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Network Working Group                                         M. CrispinRequest for Comments: 4467                      University of WashingtonUpdates:3501                                                   May 2006Category: Standards TrackInternet Message Access Protocol (IMAP) - URLAUTH ExtensionStatus 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 dataCrispin                     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 inAppendix 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 20062.  Concepts2.1.  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 insection 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 20064.  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 insection 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 20067.  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==] doneCrispin                     Standards Track                     [Page 8]

RFC 4467                IMAP - URLAUTH Extension                May 2006BASE.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 completedBASE.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 completedCrispin                     Standards Track                    [Page 11]

RFC 4467                IMAP - URLAUTH Extension                May 20068.  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 completedCrispin                     Standards Track                    [Page 12]

RFC 4467                IMAP - URLAUTH Extension                May 2006BASE.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 completed9.  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 / urlfetchresp-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 2006url-full        = astring                     ; contains authimapurlfull as defined belowurl-rump        = astring                     ; contains authimapurlrump as defined belowurlfetch        = "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 URLsauthimapurlfull = authimapurl iurlauthauthimapurlrump = authimapurl iurlauth-rumpenc-urlauth     = 32*HEXDIGenc-user        = 1*achar                     ; same as "enc_user" inRFC 2192iurlauth        = iurlauth-rump ":" mechanism ":" enc-urlauthiurlauth-rump   = [expire] ";URLAUTH=" accessaccess          = ("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 IANACrispin                     Standards Track                    [Page 14]

RFC 4467                IMAP - URLAUTH Extension                May 200610.  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 providedCrispin                     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 200612.  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.EDUCrispin                     Standards Track                    [Page 17]

RFC 4467                IMAP - URLAUTH Extension                May 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Crispin                     Standards Track                    [Page 18]