| Internet-Draft | IMAP JMAPACCESS | March 2024 |
| Gulbrandsen & Gondwana | Expires 2 September 2024 | [Page] |
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.¶
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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.¶
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.¶
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.¶
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.¶
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.¶
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 "]".¶
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¶
The IANA is requested to add the JMAPACCESS response code to the IMAP Response Codes registry, with this document as reference.¶
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.¶