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.

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

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.

   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".

   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

   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.

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.)

   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.

   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

   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].

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.