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Network Working Group N. CookRequest for Comments: 5593 CloudmarkUpdates:5092 June 2009Category: Standards TrackInternet Message Access Protocol (IMAP) -URL Access Identifier ExtensionStatus of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject toBCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document.Abstract The existing IMAP URL specification (RFC 5092) lists several <access> identifiers and <access> identifier prefixes that can be used to restrict access to URLAUTH-generated URLs. However, these identifiers do not provide facilities for new services such as streaming. This document proposes a set of new <access> identifiers as well as an IANA mechanism to register new <access> identifiers for future applications. This document updatesRFC 5092.Cook Standards Track [Page 1]

RFC 5593 IMAP URL Access Identifier June 2009Table of Contents1. Introduction ....................................................22. Conventions Used in This Document ...............................23. Additional Authorized Access Identifiers ........................33.1. Existing Access Identifiers ................................33.2. Requirement for Additional Access Identifiers ..............33.3. Additional Access Identifier Specification .................43.4. Defining an Access Identifier for Streaming ................54. Formal Syntax ...................................................55. Acknowledgements ................................................66. IANA Considerations .............................................66.1. Access Identifier Registration Template ....................76.2. Stream Application Registration ............................76.3. Submit Application Registration ............................86.4. User Application Registration ..............................86.5. Authuser Application Registration ..........................96.6. Anonymous Application Registration .........................97. Security Considerations .........................................98. References .....................................................108.1. Normative References ......................................108.2. Informative References ....................................101. Introduction The IMAP URL specification [RFC5092] provides a way to carry authorization information in IMAP URLs. Several authorization <access> identifiers are specified in the document that allow URLAUTH-authorized URLs to be used only by anonymous users, authenticated users, or message submission entities. However, there is no mechanism defined to create new <access> identifiers, and overloading the existing mechanisms has security as well as administrative implications. This document describes a new <access> identifier, "stream", to be used by message streaming entities (as described in [STREAMING]), and defines an IANA registration template, which can be used to register new <access> identifiers for future applications. IANA definitions for the existing access identifiers and prefixes fromRFC 5092 are also defined in this document -- this document updatesRFC 5092 and should be taken as the master in the event of any differences or discrepancies.2. Conventions Used in This Document 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 inRFC 2119 [RFC2119].Cook Standards Track [Page 2]

RFC 5593               IMAP URL Access Identifier              June 2009   In examples, "C:" and "S:" indicate lines sent by the client and   server, respectively.  If a single "C:" or "S:" label applies to   multiple lines, then some of the line breaks between those lines are   for editorial clarity only and may not be part of the actual protocol   exchange.3.  Additional Authorized Access Identifiers3.1.  Existing Access Identifiers   The IMAP URL specification [RFC5092] specifies the following   authorized <access> identifiers:   o  "authuser" - Indicates that use of this URL is limited to      authenticated IMAP sessions that are logged in as any non-      anonymous user.   o  "anonymous" - Indicates that use of this URL is not restricted by      session authorization identity.   Additionally, the following <access> identifier prefixes are defined   in [RFC5092]:   o  "submit+" - Followed by a userid, indicates that only a userid      authorized as a message submission entity on behalf of the      specified userid is permitted to use this URL.   o  "user+" - Followed by a userid, indicates that use of this URL is      limited to IMAP sessions that are logged in as the specified      userid.3.2.  Requirement for Additional Access Identifiers   The existing <access> identifiers are suitable for user-based   authorization, but only the "submit+" <access> identifier prefix is   suitable for entities acting on behalf of a user.  Generic support   for external entities acting on behalf of users is required for new   services such as streaming [STREAMING].   The "submit+" <access> identifier prefix is not suitable for use as a   general mechanism to grant access to entities acting on behalf of   users, for reasons that include:   o  Security - The IMAP server maintains a list of submission server      entities that are entitled to retrieve IMAP URLs specifying the      "submit+" <access> identifier prefix.  If this list is extended to      include the set of all external entities that could act on behalf      of users, then the attack surface would be increased.Cook                        Standards Track                     [Page 3]

RFC 5593 IMAP URL Access Identifier June 2009 o Administration - When URLAUTH-style IMAP URLs are presented to an IMAP server by entities acting on behalf of users, the server administrator has no way of determining the intended use of that URL from the server logs. o Resourcing - Without a mechanism to distinguish between the application for which an IMAP URL is to be used, the IMAP server has no way to prioritize resources for particular applications. For example, the server could prioritize "submit+" URL fetch requests over other access identifiers.3.3. Additional Access Identifier Specification The previous section establishes that additional access identifiers are required to support applications, such as streaming [STREAMING], that require entities to retrieve URLAUTH URLs on behalf of users. This section describes the scope and meaning of any additional <access> identifiers that are created. Additional <access> identifiers MUST take one of two forms (Section 4 gives the formal ABNF syntax): o <access> identifier - The name of the application, e.g., "exampleapp". o <access> identifier prefix - The name of the application, e.g., "exampleapp3", followed by a "+" and then a userid. For example, consider "exampleapp3+testuser". Note that an <access> identifier name can also be registered as an <access> identifier prefix. However, this would require 2 separate IANA registrations. In both cases, the semantics are the same as those for "submit+", i.e., the <access> identifier or <access> identifier prefix (which MUST be followed by a userid) indicates that only a userid authorized as an application entity for the specified application is permitted to use this URL. In the case of <access> identifier prefixes, the IMAP server SHALL NOT validate the specified userid but MUST validate that the IMAP session has an authorization identity that is authorized as an application entity for the specified application.Cook Standards Track [Page 4]

RFC 5593 IMAP URL Access Identifier June 2009 The application entity itself MAY choose to perform validation on any specified userid before attempting to retrieve the URL. The authorization granted by any <access> identifiers used as described above is self-describing, and so requires that the IMAP server provide an extensible mechanism for associating userids with new applications. For example, imagine a new application, "foo", is created that requires application entities to retrieve URLs on behalf of users. In this case, the IMAP server would need to provide a way to register the new application "foo" and to associate the set of userids to be used by those entities with the application "foo". Any attempt to retrieve URLs containing the <access> identifier "foo" would be checked for authorization against the list of userids associated with the application "foo".Section 6 provides the template required to register new <access> identifiers or prefixes with IANA.3.4. Defining an Access Identifier for Streaming One application that makes use of URLAUTH-authorized URLs is that of streaming multimedia files that are received as internet-messaging attachments. This application is described in [STREAMING]. SeeSection 6.2 for the IANA registration template for the "stream" <access> identifier.4. Formal Syntax The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in [RFC5234]. Except as noted otherwise, all alphabetic characters are case- insensitive. The use of upper- or lower-case characters to define token strings is for editorial clarity only. Implementations MUST accept these strings in a case-insensitive fashion. The ABNF specified below updates the formal syntax of <access> identifiers as defined in IMAP URL [RFC5092]. Copyright (c) 2009 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:Cook Standards Track [Page 5]

RFC 5593               IMAP URL Access Identifier              June 2009   - Redistributions of source code must retain the above copyright     notice, this list of conditions and the following disclaimer.   - Redistributions in binary form must reproduce the above copyright     notice, this list of conditions and the following disclaimer in     the documentation and/or other materials provided with the     distribution.   - Neither the name of Internet Society, IETF or IETF Trust, nor the     names of specific contributors, may be used to endorse or promote     products derived from this software without specific prior     written permission.   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR   A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT   OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.         application = 1*(ALPHA/DIGIT)         access      =/ application / (application "+" enc-user)5.  Acknowledgements   This document was inspired by discussions in the Lemonade Working   Group.6.  IANA Considerations   IANA created a new registry for IMAP URLAUTH access identifiers and   prefixes.   Access identifiers and prefixes MUST be registered using the "IETF   Review" policy [RFC5226].  This section gives the IANA registration   entries for the existing access identifiers and prefixes fromRFC5092 as well as the entry for the "stream" application.Cook                        Standards Track                     [Page 6]

RFC 5593               IMAP URL Access Identifier              June 20096.1.  Access Identifier Registration Template   To: iana@iana.org   Subject: IMAP URL Access Identifier Registration   Type:          [Either "<access> identifier" or                   "<access> identifier prefix"]   Application:   [Name of the application, e.g., "stream"]   Description:   [A description of the application and its use                   of IMAP URLs]   RFC Number:    [Number of the RFC in which the application is                   defined]   Contact:       [Email and/or physical address to contact for                   additional information]6.2.  Stream Application Registration   To: iana@iana.org   Subject: IMAP URL Access Identifier Registration   Type:          <access> identifier   Application:   stream   Description:   Used by SIP Media Servers to retrieve                  attachments for streaming to email                  clients   RFC Number:RFC 5593   Contact:       Neil Cook <neil.cook@noware.co.uk>Cook                        Standards Track                     [Page 7]

RFC 5593               IMAP URL Access Identifier              June 20096.3.  Submit Application Registration   To: iana@iana.org   Subject: IMAP URL Access Identifier Registration   Type:          <access> identifier prefix   Application:   submit   Description:   Used by message submission entities to                  retrieve attachments to be included in                  submitted messages   RFC Number:RFC 5593 andRFC 5092   Contact:       Lemonade WG <lemonade@ietf.org>6.4.  User Application Registration   To: iana@iana.org   Subject: IMAP URL Access Identifier Registration   Type:          <access> identifier prefix   Application:   user   Description:   Used to restrict access to IMAP sessions                  that are logged in as the specified userid   RFC Number:RFC 5593 andRFC 5092   Contact:       Lemonade WG <lemonade@ietf.org>Cook                        Standards Track                     [Page 8]

RFC 5593               IMAP URL Access Identifier              June 20096.5.  Authuser Application Registration   To: iana@iana.org   Subject: IMAP URL Access Identifier Registration   Type:          <access> identifier   Application:   authuser   Description:   Used to restrict access to IMAP sessions                  that are logged in as any non-anonymous                  user of that IMAP server   RFC Number:RFC 5593 andRFC 5092   Contact:       Lemonade WG <lemonade@ietf.org>6.6.  Anonymous Application Registration   To: iana@iana.org   Subject: IMAP URL Access Identifier Registration   Type:          <access> identifier   Application:   anonymous   Description:   Indicates that use of this URL is                  not restricted by session authorization                  identity   RFC Number:RFC 5593 andRFC 5092   Contact:       Lemonade WG <lemonade@ietf.org>7.  Security Considerations   The extension to <access> identifiers specified in this document   provides a mechanism for extending the semantics of the "submit+"   <access> prefix to arbitrary applications.  The use of such   additional <access> identifiers and prefixes is primarily for   security purposes, i.e., to prevent the overloading of "submit+" as a   generic mechanism to allow entities to retrieve IMAP URLs on behalf   of userids.  Other than this, the security implications are identical   to those discussed inSection 10.1 of IMAPURL [RFC5092].Cook                        Standards Track                     [Page 9]

RFC 5593               IMAP URL Access Identifier              June 20098.  References8.1.  Normative References   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC5092]   Melnikov, A., Ed., and C. Newman, "IMAP URL Scheme",RFC5092, November 2007.   [RFC5234]   Crocker, D., Ed., and P. Overell, "Augmented BNF for               Syntax Specifications: ABNF", STD 68,RFC 5234, January               2008.8.2.  Informative References   [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an               IANA Considerations Section in RFCs",BCP 26,RFC 5226,               May 2008.   [STREAMING] Cook, N.,"Streaming Internet Messaging Attachments",               Work in Progress, May 2009.Author's Address   Neil L Cook   Cloudmark   EMail: neil.cook@noware.co.ukCook                        Standards Track                    [Page 10]

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