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rfc:rfc8314

Internet Engineering Task Force (IETF) K. Moore Request for Comments: 8314 Windrock, Inc. Updates: 1939, 2595, 3501, 5068, 6186, 6409 C. Newman Category: Standards Track Oracle ISSN: 2070-1721 January 2018

Cleartext Considered Obsolete: Use of Transport Layer Security (TLS)
                  for Email Submission and Access

Abstract

 This specification outlines current recommendations for the use of
 Transport Layer Security (TLS) to provide confidentiality of email
 traffic between a Mail User Agent (MUA) and a Mail Submission Server
 or Mail Access Server.  This document updates RFCs 1939, 2595, 3501,
 5068, 6186, and 6409.

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
 https://www.rfc-editor.org/info/rfc8314.

Copyright Notice

 Copyright (c) 2018 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 Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Moore & Newman Standards Track [Page 1] RFC 8314 Use of TLS for Email Submission/Access January 2018

Table of Contents

 1. Introduction ....................................................3
    1.1. How This Document Updates Previous RFCs ....................3
 2. Conventions and Terminology Used in This Document ...............4
 3. Implicit TLS ....................................................5
    3.1. Implicit TLS for POP .......................................5
    3.2. Implicit TLS for IMAP ......................................5
    3.3. Implicit TLS for SMTP Submission ...........................6
    3.4. Implicit TLS Connection Closure for POP, IMAP, and
         SMTP Submission ............................................7
 4. Use of TLS by Mail Access Servers and Message Submission
    Servers .........................................................7
    4.1. Deprecation of Services Using Cleartext and TLS Versions
         Less Than 1.1 ..............................................8
    4.2. Mail Server Use of Client Certificate Authentication .......9
    4.3. Recording TLS Ciphersuite in "Received" Header Field .......9
    4.4. TLS Server Certificate Requirements .......................10
    4.5. Recommended DNS Records for Mail Protocol Servers .........11
         4.5.1. MX Records .........................................11
         4.5.2. SRV Records ........................................11
         4.5.3. DNSSEC .............................................11
         4.5.4. TLSA Records .......................................11
    4.6. Changes to Internet-Facing Servers ........................11
 5. Use of TLS by Mail User Agents .................................12
    5.1. Use of SRV Records in Establishing Configuration ..........13
    5.2. Minimum Confidentiality Level .............................14
    5.3. Certificate Validation ....................................15
    5.4. Certificate Pinning .......................................15
    5.5. Client Certificate Authentication .........................16
 6. Considerations Related to Antivirus/Antispam Software
    and Services ...................................................17
 7. IANA Considerations ............................................17
    7.1. POP3S Port Registration Update ............................17
    7.2. IMAPS Port Registration Update ............................18
    7.3. Submissions Port Registration .............................18
    7.4. Additional Registered Clauses for "Received" Fields .......19
 8. Security Considerations ........................................19
 9. References .....................................................20
    9.1. Normative References ......................................20
    9.2. Informative References ....................................22
 Appendix A. Design Considerations .................................24
 Acknowledgements ..................................................26
 Authors' Addresses ................................................26

Moore & Newman Standards Track [Page 2] RFC 8314 Use of TLS for Email Submission/Access January 2018

1. Introduction

 Software that provides email service via the Internet Message Access
 Protocol (IMAP) [RFC3501], the Post Office Protocol (POP) [RFC1939],
 and/or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409]
 usually has Transport Layer Security (TLS) [RFC5246] support but
 often does not use it in a way that maximizes end-user
 confidentiality.  This specification describes current
 recommendations for the use of TLS in interactions between Mail User
 Agents (MUAs) and Mail Access Servers, and also between MUAs and Mail
 Submission Servers.
 In brief, this memo now recommends that:
 o  TLS version 1.2 or greater be used for all traffic between MUAs
    and Mail Submission Servers, and also between MUAs and Mail Access
    Servers.
 o  MUAs and Mail Service Providers (MSPs) (a) discourage the use of
    cleartext protocols for mail access and mail submission and
    (b) deprecate the use of cleartext protocols for these purposes as
    soon as practicable.
 o  Connections to Mail Submission Servers and Mail Access Servers be
    made using "Implicit TLS" (as defined below), in preference to
    connecting to the "cleartext" port and negotiating TLS using the
    STARTTLS command or a similar command.
 This memo does not address the use of TLS with SMTP for message relay
 (where Message Submission [RFC6409] does not apply).  Improving the
 use of TLS with SMTP for message relay requires a different approach.
 One approach to address that topic is described in [RFC7672]; another
 is provided in [MTA-STS].
 The recommendations in this memo do not replace the functionality of,
 and are not intended as a substitute for, end-to-end encryption of
 electronic mail.

1.1. How This Document Updates Previous RFCs

 This document updates POP (RFC 1939), IMAP (RFC 3501), and Submission
 (RFC 6409, RFC 5068) in two ways:
 1.  By adding Implicit TLS ports as Standards Track ports for these
     protocols as described in Section 3.
 2.  By updating TLS best practices that apply to these protocols as
     described in Sections 4 and 5.

Moore & Newman Standards Track [Page 3] RFC 8314 Use of TLS for Email Submission/Access January 2018

 This document updates RFC 2595 by replacing Section 7 of RFC 2595
 with the preference for Implicit TLS as described in Sections 1 and 3
 of this document, as well as by updating TLS best practices that
 apply to the protocols in RFC 2595 as described in Sections 4 and 5
 of this document.
 This document updates RFC 6186 as described herein, in Section 5.1.

2. Conventions and Terminology 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.
 The term "Implicit TLS" refers to the automatic negotiation of TLS
 whenever a TCP connection is made on a particular TCP port that is
 used exclusively by that server for TLS connections.  The term
 "Implicit TLS" is intended to contrast with the use of STARTTLS and
 similar commands in POP, IMAP, SMTP Message Submission, and other
 protocols, that are used by the client and the server to explicitly
 negotiate TLS on an established cleartext TCP connection.
 The term "Mail Access Server" refers to a server for POP, IMAP, and
 any other protocol used to access or modify received messages, or to
 access or modify a mail user's account configuration.
 The term "Mail Submission Server" refers to a server for the protocol
 specified in [RFC6409] (or one of its predecessors or successors) for
 submission of outgoing messages for delivery to recipients.
 The term "Mail Service Provider" (or "MSP") refers to an operator of
 Mail Access Servers and/or Mail Submission Servers.
 The term "Mail Account" refers to a user's identity with an MSP, that
 user's authentication credentials, any user email that is stored by
 the MSP, and any other per-user configuration information maintained
 by the MSP (for example, instructions for filtering spam).  Most MUAs
 support the ability to access multiple Mail Accounts.
 For each account that an MUA accesses on its user's behalf, it must
 have the server names, ports, authentication credentials, and other
 configuration information specified by the user.  This information,
 which is used by the MUA, is referred to as "Mail Account
 Configuration".

Moore & Newman Standards Track [Page 4] RFC 8314 Use of TLS for Email Submission/Access January 2018

 This specification expresses syntax using the Augmented Backus-Naur
 Form (ABNF) as described in [RFC5234], including the core rules
 provided in Appendix B of [RFC5234] and the rules provided in
 [RFC5322].

3. Implicit TLS

 Previous standards for the use of email protocols with TLS used the
 STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501].  With
 STARTTLS, the client establishes a cleartext application session and
 determines whether to issue a STARTTLS command based on server
 capabilities and client configuration.  If the client issues a
 STARTTLS command, a TLS handshake follows that can upgrade the
 connection.  Although this mechanism has been deployed, an alternate
 mechanism where TLS is negotiated immediately at connection start on
 a separate port (referred to in this document as "Implicit TLS") has
 been deployed more successfully.  To encourage more widespread use of
 TLS and to also encourage greater consistency regarding how TLS is
 used, this specification now recommends the use of Implicit TLS for
 POP, IMAP, SMTP Submission, and all other protocols used between an
 MUA and an MSP.

3.1. Implicit TLS for POP

 When a TCP connection is established for the "pop3s" service (default
 port 995), a TLS handshake begins immediately.  Clients MUST
 implement the certificate validation mechanism described in
 [RFC7817].  Once the TLS session is established, POP3 [RFC1939]
 protocol messages are exchanged as TLS application data for the
 remainder of the TCP connection.  After the server sends an +OK
 greeting, the server and client MUST enter the AUTHORIZATION state,
 even if a client certificate was supplied during the TLS handshake.
 See Sections 5.5 and 4.2 for additional information on client
 certificate authentication.  See Section 7.1 for port registration
 information.

3.2. Implicit TLS for IMAP

 When a TCP connection is established for the "imaps" service (default
 port 993), a TLS handshake begins immediately.  Clients MUST
 implement the certificate validation mechanism described in
 [RFC7817].  Once the TLS session is established, IMAP [RFC3501]
 protocol messages are exchanged as TLS application data for the
 remainder of the TCP connection.  If a client certificate was
 provided during the TLS handshake that the server finds acceptable,
 the server MAY issue a PREAUTH greeting, in which case both the

Moore & Newman Standards Track [Page 5] RFC 8314 Use of TLS for Email Submission/Access January 2018

 server and the client enter the AUTHENTICATED state.  If the server
 issues an OK greeting, then both the server and the client enter the
 NOT AUTHENTICATED state.
 See Sections 5.5 and 4.2 for additional information on client
 certificate authentication.  See Section 7.2 for port registration
 information.

3.3. Implicit TLS for SMTP Submission

 When a TCP connection is established for the "submissions" service
 (default port 465), a TLS handshake begins immediately.  Clients MUST
 implement the certificate validation mechanism described in
 [RFC7817].  Once the TLS session is established, Message Submission
 protocol data [RFC6409] is exchanged as TLS application data for the
 remainder of the TCP connection.  (Note: The "submissions" service
 name is defined in Section 7.3 of this document and follows the usual
 convention that the name of a service layered on top of Implicit TLS
 consists of the name of the service as used without TLS, with an "s"
 appended.)
 The STARTTLS mechanism on port 587 is relatively widely deployed due
 to the situation with port 465 (discussed in Section 7.3).  This
 differs from IMAP and POP services where Implicit TLS is more widely
 deployed on servers than STARTTLS.  It is desirable to migrate core
 protocols used by MUA software to Implicit TLS over time, for
 consistency as well as for the additional reasons discussed in
 Appendix A.  However, to maximize the use of encryption for
 submission, it is desirable to support both mechanisms for Message
 Submission over TLS for a transition period of several years.  As a
 result, clients and servers SHOULD implement both STARTTLS on
 port 587 and Implicit TLS on port 465 for this transition period.
 Note that there is no significant difference between the security
 properties of STARTTLS on port 587 and Implicit TLS on port 465 if
 the implementations are correct and if both the client and the server
 are configured to require successful negotiation of TLS prior to
 Message Submission.
 Note that the "submissions" port provides access to a Message
 Submission Agent (MSA) as defined in [RFC6409], so requirements and
 recommendations for MSAs in that document, including the requirement
 to implement SMTP AUTH [RFC4954] and the requirements of Email
 Submission Operations [RFC5068], also apply to the submissions port.
 See Sections 5.5 and 4.2 for additional information on client
 certificate authentication.  See Section 7.3 for port registration
 information.

Moore & Newman Standards Track [Page 6] RFC 8314 Use of TLS for Email Submission/Access January 2018

3.4. Implicit TLS Connection Closure for POP, IMAP, and SMTP Submission

 When a client or server wishes to close the connection, it SHOULD
 initiate the exchange of TLS close alerts before TCP connection
 termination.  The client MAY, after sending a TLS close alert,
 gracefully close the TCP connection (e.g., call the close() function
 on the TCP socket or otherwise issue a TCP CLOSE ([RFC793],
 Section 3.5)) without waiting for a TLS response from the server.

4. Use of TLS by Mail Access Servers and Message Submission Servers

 The following requirements and recommendations apply to Mail Access
 Servers and Mail Submission Servers, or, if indicated, to MSPs:
 o  MSPs that support POP, IMAP, and/or Message Submission MUST
    support TLS access for those protocol servers.
 o  Servers provided by MSPs other than POP, IMAP, and/or Message
    Submission SHOULD support TLS access and MUST support TLS access
    for those servers that support authentication via username and
    password.
 o  MSPs that support POP, IMAP, and/or Message Submission SHOULD
    provide and support instances of those services that use Implicit
    TLS.  (See Section 3.)
 o  For compatibility with existing MUAs and existing MUA
    configurations, MSPs SHOULD also, in the near term, provide
    instances of these services that support STARTTLS.  This will
    permit legacy MUAs to discover new availability of TLS capability
    on servers and may increase the use of TLS by such MUAs.  However,
    servers SHOULD NOT advertise STARTTLS if the use of the STARTTLS
    command by a client is likely to fail (for example, if the server
    has no server certificate configured).
 o  MSPs SHOULD advertise their Mail Access Servers and Mail
    Submission Servers, using DNS SRV records according to [RFC6186].
    (In addition to making correct configuration easier for MUAs, this
    provides a way by which MUAs can discover when an MSP begins to
    offer TLS-based services.)  Servers supporting TLS SHOULD be
    advertised in preference to cleartext servers (if offered).  In
    addition, servers using Implicit TLS SHOULD be advertised in
    preference to servers supporting STARTTLS (if offered).  (See also
    Section 4.5.)
 o  MSPs SHOULD deprecate the use of cleartext Mail Access Servers and
    Mail Submission Servers as soon as practicable.  (See
    Section 4.1.)

Moore & Newman Standards Track [Page 7] RFC 8314 Use of TLS for Email Submission/Access January 2018

 o  MSPs currently supporting such use of cleartext SMTP (on port 25)
    as a means of Message Submission by their users (whether or not
    requiring authentication) SHOULD transition their users to using
    TLS (either Implicit TLS or STARTTLS) as soon as practicable.
 o  Mail Access Servers and Mail Submission Servers MUST support
    TLS 1.2 or later.
 o  All Mail Access Servers and Mail Submission Servers SHOULD
    implement the recommended TLS ciphersuites described in [RFC7525]
    or a future BCP or Standards Track revision of that document.
 o  As soon as practicable, MSPs currently supporting Secure Sockets
    Layer (SSL) 2.x, SSL 3.0, or TLS 1.0 SHOULD transition their users
    to TLS 1.1 or later and discontinue support for those earlier
    versions of SSL and TLS.
 o  Mail Submission Servers accepting mail using TLS SHOULD include in
    the Received field of the outgoing message the TLS ciphersuite of
    the session in which the mail was received.  (See Section 4.3.)
 o  All Mail Access Servers and Mail Submission Servers implementing
    TLS SHOULD log TLS cipher information along with any connection or
    authentication logs that they maintain.
 Additional considerations and details appear below.

4.1. Deprecation of Services Using Cleartext and TLS Versions

    Less Than 1.1
 The specific means employed for deprecation of cleartext Mail Access
 Servers and Mail Submission Servers MAY vary from one MSP to the next
 in light of their user communities' needs and constraints.  For
 example, an MSP MAY implement a gradual transition in which, over
 time, more and more users are forbidden to authenticate to cleartext
 instances of these servers, thus encouraging those users to migrate
 to Implicit TLS.  Access to cleartext servers should eventually be
 either (a) disabled or (b) limited strictly for use by legacy systems
 that cannot be upgraded.
 After a user's ability to authenticate to a server using cleartext is
 revoked, the server denying such access MUST NOT provide any
 indication over a cleartext channel of whether the user's
 authentication credentials were valid.  An attempt to authenticate as
 such a user using either invalid credentials or valid credentials
 MUST both result in the same indication of access being denied.

Moore & Newman Standards Track [Page 8] RFC 8314 Use of TLS for Email Submission/Access January 2018

 Also, users previously authenticating with passwords sent as
 cleartext SHOULD be required to change those passwords when migrating
 to TLS, if the old passwords were likely to have been compromised.
 (For any large community of users using the public Internet to access
 mail without encryption, the compromise of at least some of those
 passwords should be assumed.)
 Transition of users from SSL or TLS 1.0 to later versions of TLS MAY
 be accomplished by a means similar to that described above.  There
 are multiple ways to accomplish this.  One way is for the server to
 refuse a ClientHello message from any client sending a
 ClientHello.version field corresponding to any version of SSL or
 TLS 1.0.  Another way is for the server to accept ClientHello
 messages from some client versions that it does not wish to support
 but later refuse to allow the user to authenticate.  The latter
 method may provide a better indication to the user of the reason for
 the failure but (depending on the protocol and method of
 authentication used) may also risk exposure of the user's password
 over a channel that is known to not provide adequate confidentiality.
 It is RECOMMENDED that new users be required to use TLS version 1.1
 or greater from the start.  However, an MSP may find it necessary to
 make exceptions to accommodate some legacy systems that support only
 earlier versions of TLS or only cleartext.

4.2. Mail Server Use of Client Certificate Authentication

 Mail Submission Servers and Mail Access Servers MAY implement client
 certificate authentication on the Implicit TLS port.  Such servers
 MUST NOT request a client certificate during the TLS handshake unless
 the server is configured to accept some client certificates as
 sufficient for authentication and the server has the ability to
 determine a mail server authorization identity matching such
 certificates.  How to make this determination is presently
 implementation specific.
 If the server accepts the client's certificate as sufficient for
 authorization, it MUST enable the Simple Authentication and Security
 Layer (SASL) EXTERNAL mechanism [RFC4422].  An IMAPS server MAY issue
 a PREAUTH greeting instead of enabling SASL EXTERNAL.

4.3. Recording TLS Ciphersuite in "Received" Header Field

 The ESMTPS transmission type [RFC3848] provides trace information
 that can indicate that TLS was used when transferring mail.  However,
 TLS usage by itself is not a guarantee of confidentiality or
 security.  The TLS ciphersuite provides additional information about
 the level of security made available for a connection.  This section

Moore & Newman Standards Track [Page 9] RFC 8314 Use of TLS for Email Submission/Access January 2018

 defines a new SMTP "tls" Received header additional-registered-clause
 that is used to record the TLS ciphersuite that was negotiated for
 the connection.  This clause SHOULD be included whenever a Submission
 server generates a Received header field for a message received via
 TLS.  The value included in this additional clause SHOULD be the
 registered ciphersuite name (e.g.,
 TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) included in the "TLS Cipher
 Suite Registry".  In the event that the implementation does not know
 the name of the ciphersuite (a situation that should be remedied
 promptly), a four-digit hexadecimal ciphersuite identifier MAY be
 used.  In addition, the Diffie-Hellman group name associated with the
 ciphersuite MAY be included (when applicable and known) following the
 ciphersuite name.  The ABNF for the field follows:
 tls-cipher-clause  =  CFWS "tls" FWS tls-cipher
                       [ CFWS tls-dh-group-clause ]
 tls-cipher         =  tls-cipher-name / tls-cipher-hex
 tls-cipher-name    =  ALPHA *(ALPHA / DIGIT / "_")
 ; as registered in the IANA "TLS Cipher Suite Registry"
 ; <https://www.iana.org/assignments/tls-parameters>
 tls-cipher-hex     =  "0x" 4HEXDIG
 tls-dh-group-clause = "group" FWS dh-group
 ; not to be used except immediately after tls-cipher
 dh-group           = ALPHA *(ALPHA / DIGIT / "_" / "-")
 ; as registered in the IANA "TLS Supported Groups Registry"
 ; <https://www.iana.org/assignments/tls-parameters>

4.4. TLS Server Certificate Requirements

 MSPs MUST maintain valid server certificates for all servers.  See
 [RFC7817] for the recommendations and requirements necessary to
 achieve this.
 If a protocol server provides service for more than one mail domain,
 it MAY use a separate IP address for each domain and/or a server
 certificate that advertises multiple domains.  This will generally be
 necessary unless and until it is acceptable to impose the constraint
 that the server and all clients support the Server Name Indication
 (SNI) extension to TLS [RFC6066].  Mail servers supporting the SNI
 need to support the post-SRV hostname to interoperate with MUAs that
 have not implemented [RFC6186].  For more discussion of this problem,
 see Section 5.1 of [RFC7817].

Moore & Newman Standards Track [Page 10] RFC 8314 Use of TLS for Email Submission/Access January 2018

4.5. Recommended DNS Records for Mail Protocol Servers

 This section discusses not only the DNS records that are recommended
 but also implications of DNS records for server configuration and TLS
 server certificates.

4.5.1. MX Records

 It is recommended that MSPs advertise MX records for the handling of
 inbound mail (instead of relying entirely on A or AAAA records) and
 that those MX records be signed using DNSSEC [RFC4033].  This is
 mentioned here only for completeness, as the handling of inbound mail
 is out of scope for this document.

4.5.2. SRV Records

 MSPs SHOULD advertise SRV records to aid MUAs in determining the
 proper configuration of servers, per the instructions in [RFC6186].
 MSPs SHOULD advertise servers that support Implicit TLS in preference
 to servers that support cleartext and/or STARTTLS operation.

4.5.3. DNSSEC

 All DNS records advertised by an MSP as a means of aiding clients in
 communicating with the MSP's servers SHOULD be signed using DNSSEC if
 and when the parent DNS zone supports doing so.

4.5.4. TLSA Records

 MSPs SHOULD advertise TLSA records to provide an additional trust
 anchor for public keys used in TLS server certificates.  However,
 TLSA records MUST NOT be advertised unless they are signed using
 DNSSEC.

4.6. Changes to Internet-Facing Servers

 When an MSP changes the Internet-facing Mail Access Servers and Mail
 Submission Servers, including SMTP-based spam/virus filters, it is
 generally necessary to support the same and/or a newer version of TLS
 than the one previously used.

Moore & Newman Standards Track [Page 11] RFC 8314 Use of TLS for Email Submission/Access January 2018

5. Use of TLS by Mail User Agents

 The following requirements and recommendations apply to MUAs:
 o  MUAs SHOULD be capable of using DNS SRV records to discover Mail
    Access Servers and Mail Submission Servers that are advertised by
    an MSP for an account being configured.  Other means of
    discovering server configuration information (e.g., a database
    maintained by the MUA vendor) MAY also be supported.  (See
    Section 5.1 for more information.)
 o  MUAs SHOULD be configurable to require a minimum level of
    confidentiality for any particular Mail Account and refuse to
    exchange information via any service associated with that Mail
    Account if the session does not provide that minimum level of
    confidentiality.  (See Section 5.2.)
 o  MUAs MUST NOT treat a session as meeting a minimum level of
    confidentiality if the server's TLS certificate cannot be
    validated.  (See Section 5.3.)
 o  MUAs MAY impose other minimum confidentiality requirements in the
    future, e.g., in order to discourage the use of TLS versions or
    cryptographic algorithms in which weaknesses have been discovered.
 o  MUAs SHOULD provide a prominent indication of the level of
    confidentiality associated with an account configuration that is
    appropriate for the user interface (for example, a "lock" icon or
    changed background color for a visual interface, or some sort of
    audible indication for an audio user interface), at appropriate
    times and/or locations, in order to inform the user of the
    confidentiality of the communications associated with that
    account.  For example, this might be done whenever (a) the user is
    prompted for authentication credentials, (b) the user is composing
    mail that will be sent to a particular submission server, (c) a
    list of accounts is displayed (particularly if the user can select
    from that list to read mail), or (d) the user is asking to view or
    update any configuration data that will be stored on a remote
    server.  If, however, an MUA provides such an indication, it
    MUST NOT indicate confidentiality for any connection that does not
    at least use TLS 1.1 with certificate verification and also meet
    the minimum confidentiality requirements associated with that
    account.
 o  MUAs MUST implement TLS 1.2 [RFC5246] or later.  Earlier TLS and
    SSL versions MAY also be supported, so long as the MUA requires at
    least TLS 1.1 [RFC4346] when accessing accounts that are
    configured to impose minimum confidentiality requirements.

Moore & Newman Standards Track [Page 12] RFC 8314 Use of TLS for Email Submission/Access January 2018

 o  All MUAs SHOULD implement the recommended TLS ciphersuites
    described in [RFC7525] or a future BCP or Standards Track revision
    of that document.
 o  MUAs that are configured to not require minimum confidentiality
    for one or more accounts SHOULD detect when TLS becomes available
    on those accounts (using [RFC6186] or other means) and offer to
    upgrade the account to require TLS.
 Additional considerations and details appear below.

5.1. Use of SRV Records in Establishing Configuration

 This document updates [RFC6186] by changing the preference rules and
 adding a new SRV service label _submissions._tcp to refer to Message
 Submission with Implicit TLS.
 User-configurable MUAs SHOULD support the use of [RFC6186] for
 account setup.  However, when using configuration information
 obtained via this method, MUAs SHOULD ignore advertised services that
 do not satisfy minimum confidentiality requirements, unless the user
 has explicitly requested reduced confidentiality.  This will have the
 effect of causing the MUA to default to ignoring advertised
 configurations that do not support TLS, even when those advertised
 configurations have a higher priority than other advertised
 configurations.
 When using configuration information per [RFC6186], MUAs SHOULD NOT
 automatically establish new configurations that do not require TLS
 for all servers, unless there are no advertised configurations using
 TLS.  If such a configuration is chosen, prior to attempting to
 authenticate to the server or use the server for Message Submission,
 the MUA SHOULD warn the user that traffic to that server will not be
 encrypted and that it will therefore likely be intercepted by
 unauthorized parties.  The specific wording is to be determined by
 the implementation, but it should adequately capture the sense of
 risk, given the widespread incidence of mass surveillance of email
 traffic.
 Similarly, an MUA MUST NOT attempt to "test" a particular Mail
 Account configuration by submitting the user's authentication
 credentials to a server, unless a TLS session meeting minimum
 confidentiality levels has been established with that server.  If
 minimum confidentiality requirements have not been satisfied, the MUA
 must explicitly warn that the user's password may be exposed to
 attackers before testing the new configuration.

Moore & Newman Standards Track [Page 13] RFC 8314 Use of TLS for Email Submission/Access January 2018

 When establishing a new configuration for connecting to an IMAP, POP,
 or SMTP submission server, based on SRV records, an MUA SHOULD verify
 that either (a) the SRV records are signed using DNSSEC or (b) the
 target Fully Qualified Domain Name (FQDN) of the SRV record matches
 the original server FQDN for which the SRV queries were made.  If the
 target FQDN is not in the queried domain, the MUA SHOULD verify with
 the user that the SRV target FQDN is suitable for use, before
 executing any connections to the host.  (See Section 6 of [RFC6186].)
 An MUA MUST NOT consult SRV records to determine which servers to use
 on every connection attempt, unless those SRV records are signed by
 DNSSEC and have a valid signature.  However, an MUA MAY consult SRV
 records from time to time to determine if an MSP's server
 configuration has changed and alert the user if it appears that this
 has happened.  This can also serve as a means to encourage users to
 upgrade their configurations to require TLS if and when their MSPs
 support it.

5.2. Minimum Confidentiality Level

 MUAs SHOULD, by default, require a minimum level of confidentiality
 for services accessed by each account.  For MUAs supporting the
 ability to access multiple Mail Accounts, this requirement SHOULD be
 configurable on a per-account basis.
 The default minimum expected level of confidentiality for all new
 accounts MUST require successful validation of the server's
 certificate and SHOULD require negotiation of TLS version 1.1 or
 greater.  (Future revisions to this specification may raise these
 requirements or impose additional requirements to address newly
 discovered weaknesses in protocols or cryptographic algorithms.)
 MUAs MAY permit the user to disable this minimum confidentiality
 requirement during initial account configuration or when subsequently
 editing an account configuration but MUST warn users that such a
 configuration will not assure privacy for either passwords or
 messages.
 An MUA that is configured to require a minimum level of
 confidentiality for a Mail Account MUST NOT attempt to perform any
 operation other than capability discovery, or STARTTLS for servers
 not using Implicit TLS, unless the minimum level of confidentiality
 is provided by that connection.
 MUAs SHOULD NOT allow users to easily access or send mail via a
 connection, or authenticate to any service using a password, if that
 account is configured to impose minimum confidentiality requirements
 and that connection does not meet all of those requirements.  An

Moore & Newman Standards Track [Page 14] RFC 8314 Use of TLS for Email Submission/Access January 2018

 example of "easy access" would be to display a dialog informing the
 user that the security requirements of the account were not met by
 the connection but allowing the user to "click through" to send mail
 or access the service anyway.  Experience indicates that users
 presented with such an option often "click through" without
 understanding the risks that they're accepting by doing so.
 Furthermore, users who frequently find the need to "click through" to
 use an insecure connection may become conditioned to do so as a
 matter of habit, before considering whether the risks are reasonable
 in each specific instance.
 An MUA that is not configured to require a minimum level of
 confidentiality for a Mail Account SHOULD still attempt to connect to
 the services associated with that account using the most secure means
 available, e.g., by using Implicit TLS or STARTTLS.

5.3. Certificate Validation

 MUAs MUST validate TLS server certificates according to [RFC7817] and
 PKIX [RFC5280].
 MUAs MAY also support DNS-Based Authentication of Named Entities
 (DANE) [RFC6698] as a means of validating server certificates in
 order to meet minimum confidentiality requirements.
 MUAs MAY support the use of certificate pinning but MUST NOT consider
 a connection in which the server's authenticity relies on certificate
 pinning as providing the minimum level of confidentiality.  (See
 Section 5.4.)

5.4. Certificate Pinning

 During account setup, the MUA will identify servers that provide
 account services such as mail access and mail submission (Section 5.1
 describes one way to do this).  The certificates for these servers
 are verified using the rules described in [RFC7817] and PKIX
 [RFC5280].  In the event that the certificate does not validate due
 to an expired certificate, a lack of an appropriate chain of trust,
 or a lack of an identifier match, the MUA MAY offer to create a
 persistent binding between that certificate and the saved hostname
 for the server, for use when accessing that account's servers.  This
 is called "certificate pinning".
 (Note: This use of the term "certificate pinning" means something
 subtly different than HTTP Public Key Pinning as described in
 [RFC7469].  The dual use of the same term is confusing, but
 unfortunately both uses are well established.)

Moore & Newman Standards Track [Page 15] RFC 8314 Use of TLS for Email Submission/Access January 2018

 Certificate pinning is only appropriate during Mail Account setup and
 MUST NOT be offered as an option in response to a failed certificate
 validation for an existing Mail Account.  An MUA that allows
 certificate pinning MUST NOT allow a certificate pinned for one
 account to validate connections for other accounts.  An MUA that
 allows certificate pinning MUST also allow a user to undo the
 pinning, i.e., to revoke trust in a certificate that has previously
 been pinned.
 A pinned certificate is subject to a man-in-the-middle attack at
 account setup time and typically lacks a mechanism to automatically
 revoke or securely refresh the certificate.  Note also that a man-in-
 the-middle attack at account setup time will expose the user's
 password to the attacker (if a password is used).  Therefore, the use
 of a pinned certificate does not meet the requirement for a minimum
 confidentiality level, and an MUA MUST NOT indicate to the user that
 such confidentiality is provided.  Additional advice on certificate
 pinning is presented in [RFC6125].

5.5. Client Certificate Authentication

 MUAs MAY implement client certificate authentication on the Implicit
 TLS port.  An MUA MUST NOT provide a client certificate during the
 TLS handshake unless the server requests one and the MUA has been
 authorized to use that client certificate with that account.  Having
 the end user explicitly configure a client certificate for use with a
 given account is sufficient to meet this requirement.  However,
 installing a client certificate for use with one account MUST NOT
 automatically authorize the use of that certificate with other
 accounts.  This is not intended to prohibit site-specific
 authorization mechanisms, such as (a) a site-administrator-controlled
 mechanism to authorize the use of a client certificate with a given
 account or (b) a domain-name-matching mechanism.
 Note: The requirement that the server request a certificate is just a
 restatement of the TLS protocol rules, e.g., Section 7.4.6 of
 [RFC5246].  The requirement that the client not send a certificate
 not known to be acceptable to the server is pragmatic in multiple
 ways: the current TLS protocol provides no way for the client to know
 which of the potentially multiple certificates it should use; also,
 when the client sends a certificate, it is potentially disclosing its
 identity (or its user's identity) to both the server and any party
 with access to the transmission medium, perhaps unnecessarily and for
 no useful purpose.

Moore & Newman Standards Track [Page 16] RFC 8314 Use of TLS for Email Submission/Access January 2018

 A client supporting client certificate authentication with Implicit
 TLS MUST implement the SASL EXTERNAL mechanism [RFC4422], using the
 appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for
 SMTP Submission [RFC4954], or AUTHENTICATE for IMAP [RFC3501]).

6. Considerations Related to Antivirus/Antispam Software and Services

 There are multiple ways to connect an AVAS service (e.g., "Antivirus
 & Antispam") to a mail server.  Some mechanisms, such as the de facto
 "milter" protocol, are out of scope for this specification.  However,
 some services use an SMTP relay proxy that intercepts mail at the
 application layer to perform a scan and proxy or forward to another
 Mail Transfer Agent (MTA).  Deploying AVAS services in this way can
 cause many problems [RFC2979], including direct interference with
 this specification, and other forms of confidentiality or security
 reduction.  An AVAS product or service is considered compatible with
 this specification if all IMAP, POP, and SMTP-related software
 (including proxies) it includes are compliant with this
 specification.
 Note that end-to-end email encryption prevents AVAS software and
 services from using email content as part of a spam or virus
 assessment.  Furthermore, although a minimum confidentiality level
 can prevent a man-in-the-middle from introducing spam or virus
 content between the MUA and Submission server, it does not prevent
 other forms of client or account compromise.  The use of AVAS
 services for submitted email therefore remains necessary.

7. IANA Considerations

7.1. POP3S Port Registration Update

 IANA has updated the registration of the TCP well-known port 995
 using the following template [RFC6335]:
   Service Name: pop3s
   Transport Protocol: TCP
   Assignee: IESG <iesg@ietf.org>
   Contact: IETF Chair <chair@ietf.org>
   Description: POP3 over TLS protocol
   Reference: RFC 8314
   Port Number: 995

Moore & Newman Standards Track [Page 17] RFC 8314 Use of TLS for Email Submission/Access January 2018

7.2. IMAPS Port Registration Update

 IANA has updated the registration of the TCP well-known port 993
 using the following template [RFC6335]:
   Service Name: imaps
   Transport Protocol: TCP
   Assignee: IESG <iesg@ietf.org>
   Contact: IETF Chair <chair@ietf.org>
   Description: IMAP over TLS protocol
   Reference: RFC 8314
   Port Number: 993
 No changes to existing UDP port assignments for pop3s or imaps are
 being requested.

7.3. Submissions Port Registration

 IANA has assigned an alternate usage of TCP port 465 in addition to
 the current assignment using the following template [RFC6335]:
   Service Name: submissions
   Transport Protocol: TCP
   Assignee: IESG <iesg@ietf.org>
   Contact: IETF Chair <chair@ietf.org>
   Description: Message Submission over TLS protocol
   Reference: RFC 8314
   Port Number: 465
 This is a one-time procedural exception to the rules in [RFC6335].
 This requires explicit IESG approval and does not set a precedent.
 Note: Since the purpose of this alternate usage assignment is to
 align with widespread existing practice and there is no known usage
 of UDP port 465 for Message Submission over TLS, IANA has not
 assigned an alternate usage of UDP port 465.
 Historically, port 465 was briefly registered as the "smtps" port.
 This registration made no sense, as the SMTP transport MX
 infrastructure has no way to specify a port, so port 25 is always
 used.  As a result, the registration was revoked and was subsequently
 reassigned to a different service.  In hindsight, the "smtps"
 registration should have been renamed or reserved rather than
 revoked.  Unfortunately, some widely deployed mail software
 interpreted "smtps" as "submissions" [RFC6409] and used that port for
 email submission by default when an end user requested security
 during account setup.  If a new port is assigned for the submissions
 service, either (a) email software will continue with unregistered
 use of port 465 (leaving the port registry inaccurate relative to

Moore & Newman Standards Track [Page 18] RFC 8314 Use of TLS for Email Submission/Access January 2018

 de facto practice and wasting a well-known port) or (b) confusion
 between the de facto and registered ports will cause harmful
 interoperability problems that will deter the use of TLS for Message
 Submission.  The authors of this document believe that both of these
 outcomes are less desirable than a "wart" in the registry documenting
 real-world usage of a port for two purposes.  Although STARTTLS on
 port 587 has been deployed, it has not replaced the deployed use of
 Implicit TLS submission on port 465.

7.4. Additional Registered Clauses for "Received" Fields

 Per the provisions in [RFC5321], IANA has added two additional-
 registered-clauses for Received fields as defined in Section 4.3 of
 this document:
 o  "tls": Indicates the TLS cipher used (if applicable)
 o  "group": Indicates the Diffie-Hellman group used with the TLS
    cipher (if applicable)
 The descriptions and syntax of these additional clauses are provided
 in Section 4.3 of this document.

8. Security Considerations

 This entire document is about security considerations.  In general,
 this is targeted to improve mail confidentiality and to mitigate
 threats external to the email system such as network-level snooping
 or interception; this is not intended to mitigate active attackers
 who have compromised service provider systems.
 Implementers should be aware that the use of client certificates with
 TLS 1.2 reveals the user's identity to any party with the ability to
 read packets from the transmission medium and therefore may
 compromise the user's privacy.  There seems to be no easy fix with
 TLS 1.2 or earlier versions, other than to avoid presenting client
 certificates except when there is explicit authorization to do so.
 TLS 1.3 [TLS-1.3] appears to reduce this privacy risk somewhat.

Moore & Newman Standards Track [Page 19] RFC 8314 Use of TLS for Email Submission/Access January 2018

9. References

9.1. Normative References

 [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, DOI 10.17487/RFC0793, September 1981,
            <https://www.rfc-editor.org/info/rfc793>.
 [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
            STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
            <https://www.rfc-editor.org/info/rfc1939>.
 [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>.
 [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
            Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
            February 2002, <https://www.rfc-editor.org/info/rfc3207>.
 [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
            VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501,
            March 2003, <https://www.rfc-editor.org/info/rfc3501>.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements",
            RFC 4033, DOI 10.17487/RFC4033, March 2005,
            <https://www.rfc-editor.org/info/rfc4033>.
 [RFC5034]  Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
            (POP3) Simple Authentication and Security Layer (SASL)
            Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034,
            July 2007, <https://www.rfc-editor.org/info/rfc5034>.
 [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
            Syntax Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
            <https://www.rfc-editor.org/info/rfc5234>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <https://www.rfc-editor.org/info/rfc5246>.

Moore & Newman Standards Track [Page 20] RFC 8314 Use of TLS for Email Submission/Access January 2018

 [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
            <https://www.rfc-editor.org/info/rfc5280>.
 [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
            DOI 10.17487/RFC5322, October 2008,
            <https://www.rfc-editor.org/info/rfc5322>.
 [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
            Submission/Access Services", RFC 6186,
            DOI 10.17487/RFC6186, March 2011,
            <https://www.rfc-editor.org/info/rfc6186>.
 [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
            STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
            <https://www.rfc-editor.org/info/rfc6409>.
 [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
            of Named Entities (DANE) Transport Layer Security (TLS)
            Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
            August 2012, <https://www.rfc-editor.org/info/rfc6698>.
 [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
            "Recommendations for Secure Use of Transport Layer
            Security (TLS) and Datagram Transport Layer Security
            (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525,
            May 2015, <https://www.rfc-editor.org/info/rfc7525>.
 [RFC7672]  Dukhovni, V. and W. Hardaker, "SMTP Security via
            Opportunistic DNS-Based Authentication of Named Entities
            (DANE) Transport Layer Security (TLS)", RFC 7672,
            DOI 10.17487/RFC7672, October 2015,
            <https://www.rfc-editor.org/info/rfc7672>.
 [RFC7817]  Melnikov, A., "Updated Transport Layer Security (TLS)
            Server Identity Check Procedure for Email-Related
            Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016,
            <https://www.rfc-editor.org/info/rfc7817>.
 [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>.

Moore & Newman Standards Track [Page 21] RFC 8314 Use of TLS for Email Submission/Access January 2018

9.2. Informative References

 [CERT-555316]
            CERT, "Vulnerability Note VU#555316: STARTTLS plaintext
            command injection vulnerability", Carnegie Mellon
            University Software Engineering Institute, September 2011,
            <https://www.kb.cert.org/vuls/id/555316>.
 [Email-TLS]
            Moore, K., "Recommendations for use of TLS by Electronic
            Mail Access Protocols", Work in Progress, draft-moore-
            email-tls-00, October 2013.
 [MTA-STS]  Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
            and J. Jones, "SMTP MTA Strict Transport Security
            (MTA-STS)", Work in Progress, draft-ietf-uta-mta-sts-14,
            January 2018.
 [POP3-over-TLS]
            Melnikov, A., Newman, C., and M. Yevstifeyev, Ed., "POP3
            over TLS", Work in Progress, draft-melnikov-pop3-
            over-tls-02, August 2011.
 [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
            RFC 2595, DOI 10.17487/RFC2595, June 1999,
            <https://www.rfc-editor.org/info/rfc2595>.
 [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
            Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000,
            <https://www.rfc-editor.org/info/rfc2979>.
 [RFC3848]  Newman, C., "ESMTP and LMTP Transmission Types
            Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004,
            <https://www.rfc-editor.org/info/rfc3848>.
 [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.1", RFC 4346,
            DOI 10.17487/RFC4346, April 2006,
            <https://www.rfc-editor.org/info/rfc4346>.
 [RFC4422]  Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
            Authentication and Security Layer (SASL)", RFC 4422,
            DOI 10.17487/RFC4422, June 2006,
            <https://www.rfc-editor.org/info/rfc4422>.

Moore & Newman Standards Track [Page 22] RFC 8314 Use of TLS for Email Submission/Access January 2018

 [RFC4954]  Siemborski, R., Ed., and A. Melnikov, Ed., "SMTP Service
            Extension for Authentication", RFC 4954,
            DOI 10.17487/RFC4954, July 2007,
            <https://www.rfc-editor.org/info/rfc4954>.
 [RFC5068]  Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
            Finch, "Email Submission Operations: Access and
            Accountability Requirements", BCP 134, RFC 5068,
            DOI 10.17487/RFC5068, November 2007,
            <https://www.rfc-editor.org/info/rfc5068>.
 [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            DOI 10.17487/RFC5321, October 2008,
            <https://www.rfc-editor.org/info/rfc5321>.
 [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
            Extensions: Extension Definitions", RFC 6066,
            DOI 10.17487/RFC6066, January 2011,
            <https://www.rfc-editor.org/info/rfc6066>.
 [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
            Verification of Domain-Based Application Service Identity
            within Internet Public Key Infrastructure Using X.509
            (PKIX) Certificates in the Context of Transport Layer
            Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
            March 2011, <https://www.rfc-editor.org/info/rfc6125>.
 [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
            Cheshire, "Internet Assigned Numbers Authority (IANA)
            Procedures for the Management of the Service Name and
            Transport Protocol Port Number Registry", BCP 165,
            RFC 6335, DOI 10.17487/RFC6335, August 2011,
            <https://www.rfc-editor.org/info/rfc6335>.
 [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
            Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469,
            April 2015, <https://www.rfc-editor.org/info/rfc7469>.
 [TLS-1.3]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
            Version 1.3", Work in Progress, draft-ietf-tls-tls13-23,
            January 2018.

Moore & Newman Standards Track [Page 23] RFC 8314 Use of TLS for Email Submission/Access January 2018

Appendix A. Design Considerations

 This section is not normative.
 The first version of this document was written independently from the
 October 2013 version of [Email-TLS] ("Recommendations for use of TLS
 by Electronic Mail Access Protocols").  Subsequent versions merge
 ideas from both documents.
 One author of this document was also the author of RFC 2595, which
 became the standard for TLS usage with POP and IMAP, and the other
 author was perhaps the first to propose that idea.  In hindsight,
 both authors now believe that that approach was a mistake.  At this
 point, the authors believe that while anything that makes it easier
 to deploy TLS is good, the desirable end state is that these
 protocols always use TLS, leaving no need for a separate port for
 cleartext operation except to support legacy clients while they
 continue to be used.  The separate-port model for TLS is inherently
 simpler to implement, debug, and deploy.  It also enables a "generic
 TLS load-balancer" that accepts secure client connections for
 arbitrary foo-over-TLS protocols and forwards them to a server that
 may or may not support TLS.  Such load-balancers cause many problems
 because they violate the end-to-end principle and the server loses
 the ability to log security-relevant information about the client
 unless the protocol is designed to forward that information (as this
 specification does for the ciphersuite).  However, they can result in
 TLS deployment where it would not otherwise happen, which is a
 sufficiently important goal that it overrides any problems.
 Although STARTTLS appears only slightly more complex than
 separate-port TLS, we again learned the lesson that complexity is the
 enemy of security in the form of the STARTTLS command injection
 vulnerability (Computer Emergency Readiness Team (CERT) vulnerability
 ID #555316 [CERT-555316]).  Although there's nothing inherently wrong
 with STARTTLS, the fact that it resulted in a common implementation
 error (made independently by multiple implementers) suggests that it
 is a less secure architecture than Implicit TLS.
 Section 7 of RFC 2595 critiques the separate-port approach to TLS.
 The first bullet was a correct critique.  There are proposals in the
 HTTP community to address that, and the use of SRV records as
 described in RFC 6186 resolves that critique for email.  The second
 bullet is correct as well but is not very important because useful
 deployment of security layers other than TLS in email is small enough
 to be effectively irrelevant.  (Also, it's less correct than it used
 to be because "export" ciphersuites are no longer supported in modern
 versions of TLS.)  The third bullet is incorrect because it misses
 the desirable option of "use TLS for all subsequent connections to

Moore & Newman Standards Track [Page 24] RFC 8314 Use of TLS for Email Submission/Access January 2018

 this server once TLS is successfully negotiated".  The fourth bullet
 may be correct, but it is not a problem yet with current port
 consumption rates.  The fundamental error was prioritizing a
 perceived better design based on a mostly valid critique over
 real-world deployability.  But getting security and confidentiality
 facilities actually deployed is so important that it should trump
 design purity considerations.
 Port 465 is presently used for two purposes: for submissions by a
 large number of clients and service providers and for the "urd"
 protocol by one vendor.  Actually documenting this current state is
 controversial, as discussed in the IANA Considerations section.
 However, there is no good alternative.  Registering a new port for
 submissions when port 465 is already widely used for that purpose
 will just create interoperability problems.  Registering a port
 that's only used if advertised by an SRV record (RFC 6186) would not
 create interoperability problems but would require all client
 deployments, server deployments, and software to change
 significantly, which is contrary to the goal of promoting the
 increased use of TLS.  Encouraging the use of STARTTLS on port 587
 would not create interoperability problems, but it is unlikely to
 have any impact on the current undocumented use of port 465 and makes
 the guidance in this document less consistent.  The remaining option
 is to document the current state of the world and support future use
 of port 465 for submission, as this increases consistency and ease of
 deployment for TLS email submission.

Moore & Newman Standards Track [Page 25] RFC 8314 Use of TLS for Email Submission/Access January 2018

Acknowledgements

 Thanks to Ned Freed for discussion of the initial concepts in this
 document.  Thanks to Alexey Melnikov for [POP3-over-TLS], which was
 the basis of the POP3 Implicit TLS text.  Thanks to Russ Housley,
 Alexey Melnikov, and Dan Newman for review feedback.  Thanks to
 Paul Hoffman for interesting feedback in initial conversations about
 this idea.

Authors' Addresses

 Keith Moore
 Windrock, Inc.
 PO Box 1934
 Knoxville, TN  37901
 United States of America
 Email: moore@network-heretics.com
 Chris Newman
 Oracle
 440 E. Huntington Dr., Suite 400
 Arcadia, CA  91006
 United States of America
 Email: chris.newman@oracle.com

Moore & Newman Standards Track [Page 26]

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