response="...",
opaque="..."
<?xml version="1.0" encoding="utf-8" ?>
<D:acl xmlns:D="DAV:">
<D:ace>
<D:principal>
<D:href>http://www.foo.org/users/friends/</D:href>
</D:principal>
<D:grant><D:read/></D:grant>
<D:principal>
<D:href>http://www.foo.org/users/ygoland-so</D:href>
</D:principal>
<D:deny><D:read/></D:deny>
</D:ace>
</D:acl>
>> Response <<
HTTP/1.1 400 Bad Request
Content-Length: 0
Note that if the request had been divided into two ACEs, one to
grant, and one to deny, the request would have been syntactically
well formed.
9 INTERNATIONALIZATION CONSIDERATIONS
In this specification, the only human-readable content can be found
in the DAV:authentication-id property, found on principal resources.
This property contains the name used to authenticate a principal,
typically by a user entering this name into a password entry screen.
As a result, the authentication-id must be capable of representing
names in multiple character sets. Since DAV:authentication-id is a
WebDAV property, it is represented on-the-wire as XML [REC-XML], and
hence can leverage XML's language tagging and character set encoding
capabilities. Specifically, XML processors must, at minimum, be able
to read XML elements encoded using the UTF-8 [UTF-8] encoding of the
ISO 10646 multilingual plane. XML examples in this specification
demonstrate use of the charset parameter of the Content-Type header,
as defined in [RFC3023], as well as the XML "encoding" attribute,
which together provide charset identification information for MIME
and XML processors.
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For properties other than DAV:authentication-id, it is expected that
implementations will treat the property names and values as tokens,
and convert these tokens into human-readable text in the user's
language and character set when displayed to a person. Only a
generic WebDAV property display utility would display these values
in their raw form.
For error reporting, we follow the convention of HTTP/1.1 status
codes, including with each status code a short, English description
of the code (e.g., 200 (OK)). While the possibility exists that a
poorly crafted user agent would display this message to a user,
internationalized applications will ignore this message, and display
an appropriate message in the user's language and character set.
Further internationalization considerations for this protocol are
described in the WebDAV Distributed Authoring protocol specification
[RFC2518].
10 SECURITY CONSIDERATIONS
Applications and users of this access control protocol should be
aware of several security considerations, detailed below. In
addition to the discussion in this document, the security
considerations detailed in the HTTP/1.1 specification [RFC2616], the
WebDAV Distributed Authoring Protocol specification [RFC2518], and
the XML Media Types specification [RFC3023] should be considered in
a security analysis of this protocol.
10.1 Increased Risk of Compromised Users
In the absence of a mechanism for remotely manipulating access
control specifications, if a single user's authentication
credentials are compromised, only those resources for which the user
has access permission can be read, modified, moved, or deleted. With
the introduction of this access control protocol, if a single
compromised user has the ability to change ACLs for a broad range of
other users (e.g., a super-user), the number of resources that could
be altered by a single compromised user increases. This risk can be
mitigated by limiting the number of people who have write-acl
privileges across a broad range of resources.
10.2 Authentication-id Property and Dictionary Attacks
Every principal has a DAV:authentication-id property defined on it,
which provides the name used to authenticate this principal,
typically the username portion of a username/password authentication
scheme. An attacker can use the information in this property when
attempting either a brute-force, or a dictionary attack to guess the
principal's identifying password. By providing the username in
DAV:authentication-id, the scope of an attack can be reduced to a
single, valid username. Furthermore, it is possible that principals
can potentially belong to a collection. In this case, it is possible
to use the PROPFIND method to retrieve the DAV:authentication-id
property from all of the principals in a collection, thus providing
multiple usernames that can be the focus of attack.
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To reduce this risk, the DAV:authentication-id property should not
be world-readable. Which principals are granted default read
privilege for DAV:authentication-id should be carefully considered
in any deployment of this protocol.
10.3 Risks of the read-acl Privilege
The ability to read the access privileges (stored in the DAV:acl
property), or the privileges permitted the currently authenticated
user (stored in the DAV:current-user-privilege-set property) on a
resource may seem innocuous, since reading an ACL cannot possibly
affect the resource's state. However, if all resources have world-
readable ACLs, it is possible to perform an exhaustive search for
those resources that have inadvertently left themselves in a
vulnerable state, such as being world-writeable. In particular, the
property retrieval method PROPFIND, executed with Depth infinity on
an entire hierarchy, is a very efficient way to retrieve the DAV:acl
or DAV:current-user-privilege-set properties. Once found, this
vulnerability can be exploited by a denial of service attack in
which the open resource is repeatedly overwritten. Alternately,
writeable resources can be modified in undesirable ways.
To reduce this risk, read-acl privileges should not be granted to
unauthenticated principals, and restrictions on read-acl privileges
for authenticated principals should be carefully analyzed when
deploying this protocol.
11 AUTHENTICATION
Authentication mechanisms defined in WebDAV also apply to this
WebDAV Access Control Protocol, in particular the Basic and Digest
authentication mechanisms defined in [RFC2617].
12 IANA CONSIDERATIONS
This document uses the namespace defined by [RFC2518] for XML
elements. All other IANA considerations mentioned in [RFC2518] also
applicable to WebDAV ACL.
13 INTELLECTUAL PROPERTY
The following notice is copied from RFC 2026, section 10.4, and
describes the position of the IETF concerning intellectual property
claims made against this document.
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use other technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
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to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification
can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
14 ACKNOWLEDGEMENTS
This protocol is the collaborative product of the WebDAV ACL design
team: Bernard Chester, Geoff Clemm (Rational), Anne Hopkins
(Microsoft), Barry Lind (Xythos), Sean Lyndersay (Microsoft), Eric
Sedlar (Oracle), Greg Stein (Apache.org), and Jim Whitehead (UC
Santa Cruz). The authors are grateful for the detailed review and
comments provided by Jim Amsden, Gino Basso, Murthy Chintalapati,
Dennis Hamilton, Ron Jacobs, Chris Knight, and Remy Maucherat. Prior
work on WebDAV access control protocols has been performed by Yaron
Goland, Paul Leach, Lisa Dusseault, Howard Palmer, and Jon Radoff.
We would like to acknowledge the foundation laid for us by the
authors of the WebDAV and HTTP protocols upon which this protocol is
layered, and the invaluable feedback from the WebDAV working group.
15 REFERENCES
15.1 Normative References
[RFC2119] S.Bradner, "Key words for use in RFCs to Indicate
Requirement Levels." RFC 2119, BCP 14, Harvard, March, 1997.
[REC-XML] T. Bray, J. Paoli, C.M. Sperberg-McQueen, "Extensible
Markup Language (XML)." World Wide Web Consortium Recommendation
REC-xml-19980210. http://www.w3.org/TR/REC-xml-19980210.
[RFC2616] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, L.
Masinter, P. Leach, and T. Berners-Lee, "Hypertext Transfer Protocol
-- HTTP/1.1." RFC 2616. U.C.Irvine, Compaq, Xerox, Microsoft,
MIT/LCS, June, 1999.
[RFC2617] J. Franks, P. Hallam-Baker, J. Hostetler, S. Lawrence, P.
Leach, A. Luotonen, L. Stewart, "HTTP Authentication: Basic and
Digest Access Authentication. " RFC 2617. Northwestern University,
Verisign, AbiSource, Agranat, Microsoft, Netscape, Open Market,
June, 1999.
[RFC2518] Y. Goland, E. Whitehead, A. Faizi, S. R. Carter, D.
Jensen, "HTTP Extensions for Distributed Authoring -- WEBDAV." RFC
2518. Microsoft, U.C.Irvine, Netscape, Novell, February, 1999.
[RFC3023] M. Murata, S. St.Laurent, D. Kohn, "XML Media Types." RFC
3023. IBM Tokyo Research Laboratory, simonstl.com, Skymoon Ventures,
January, 2001.
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[UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode and
ISO 10646." RFC 2279. Alis Technologies. January, 1998.
15.2Informational References
[RFC2026] S.Bradner, "The Internet Standards Process û Revision 3."
RFC 2026, BCP 9. Harvard, October, 1996.
16 AUTHORS' ADDRESSES
Geoffrey Clemm
Rational Software
20 Maguire Road
Lexington, MA 02421
Email: geoffrey.clemm@rational.com
Anne Hopkins
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Email: annehop@microsoft.com
Eric Sedlar
Oracle Corporation
500 Oracle Parkway
Redwood Shores, CA 94065
Email: esedlar@us.oracle.com
Jim Whitehead
U.C. Santa Cruz
Dept. of Computer Science
Baskin Engineering
1156 High Street
Santa Cruz, CA 95064
Email: ejw@cse.ucsc.edu
17 APPENDICIES
17.1XML Document Type Definition
<!-- Privileges -->
<!ELEMENT read EMPTY>
<!ELEMENT write EMPTY>
<!ELEMENT read-acl EMPTY>
<!ELEMENT write-acl EMPTY>
<!ELEMENT all EMPTY>
<!-- Principal Properties (Section 4) -->
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<!ELEMENT is-principal (#PCDATA)>
<!ELEMENT authentication-id (#PCDATA)>
<!-- Access Control Properties (Section 5) -->
<!-- DAV:owner Property (Section 5.1) -->
<!ELEMENT owner (href prop?)>
<!ELEMENT prop (see [RFC2518], section 12.11)>
<!-- DAV:supported-privilege-set Property (Section 5.2) -->
<!ELEMENT supported-privilege-set (supported-privilege*)>
<!ELEMENT supported-privilege
(privilege, abstract?, description, supported-privilege*)>
<!ELEMENT privilege ANY>
<!ELEMENT abstract EMPTY>
<!ELEMENT description #PCDATA>
<!ELEMENT privilege ANY>
<!-- DAV:current-user-privilege-set Property (Section 5.3) -->
<!ELEMENT current-user-privilege-set (privilege*)>
<!-- DAV:acl Property (Section 5.4) -->
<!ELEMENT acl (ace*)>
<!ELEMENT ace (principal, (grant|deny), protected?, inherited?)>
<!ELEMENT principal ((href, prop?)
| all | authenticated | unauthenticated
| property | self)>
<!ELEMENT prop (see [RFC2518], section 12.11)>
<!ELEMENT all EMPTY>
<!ELEMENT authenticated EMPTY>
<!ELEMENT unauthenticated EMPTY>
<!ELEMENT property ANY>
<!ELEMENT self EMPTY>
<!ELEMENT grant (privilege+)>
<!ELEMENT deny (privilege+)>
<!ELEMENT privilege ANY>
<!ELEMENT protected EMPTY>
<!ELEMENT inherited (href)>
<!-- DAV:principal-collection-set Property (Section 5.6) -->
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<!ELEMENT principal-collection-set (href*)>
<!-- DAV:acl-semantics Property (Section 6) -->
<!ELEMENT acl-semantics acl-sem*>
<!ELEMENT acl-sem (ace-combination, ace-ordering, required-
principal)>
<!ELEMENT ace-combination
(first-match | all-grant-before-any-deny | no-deny)>
<!ELEMENT first-match EMPTY>
<!ELEMENT all-grant-before-any-deny EMPTY>
<!ELEMENT no-deny EMPTY>
<!ELEMENT ace-ordering (deny-before-grant)? >
<!ELEMENT deny-before-grant EMPTY>
<!ELEMENT required-principal
(href | all | authenticated | unauthenticated | property |
self)>
<!-- ACL method preconditions (Section 8.1.1) -->
<!ELEMENT protected EMPTY>
<!ELEMENT too-many-aces EMPTY>
<!ELEMENT non-inherited-must-precede-inherited EMPTY>
<!ELEMENT deny-must-precede-grant EMPTY>
<!ELEMENT acl-requires-lock-token EMPTY>
Clemm, Hopkins, Sedlar, Whitehead [Page 27]| PAFTECH AB 2003-2026 | 2026-04-24 10:29:21 |