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docker-offlineimap/docs/rfcs/rfc4467.IMAP_URLAUTH_extension.txt
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Network Working Group M. Crispin
Request for Comments: 4467 University of Washington
Updates: 3501 May 2006
Category: Standards Track
Internet Message Access Protocol (IMAP) - URLAUTH 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) The Internet Society (2006).
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 use URLs carrying authorization to access limited message data on
the IMAP server.
An IMAP server that supports this extension indicates this with a
capability name of "URLAUTH".
1. Introduction
In [IMAPURL], a URL of the form imap://fred@example.com/INBOX/;uid=20
requires authorization as userid "fred". However, [IMAPURL] implies
that it only supports authentication and confuses the concepts of
authentication and authorization.
The URLAUTH extension defines an authorization mechanism for IMAP
URLs to replace [IMAPURL]'s authentication-only mechanism. URLAUTH
conveys authorization in the URL string itself and reuses a portion
of the syntax of the [IMAPURL] authentication mechanism to convey the
authorization identity (which also defines the default namespace in
[IMAP]).
The URLAUTH extension provides a means by which an authorized user of
an IMAP server can create URLAUTH-authorized IMAP URLs. A URLAUTH-
authorized URL conveys authorization (not authentication) to the data
Crispin Standards Track [Page 1]
RFC 4467 IMAP - URLAUTH Extension May 2006
addressed by that URL. This URL can be used in another IMAP session
to access specific content on the IMAP server, without otherwise
providing authorization to any other data (such as other data in the
mailbox specified in the URL) owned by the authorizing user.
Conceptually, a URLAUTH-authorized URL can be thought of as a "pawn
ticket" that carries no authentication information and can be
redeemed by whomever presents it. However, unlike a pawn ticket,
URLAUTH has optional mechanisms to restrict the usage of a URLAUTH-
authorized URL. Using these mechanisms, URLAUTH-authorized URLs can
be usable by:
. anonymous (the "pawn ticket" model)
. authenticated users only
. a specific authenticated user only
. message submission acting on behalf of a specific user only
There is also a mechanism for expiration.
A URLAUTH-authorized URL can be used in the argument to the BURL
command in message composition, as described in [BURL], for such
purposes as allowing a client (with limited memory or other
resources) to submit a message forward or to resend from an IMAP
mailbox without requiring the client to fetch that message data.
The URLAUTH is generated using an authorization mechanism name and an
authorization token, which is generated using a secret mailbox access
key. An IMAP client can request that the server generate and assign
a new mailbox access key (thus effectively revoking all current URLs
using URLAUTH with the old mailbox access key) but cannot set the
mailbox access key to a key of its own choosing.
1.1. 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 uses 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.
Crispin Standards Track [Page 2]
RFC 4467 IMAP - URLAUTH Extension May 2006
2. Concepts
2.1. URLAUTH
The URLAUTH is a component, appended at the end of a URL, that
conveys authorization to access the data addressed by that URL. It
contains an authorized access identifier, an authorization mechanism
name, and an authorization token. The authorization token is
generated from the URL, the authorized access identifier, the
authorization mechanism name, and a mailbox access key.
2.2. Mailbox Access Key
The mailbox access key is a random string with at least 128 bits of
entropy. It is generated by software (not by the human user) and
MUST be unpredictable.
Each user has a table of mailboxes and an associated mailbox access
key for each mailbox. Consequently, the mailbox access key is per-
user and per-mailbox. In other words, two users sharing the same
mailbox each have a different mailbox access key for that mailbox,
and each mailbox accessed by a single user also has a different
mailbox access key.
2.3. Authorized Access Identifier
The authorized access identifier restricts use of the URLAUTH
authorized URL to certain users authorized on the server, as
described in section 3.
2.4. Authorization Mechanism
The authorization mechanism is the algorithm by which the URLAUTH is
generated and subsequently verified, using the mailbox access key.
2.4.1. INTERNAL Authorization Mechanism
This specification defines the INTERNAL mechanism, which uses a token
generation algorithm of the server's choosing and does not involve
disclosure of the mailbox access key to the client.
Note: The token generation algorithm chosen by the server
implementation should be modern and reasonably secure. At the
time of the writing of this document, an [HMAC] such as HMAC-SHA1
is recommended.
Crispin Standards Track [Page 3]
RFC 4467 IMAP - URLAUTH Extension May 2006
If it becomes necessary to change the token generation algorithm
of the INTERNAL mechanism (e.g., because an attack against the
current algorithm has been discovered), all currently existing
URLAUTH-authorized URLs are invalidated by the change in
algorithm. Since this would be an unpleasant surprise to
applications that depend upon the validity of a URLAUTH-authorized
URL, and there is no good way to do a bulk update of existing
deployed URLs, it is best to avoid this situation by using a
secure algorithm as opposed to one that is "good enough".
Server implementations SHOULD consider the possibility of changing
the algorithm. In some cases, it may be desirable to implement
the change of algorithm in a way that newly-generated tokens use
the new algorithm, but that for a limited period of time tokens
using either the new or old algorithm can be validated.
Consequently, the server SHOULD incorporate some means of
identifying the token generation algorithm within the token.
Although this specification is extensible for other mechanisms, none
are defined in this document. In addition to the mechanism name
itself, other mechanisms may have mechanism-specific data, which is
to be interpreted according to the definition of that mechanism.
2.5. Authorization Token
The authorization token is a deterministic string of at least 128
bits that an entity with knowledge of the secret mailbox access key
and URL authorization mechanism can use to verify the URL.
3. IMAP URL Extensions
[IMAPURL] is extended by allowing the addition of
";EXPIRE=<datetime>" and ";URLAUTH=<access>:<mech>:<token>" to IMAP
URLs that refer to a specific message or message parts.
The URLAUTH is comprised of ";URLAUTH=<access>:<mech>:<token>" and
MUST be at the end of the URL.
URLAUTH does not apply to, and MUST NOT be used with, any IMAP URL
that refers to an entire IMAP server, a list of mailboxes, an entire
IMAP mailbox, or IMAP search results.
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.
Crispin Standards Track [Page 4]
RFC 4467 IMAP - URLAUTH Extension May 2006
The URLAUTH takes the form ";URLAUTH=<access>:<mech>:<token>". It is
composed of three parts. The <access> portion provides the
authorized access identifiers, which may constrain the operations and
users that are permitted to use this URL. The <mech> portion
provides the authorization mechanism used by the IMAP server to
generate the authorization token that follows. The <token> portion
provides the authorization token.
The "submit+" access identifier prefix, 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. The IMAP server does not validate the specified userid but does
validate that the IMAP session has an authorization identity that is
authorized as a message submission entity. The authorized message
submission entity MUST validate the userid prior to contacting the
IMAP server.
The "user+" access identifier prefix, followed by a userid, indicates
that use of this URL is limited to IMAP sessions that are logged in
as the specified userid (that is, have authorization identity as that
userid).
Note: If a SASL mechanism that provides both authorization and
authentication identifiers is used to authenticate to the IMAP
server, the "user+" access identifier MUST match the authorization
identifier.
The "authuser" access identifier indicates that use of this URL is
limited to IMAP sessions that are logged in as an authorized user
(that is, have authorization identity as an authorized user) of that
IMAP server. Use of this URL is prohibited to anonymous IMAP
sessions.
The "anonymous" access identifier indicates that use of this URL is
not restricted by session authorization identity; that is, any IMAP
session in authenticated or selected state (as defined in [IMAP]),
including anonymous sessions, may issue a URLFETCH using this URL.
The authorization token is represented as an ASCII-encoded
hexadecimal string, which is used to authorize the URL. The length
and the calculation of the authorization token depends upon the
mechanism used; but, in all cases, the authorization token is at
least 128 bits (and therefore at least 32 hexadecimal digits).
Crispin Standards Track [Page 5]
RFC 4467 IMAP - URLAUTH Extension May 2006
4. Discussion of URLAUTH Authorization Issues
In [IMAPURL], the userid before the "@" in the URL has two purposes:
1) It provides context for user-specific mailbox paths such as
"INBOX".
2) It specifies that resolution of the URL requires logging in as
that user and limits use of that URL to only that user.
An obvious limitation of using the same field for both purposes is
that the URL can only be resolved by the mailbox owner.
URLAUTH overrides the second purpose of the userid in the IMAP URL
and by default permits the URL to be resolved by any user permitted
by the access identifier.
The "user+<userid>" access identifier limits resolution of that URL
to a particular userid, whereas the "submit+<userid>" access
identifier is more general and simply requires that the session be
authorized by a user that has been granted a "submit" role within the
authentication system. Use of either of these access identifiers
makes it impossible for an attacker, spying on the session, to use
the same URL, either directly or by submission to a message
submission entity.
The "authuser" and "anonymous" access identifiers do not have this
level of protection and should be used with caution. These access
identifiers are primarily useful for public export of data from an
IMAP server, without requiring that it be copied to a web or
anonymous FTP server. Refer to the Security Considerations for more
details.
5. Generation of URLAUTH-Authorized URLs
A URLAUTH-authorized URL is generated from an initial URL as follows:
An initial URL is built, ending with ";URLAUTH=<access>" but without
the ":<mech>:<token>" components. An authorization mechanism is
selected and used to calculate the authorization token, with the
initial URL as the data and a secret known to the IMAP server as the
key. The URLAUTH-authorized URL is generated by taking the initial
URL and appending ":", the URL authorization mechanism name, ":", and
the ASCII-encoded hexadecimal representation of the authorization
token.
Crispin Standards Track [Page 6]
RFC 4467 IMAP - URLAUTH Extension May 2006
Note: ASCII-encoded hexadecimal is used instead of BASE64 because
a BASE64 representation may have "=" padding characters, which
would be problematic in a URL.
In the INTERNAL mechanism, the mailbox access key for that mailbox is
the secret known to the IMAP server, and a server-selected algorithm
is used as described in section 2.4.1.
6. Validation of URLAUTH-authorized URLs
A URLAUTH-authorized URL is validated as follows:
The URL is split at the ":" that separates "<access>" from
"<mech>:<token>" in the ";URLAUTH=<access>:<mech>:<token>" portion of
the URL. The "<mech>:<token>" portion is first parsed and saved as
the authorization mechanism and the authorization token. The URL is
truncated, discarding the ":" described above, to create a "rump URL"
(the URL minus the ":" and the "<mech>:<token>" portion). The rump
URL is then analyzed to identify the mailbox.
If the mailbox cannot be identified, an authorization token is
calculated on the rump URL, using random "plausible" keys (selected
by the server) as needed, before returning a validation failure.
This prevents timing attacks aimed at identifying mailbox names.
If the mailbox can be identified, the authorization token is
calculated on the rump URL and a secret known to the IMAP server
using the given URL authorization mechanism. Validation is
successful if, and only if, the calculated authorization token for
that mechanism matches the authorization token supplied in
";URLAUTH=<access>:<mech>:<token>".
Removal of the ":<mech>:<token>" portion of the URL MUST be the only
operation applied to the URLAUTH-authorized URL to get the rump URL.
In particular, URL percent escape decoding and case-folding
(including to the domain part of the URL) MUST NOT occur.
In the INTERNAL mechanism, the mailbox access key for that mailbox is
used as the secret known to the IMAP server, and the same server-
selected algorithm used for generating URLs is used to calculate the
authorization token for verification.
Crispin Standards Track [Page 7]
RFC 4467 IMAP - URLAUTH Extension May 2006
7. Additional Commands
These commands are extensions to the [IMAP] base protocol.
The section headings of these commands are intended to correspond
with where they would be located in the base protocol document if
they were part of that document.
BASE.6.3.RESETKEY. RESETKEY Command
Arguments: optional mailbox name
optional mechanism name(s)
Responses: none other than in result
Result: OK - RESETKEY completed, URLMECH containing new data
NO - RESETKEY error: can't change key of that mailbox
BAD - command unknown or arguments invalid
The RESETKEY command has two forms.
The first form accepts a mailbox name as an argument and generates a
new mailbox access key for the given mailbox in the user's mailbox
access key table, replacing any previous mailbox access key (and
revoking any URLs that were authorized with a URLAUTH using that key)
in that table. By default, the mailbox access key is generated for
the INTERNAL mechanism; other mechanisms can be specified with the
optional mechanism argument.
The second form, with no arguments, removes all mailbox access keys
in the user's mailbox access key table, revoking all URLs currently
authorized using URLAUTH by the user.
Any current IMAP session logged in as the user that has the mailbox
selected will receive an untagged OK response with the URLMECH status
response code (see section BASE.7.1.URLMECH for more details about
the URLMECH status response code).
Example:
C: a31 RESETKEY
S: a31 OK All keys removed
C: a32 RESETKEY INBOX
S: a32 OK [URLMECH INTERNAL] mechs
C: a33 RESETKEY INBOX XSAMPLE
S: a33 OK [URLMECH INTERNAL XSAMPLE=P34OKhO7VEkCbsiYY8rGEg==] done
Crispin Standards Track [Page 8]
RFC 4467 IMAP - URLAUTH Extension May 2006
BASE.6.3.GENURLAUTH. GENURLAUTH Command
Argument: one or more URL/mechanism pairs
Response: untagged response: GENURLAUTH
Result: OK - GENURLAUTH completed
NO - GENURLAUTH error: can't generate a URLAUTH
BAD - command unknown or arguments invalid
The GENURLAUTH command requests that the server generate a URLAUTH-
authorized URL for each of the given URLs using the given URL
authorization mechanism.
The server MUST validate each supplied URL as follows:
(1) The mailbox component of the URL MUST refer to an existing
mailbox.
(2) The server component of the URL MUST contain a valid userid
that identifies the owner of the mailbox access key table that
will be used to generate the URLAUTH-authorized URL. As a
consequence, the iserver rule of [IMAPURL] is modified so that
iuserauth is mandatory.
Note: the server component of the URL is generally the
logged in userid and server. If not, then the logged in
userid and server MUST have owner-type access to the
mailbox access key table owned by the userid and server
indicated by the server component of the URL.
(3) There is a valid access identifier that, in the case of
"submit+" and "user+", will contain a valid userid. This
userid is not necessarily the same as the owner userid
described in (2).
(4) The server MAY also verify that the iuid and/or isection
components (if present) are valid.
If any of the above checks fail, the server MUST return a tagged BAD
response with the following exception. If an invalid userid is
supplied as the mailbox access key owner and/or as part of the access
identifier, the server MAY issue a tagged OK response with a
generated mailbox key that always fails validation when used with a
URLFETCH command. This exception prevents an attacker from
validating userids.
Crispin Standards Track [Page 9]
RFC 4467 IMAP - URLAUTH Extension May 2006
If there is currently no mailbox access key for the given mailbox in
the owner's mailbox access key table, one is automatically generated.
That is, it is not necessary to use RESETKEY prior to first-time use
of GENURLAUTH.
If the command is successful, a GENURLAUTH response code is returned
listing the requested URLs as URLAUTH-authorized URLs.
Examples:
C: a775 GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2" INTERNAL
S: a775 BAD missing access identifier in supplied URL
C: a776 GENURLAUTH "imap://example.com/Shared/;uid=20/
;section=1.2;urlauth=submit+fred" INTERNAL
S: a776 BAD missing owner username in supplied URL
C: a777 GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2;urlauth=submit+fred" INTERNAL
S: * GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/;section=1.2
;urlauth=submit+fred:internal:91354a473744909de610943775f92038"
S: a777 OK GENURLAUTH completed
BASE.6.3.URLFETCH. URLFETCH Command
Argument: one or more URLs
Response: untagged response: 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 the specified 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 also does not in any way interfere with any selected
mailbox.
Crispin Standards Track [Page 10]
RFC 4467 IMAP - URLAUTH Extension May 2006
The URLFETCH command effectively executes with the access of the
userid in the server component of the URL (which is generally the
userid that issued the GENURLAUTH). By itself, the URLAUTH does NOT
grant access to the data; once validated, it grants whatever access
to the data is held by the userid in the server component of the URL.
That access may have changed since the GENURLAUTH was done.
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 that may be
resolved on later retry.
Note: The possibility of a NO response is to accommodate
implementations that 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.
The server MUST return NIL for any IMAP URL that references an entire
IMAP server, a list of mailboxes, an entire IMAP mailbox, or IMAP
search results.
Example:
Note: For clarity, this example uses the LOGIN command, which
SHOULD NOT be used over a non-encrypted communication path.
This example is of a submit server, obtaining a message segment
for a message that it has already validated was submitted by
"fred".
S: * OK [CAPABILITY IMAP4REV1 URLAUTH] example.com IMAP server
C: a001 LOGIN submitserver secret
S: a001 OK submitserver logged in
C: a002 URLFETCH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2;urlauth=submit+fred:internal
:91354a473744909de610943775f92038"
S: * URLFETCH "imap://joe@example.com/INBOX/;uid=20/;section=1.2
;urlauth=submit+fred:internal
:91354a473744909de610943775f92038" {28}
S: Si vis pacem, para bellum.
S:
S: a002 OK URLFETCH completed
Crispin Standards Track [Page 11]
RFC 4467 IMAP - URLAUTH Extension May 2006
8. Additional Responses
These responses are extensions to the [IMAP] base protocol.
The section headings of these responses are intended to correspond
with where they would be located in the base protocol document if
they were part of that document.
BASE.7.1.URLMECH. URLMECH Status Response Code
The URLMECH status response code is followed by a list of URL
authorization mechanism names. Mechanism names other than INTERNAL
may be appended with an "=" and BASE64-encoded form of mechanism-
specific data.
This status response code is returned in an untagged OK response in
response to a RESETKEY, SELECT, or EXAMINE command. In the case of
the RESETKEY command, this status response code can be sent in the
tagged OK response instead of requiring a separate untagged OK
response.
Example:
C: a33 RESETKEY INBOX XSAMPLE
S: a33 OK [URLMECH INTERNAL XSAMPLE=P34OKhO7VEkCbsiYY8rGEg==] done
In this example, the server supports the INTERNAL mechanism and an
experimental mechanism called XSAMPLE, which also holds some
mechanism-specific data (the name "XSAMPLE" is for illustrative
purposes only).
BASE.7.4.GENURLAUTH. GENURLAUTH Response
Contents: One or more URLs
The GENURLAUTH response returns the URLAUTH-authorized URL(s)
requested by a GENURLAUTH command.
Example:
C: a777 GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2;urlauth=submit+fred" INTERNAL
S: * GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/;section=1.2
;urlauth=submit+fred:internal:91354a473744909de610943775f92038"
S: a777 OK GENURLAUTH completed
Crispin Standards Track [Page 12]
RFC 4467 IMAP - URLAUTH Extension May 2006
BASE.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.
Note: This command does not require that the URL refer to the
selected mailbox; nor does it require that any mailbox be
selected. It also does not in any way interfere with any selected
mailbox.
Example:
C: a002 URLFETCH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2;urlauth=submit+fred:internal
:91354a473744909de610943775f92038"
S: * URLFETCH "imap://joe@example.com/INBOX/;uid=20/;section=1.2
;urlauth=submit+fred:internal
:91354a473744909de610943775f92038" {28}
S: Si vis pacem, para bellum.
S:
S: a002 OK URLFETCH completed
9. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [ABNF].
The following modifications are made to the Formal Syntax in [IMAP]:
resetkey = "RESETKEY" [SP mailbox *(SP mechanism)]
capability =/ "URLAUTH"
command-auth =/ resetkey / genurlauth / urlfetch
resp-text-code =/ "URLMECH" SP "INTERNAL" *(SP mechanism ["=" base64])
genurlauth = "GENURLAUTH" 1*(SP url-rump SP mechanism)
genurlauth-data = "*" SP "GENURLAUTH" 1*(SP url-full)
Crispin Standards Track [Page 13]
RFC 4467 IMAP - URLAUTH Extension May 2006
url-full = astring
; contains authimapurlfull as defined below
url-rump = astring
; contains authimapurlrump as defined below
urlfetch = "URLFETCH" 1*(SP url-full)
urlfetch-data = "*" SP "URLFETCH" 1*(SP url-full SP nstring)
The following extensions are made to the Formal Syntax in [IMAPURL]:
authimapurl = "imap://" enc-user [iauth] "@" hostport "/"
imessagepart
; replaces "imapurl" and "iserver" rules for
; URLAUTH authorized URLs
authimapurlfull = authimapurl iurlauth
authimapurlrump = authimapurl iurlauth-rump
enc-urlauth = 32*HEXDIG
enc-user = 1*achar
; same as "enc_user" in RFC 2192
iurlauth = iurlauth-rump ":" mechanism ":" enc-urlauth
iurlauth-rump = [expire] ";URLAUTH=" access
access = ("submit+" enc-user) / ("user+" enc-user) /
"authuser" / "anonymous"
expire = ";EXPIRE=" date-time
; date-time defined in [DATETIME]
mechanism = "INTERNAL" / 1*(ALPHA / DIGIT / "-" / ".")
; case-insensitive
; new mechanisms MUST be registered with IANA
Crispin Standards Track [Page 14]
RFC 4467 IMAP - URLAUTH Extension May 2006
10. Security Considerations
Security considerations are discussed throughout this memo.
The mailbox access key SHOULD have at least 128 bits of entropy
(refer to [RANDOM] for more details) and MUST be unpredictable.
The server implementation of the INTERNAL mechanism SHOULD consider
the possibility of needing to change the token generation algorithm,
and SHOULD incorporate some means of identifying the token generation
algorithm within the token.
The URLMECH status response code may expose sensitive data in the
mechanism-specific data for mechanisms other than INTERNAL. A server
implementation MUST implement a configuration that will not return a
URLMECH status response code unless some mechanism is provided that
protects the session from snooping, such as a TLS or SASL security
layer that provides confidentiality protection.
The calculation of an authorization token with a "plausible" key if
the mailbox can not be identified 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 versus some other name.
To protect against a computational denial-of-service attack, a server
MAY impose progressively longer delays on multiple URL requests that
fail validation.
The decision to use the "authuser" access identifier should be made
with caution. An "authuser" access identifier can be used by any
authorized user of the IMAP server; therefore, use of this access
identifier should be limited to content that may be disclosed to any
authorized user of the IMAP server.
The decision to use the "anonymous" access identifier should be made
with extreme caution. An "anonymous" access identifier can be used
by anyone; therefore, use of this access identifier should be limited
to content that may be disclosed to anyone. Many IMAP servers do not
permit anonymous access; in this case, the "anonymous" access
identifier is equivalent to "authuser", but this MUST NOT be relied
upon.
Although this specification does not prohibit the theoretical
capability to generate a URL with a server component other than the
logged in userid and server, this capability should only be provided
Crispin Standards Track [Page 15]
RFC 4467 IMAP - URLAUTH Extension May 2006
when the logged in userid/server has been authorized as equivalent to
the server component userid/server, or otherwise has access to that
userid/server mailbox access key table.
11. IANA Considerations
This document constitutes registration of the URLAUTH capability in
the imap4-capabilities registry.
URLAUTH authorization mechanisms are registered by publishing a
standards track or IESG-approved experimental RFC. The registry is
currently located at:
http://www.iana.org/assignments/urlauth-authorization-mechanism-registry
This registry is case-insensitive.
This document constitutes registration of the INTERNAL URLAUTH
authorization mechanism.
IMAP URLAUTH Authorization Mechanism Registry
Mechanism Name Reference
-------------- ---------
INTERNAL [RFC4467]
Crispin Standards Track [Page 16]
RFC 4467 IMAP - URLAUTH Extension May 2006
12. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[BURL] Newman, C., "Message Submission BURL Extension", RFC 4468,
May 2006.
[DATETIME] Klyne, G. and C. Newman, "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.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
13. Informative References
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RANDOM] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
Author's Address
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
Crispin Standards Track [Page 17]
RFC 4467 IMAP - URLAUTH Extension May 2006
Full Copyright Statement
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Crispin Standards Track [Page 18]
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