ICANN

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

Fastmail

Level 2, 114 William St. Melbourne VIC 3000 Australia brong@fastmailteam.com https://fastmail.com

ART extra IMAP JMAP This document defines an IMAP extension to let clients know that the messages in this IMAP server are also available via the JSON Meta Application Protocol (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 This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

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Table of Contents

• .  Introduction
• .  Requirements Language
• .  Details
• .  The GETJMAPACCESS Command and the JMAPACCESS Response
• .  Examples
• .  IANA Considerations
• .  Security Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Authors' Addresses

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).

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 when, and only when, they appear in all capitals, as shown here.

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 , , and ), 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 . 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. 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 include a JMAPACCESS capability in any capability list sent after that point. This includes the capability list that some servers send immediately when authentication succeeds. 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 a mailbox ID. However, the JMAP specification regulates that in the text about the name and role properties described in . Note that all JMAP servers support internationalized email addresses (see ). If this IMAP server does not or if the IMAP client does not issue ENABLE UTF8=ACCEPT (see ), then it is possible that the client will receive accurate address fields via JMAP and downgraded fields via IMAP (see and for examples). Issuing ENABLE UTF8=ACCEPT is a simple way to sidestep the issue.

The GETJMAPACCESS Command and the JMAPACCESS Response The GETJMAPACCESS command requests that the server respond with the session URL for the JMAP server that provides access to the same mail. If such a JMAP server is known to this server, the server MUST respond with an untagged JMAPACCESS response containing the JMAP server's session resource (a URL) followed by a tagged OK response. If such a JMAP server is not known, the server MUST respond with a tagged BAD response (and MUST NOT include JMAPACCESS in the capability list). The JMAPACCESS response is followed by a single link to a JMAP session resource. The formal syntax in is extended as follows: command-auth =/ "GETJMAPACCESS" mailbox-data =/ resp-jmapaccess resp-jmapaccess = "JMAPACCESS" SP quoted The syntax in is extended similarly (this extension may be used with IMAP4rev1 as well as IMAP4rev2).

Examples Lines sent by the client are preceded by C: and lines sent by the server are preceded 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. However, these examples focus on JMAPACCESS. 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 includes a JMAPACCESS capability in its reply (again, real capability lists are much longer): S: 1 OK [CAPABILITY IMAP4rev1 JMAPACCESS] done C: 1b GETJMAPACCESS S: * JMAPACCESS "https://example.com/.well-known/jmap" S: 1b OK done SASL OAuth is specified by , and the argument in this example is abbreviated from the more realistic length used in RFC 7628. Example 2: A client connects, sees no SASL method it recognizes, 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 capability: S: * OK [CAPABILITY IMAP4rev2 JMAPACCESS] done S: 2 OK done C: 2b JMAPACCESS S: * JMAPACCESS "https://example.com/.well-known/jmap" S: 2b OK done The URL uses the same quoting rules as most other IMAP strings. Example 3: A client connects, sees no SASL method it recognizes, 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. Therefore, the login succeeds, but there is no JMAPACCESS capability. S: 3 OK done Example 4: A client connects, sees no SASL method it recognizes, 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 mention JMAPACCESS. It replies curtly: S: 4 NO done

IANA Considerations The IANA has added the JMAPACCESS capability to the "Internet Message Access Protocol (IMAP) Capabilities Registry" and listed this document as the reference.

Security Considerations JMAPACCESS 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 reveal anything at all about authentication. However, if the client is an attacker, then the attacker is known to have valid credentials, and tells the attacker how to find the revealed URL without the help of this extension. Therefore, it is believed that this document does not benefit an attacker noticeably, and its value for migration far outweighs its risk.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Internet Message Access Protocol, Version 4rev1 (IMAP4rev1) allows a client to access and manipulate electronic mail messages on a server. IMAP4rev1 permits manipulation of mailboxes (remote message folders) in a way that is functionally equivalent to local folders. IMAP4rev1 also provides the capability for an offline client to resynchronize with the server. IMAP4rev1 includes operations for creating, deleting, and renaming mailboxes, checking for new messages, permanently removing messages, setting and clearing flags, RFC 2822 and RFC 2045 parsing, searching, and selective fetching of message attributes, texts, and portions thereof. Messages in IMAP4rev1 are accessed by the use of numbers. These numbers are either message sequence numbers or unique identifiers. IMAP4rev1 supports a single server. A mechanism for accessing configuration information to support multiple IMAP4rev1 servers is discussed in RFC 2244. IMAP4rev1 does not specify a means of posting mail; this function is handled by a mail transfer protocol such as RFC 2821. [STANDARDS-TRACK] RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document updates RFC 3501 (IMAP4rev1) with persistent identifiers on mailboxes and messages to allow clients to more efficiently reuse cached data when resources have changed location on the server. The Internet Message Access Protocol Version 4rev2 (IMAP4rev2) allows a client to access and manipulate electronic mail messages on a server. IMAP4rev2 permits manipulation of mailboxes (remote message folders) in a way that is functionally equivalent to local folders. IMAP4rev2 also provides the capability for an offline client to resynchronize with the server. IMAP4rev2 includes operations for creating, deleting, and renaming mailboxes; checking for new messages; removing messages permanently; setting and clearing flags; parsing per RFCs 5322, 2045, and 2231; searching; and selective fetching of message attributes, texts, and portions thereof. Messages in IMAP4rev2 are accessed by the use of numbers. These numbers are either message sequence numbers or unique identifiers. IMAP4rev2 does not specify a means of posting mail; this function is handled by a mail submission protocol such as the one specified in RFC 6409. Informative References Full use of electronic mail throughout the world requires that (subject to other constraints) people be able to use close variations on their own names (written correctly in their own languages and scripts) as mailbox names in email addresses. This document introduces a series of specifications that define mechanisms and protocol extensions needed to fully support internationalized email addresses. These changes include an SMTP extension and extension of email header syntax to accommodate UTF-8 data. The document set also includes discussion of key assumptions and issues in deploying fully internationalized email. This document is a replacement for RFC 4952; it reflects additional issues identified since that document was published. [STANDARDS-TRACK] This specification extends the Internet Message Access Protocol (IMAP) to support UTF-8 encoded international characters in user names, mail addresses, and message headers. This specification replaces RFC 5738. The Email Address Internationalization (SMTPUTF8) extension to SMTP allows Unicode characters encoded in UTF-8 and outside the ASCII repertoire in mail header fields. Upgraded POP and IMAP servers support internationalized messages. If a POP or IMAP client does not support Email Address Internationalization, a POP or IMAP server cannot deliver internationalized messages to the client and cannot remove the message. To avoid that situation, this document describes a mechanism for converting internationalized messages into the traditional message format. As part of the conversion process, message elements that require internationalized treatment are recoded or removed, and receivers are able to recognize that they received messages containing such elements, even if they cannot process the internationalized elements. This document specifies a method for IMAP and POP servers to serve internationalized messages to conventional clients. The specification is simple, easy to implement, and provides only rudimentary results. OAuth enables a third-party application to obtain limited access to a protected resource, either on behalf of a resource owner by orchestrating an approval interaction or by allowing the third-party application to obtain access on its own behalf. This document defines how an application client uses credentials obtained via OAuth over the Simple Authentication and Security Layer (SASL) to access a protected resource at a resource server. Thereby, it enables schemes defined within the OAuth framework for non-HTTP-based application protocols. Clients typically store the user's long-term credential. This does, however, lead to significant security vulnerabilities, for example, when such a credential leaks. A significant benefit of OAuth for usage in those clients is that the password is replaced by a shared secret with higher entropy, i.e., the token. Tokens typically provide limited access rights and can be managed and revoked separately from the user's long-term password. This document specifies a protocol for clients to efficiently query, fetch, and modify JSON-based data objects, with support for push notification of changes and fast resynchronisation and for out-of- band binary data upload/download.

Authors' Addresses ICANN

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

Fastmail

Level 2, 114 William St. Melbourne VIC 3000 Australia brong@fastmailteam.com https://fastmail.com