The JMAPACCESS Extension for IMAP
draft-ietf-extra-jmapaccess-08

EXTRA                                                     A. Gulbrandsen
Internet-Draft                                                     ICANN
Intended status: Standards Track                             B. Gondwana
Expires: 2 September 2024                                       Fastmail
                                                            1 March 2024

                   The JMAPACCESS Extension for IMAP
                     draft-ietf-extra-jmapaccess-08

Abstract

   This document defines an IMAP extension to let clients know that the
   messages in this IMAP server are also available via JMAP, and how.
   It is intended for clients that want to migrate gradually to JMAP or
   use JMAP extensions within an IMAP client.

Status of This Memo

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   This Internet-Draft will expire on 2 September 2024.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Details . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   4.  The JMAPACCESS Response Code  . . . . . . . . . . . . . . . .   3
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   An IMAP server can declare that the messages in its mailstore are
   also available via JMAP.  For simplicity, only a complete equivalence
   is supported (the same set of messages are available via both IMAP
   and JMAP).

   This document also defines a way to provide debugging information
   that can be forwarded to client developers without privacy concerns,
   which is used by JMAPACCESS but can also be used by others.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Details

   By advertising the JMAPACCESS capability, the server asserts that if
   a mailbox or message has a particular object ID when accessed via
   either IMAP or JMAP (see [RFC3501], [RFC9051] and [RFC8620]), then
   the same mailbox or message is accessible via the other protocol, and
   it has the same ID.

   The server MUST also advertise the OBJECTID extension, defined by
   [RFC8474].  The JMAP session resource that allows access to the same
   messages is called "the JMAP server" below.

   This specification does not affect message lifetime: If a client
   accesses a message via IMAP and half a second later via JMAP, then
   the message may have been deleted between the two accesses.

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   When the server processes the client's LOGIN/AUTHENTICATE command and
   enters Authenticated state, the server considers the way the client
   authenticated.  If the IMAP server can infer from the client's
   authentication process that its credentials suffice to authenticate
   via JMAP, then the server MUST also send a JMAPACCESS response code
   containing a link to the JMAP server.

   Servers are encouraged to report the same message flags and other
   data via both protocols, as far as possible.

   This specification does not require mailboxes to have the same name
   in IMAP and JMAP, even if they share mailbox ID.  However, the JMAP
   specification regulates that, in the text about the name and role
   properties in [RFC8620] section 2.

   Note that all JMAP servers support internationalized email addresses
   (see [RFC6530]).  If this IMAP server does not, or the IMAP client
   does not issue ENABLE UTF8=ACCEPT (see [RFC6855]), then there is a
   possibility that the client receives accurate address fields via JMAP
   and downgraded fields via IMAP (see (see [RFC6857] and [RFC6858] for
   examples).  Issuing ENABLE UTF8=ACCEPT is a simple way to sidestep
   the issue.

4.  The JMAPACCESS Response Code

   The JMAPACCESS response code is followed by a single link to a JMAP
   session resource.  The server/mailstore at that location is
   referenced as "the JMAP server" in this document.

   The formal syntax in [RFC9051] is extended thus:

   resp-code-jmapaccess = "JMAPACCESS" SP quoted

   resp-text-code =/ resp-code-jmapaccess

   The syntax in [RFC3501] is extended similarly (this extension may be
   used with IMAP4rev1 as well as IMAP4rev2).

   Note that some clients parse response codes from the outside, ie.
   scanning for the following ']' before they parse the contents of the
   response code.  Sending a URL that contains either '"' or ']' may be
   risky.

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5.  Examples

   Lines sent by the client are preceded by C:, lines sent by the server
   by S:. Each example starts with the IMAP banner issued by the server
   on connection, and generally abbreviates the capability lists to
   what's required by the example itself.

   Real connections use longer capability lists, much longer
   AUTHENTICATE arguments and of course use TLS.  These examples focus
   on JMAPACCESS, though.

   Example 1.  A client connects, sees that SASL OAUTH is available, and
   authenticates in that way.

   S: * OK [CAPABILITY IMAP4rev1 AUTH=OAUTHBEARER SASL-IR] example1
   C: 1 AUTHENTICATE OAUTHBEARER bixhPXVzZ...QEB

   The server processes the command successfully.  It knows that the
   client used Oauth, and that it and its JMAP alter ego use the same
   Oauth backend subsystem.  Because of that it infers that the (next)
   access token is just as usable via JMAP as via IMAP.  It issues a
   JMAPACCESS response code in its reply:

   S: 1 OK [JMAPACCESS "https://example.com/jmap"] done

   SASL OAUTH is specified by [RFC7628], and the argument in this
   example is abbreviated from the more realistic length used in
   RFC7628.

   Example 2.  A client connects, sees no SASL method it recognises, and
   issues a LOGIN command.

   S: * OK [CAPABILITY IMAP4rev2] example2
   C: 2 LOGIN "arnt" "trondheim"

   The server sees that the password is accepted, knows that it and its
   JMAP alter ego use the same password database, and issues a
   JMAPACCESS response code:

   S: * OK [JMAPACCESS "https://example.com/.s/[jmap]"] For JMAP access
   S: 2 OK done

   The URL uses the same quoting rules as most other IMAP strings, and
   "]" is permitted in quoted strings.  Permitted but in this case not
   encouraged, since some clients are known to scan for the "]" before
   parsing the string inside "[]".  Luckily, few URLs contain "]".

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   Example 3.  A client connects, sees no SASL method it recognises, and
   issues a LOGIN command with a correct password.

   S: * OK [CAPABILITY IMAP4rev1 IMAP4rev2] example3
   C: 3 LOGIN "arnt" "trondheim"

   The server operator has decided to disable password use with JMAP,
   but allow it for a while with IMAP to cater to older clients, so the
   login succeeds, but there is no JMAPACCESS response code.

   S: 3 OK done

   Example 4.  A client connects, sees no SASL method it recognises, and
   issues a LOGIN command.  Its password is incorrect.

   S: * OK [CAPABILITY IMAP4rev2 AUTH=GSS] example4
   C: 4 LOGIN "arnt" "oslo"

   The server does not enter Authenticated state, so nothing requires it
   to issue JMAPACCESS.  It replies curtly:

   S: 4 NO done

6.  IANA Considerations

   The IANA is requested to add the JMAPACCESS response code to the IMAP
   Response Codes registry, with this document as reference.

7.  Security Considerations

   The JMAPACCESS response code reveals to authenticated IMAP clients
   that they would be able to authenticate via JMAP using the same
   credentials, and that the object IDs match.

   One does not normally wish reveal anything at all about
   authentication.  However, in this case information is revealed to an
   authenticated client, the revealed URL can usually be found via JMAP
   autodiscovery, and an attacker would only need to try the credentials
   used once anyway (a matter of a second or two).  Therefore, it is
   believed that this document does not benefit an attacker noticeably,
   and its value for migration far outweighs its risk.

8.  References

8.1.  Normative References

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   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/rfc/rfc3501>.

   [RFC8474]  Gondwana, B., Ed., "IMAP Extension for Object
              Identifiers", RFC 8474, DOI 10.17487/RFC8474, September
              2018, <https://www.rfc-editor.org/rfc/rfc8474>.

   [RFC9051]  Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message
              Access Protocol (IMAP) - Version 4rev2", RFC 9051,
              DOI 10.17487/RFC9051, August 2021,
              <https://www.rfc-editor.org/rfc/rfc9051>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

8.2.  Informative References

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
              February 2012, <https://www.rfc-editor.org/rfc/rfc6530>.

   [RFC6855]  Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP
              Support for UTF-8", RFC 6855, DOI 10.17487/RFC6855, March
              2013, <https://www.rfc-editor.org/rfc/rfc6855>.

   [RFC6857]  Fujiwara, K., "Post-Delivery Message Downgrading for
              Internationalized Email Messages", RFC 6857,
              DOI 10.17487/RFC6857, March 2013,
              <https://www.rfc-editor.org/rfc/rfc6857>.

   [RFC6858]  Gulbrandsen, A., "Simplified POP and IMAP Downgrading for
              Internationalized Email", RFC 6858, DOI 10.17487/RFC6858,
              March 2013, <https://www.rfc-editor.org/rfc/rfc6858>.

   [RFC7628]  Mills, W., Showalter, T., and H. Tschofenig, "A Set of
              Simple Authentication and Security Layer (SASL) Mechanisms
              for OAuth", RFC 7628, DOI 10.17487/RFC7628, August 2015,
              <https://www.rfc-editor.org/rfc/rfc7628>.

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   [RFC8620]  Jenkins, N. and C. Newman, "The JSON Meta Application
              Protocol (JMAP)", RFC 8620, DOI 10.17487/RFC8620, July
              2019, <https://www.rfc-editor.org/rfc/rfc8620>.

Authors' Addresses

   Arnt Gulbrandsen
   ICANN
   6 Rond Point Schumann, Bd. 1
   1040 Brussels
   Belgium
   Email: arnt@gulbrandsen.priv.no
   URI:   https://icann.org/ua

   Bron Gondwana
   Fastmail
   Level 2, 114 William St.
   Melbourne VIC  3000
   Australia
   Email: brong@fastmailteam.com
   URI:   https://fastmail.com

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