Network Working Group N. Cook
Request for Comments: 5593 Cloudmark
Updates: 5092 June 2009
Category: Standards Track
Internet Message Access Protocol (IMAP) -
URL Access Identifier Extension
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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document authors. All rights reserved.
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Abstract
The existing IMAP URL specification (RFC 5092) lists several
identifiers and 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 identifiers
as well as an IANA mechanism to register new identifiers for
future applications.
This document updates RFC 5092.
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RFC 5593 IMAP URL Access Identifier June 2009
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. Additional Authorized Access Identifiers ........................3
3.1. Existing Access Identifiers ................................3
3.2. Requirement for Additional Access Identifiers ..............3
3.3. Additional Access Identifier Specification .................4
3.4. Defining an Access Identifier for Streaming ................5
4. Formal Syntax ...................................................5
5. Acknowledgements ................................................6
6. IANA Considerations .............................................6
6.1. Access Identifier Registration Template ....................7
6.2. Stream Application Registration ............................7
6.3. Submit Application Registration ............................8
6.4. User Application Registration ..............................8
6.5. Authuser Application Registration ..........................9
6.6. Anonymous Application Registration .........................9
7. Security Considerations .........................................9
8. References .....................................................10
8.1. Normative References ......................................10
8.2. Informative References ....................................10
1. Introduction
The IMAP URL specification [RFC5092] provides a way to carry
authorization information in IMAP URLs. Several authorization
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 identifiers, and
overloading the existing mechanisms has security as well as
administrative implications.
This document describes a new 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 identifiers for future applications. IANA definitions
for the existing access identifiers and prefixes from RFC 5092 are
also defined in this document -- this document updates RFC 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 in RFC 2119 [RFC2119].
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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 Identifiers
3.1. Existing Access Identifiers
The IMAP URL specification [RFC5092] specifies the following
authorized 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 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 identifiers are suitable for user-based
authorization, but only the "submit+" 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+" 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+" 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.
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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
identifiers that are created.
Additional identifiers MUST take one of two forms (Section 4
gives the formal ABNF syntax):
o identifier - The name of the application, e.g.,
"exampleapp".
o 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 identifier name can also be registered as an
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 identifier or 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 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.
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The application entity itself MAY choose to perform validation on any
specified userid before attempting to retrieve the URL.
The authorization granted by any 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 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
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].
See Section 6.2 for the IANA registration template for the "stream"
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
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:
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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 from RFC
5092 as well as the entry for the "stream" application.
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6.1. Access Identifier Registration Template
To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: [Either " identifier" or
" 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: identifier
Application: stream
Description: Used by SIP Media Servers to retrieve
attachments for streaming to email
clients
RFC Number: RFC 5593
Contact: Neil Cook
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6.3. Submit Application Registration
To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: identifier prefix
Application: submit
Description: Used by message submission entities to
retrieve attachments to be included in
submitted messages
RFC Number: RFC 5593 and RFC 5092
Contact: Lemonade WG
6.4. User Application Registration
To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: identifier prefix
Application: user
Description: Used to restrict access to IMAP sessions
that are logged in as the specified userid
RFC Number: RFC 5593 and RFC 5092
Contact: Lemonade WG
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6.5. Authuser Application Registration
To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: 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 and RFC 5092
Contact: Lemonade WG
6.6. Anonymous Application Registration
To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: identifier
Application: anonymous
Description: Indicates that use of this URL is
not restricted by session authorization
identity
RFC Number: RFC 5593 and RFC 5092
Contact: Lemonade WG
7. Security Considerations
The extension to identifiers specified in this document
provides a mechanism for extending the semantics of the "submit+"
prefix to arbitrary applications. The use of such
additional 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 in Section 10.1 of IMAPURL [RFC5092].
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8. References
8.1. 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", RFC
5092, 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.uk
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RFC, FYI, BCP