IMAP URL access IDs: Internet Message Access Protocol (IMAP) - URL Access Identifier Extension
New access identifiers for URLAUTH IMAP URLs, so services such as streaming can fetch mail on a user's behalf without abusing submit+.
Cook, N., "Internet Message Access Protocol (IMAP) - URL Access Identifier Extension", RFC 5593, DOI 10.17487/RFC5593, June 2009, <https://www.rfc-editor.org/info/rfc5593>.
@misc{rfc5593,
series = {Request for Comments},
number = 5593,
howpublished = {RFC 5593},
publisher = {RFC Editor},
doi = {10.17487/RFC5593},
url = {https://www.rfc-editor.org/info/rfc5593},
author = {N. Cook},
title = {{Internet Message Access Protocol (IMAP) - URL Access Identifier Extension}},
pagetotal = 10,
year = 2009,
month = jun,
} RFC text
Contents
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.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 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 updates RFC 5092.
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
<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 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].
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 <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.
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.
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].
See Section 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:
- 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
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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,
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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.
6.1. 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>
6.3. 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 and RFC 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 and RFC 5092
Contact: Lemonade WG <lemonade@ietf.org>
6.5. 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 and RFC 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 and RFC 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 in Section 10.1 of IMAPURL [RFC5092].
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
AI ELI5
On this page
§1Why More Access Identifiers
The IMAP URL specification (RFC 5092) lets URLs carry authorization information through URLAUTH. An <access> identifier in the URL limits who may use it, for example only anonymous users, authenticated users or message submission entities.
RFC 5593 points out that there was no way to create new identifiers, and that overloading the existing ones has security and administrative implications. It adds a new identifier, stream, for message streaming entities, and an IANA registration template for future ones.
It updates RFC 5092 and should be taken as the master if the two differ on the IANA definitions of the existing identifiers.
A ticket to a mail item can already say who may use it. This adds a way to invent new kinds of ticket for new jobs.
§3.1The Existing Identifiers
RFC 5092 defines two identifiers. authuser limits a URL to authenticated IMAP sessions logged in as any non-anonymous user. anonymous means use is not restricted by the session's authorization identity.
It also defines two prefixes that are followed by a userid. submit+ means only a userid authorized as a message submission entity on behalf of that user may use the URL. user+ limits use to IMAP sessions logged in as the specified userid.
There were four kinds of ticket: anyone, any logged-in person, one named person, or the post office working for a named person.
| Application | Type | Used for |
|---|---|---|
| stream THIS RFC | identifier | SIP Media Servers retrieving attachments for streaming to email clients |
| submit | prefix | Message submission entities retrieving attachments for submitted messages |
| user | prefix | Sessions logged in as the specified userid |
| authuser | identifier | Sessions logged in as any non-anonymous user |
| anonymous | identifier | No restriction by session authorization identity |
§3.2Why submit+ Is Not Enough
The existing identifiers suit user-based authorization, but only submit+ covers entities acting for a user. New services such as streaming need generic support for such external entities, and the RFC gives three reasons not to stretch submit+ for that.
- Security: the IMAP server keeps a list of submission servers allowed to fetch
submit+URLs. Adding every possible external entity to that list would increase the attack surface. - Administration: when such an entity presents a URLAUTH URL, the server logs do not show what it was for.
- Resourcing: without a way to tell applications apart, the server cannot prioritize resources for particular ones.
If every helper had to use the post office's ticket, the post office's guest list would get long and risky, and nobody could tell what each helper was doing.
§3.3How New Identifiers Work
An additional identifier MUST take one of two forms. It is either an access identifier, which is the application name such as exampleapp, or an access identifier prefix, which is the name followed by + and a userid, such as exampleapp3+testuser. A name can be registered in both forms, but that needs two separate IANA registrations.
The semantics match submit+: only a userid authorized as an application entity for that application may use the URL. For prefixes, the IMAP server SHALL NOT validate the userid in the URL, but MUST validate that the IMAP session's authorization identity is authorized as an application entity for that application. The application entity itself MAY validate the userid before fetching.
The server therefore needs an extensible way to register applications and associate userids with them. If an application foo is created, URLs with the identifier foo are checked against the list of userids associated with foo.
Each helper job gets its own named guest list. A ticket stamped with that job's name only works for people on that job's list.
§4Formal Syntax and the stream Identifier
The ABNF extends the access rule from RFC 5092 with an application made of one or more letters or digits, either on its own or followed by + and an encoded user. Strings are case-insensitive, and implementations MUST accept them that way.
The first application is streaming of multimedia files received as messaging attachments. The stream identifier is registered for it, and it is described as used by SIP Media Servers to retrieve attachments for streaming to email clients.
The rule is simple: a job name, optionally followed by a plus and a person's name. The first new job name is stream.
§6The IANA Registry
IANA created a registry for IMAP URLAUTH access identifiers and prefixes. New entries MUST be registered under the IETF Review policy. A registration gives the type (identifier or prefix), the application name, a description, the RFC number and a contact.
The RFC also supplies the entries for the existing identifiers and prefixes from RFC 5092 and for stream.
There is now an official book where each new ticket kind is written down, with what it is for and who to ask about it.
§7Security Considerations
The extension generalizes the submit+ idea to arbitrary applications. Its main purpose is security: to avoid overloading submit+ as a generic way for entities to fetch IMAP URLs on behalf of userids. Otherwise the security implications are the same as those in Section 10.1 of RFC 5092.
Giving each job its own list is safer than putting every helper on one big list.