Network Working Group M. Crispin
INTERNET-DRAFT: IMAP URLAUTH University of Washington
C. Newman
Sun Microsystems
Document: internet-drafts/draft-crispin-imap-urlauth-02.txt August 2003
Internet Message Access Protocol (IMAP) - URLAUTH Extension
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Abstract
This document describes the URLAUTH extension to the Internet
Message Access Protocol (IMAP) (RFC 3501) and the IMAP URL Scheme
(IMAPURL) (RFC 2192). This extension provides a means by which an
IMAP client can create "signed" URLs carrying authorization to
access limited message data on the IMAP server.
An IMAP server which supports this extension indicates this with a
capability name of "URLAUTH".
Conventions Used in this Document
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in [KEYWORDS].
The formal syntax use the Augmented Backus-Naur Form (ABNF)
notation including the core rules defined in Appendix A of [ABNF].
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 the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange.
Introduction
The URLAUTH extension provides a mechanism by which an authorized
user of an IMAP server can create signed IMAP URLs. A signed URL
conveys authorization (not authentication) to its data, and can be
used by others to access specific content on the IMAP server without
being provided with unlimited access to the user's IMAP data.
URL authentication is performed as specified in [IMAPURL]; for example,
a URL of the form imap://fred@example.com requires authentication as
userid "fred". The absense of [IMAPURL] authentication indicates that
the URL is available to anonymous sessions.
A URLAUTH signed URL can be used in the argument to the BURL command
in message composition, as described in [COMPOSE], for such purposes
as a small client submitting a message forward or resend from an IMAP
mailbox without requiring the client to fetch that message data.
IMAP URL Extensions
[IMAPURL] is extended by allowing the addition of ";EXPIRE=<datetime>",
";BEHALF=<userid>", and ";URLAUTH=<authkey>" to the end of IMAP URLs
which refer to specific message or message parts.
This facility does not apply to IMAP URLs that refer to lists of
mailboxes or messages.
When ";EXPIRE=<datetime>" is used, this indicates the latest date and
time that the URL is valid. After that date and time, the URL has
expired and server implementations MUST reject the URL. If
";EXPIRE=<datetime>" is not used, the URL has no expiration, but still
can be revoked as discussed below.
When ";BEHALF=<userid>" is used, this indicates that this URL is only
to be used on behalf of the indicated user. This is not enforced by
the IMAP server, but rather by the client that issues the URLFETCH (see
below) such as a [COMPOSE] server.
DISCUSSION: Consider an environment in which userid fred issues a
URLAUTH signed URL allowing user outbox to access the data. User
"outbox", for the purpose of this discussion, is the well-known
userid used by the [COMPOSE] server to authenticate to the IMAP
server. Userid joe intercepts the submission of the signed URL to
the [COMPOSE] server, and submits a request to [COMPOSE] server
with that URL and a destination of joe's choosing. If the URL
included ";BEHALF=fred", this attack would not work, since the
[COMPOSE] server would not accept a URL on fred's behalf from joe.
When ";URLAUTH=<authkey>" is used, this indicates a BASE64-encoded
authentication key which is used to authorize the URL. The calculation
of the authentication key is discussed below.
Authentication Key Calculation and Validation
Associated with every mailbox is a mailbox access generation key, which is
automatically generated and assigned by the IMAP server. An IMAP
client can request the server to generate and assign a new mailbox
access geneation key (thus effectively revoking all current URLs using
URLAUTH with that mailbox access generation key) but can not set it to
a key of its own choosing.
The authentication key is calculated as follows. An initial URL is
built with no ";URLAUTH=<authkey>" field. An [HMAC-MD5] key is
calculated, with the initial URL as the data and the mailbox private
key as the key. The signed URL is created by taking the initial URL
and appending ";URLAUTH=" followed by the BASE64 (as described in
[MIME-IMB]) representation of the first 120 bits of the calculated
[HMAC-MD5] key.
Note: only the first 120 bits of the [HMAC-MD5] key, rather than
the full 128 bits, is used so that the BASE64 representation does
not have any BASE64 "=" padding characters (which would be
problematic in a URL).
A signed URL is validated as follows. The ";URLAUTH=<authkey>"
portion of the URL is extracted, and the <authkey> value saved as
the signature. The remainder of the URL minus the ";URLAUTH=<authkey>"
(the "rump URL") is analyzed to identify the mailbox. If the mailbox
can not be identified, validation fails with no further checking. If
the mailbox can be identified, the mailbox access generation key for
the mailbox is appended to the rump URL, and an [HMAC-MD5] checksum
is calculated on the resulting string. Validation is successful if,
and only if, the supplied signature matches the first 120 bits of the
calculated [HMAC-MD5] checksum.
Note: removal of the ";URLAUTH=<authkey>" portion of the URL is
the only operation applied to the signed URL to get the rump URL.
In particular, URL percent escape decoding and case-folding MUST
NOT occur.
6.3.CHANGEKEY. CHANGEKEY Command
Arguments: mailbox name
Responses: none other than in result
Result: OK - changekey completed, new key in MAILBOXKEY status
response code
NO - changekey error: can't change key of that mailbox
BAD - command unknown or arguments invalid
The CHANGEKEY command requests the server to generate a new mailbox
access generation key for the given mailbox. If the command is
successful, a MAILBOXKEY status response code is returned in the
tagged OK result.
Any current IMAP session which has the mailbox selected will also
received an untagged OK response with the MAILBOXKEY status response
code.
If the [ACL] or [ACL2] extension is present, the user must have "a"
(administrative) rights to the mailbox in order to perform a CHANGEKEY.
CHANGEKEY has the effect of revoking all current URLAUTH signed URLs
on that mailbox.
6.3.URLFETCH. URLFETCH Command
Arguments: one or more URLs
Responses: untagged responses: URLFETCH
Result: OK - urlfetch completed
NO - urlfetch failed due to server internal error
BAD - command unknown or arguments invalid
The URLFETCH command requests that the server return the text data
associated with one or more IMAP URLs, as described in [IMAPURL]
and extended by this document. The data is returned for all
validated URLs, regardless of whether or not the session would
otherwise be able to access the mailbox containing that data via
SELECT or examine.
Note: This command does not require that the URL refer to the
selected mailbox; nor does it require that any mailbox be
selected. It does not in any way interfere with any selected
mailbox either.
The URLFETCH command MUST return an untagged URLFETCH response and
a tagged OK response to any URLFETCH command that is syntactically
valid. A NO response indicates a server internal failure which may
be resolved on later retry.
Note: the possibility of a NO response is to accomodate
implementations which would otherwise have to issue an
untagged BYE with a fatal error due to an inability to
respond to a valid request. In an ideal world, a server
SHOULD NOT issue a NO response.
If any of the URLs could not be fetched (the returned data string is
NIL for that URL in the URLFETCH response) then this command MUST
have a time delay of a minimum of three seconds before returning the
untagged URLFETCH response and the tagged OK response.
7.1.MAILBOXKEY. MAILBOXKEY Status Response Code
The MAILBOXKEY status response code is followed by the BASE64 encoded
form of the mailbox access generation key. This status response code
is returned in an untagged OK response in response to a CHANGEKEY,
SELECT, or EXAMINE command.
7.4.URLFETCH. URLFETCH Response
Contents: one or more URL/nstring pairs
The URLFETCH response returns the message text data associated with one or
more IMAP URLs, as described in [IMAPURL] and extended by this document.
This response occurs as the result of a URLFETCH command.
The returned data string is NIL if the URL is invalid for any reason
(including validation failure). If the URL is valid, but the IMAP
fetch of the body part returned NIL (this should not happen), the
returned data string should be the empty string ("") and not NIL.
Example
Note that lines which do not begin with "C:" or "S:" are continuations
of the previous line for typographical clarity and are not separate
lines in the protocol
C: a002 URLFETCH
"imap://joe@example.com/INBOX;uid=20;section=1.2;URLAUTH=kjsaddfkl"
S: * URLFETCH
"imap://joe@example.com/INBOX;uid=20;section=1.2;URLAUTH=kjsaddfkl"
{6}
S: foobar
S: a002 OK URLFETCH completed
Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [ABNF].
base64 = *(4base64-char)
base64-char = ALPHA / DIGIT / "+" / "/"
; Case-sensitive
The following modifications are made to the Formal Syntax in [IMAP]:
changekey = "CHANGEKEY" SP mailbox
resp-text-code =/ "MAILBOXKEY" SP base64
url = astring
; contains imapurl as defined below
urlfetch = "URLFETCH" SP 1*url
urlfetch-data = "*" SP "URLFETCH" 1*(url nstring)
The following modifications are made to the Formal Syntax in [IMAPURL]:
enc-urlauth = base64
; BASE64 as described in [MIME-IMB]
imapurl = "imap://" iserver "/" [icommand] [iextend]
iextend = [ibehalf] [iexpire] [iurlauth]
iexpire = ";EXPIRE=" date-time
; date-time defined in [DATETIME]
iurlauth = ";URLAUTH=" enc-urlauth
Security Considerations
Security considerations are discussed throughout this memo.
URLAUTH signed URLs with anonymous authentication can be used by anyone
to access the authorized data. Unless it is intended to provide public
access to the data authorized by the URLAUTH signed URL, anonymous
authenticated URLAUTH signed URLs SHOULD NOT be used, and MUST NOT be
transmitted over a non-encrypted communication path.
To simplify interoperability with a [COMPOSE] server, and to ensure
that only the [COMPOSE] server should use a URLAUTH signed URL, a
well-known authenticated userid for the URL SHOULD be defined and
used instead of anonymous authentication. The ";BEHALF=<userid>"
facility SHOULD also be used.
The MAILBOXKEY status response code exposes sensitive data. A server
implementation MUST implement a configuration that will not return
a MAILBOXKEY status response code unless:
(1) The STARTTLS command has been negotiated.
OR
(2) Some other mechanism is provided that protects the session
from snooping.
The three second time delay for a URLFETCH failure is necessary to
avoid attacks in which the server is probed to see if a particular
mailbox exists on the server by measuring the amount of time taken
to reject a known bad name vs. some other name. The time delay
SHOULD be variable to further frustrate analysis.
References
All references in this document are normative.
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[ACL] Myers, J., "IMAP4 ACL Extension", RFC 2086, January 1997.
[ACL2] Melnikov, A., "IMAP4 ACL Extension",
draft-ietf-imapext-acl-08.txt (work in progress), June 2003.
[COMPOSE] Newman, C., "Message Composition",
draft-newman-lemonade-compose-00.txt (work in progress),
June 2003.
[DATETIME] Klyne, G., and Newman, C., "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[IMAP] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
[IMAPURL] Newman, C., "IMAP URL Scheme", RFC 2192, September 1997.
[HMAC-MD5] Krawczyk, H., Bellare, M., and Canetti, R. "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[MD5] Rivest, R., "The MD5 Message Digest Algorithm",
RFC 1321, MIT Laboratory for Computer Science, April 1992.
[MIME-IMB] Freed, N. and N. Borenstein, "MIME (Multipurpose Internet
Mail Extensions) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
Author's Addresses
Mark R. Crispin
Networks and Distributed Computing
University of Washington
4545 15th Avenue NE
Seattle, WA 98105-4527
Phone: (206) 543-5762
EMail: MRC@CAC.Washington.EDU
Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790
EMail: chris.newman@sun.com
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