INTERNET-DRAFT Geoffrey Clemm, Rational Software draft-ietf-webdav-acl-03 Anne Hopkins, Microsoft Corporation Eric Sedlar, Oracle Corporation Jim Whitehead, U.C. Santa Cruz Expires May 24, 2001 November 24, 2000 WebDAV Access Control Protocol Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts 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/. Clemm, Hopkins, Sedlar, Whitehead [Page 1] INTERNET-DRAFT WebDAV ACL October 16, 2000 Table of Contents 1 INTRODUCTION ............................................3 1.1 Terms .................................................3 1.2 Notational Conventions ................................4 2 PRINCIPALS ..............................................4 3 PRIVILEGES ..............................................5 3.1 DAV:read Privilege ....................................5 3.2 DAV:write Privilege ...................................6 3.3 DAV:read-acl Privilege ................................6 3.4 DAV:write-acl Privilege ...............................6 3.5 DAV:all Privilege .....................................6 4 PRINCIPAL PROPERTIES ....................................6 4.1 DAV:is-principal ......................................6 4.2 DAV:authentication-id .................................6 5 ACCESS CONTROL PROPERTIES ...............................7 5.1 DAV:owner .............................................7 5.2 DAV:supported-privilege-set ...........................7 5.3 DAV:current-user-privilege-set ........................8 5.4 DAV:acl ...............................................8 5.4.1 ACE Principal .....................................8 5.4.2 ACE Grant and Deny ................................9 5.4.3 ACE Protection ...................................10 5.4.4 ACE Inheritance ..................................10 5.5 DAV:acl-semantics ....................................10 5.5.1 first-match Semantics ............................14 5.5.2 all-grant-before-any-deny Semantics ..............14 5.5.3 no-deny Semantics ................................14 5.6 DAV:principal-collection-set .........................10 5.7 Example: PROPFIND to retrieve access control properties11 6 ACCESS CONTROL AND EXISTING METHODS ....................14 6.1 OPTIONS ..............................................15 6.1.1 Example - OPTIONS ................................15 7 ACCESS CONTROL METHODS .................................16 7.1 ACL ..................................................16 7.1.1 ACL Preconditions ................................16 7.1.2 Example: the ACL method ..........................17 7.1.3 Example: ACL method failure ......................17 8 INTERNATIONALIZATION CONSIDERATIONS ....................18 9 SECURITY CONSIDERATIONS ................................19 10 AUTHENTICATION .......................................20 11 IANA CONSIDERATIONS ..................................20 12 INTELLECTUAL PROPERTY ................................20 13 ACKNOWLEDGEMENTS .....................................21 Clemm, Hopkins, Sedlar, Whitehead [Page 2] INTERNET-DRAFT WebDAV ACL October 16, 2000 14 REFERENCES ...........................................21 15 AUTHORS' ADDRESSES ...................................22 16 STILL TO DO ..........................................22 1 INTRODUCTION The goal of the WebDAV access control extensions is to provide an interoperable mechanism for handling discretionary access control for content in WebDAV servers. WebDAV access control can be implemented on content repositories with security as simple as that of a UNIX file system, as well as more sophisticated models. The underlying principle of access control is that who you are determines how you can access a resource. The "who you are" is defined by a "principal" identifier; users, client software, servers, and groups of the previous have principal identifiers. The "how" is determined by a single "access control list" (ACL) associated with a resource. An ACL contains a set of "access control entries" (ACEs), where each ACE specifies a principal and a set of rights that are either granted or denied to that principal. When a principal submits an operation (such as an HTTP or WebDAV method) to a resource for execution, the server evaluates the ACEs in the ACL to determine if the principal has permission for that operation. This specification has intentionally omits discussion of authentication, as the HTTP protocol already has a number of authentication mechanisms[RFC2617] . Some authentication mechanism (such as HTTP Digest Authentication, which all WebDAV compliant implementations are required to support) must be availableto validate the identity of a principal. In the interests of timeliness, the following set of security mechanisms is currently viewed as out of scope of this document: * 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 1.1 Terms This draft uses the terms defined in HTTP [RFC2616] and WebDAV [RFC2518]. In addition, the following terms are defined: Clemm, Hopkins, Sedlar, Whitehead [Page 3] INTERNET-DRAFT WebDAV ACL October 16, 2000 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. privilege A "privilege" controls access to a particular set of HTTP operations on a resource. aggregate privilege An "aggregate privilege " is a privilege that comprises a set of other privileges. 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 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. Clemm, Hopkins, Sedlar, Whitehead [Page 4] INTERNET-DRAFT WebDAV ACL October 16, 2000 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 define new HTTP methods SHOULD specify which privileges (by defining new privileges, or mapping to ones below) are required to perform the method. A principal with no privileges to a resource SHOULD be denied any HTTP access to that resource. Privileges may be aggregates of other 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. The set of privileges that apply to a particular resource may vary with the DAV:resourcetype of the resource, as well as between different server implementations. To promote interoperability, however, WebDAV defines a set of well-known privileges (e.g. DAV:read and DAV:write), which can at least be used to classify the other privileges defined on a particular resource. 3.1 DAV:read Privilege The read privilege controls methods that return information about the state of the resource, including the resource's properties. Affected methods include GET and PROPFIND. The read privilege does not control the OPTIONS method since the OPTIONS method returns capabilities rather than state. Clemm, Hopkins, Sedlar, Whitehead [Page 5] INTERNET-DRAFT WebDAV ACL October 16, 2000 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 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. 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. 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. 3.5 DAV:all Privilege The DAV:all privilege controls all privileges on the resource. 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.) 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). PCDATA value: any string Clemm, Hopkins, Sedlar, Whitehead [Page 6] INTERNET-DRAFT WebDAV ACL October 16, 2000 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. 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. Each privilege appears as an XML element, where aggregate privileges list as sub-elements all of the privileges that they aggregate. An abstract privilege is used to classify the non-abstract privilege elements. An abstract privilege of a resource MUST NOT be used in an ACE for that resource. Servers MUST fail an attempt to set an abstract privilege. A description is a human-readable description of what this privilege controls access to. It is envisioned that a WebDAV ACL-aware administrative client would list the supported privileges in a dialog box, and allow the user to choose non-abstract privileges to apply in an ACE. The privileges tree is useful programmatically to map well- Clemm, Hopkins, Sedlar, Whitehead [Page 7] INTERNET-DRAFT WebDAV ACL October 16, 2000 known privileges (defined by WebDAV or other standards groups) into privileges that are supported by any particular server implementation. The privilege tree also serves to hide complexity in implementations allowing large number of privileges to be defined by displaying aggregates to the user. 5.3 DAV:current-user-privilege-set This is a read-only property containing a list of privileges granted to the currently authenticated HTTP user. The current user has no access privileges to an object protected by an ACL unless that user matches one or more of the principals specified in the ACEs. 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. 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. 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. 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. An implementation MAY include a DAV:prop element after the DAV:href element, containing a list of selected properties of that principal resource. Which properties (if any) are Clemm, Hopkins, Sedlar, Whitehead [Page 8] INTERNET-DRAFT WebDAV ACL October 16, 2000 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. 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. 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 , 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. 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. 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. Clemm, Hopkins, Sedlar, Whitehead [Page 9] INTERNET-DRAFT WebDAV ACL October 16, 2000 5.4.3 ACE Protection If an ACE contains a DAV:protected element, an ACL request without that ACE MUST fail. 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. 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 Often a versioning implementation constrains where a principal can be located in the URL space. The DAV:principal- collection-set enumerates which collections may contain principals. 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. Clemm, Hopkins, Sedlar, Whitehead [Page 10] INTERNET-DRAFT WebDAV ACL October 16, 2000 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="..." >> Response << HTTP/1.1 207 Multi-Status Content-Type: text/xml; charset="utf-8" Content-Length: xxx HTTP/1.1 200 OK http://www.foo.org/users/gclemm Any operation Read any object Write any object Clemm, Hopkins, Sedlar, Whitehead [Page 11] INTERNET-DRAFT WebDAV ACL October 16, 2000 Create an object Update an object Delete an object Read the ACL Write the ACL http://www.foo.org/users/esedlar esedlar Eric Sedlar http://www.foo.org/groups/marketing/ Clemm, Hopkins, Sedlar, Whitehead [Page 12] INTERNET-DRAFT WebDAV ACL October 16, 2000 http://www.foo.org/top/ The value of the DAV:owner property is a single DAV:href XML element containing the URL of the principal that owns this resource. The value of the DAV:supported-privilege-set property is a tree of supported privileges: DAV:acl (abstract) | +-- DAV:read +-- DAV:write (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. ACE #3: In this ACE, the principal is a property principal, specifically the DAV:owner property. When evaluating this ACE, the value of the DAV:owner property is retrieved, and is examined to see if it contains a DAV:href XML element. If so, the URL within the DAV:href element is read, and identifies a principal. In this ACE, the owner is granted DAV:read-acl, and DAV:write-acl privileges. ACE #4: This ACE grants the DAV:all principal (all users) the DAV:read privilege. This ACE is inherited from the resource Clemm, Hopkins, Sedlar, Whitehead [Page 13] INTERNET-DRAFT WebDAV ACL October 16, 2000 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. ANY value: zero or more of the ACL semantic elements 6.1 ACE Combination The DAV:ace-combination element defines how privileges from multiple ACEs that match the current user will be combined to determine the access rights 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. 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. 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. 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 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. Clemm, Hopkins, Sedlar, Whitehead [Page 14] INTERNET-DRAFT WebDAV ACL October 16, 2000 6.2 ACE Ordering The DAV:ace-ordering element defines a constraint on how the ACEs can be ordered in the ACL. 6.2.1 DAV:deny-before-grant ACE Ordering This element indicates that all deny ACEs must precede all grant ACEs. 6.3 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 ACE contain a DAV:owner property ACE: 7 ACCESS CONTROL AND EXISTING METHODS This section defines the impact of access control functionality on existing methods. 7.1 OPTIONS If the server supports access control, it MUST return "access- control" as a field in the DAV response header from an OPTIONS request on any resource implemented by that server. 7.1.1 Example - OPTIONS >>REQUEST OPTIONS /foo.html HTTP/1.1 Host: www.webdav.org Content-Length: 0 >>RESPONSE HTTP/1.1 200 OK DAV: 1, 2, access-control Clemm, Hopkins, Sedlar, Whitehead [Page 15] INTERNET-DRAFT WebDAV ACL October 16, 2000 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. : 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. : 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. : All non-inherited ACEs MUST precede all inherited ACEs. : All non-inherited deny ACEs MUST precede all non-inherited grant ACEs. : If a resource is locked, the lock token MUST be specified in the ACL request. Clemm, Hopkins, Sedlar, Whitehead [Page 16] INTERNET-DRAFT WebDAV ACL October 16, 2000 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.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="..." http://www.foo.org/users/esedlar http://www.foo.org/top/ >> Response << HTTP/1.1 200 OK 8.1.3Example: ACL method failure In the following request, user "fielding", authenticated by information in the Authorization header, attempts to grant principal identified by the URL http://www.foo.org/users/esedlar (i.e., the user "esedlar") read privileges, but fails because an implicit ACE has been Clemm, Hopkins, Sedlar, Whitehead [Page 17] INTERNET-DRAFT WebDAV ACL October 16, 2000 omitted (e.g. the ACE granting the DAV:owner DAV:read-acl and DAV:write-acl privileges). >> Request << ACL /top/container HTTP/1.1 Host: www.foo.bar Content-Type: text/xml; charset="utf-8" Content-Length: xxxx Authorization: Digest username="fielding", realm="users@foo.org", nonce="...", uri="/top/container/", response="...", opaque="..." http://www.foo.bar/users/esedlar >> Response << HTTP/1.1 403 Forbidden Content-Type: text/xml; charset="utf-8" Content-Length: xxx 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 [RFC2376], as well as the XML "encoding" attribute, which together provide charset identification information for MIME and XML processors. For properties other than DAV:authentication-id, it is expected that implementations will treat the property names and values as tokens, and convert these tokens into human- Clemm, Hopkins, Sedlar, Whitehead [Page 18] INTERNET-DRAFT WebDAV ACL October 16, 2000 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 specification (discussed in [RFC2376]) 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 Clemm, Hopkins, Sedlar, Whitehead [Page 19] INTERNET-DRAFT WebDAV ACL October 16, 2000 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. To reduce this risk, the DAV:authentication-id property should not be world-readable. Which principals are granted default read permission for DAV:authentication-id should be carefully considered in any deployment of this protocol. 10.3 Risks of the read-acl Privilege The ability to read the access permissions (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. Once found, this vulnerability can be exploited by a denial of service attack in which the open resource is repeatedly overwritten. Alternately, writeable resources can be modified in undesirable ways. To reduce this risk, read-acl privileges should not be granted to unauthenticated principals, and restrictions on read-acl privileges for authenticated principals should be carefully analysed when deploying this protocol. 11 AUTHENTICATION Authentication mechanisms defined in WebDAV will also apply to WebDAV ACL. 12 IANA CONSIDERATIONS This document uses the namespace defined by [RFC2518] for XML elements. All other IANA considerations mentioned in [RFC2518] also applicable to WebDAV ACL. 13 INTELLECTUAL PROPERTY The following notice is copied from RFC 2026, section 10.4, and describes the position of the IETF concerning intellectual property claims made against this document. The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use other technology described in this document or the extent to which any license under such rights might or might not be available; Clemm, Hopkins, Sedlar, Whitehead [Page 20] INTERNET-DRAFT WebDAV ACL October 16, 2000 neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to practice 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). 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. Clemm, Hopkins, Sedlar, Whitehead [Page 21] INTERNET-DRAFT WebDAV ACL October 16, 2000 [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. [UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode and ISO 10646." RFC 2279. Alis Technologies. January, 1998. 15.2 Informational References [RFC2026] S.Bradner, "The Internet Standards Process _ Revision 3." RFC 2026, BCP 9. Harvard, October, 1996. [RFC2396] E. Whitehead, M. Murata, "XML Media Types." RFC 2376. U.C. Irvine, Fuji Xerox Info. Systems. July, 1998. (This RFC will soon be superseded by , which has been approved by the IESG as a Proposed Standard, but not yet issued as an RFC.) 16 AUTHORS' ADDRESSES Geoffrey Clemm Rational Software 20 Maguire Road Lexington, MA 02421 Email: geoffrey.clemm@rational.com Anne Hopkins Microsoft Corporation One Microsoft Way Redmond, WA 98052 Email: annehop@microsoft.com Eric Sedlar Oracle Corporation 500 Oracle Parkway Redwood Shores, CA 94065 Email: esedlar@us.oracle.com Jim Whitehead U.C. Santa Cruz Dept. of Computer Science Baskin Engineering 1156 High Street Santa Cruz, CA 95064 Email: ejw@cse.ucsc.edu 17 STILL TO DO * If we can add more elements to DAV:resourcetype, we can eliminate DAV:is-principal. Clemm, Hopkins, Sedlar, Whitehead [Page 22] INTERNET-DRAFT WebDAV ACL October 16, 2000 * Add back the XML schema if provides info not in the DTD's. * Consider adding a DAV:matching-principals, which identifies all ACL principals that match the current user. * Add DAV:ordering-constraints, DAV:required-principals, and DAV:ace-combination-semantics as sub-elements of DAV:acl- semantics. Clemm, Hopkins, Sedlar, Whitehead [Page 23]