IMAP Service Extension for Client Identity
draft-yu-imap-client-id-15

Internet Engineering Task Force D. Yu, Ed.
Internet-Draft S. Johnson, Ed.
Intended status: Standards Track LinuxMagic
Expires: 1 June 2026 28 November 2025
 IMAP Service Extension for Client Identity
 draft-yu-imap-client-id-15.txt
Abstract
 Multi-Factor Authentication has rapidly become a driving requirement
 for any internet based technology that requires authentication.
 While a large number of initiatives are active for providing
 solutions to this requirement for Web Browser based applications that
 can generally support real time human interaction for providing a
 secondary method of identification, legacy protocols such as [IMAP]
 have not yet been revised to provide such support despite being a
 high-risk target for business email compromise, possibly as a result
 of [IMAP] activity generally expecting to be non-interactive in
 nature outside of Webmail logins.
 This document defines an extension to the [IMAP] service protocol
 called "CLIENTID" that an [IMAP] client can provide an additional
 unique identification token prior to standard credentials
 authentication that the server may then apply as an identity
 verification method in a similar manner to other Multi-Factor
 authentication techniques.
Status of This Memo
 This Internet-Draft is submitted in full conformance with the
 provisions of BCP 78 and BCP 79.
 Internet-Drafts are working documents of the Internet Engineering
 Task Force (IETF). Note that other groups may also distribute
 working documents as Internet-Drafts. The list of current Internet-
 Drafts is at https://datatracker.ietf.org/drafts/current/.
 Internet-Drafts are draft documents valid for a maximum of six months
 and may be updated, replaced, or obsoleted by other documents at any
 time. It is inappropriate to use Internet-Drafts as reference
 material or to cite them other than as "work in progress."
 This Internet-Draft will expire on 1 June 2026.
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Copyright Notice
 Copyright (c) 2025 IETF Trust and the persons identified as the
 document authors. All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents (https://trustee.ietf.org/
 license-info) in effect on the date of publication of this document.
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document. Code Components
 extracted from this document must include Revised BSD License text as
 described in Section 4.e of the Trust Legal Provisions and are
 provided without warranty as described in the Revised BSD License.
Table of Contents
 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
 1.1. Conventions Used in This Document . . . . . . . . . . . . 4
 2. CLIENTID . . . . . . . . . . . . . . . . . . . . . . . . . . 4
 2.1. CLIENTID Command . . . . . . . . . . . . . . . . . . . . 4
 2.2. CLIENTID Arguments . . . . . . . . . . . . . . . . . . . 5
 2.3. Advertising the CLIENTID capability . . . . . . . . . . . 5
 2.4. Restrictions on the CLIENTID command . . . . . . . . . . 5
 3. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 6
 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6
 4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 6
 4.2. Applying heuristics to CLIENTID . . . . . . . . . . . . . 7
 4.3. Utility of CLIENTID . . . . . . . . . . . . . . . . . . . 8
 4.4. Use Cases of CLIENTID . . . . . . . . . . . . . . . . . . 9
 4.5. Other IMAP Client Identifiers . . . . . . . . . . . . . . 10
 4.6. Alternative Considerations . . . . . . . . . . . . . . . 11
 4.7. Future Considerations . . . . . . . . . . . . . . . . . . 11
 5. Client Identity Types . . . . . . . . . . . . . . . . . . . . 12
 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 13
 6.1. UUID as Client Identity . . . . . . . . . . . . . . . . . 13
 6.2. Malformed CLIENTID Command . . . . . . . . . . . . . . . 14
 6.3. Client Identity without TLS/SSL Session . . . . . . . . . 14
 6.4. Client Identity Leading to Rejection . . . . . . . . . . 14
 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
 8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
 9.1. Normative References . . . . . . . . . . . . . . . . . . 16
 Appendix A. Appendix A. CLIENTID Product Support . . . . . . . 17
 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 17
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
 The [IMAP] protocol and its extensions describe methods whereby a
 client may provide identity and/or authentication information to an
 IMAP server, traditionally in the form of a [username] or an [email
 address] in combination with a [password] string. Historically, this
 was the primary form of security controlling who could access the
 IMAP resources. While newer forms of communication may have more
 complex forms of authentication, and web browser applications have
 received heightened attention to multi-factor authentication, legacy
 email protocols are limited in how they can change, while still
 offering a means for staged adoption via backwards compatibility with
 all the existing email clients.
 The approaches taken for other protocols do not lend themselves well
 to the [IMAP] protocol, and general usage. IMAP users expect their
 client to automatically log in, fetch their messages, and notify them
 of new messages in a non-interactive procedure.
 The challenges with introducing novel methods of enhanced security
 for IMAP without upsetting the non-interactive flow of usage has
 limited the ability for IMAP to address the security concerns with
 the traditional use of username/password combinations.
 A core of the problem is determining the 'identity' of the client or
 person presenting authentication information in a manner that does
 not require active participation on the part of the user. There are
 many ways in which a persons authentication credentials can be
 obtained fraudulently; for instance accidental exposure in plain text
 across insecure or compromised networks, reuse of the same
 credentials across multiple services (password reuse) where the theft
 or data breach at one service allows access to the rest of the
 services tied to that email identifier, or theft of stored
 credentials in browsers and password managers.
 Existing methods, such as the use of [IP], [ID], Geolocation and
 other tools available in the standard [IMAP] protocol currently are
 subject to limitations and none offer a way to sufficiently identify
 the source of the IMAP client with confidence. This has led to an
 environment of extreme risk of threat actors accessing IMAP
 resources, which can lead to serious financial impact and losses.
 Over 14 Billion email credentials have been exposed or stolen, and
 without methods to identify who or what is presenting authentication
 credentials, there is little defense against malicious or
 unauthorized use of those credentials.
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 This document defines an IMAP service extension to provide an
 additional identity token which can help represent the IMAP client
 with a higher degree of certainty than normally available in the
 current IMAP standards and protocols when accessing the IMAP server.
 It should be noted that other proposed alternative methods of
 improving the situation, and the resulting threats and risks, such as
 [oAUTH] have been examined as alternatives, (see discussion) but did
 not satisfy the requirements that led to CLIENTID implementations.
 Using the CLIENTID extension, an IMAP client can provide an
 additional identity token to the server called its "client identity".
 The client identity can provide unique characteristics about the
 client accessing the IMAP service and may be combined with existing
 identification mechanisms in order to identify the client. An IMAP
 server may then apply additional security policies using this
 identity such as restricting use of the service to clients presenting
 recognized client identities or only allowing use of authorized
 identities that match previously established client identities.
 The CLIENTID extension is present in any IMAP implementation that
 returns "CLIENTID" as one of the supported capabilities to the
 CAPABILITY command.
1.1. Conventions Used in This Document
 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.
2. CLIENTID
2.1. CLIENTID Command
 Arguments: client identity type
 client identity token
 Responses: no specific responses for this command
 Result: OK - clientid completed, client identity stored
 BAD - command unknown or arguments invalid
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 Note that a valid CLIENTID command will never return the NO result
 because heuristics MUST NOT be applied to the CLIENTID arguments at
 this stage. Instead the client identity information SHOULD be stored
 within the session and passed along to any and all [SASL]
 authentication mechanisms.
2.2. CLIENTID Arguments
 The CLIENTID command takes the following two arguments:
 1. client identity type: A string identifying the identity type the
 client is providing. It MUST be between 1 and 16 alphanumeric
 and dash characters.
 2. client identity token: A string identifying the client. It MUST
 be between 1 and 128 printable characters.
 The IMAP server MUST reject any CLIENTID command with badly formatted
 arguments. The IMAP server MUST accept the arguments from a valid
 CLIENTID command and SHOULD store it at the minimum for the remaining
 duration of the IMAP connection.
2.3. Advertising the CLIENTID capability
 The CLIENTID capability is used to tell the IMAP client that the IMAP
 server supports the CLIENTID extension. However, certain conditions
 MUST be met before the IMAP server advertises the CLIENTID
 capability.
 1. The IMAP server and IMAP client MUST negotiate encryption via
 STARTTLS/SSL or some other secure mechanism.
 2. The IMAP server MUST be in the non-authenticated state.
 3. The IMAP server MUST have the CLIENTID extension support enabled.
 When all the conditions are met, the IMAP server MUST advertise the
 CLIENTID capability in all proceeding CAPABILITY commands for as long
 as all conditions are met.
2.4. Restrictions on the CLIENTID command
 Under certain circumstances, the use of the CLIENTID command will be
 restricted:
 1. Before the CLIENTID capability has been advertised, the IMAP
 server MUST reject any issued CLIENTID command and the IMAP
 client MUST NOT issue the CLIENTID command.
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 2. Outside of the non-authenticated state, the IMAP server MUST
 reject any CLIENTID command issued by the IMAP client and the
 IMAP client MUST NOT issue the CLIENTID command.
 3. Once a valid CLIENTID command has been issued, the IMAP server
 MUST reject any further CLIENTID command issued by the IMAP
 client and the IMAP client MUST NOT issue any subsequent CLIENTID
 commands.
3. Formal Syntax
 The following syntax specification uses the Augmented Backus-Naur
 Form notation as specified in [ABNF]. [IMAP] defines the non-
 terminals "capability" and "command-nonauth".
 Except as noted otherwise, all alphabetic characters are case-
 insensitive. The use of upper or lower case characters to define
 token strings is for editorial clarity only. Implementations MUST
 accept these strings in a case-insensitive fashion.
 capability =/ "CLIENTID"
 command-nonauth =/ client-id
 client-id = "CLIENTID" SP client-id-type SP client-id-token
 client-id-type = 1*16 ALPHA / DIGIT / "-"
 ;; alphanumeric with dash character
 client-id-token = 1*128 VCHAR
 ;; any printable US-ASCII character
4. Discussion
4.1. Background
 The historical standard of using the user and password combination as
 a means of authentication is no longer effective in this day and age
 with recent developments in the world.
 1. ISPs transitioning to Carrier-grade NAT due to IPv4 address
 exhaustion placing multiple devices behind the same IP address.
 2. Numerous large scale data breaches exposing billions of user and
 password combinations.
 3. Continued propensity for user to use same simple passwords across
 multiple accounts and services.
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 4. Botnets growing larger and more sophisticated due to the
 proliferation of IoT devices.
 As a result, brute force attacks against internet services have
 become increasingly effective as malicious actors have easy access to
 billions of email addresses, commonly used passwords and massive
 botnets while the safety practices of users have not improved.
 The traditional methods of defending against these types of attacks
 such as tracking the rate of failed password attempts and
 subsequently blocking by IP address are no longer viable without
 collateral damage as thousands of devices could potentially be behind
 the same IP address as more ISPs adopt the CGN/LSN/NAT444 standard,
 i.e. blocking an IP address due to the actions of a single malicious
 actor bears the risk of blocking legitimate users.
 By introducing CLIENTID as another non-public factor to be used in
 tandem with the user and password combination, authentication becomes
 much more resilient against brute force attacks. The email addresses
 and passwords exposed from the data breaches will no longer be
 sufficient to authenticate. Security and mitigation methods such as
 limiting the rate in which authentication attempts can be made from a
 single IP address can be carried out by the CLIENTID identifier
 instead of the IP address, reducing the risk of falsely blocking
 traffic for clients behind a CGN/LSN/NAT444 connection. CLIENTID
 would also be backwards compatible with existing authentication
 protocols encouraging adoption in a viable manner.
4.2. Applying heuristics to CLIENTID
 This section discusses the possible heuristics that can be applied to
 the information that is presented via the CLIENTID command. This
 information includes whether a valid CLIENTID command was issued, the
 client identity type and the client identity token.
 1. The IMAP server MAY choose to require that a successful CLIENTID
 command be issued or that a particular client identity type be
 presented before processing or accepting an authentication
 request.
 2. The IMAP server MAY reject any authentication request not
 preceded with a client identity type that matches ACL's or rules
 as defined in the IMAP server.
 3. An IMAP server MAY reject any authentication request preceded by
 a CLIENTID command that contains a client identity type or client
 identity token that the server chooses not to accept for any
 reason such as by policy.
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 4. An IMAP server MAY reject any authentication request preceded by
 a CLIENTID command that contains a client identity type or client
 identity token that the server has chosen to disable or revoke
 use of either temporarily or permanently.
 The IMAP server SHOULD only ever reject an IMAP client based on
 CLIENTID information during or after the authentication process/
 handler. In the interest of limiting the amount of information being
 revealed, the rejection message SHOULD be as generic as possible and
 SHOULD NOT reveal any information on the heuristics.
 Even if the client identity type and/or client identity token are not
 recognized, supported or permitted by the server and/or the owner of
 the authentication credentials, the presented information may still
 be useful for analysis.
4.3. Utility of CLIENTID
 Regardless of how frowned upon, users commonly reuse authorization
 information (like the username and password pair) across multiple
 services. When one service is compromised, malicious actors can also
 gain access to other services where the user also used the same
 credentials. Based on this representative problem alone, the utility
 of CLIENTID as an additional layer of determining the rights to
 present such authorization information becomes quickly apparent.
 The utility of CLIENTID may be seen by considering the following:
 1. An IMAP server could recognize a device not historically known to
 have presented the authentication credentials before.
 2. An IMAP server could restrict authentication from actors not
 presenting a valid CLIENTID, or an account holder that the IMAP
 server provides service for could restrict authentication to only
 those devices that present valid CLIENTID.
 3. An IMAP server could restrict authentication to only devices
 which present a CLIENTID containing a client type identifier
 which the account holder or operator of the server deems to be
 permitted. (e.g. Only allow vendor A's devices)
 4. An IMAP server could alert an account holder that an attempt to
 present their authorization credentials came from an unknown,
 unrecognized, or different device.
 However, this extends beyond just the restriction of authentication.
 While it might be argued that this can be served as a special form of
 SASL, by implementing this in the IMAP service itself, the IMAP
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 service can choose before allowing a connection to be passed to a
 SASL implementation, allowing it to perform other heuristics, such as
 brute force attacks, more efficiently.
 While 'forgery' and/or the use of random client identifier is
 possible, such behavior is also more readily detectable when a device
 identifier is presented.
 1. The IMAP server, when faced with hundreds of devices behind the
 same IP address, during an attack can restrict authentication
 attempts to only connections presenting a valid client identifier
 token.
 2. The IMAP server, during an attack, can restrict authentication to
 only historically known devices.
 3. The IMAP server can differentiate between many different devices
 behind the same IP, and apply maximum connections per device,
 rather than maximum connections per IP.
 4. While a person may present authentication credentials from many
 different geographical locations, e.g. home, office, and travel,
 a single device will not in general be able to be in two
 geographical locations at the same time. The IMAP server will
 have new information to apply to threat detection heuristics,
 i.e. to treat the use of the same client identifier token from
 two locations, as a possible brute force or forgery situation.
4.4. Use Cases of CLIENTID
 With CLIENTID the IMAP server has additional information it may use
 in its interactions with the client. It may:
 1. Restrict use of an authorization tokens to a set of client
 identity token identities, thereby offering an added level of
 security. For example the use of authorization credentials may
 only be accompanied by a specified set of CLIENTID tokens and/or
 types for a specific account holder, or set of account holders
 2. Identify that the same CLIENTID token is used to access multiple
 authorized identities, and restrict access to the IMAP service.
 For example a malicious client that has attempted to gain access
 using multiple authorization tokens may be identified through its
 unusual behavior.
 3. Retain knowledge of CLIENTID tokens previously presented with
 specific authorization credentials, and if the token has not been
 previously seen, restrict access to the IMAP service.
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 4. Require that the IMAP client present a token such as a license
 key established outside of the IMAP session in order to make use
 of any authorized identity.
 5. Apply different security policies to clients that provide a
 CLIENTID token versus those which do not. For example, provide
 clients providing such an identity with additional trust.
 6. Ability to rate limit or block based on the presented client-
 identifier-token, when multiple devices use a shared IP address,
 without affecting other devices.
 7. Ability to detect distributed and localized dictionary attacks
 and brute force attacks.
 8. Use the client-identifier-token as a third factor to be passed to
 authentication methods eg. [SASL]
4.5. Other IMAP Client Identifiers
 The [IMAP] protocol and its extensions describe methods whereby an
 IMAP client may provide identity information to an IMAP server. Some
 of these identifiers are listed for contrast:
 1. The client connection provides a source IP address associated
 with the IMAP session. This may be accompanied by a PTR record
 and/or GeoIP information. This information is growing
 increasingly unreliable as IMAP Client softwares are increasingly
 configured to route traffic through 3rd party networks such as
 Virtual Private Networks.
 2. The AUTHENTICATE and LOGIN command allows the client to present a
 user and/or password/authentication mechanism for an IMAP
 session.
 3. The IMAP4 [ID] extension allows the server and client to exchange
 identification information for bug reports and usage statistics
 purposes. This specification however has limitations for the use
 of acting as an additional identification token for security:
 * The command is valid in any state - non encrypted, authenticated,
 or other.
 * Popular client side implementations of the [ID extension]
 implement the action after authentication is already complete -
 for ID to act as a security identifier with authentication would
 require a critical change to the specification that could cause
 conflict with existing implementations.
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4.6. Alternative Considerations
 Discussion on why [OAUTH] is not a consideration vs CLIENTID. Some
 critics have suggested that instead of CLIENTID, that the use of
 [OAUTH] should be improved or considered to address the problems
 surrounding authentication in legacy protocols. The authors of this
 draft considered that carefully, and researched why more adoption
 wasn't supported by those in the industry.
 The main objections stem from two new perceived problems that [OAUTH]
 is seen to create. One is the requirement of an [OAUTH] service,
 separate from the client -> server used in traditional protocols,
 which is perceived to be complicated, and introduces another
 potential point of failure in the process. Email operators, even
 large ones such as ISPs and Telcos are reluctant to build out and
 maintain [OAUTH] services.
 The alternative, eg using 3rd party [OAUTH] services is also seen to
 be a concern, due to adding extra risks to a stable environment,
 which may not be under their control, as well as concern over privacy
 problems when using 3rd party services for something like
 authentication. Concern also that the main providers of [OAUTH]
 services currently perform data collection as part of their business
 model, and the use of those services for [OAUTH] would share customer
 behavior inappropriately.
 CLIENTID is seen to be easier to implement at the client and server
 level, more transparent, and less of a risk of sharing customer
 behavior outside of the client->server relationship. CLIENTID
 support is also seen to require less change in customer behavior
 compared to legacy usage, allows for easier transition to a more
 secure environment transparently.
4.7. Future Considerations
 In the future there may be a demand for being able to provide
 multiple CLIENTID commands with different client identity types. For
 instance, it may be desirable for a device to identify itself, both
 with a hardware device identifier, and a software identifier. We
 believe this to be out of scope, and can be accommodated with a
 special client-identifier-token which encapsulates both.
 In future, there may be a demand to formalize the methods to describe
 how the CLIENTID command should be handled for any given client
 identity type. We believe this to currently be out of scope, but
 have created a successful implementation in which a server associates
 a set of flags to describe how it should behave:
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 1. Handled but treat as not presented (ignored, no persistence)
 2. Store in SMTP session but treat as not presented (for debug)
 3. Store in the SMTP session, so it is available to System log
 4. Store in the SMTP session, so it is available to User log
 5. Use for authentication
 6. Use for alert when authentication fails
 7. Use for alert when authentication succeeds
 8. Unused
5. Client Identity Types
 This document does not specify any CLIENTID identity type that MUST
 be supported. The client identity type is meant to be defined by the
 client implementation that is designed to access the IMAP server and
 protocol. For instance, many IMAP client software implementations
 already create a distinct Universally Unique Identifer [UUID] for
 each account. Some commercial email clients have a license key.
 Some physical devices that need to interact with IMAP might have a
 unique hardware ID. While there is no pre-defined list of client
 identity type defined by this RFC, and all IMAP servers should be
 prepared to accept any form of client identity type that conforms to
 the definition, it is suggested that IMAP client developers carefully
 consider the name of the client identity type. For example, rather
 that using a client identity type of UUID, consider the advantages of
 making it more distinct, e.g. "<product_short_code>UUID". This way
 the IMAP server can better record histories, e.g. the difference
 between say a Thunderbird generated unique id, and a Mutt generated
 unique id.
 Some examples of identity type might be UUID, LICENSE, DEVICE_ID,
 and/or COOKIE. It is expected that the most common types might be
 related to distinct UUID, LICENSEKEY, or HARDWAREID.
 An IMAP server SHOULD NOT reject an unidentified CLIENTID type,
 except for specific policy use cases.
 It is envisioned that in the future it will be useful to propose a
 set of standardized client-identity-type to help with validation, or
 to allow the IMAP server to apply ACL rules on expected types, this
 would be an extension to this RFC.
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 1.UUID
 UUID is a common practice to represent either a individual user,
 hardware device or software installation associated with a
 specific individual. The support of UUID enables existing UUID
 implementations to be used to semi-uniquely identify a device
 associated with an individual. A definition of the format should
 be considered. Otherwise non-standard UUID might be a separate
 type specific to the software implementation, for instance TBIRD-
 UUID.
 2.LICENSE
 An IMAP client may find it useful to identify the license key of
 software it is using. Such licenses are typically crafted such
 that they are unique and useful to identify a software
 installation. This is more normally suited for a software
 designed for a single-user. While LICENSE could be standard type
 again, it might more more helpful to specify a vendor specific
 type such as BBLICENSEKEY.
 3.DEVICE_ID
 Many hardware devices are designed to be used by a single
 individual and already have an associated hardware device id.
 While a standard type might be defined, it also might be more
 helpful to use a vendor specific type, such as ATOM-DEVICEID.
 4.COOKIE
 While not guaranteed to be consistent many web applications are
 designed to access IMAP directly and may need to have a semi-
 unique identifier available as part of the web based transaction.
 It is assumed that COOKIE encompasses the group of web based
 tokens known to persist from session to session. A specific web
 based application can provide sufficient information in the actual
 client-identifier-token to differentiate between applications and
 or websites, and are convenient as they can be related to very
 specific domains, and are universally available to web application
 designers.
 As a reminder, an IMAP server SHOULD NOT retain and/or store the
 CLIENTID information WITH authentication credentials or
 authentication systems directly, but the IMAP service MAY associate
 the CLIENTID with a specific account holder, e.g. to create a history
 file of known CLIENTID tokens associated or permitted to access or
 present authentication credentials for that account holder.
6. Examples
6.1. UUID as Client Identity
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 C: [connection established over a plaintext connection]
 C: a001 CAPABILITY
 S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI LOGINDISABLED
 S: a001 OK CAPABILITY completed
 C: a002 STARTTLS
 S: a002 OK STARTTLS completed
 <TLS negotiation, further commands are under [TLS] layer>
 C: a003 CAPABILITY
 S: * CAPABILITY IMAP4rev1 AUTH=GSSAPI AUTH=PLAIN CLIENTID
 S: a003 OK CAPABILITY completed
 C: a004 CLIENTID UUID 23bf83be-aad7-46aa-9e0f-39191ccf402f
 S: a004 OK CLIENTID completed
 C: a005 LOGIN joe password
 S: a005 OK LOGIN completed
6.2. Malformed CLIENTID Command
 C: [connection established over a plaintext connection]
 C: a001 CAPABILITY
 S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI LOGINDISABLED
 S: a001 OK CAPABILITY completed
 C: a002 STARTTLS
 S: a002 OK STARTTLS completed
 <TLS negotiation, further commands are under [TLS] layer>
 C: a003 CAPABILITY
 S: * CAPABILITY IMAP4rev1 AUTH=GSSAPI AUTH=PLAIN CLIENTID
 S: a003 OK CAPABILITY completed
 C: a004 CLIENTID UUID
 S: a004 BAD Error in IMAP command received by server
 The IMAP server rejects the CLIENTID command as it is not well formed
 due to there being only a single parameter provided.
6.3. Client Identity without TLS/SSL Session
 C: [connection established over a plaintext connection]
 C: a001 CAPABILITY
 S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI LOGINDISABLED
 S: a001 OK CAPABILITY completed
 C: a002 CLIENTID UUID 23bf83be-aad7-46aa-9e0f-39191ccf402f
 S: a002 BAD Unknown IMAP command received by server
 The IMAP server rejects use of the CLIENTID command as the CLIENTID
 capability had not been advertised because no encryption was
 negotiated between the IMAP server and IMAP client.
6.4. Client Identity Leading to Rejection
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 C: [connection established over a plaintext connection]
 C: a001 CAPABILITY
 S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI LOGINDISABLED
 S: a001 OK CAPABILITY completed
 C: a002 STARTTLS
 S: a002 OK STARTTLS completed
 <TLS negotiation, further commands are under [TLS] layer>
 C: a003 CAPABILITY
 S: * CAPABILITY IMAP4rev1 AUTH=GSSAPI AUTH=PLAIN CLIENTID
 S: a003 OK CAPABILITY completed
 C: a004 CLIENTID UUID 23bf83be-aad7-46aa-9e0f-39191ccf402f
 S: a004 OK CLIENTID completed
 C: a005 LOGIN joe password
 S: a005 BAD Failed to authenticate
 The IMAP server rejects use of the system during the LOGIN command
 after deciding that the provided client identity does not establish
 sufficient privileges. Note that the error message that's returned
 to the client is very generic and does not reveal any information
 about CLIENTID and/or the existence of 'joe' and/or the validity of
 the password.
7. IANA Considerations
 The IANA is requested to add CLIENTID to the "IMAP 4 Capabilities"
 registry, http://www.iana.org/assignments/imap4-capabilities.
8. Security Considerations
 As this extension provides an additional means of communicating
 information from a client to a server, it is clear that there is
 additional information divulged to the server. This may have privacy
 considerations depending on the client identity type or its contents.
 For example, it may reveal a MAC address of the device used to
 communicate with a server that would not previously have been
 revealed. While it has been useful to use identifier such as email
 address for authentication, it is easy for these authentication
 tokens to be shared and/or reused and/or be publicly available for
 other purposes. An IMAP server and/or its operators SHOULD NOT share
 any CLIENTID information presented with a third party as it may
 represent or be linked to an individual and SHOULD never be shared in
 association with authentication tokens.
 In essence, this provides a transparent method of multi-factor
 authentication requiring no modification to the IMAP resource of the
 client, where the traditional username and password along with any
 one of the unique identifiers can be used to identify a device you
 'own'. However, great care should be taken by the client when
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 deciding on the unique identifier to use and select one that cannot
 be easily discovered. For example, one could use the MAC address but
 such an identifier may be elementary to discover and be forged by
 another device. While any compromise of a device MAY reveal the
 unique identifier, that problem is beyond the scope of the problem
 that this RFC is designed to solve. Consider using a different
 unique identifier for each service to avoid having a compromised
 service expose identifiers that can then be used to access another
 service.
 Also, while this service extension requires that the identity
 information only be transmitted over an encrypted channel to reduce
 the risk of eavesdropping, it does not specify any policies or
 practices required in the establishment of such a channel, and so it
 is the responsibility of the client and the server to determine that
 the communication medium meets their requirements.
 An example of service specific device identifiers can be seen in the
 implementation of CLIENTID in the Thunderbird email client.
9. References
9.1. Normative References
 [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/info/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/info/rfc8174>.
 [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
 Specifications: ABNF", RFC 5234, January 2008,
 <https://www.rfc-editor.org/rfc/rfc5234>.
 [IMAP] Melnikov, A. and B. Leiba, "INTERNET MESSAGE ACCESS
 PROTOCOL - VERSION 4rev2", RFC 9051, August 2021,
 <https://www.rfc-editor.org/rfc/rfc9051>.
 [OAUTH] Hardt, D., "The OAuth 2.0 Authorization Framework",
 RFC 6749, October 2012,
 <https://www.rfc-editor.org/rfc/rfc6749>.
 [SASL] Melnikov, A. and K. Zeilenga, "Simple Authentication and
 Security Layer (SASL)", RFC 4422, June 2006,
 <https://www.rfc-editor.org/rfc/rfc4422>.
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Internet-Draft IMAP Client Identity November 2025
 [TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
 Version 1.3", RFC 8846, August 2018,
 <https://www.rfc-editor.org/rfc/rfc8846>.
Appendix A. Appendix A. CLIENTID Product Support
 Since publishing the IMAP Client Identity RFC draft, multiple email
 server and client vendors have implemented CLIENTID support into
 their products, e.g. MailEnable, MagicMail, SaneBox, BlueMail,
 emClient, and Thunderbird.
 Given the current usage and adoption of CLIENTID in the public
 domain, this RFC should be considered for a Standards track.
Contributors
 Michael Peddemors
 LinuxMagic
Authors' Addresses
 Deion Yu (editor)
 LinuxMagic
 #405 - 860 Homer St.
 Vancouver British Columbia V6B 2W5
 Canada
 Email: deiony@linuxmagic.com
 Shaun Johnson (editor)
 LinuxMagic
 #405 - 860 Homer St.
 Vancouver British Columbia V6B 2W5
 Canada
 Email: shaun@linuxmagic.com
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