Lemonade N. Cook
Internet-Draft Cloudmark
Intended status: Standards Track December 8, 2008
Expires: June 11, 2009
Internet Message Access Protocol (IMAP) - URL Access Identifier
Extension
draft-ncook-urlauth-accessid-00
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Abstract
The existing IMAP URL specification (RFC5092) 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in this Document . . . . . . . . . . . . . . . 3
3. Additional Authorized Access Identifiers . . . . . . . . . . . 3
3.1. Existing Access Identifiers . . . . . . . . . . . . . . . . 3
3.2. Requirement for Additional Access Identifiers . . . . . . . 4
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 . . . . . . . . . . 6
6.2. Stream Application Registration . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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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, which 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.
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, IMAPURL [RFC5092], specifies the
following authorized <access> identifiers:
o "authuser" - Indicating that use of this URL is limited to
authenticated IMAP sessions that are logged in as any non-
anonymous user
o "anonymous" - Indicating that use of this URL is not restricted by
session authorization identity
Additionally the following <access> identifier prefixes are defined:
o "submit+" - Followed by a userid, indicating that only a userid
authorized as a message submission entity on behalf of the
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specified userid is permitted to use this URL
o "user+" - Followed by a userid, indicating 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 users, which 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+" <ccess> 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 established 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. "stream"
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o <access> identifier prefix - The name of the application e.g.
"stream+" followed by a user id. For example "stream+ncook".
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. The IMAP server SHALL NOT validate any
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 the IMAP server
to provide an extensible mechanism for associating userids with new
applications. For example, imgine a new application "foo" is
created, which requires application entities to retrieve URLs on
behalf of users. In this case, the IMAP server would need to provide
a way to register a 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 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 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.
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The ABNF specified below updates and replaces the formal syntax of
<access> identifier as defined in IMAP URL [RFC5092].
application = *ALPHA
access = ("submit+" enc-user) / ("user+" enc-user) /
"authuser" / "anonymous" / application /
(application "+" enc-user)
5. Acknowledgements
This document was inspired by discussions in the Lemonade Working
Group.
6. IANA Considerations
Access identifiers and prefixes MUST be specified in a standards
track or IESG approved experimental RFC.
6.1. Access Identifier Registration Template
To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: [Either "access identifier" or
"access 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 that the application was
defined in ]
Contact: [email and/or physical address to contact for
additional information]
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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: This RFC
Contact: Neil Cook mailto:neil.cook@noware.co.uk
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. However, the security implications are identical to that
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. and C. Newman, "IMAP URL Scheme", RFC 5092,
November 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
8.2. Informative References
[STREAMING]
Cook, N., "Streaming Internet Messaging Attachments",
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draft-ietf-lemonade-streaming-08.txt (Work in Progress) ,
Dec 2008.
Author's Address
Neil L Cook
Cloudmark
Email: neil.cook@noware.co.uk
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