draft-ietf-uta-email-deep-12.txt   rfc8314.txt 
Network Working Group K. Moore Internet Engineering Task Force (IETF) K. Moore
Internet-Draft Windrock, Inc. Request for Comments: 8314 Windrock, Inc.
Updates: 1939, 2595, 3464, 3501, 5068, C. Newman Updates: 1939, 2595, 3501, 5068, 6186, 6409 C. Newman
6186, 6409 (if approved) Oracle Category: Standards Track Oracle
Intended status: Standards Track December 6, 2017 ISSN: 2070-1721 January 2018
Expires: June 9, 2018
Cleartext Considered Obsolete: Use of TLS for Email Submission and Cleartext Considered Obsolete: Use of Transport Layer Security (TLS)
Access for Email Submission and Access
draft-ietf-uta-email-deep-12
Abstract Abstract
This specification outlines current recommendations for the use of This specification outlines current recommendations for the use of
Transport Layer Security (TLS) to provide confidentiality of email Transport Layer Security (TLS) to provide confidentiality of email
traffic between a mail user agent (MUA) and a mail submission or mail traffic between a Mail User Agent (MUA) and a Mail Submission Server
access server. or Mail Access Server. This document updates RFCs 1939, 2595, 3501,
5068, 6186, and 6409.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
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 This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on June 9, 2018. 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 Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction ....................................................3
2. Conventions and Terminology Used in This Document . . . . . . 3 1.1. How This Document Updates Previous RFCs ....................3
3. Implicit TLS . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions and Terminology Used in This Document ...............4
3.1. Implicit TLS for POP . . . . . . . . . . . . . . . . . . 5 3. Implicit TLS ....................................................5
3.2. Implicit TLS for IMAP . . . . . . . . . . . . . . . . . . 5 3.1. Implicit TLS for POP .......................................5
3.3. Implicit TLS for SMTP Submission . . . . . . . . . . . . 5 3.2. Implicit TLS for IMAP ......................................5
3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP 3.3. Implicit TLS for SMTP Submission ...........................6
Submission . . . . . . . . . . . . . . . . . . . . . . . 6 3.4. Implicit TLS Connection Closure for POP, IMAP, and
4. Use of TLS by Mail Access Services and SMTP Submission ............................................7
Message Submission Services . . . . . . . . . . . . . . . . . 6 4. Use of TLS by Mail Access Servers and Message Submission
4.1. Deprecation of Services Using Cleartext and TLS Versions Servers .........................................................7
< 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Deprecation of Services Using Cleartext and TLS Versions
4.2. Mail Server Use of Client Certificate Authentication . . 9 Less Than 1.1 ..............................................8
4.3. Recording TLS Cipher Suite in Received Header . . . . . . 9 4.2. Mail Server Use of Client Certificate Authentication .......9
4.4. TLS Server Certificate Requirements . . . . . . . . . . . 10 4.3. Recording TLS Ciphersuite in "Received" Header Field .......9
4.5. Recommended DNS records for mail protocol servers . . . . 10 4.4. TLS Server Certificate Requirements .......................10
4.5.1. MX records . . . . . . . . . . . . . . . . . . . . . 10 4.5. Recommended DNS Records for Mail Protocol Servers .........11
4.5.2. SRV records . . . . . . . . . . . . . . . . . . . . . 10 4.5.1. MX Records .........................................11
4.5.3. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 10 4.5.2. SRV Records ........................................11
4.5.4. TLSA records . . . . . . . . . . . . . . . . . . . . 11 4.5.3. DNSSEC .............................................11
4.6. Changes to Internet Facing Servers . . . . . . . . . . . 11 4.5.4. TLSA Records .......................................11
5. Use of TLS by Mail User Agents . . . . . . . . . . . . . . . 11 4.6. Changes to Internet-Facing Servers ........................11
5.1. Use of SRV records in Establishing Configuration . . . . 12 5. Use of TLS by Mail User Agents .................................12
5.2. Minimum Confidentiality Level . . . . . . . . . . . . . . 13 5.1. Use of SRV Records in Establishing Configuration ..........13
5.3. Certificiate Validation . . . . . . . . . . . . . . . . . 14 5.2. Minimum Confidentiality Level .............................14
5.4. Certificate Pinning . . . . . . . . . . . . . . . . . . . 15 5.3. Certificate Validation ....................................15
5.5. Client Certificate Authentication . . . . . . . . . . . . 15 5.4. Certificate Pinning .......................................15
6. Considerations related to Anti-Virus/Anti-Spam Software and 5.5. Client Certificate Authentication .........................16
Services . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6. Considerations Related to Antivirus/Antispam Software
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 and Services ...................................................17
7.1. POP3S Port Registration Update . . . . . . . . . . . . . 17 7. IANA Considerations ............................................17
7.2. IMAPS Port Registration Update . . . . . . . . . . . . . 17 7.1. POP3S Port Registration Update ............................17
7.3. Submissions Port Registration . . . . . . . . . . . . . . 17 7.2. IMAPS Port Registration Update ............................18
7.4. Additional registered clauses for Received fields . . . . 18 7.3. Submissions Port Registration .............................18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 7.4. Additional Registered Clauses for "Received" Fields .......19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 8. Security Considerations ........................................19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19 9. References .....................................................20
9.2. Informative References . . . . . . . . . . . . . . . . . 21 9.1. Normative References ......................................20
Appendix A. Design Considerations . . . . . . . . . . . . . . . 22 9.2. Informative References ....................................22
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 24 Appendix A. Design Considerations .................................24
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 29 Acknowledgements ..................................................26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 Authors' Addresses ................................................26
1. Introduction 1. Introduction
Software that provides email service via Internet Message Access Software that provides email service via the Internet Message Access
Protocol (IMAP) [RFC3501], Post Office Protocol (POP) [RFC1939] and/ Protocol (IMAP) [RFC3501], the Post Office Protocol (POP) [RFC1939],
or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409] usually and/or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409]
has Transport Layer Security (TLS) [RFC5246] support but often does usually has Transport Layer Security (TLS) [RFC5246] support but
not use it in a way that maximizes end-user confidentiality. This often does not use it in a way that maximizes end-user
specification describes current recommendations for the use of TLS in confidentiality. This specification describes current
interactions between Mail User Agents and Mail Access Services, and recommendations for the use of TLS in interactions between Mail User
between Mail User Agents and Mail Submission Services. Agents (MUAs) and Mail Access Servers, and also between MUAs and Mail
Submission Servers.
In brief, this memo now recommends that: In brief, this memo now recommends that:
o TLS version 1.2 or greater be used for all traffic between mail o TLS version 1.2 or greater be used for all traffic between MUAs
user agents (MUAs) and mail submission servers, and also between and Mail Submission Servers, and also between MUAs and Mail Access
MUAs and mail access servers. Servers.
o MUAs and mail service providers discourage use of cleartext o MUAs and Mail Service Providers (MSPs) (a) discourage the use of
protocols for mail access and mail submission, and deprecate use cleartext protocols for mail access and mail submission and
of cleartext protocols for these purposes as soon as practicable. (b) deprecate the use of cleartext protocols for these purposes as
soon as practicable.
o Use of "Implicit TLS" on ports reserved for that purpose, in o Connections to Mail Submission Servers and Mail Access Servers be
preference to STARTTLS on a port that otherwise supports made using "Implicit TLS" (as defined below), in preference to
cleartext. connecting to the "cleartext" port and negotiating TLS using the
STARTTLS command or a similar command.
This memo does not address use of TLS with SMTP for message relay This memo does not address the use of TLS with SMTP for message relay
(where Message Submission [RFC6409] does not apply). Improved use of (where Message Submission [RFC6409] does not apply). Improving the
TLS with SMTP for message relay requires a different approach. One use of TLS with SMTP for message relay requires a different approach.
approach to address that topic is described in [RFC7672]; another is One approach to address that topic is described in [RFC7672]; another
in [I-D.ietf-uta-mta-sts]. is provided in [MTA-STS].
The recommendations in this memo do not replace the functionality of, The recommendations in this memo do not replace the functionality of,
and are not intended as a substitute for, end-to-end encryption of and are not intended as a substitute for, end-to-end encryption of
electronic mail. 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 2. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119][RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
The term "Implicit TLS" refers to the automatic negotiation of TLS The term "Implicit TLS" refers to the automatic negotiation of TLS
whenever a TCP connection is made on a particular TCP port that is whenever a TCP connection is made on a particular TCP port that is
used exclusively by that server for TLS connections. The term used exclusively by that server for TLS connections. The term
"Implicit TLS" is intended to contrast with use of STARTTLS and "Implicit TLS" is intended to contrast with the use of STARTTLS and
similar commands in POP, IMAP, SMTP message submission, and other similar commands in POP, IMAP, SMTP Message Submission, and other
protocols, that are used by client and server to explicitly negotiate protocols, that are used by the client and the server to explicitly
TLS on an established cleartext TCP connection. negotiate TLS on an established cleartext TCP connection.
The term "Mail Access Services" includes POP, IMAP and any other The term "Mail Access Server" refers to a server for POP, IMAP, and
protocol used to access or modify received messages, or to access or any other protocol used to access or modify received messages, or to
modify a mail user's account configuration. access or modify a mail user's account configuration.
"Mail Submission Service" refers to the use of the protocol specified The term "Mail Submission Server" refers to a server for the protocol
in [RFC6409] (or one of its predecessors or successors) for specified in [RFC6409] (or one of its predecessors or successors) for
submission of outgoing messages for delivery to recipients. submission of outgoing messages for delivery to recipients.
The term "Mail Service Provider" (MSP) refers to a provider of Mail The term "Mail Service Provider" (or "MSP") refers to an operator of
Access Services and/or Mail Submission Services. Mail Access Servers and/or Mail Submission Servers.
The term "Mail Account" refers to a user's identity with a Mail The term "Mail Account" refers to a user's identity with an MSP, that
Service Provider, that user's authentication credentials, any user user's authentication credentials, any user email that is stored by
email that is stored by the MSP, and any other per-user configuration the MSP, and any other per-user configuration information maintained
information maintained by the MSP (for example, spam filtering by the MSP (for example, instructions for filtering spam). Most MUAs
instructions). Most Mail User Agents (MUAs) support the ability to support the ability to access multiple Mail Accounts.
access multiple Mail Accounts.
For each account that an MUA accesses on its user's behalf, it must For each account that an MUA accesses on its user's behalf, it must
have the server names, ports, authentication credentials, and other have the server names, ports, authentication credentials, and other
configuration information specified by the user. This information configuration information specified by the user. This information,
which is used by the MUA is referred to as "Mail Account which is used by the MUA, is referred to as "Mail Account
Configuration" Configuration".
This specification expresses syntax using the Augmented Backus-Naur This specification expresses syntax using the Augmented Backus-Naur
Form (ABNF) as described in [RFC5234], including the core rules in Form (ABNF) as described in [RFC5234], including the core rules
Appendix B and rules from [RFC5322]. provided in Appendix B of [RFC5234] and the rules provided in
[RFC5322].
3. Implicit TLS 3. Implicit TLS
Previous standards for use of email protocols with TLS used the Previous standards for the use of email protocols with TLS used the
STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501]. With STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501]. With
STARTTLS, the client establishes a cleartext application session and STARTTLS, the client establishes a cleartext application session and
determines whether to issue a STARTTLS command based on server determines whether to issue a STARTTLS command based on server
capabilities and client configuration. If the client issues a capabilities and client configuration. If the client issues a
STARTTLS command, a TLS handshake follows that can upgrade the STARTTLS command, a TLS handshake follows that can upgrade the
connection. While this mechanism has been deployed, an alternate connection. Although this mechanism has been deployed, an alternate
mechanism where TLS is negotiated immediately at connection start on mechanism where TLS is negotiated immediately at connection start on
a separate port (referred to in this document as "Implicit TLS") has a separate port (referred to in this document as "Implicit TLS") has
been deployed more successfully. To encourage more widespread use of been deployed more successfully. To encourage more widespread use of
TLS, and to encourage a greater consistency for how TLS is used, this TLS and to also encourage greater consistency regarding how TLS is
specification now recommends use of Implicit TLS for POP, IMAP, SMTP used, this specification now recommends the use of Implicit TLS for
Submission, and all other protocols used between a Mail User Agent POP, IMAP, SMTP Submission, and all other protocols used between an
and a mail service. MUA and an MSP.
3.1. Implicit TLS for POP 3.1. Implicit TLS for POP
When a TCP connection is established for the "pop3s" service (default When a TCP connection is established for the "pop3s" service (default
port 995), a TLS handshake begins immediately. Clients MUST port 995), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in implement the certificate validation mechanism described in
[RFC7817]. Once the TLS session is established, POP3 [RFC1939] [RFC7817]. Once the TLS session is established, POP3 [RFC1939]
protocol messages are exchanged as TLS application data for the protocol messages are exchanged as TLS application data for the
remainder of the TCP connection. After the server sends a +OK remainder of the TCP connection. After the server sends an +OK
greeting, the server and client MUST enter AUTHORIZATION state, even greeting, the server and client MUST enter the AUTHORIZATION state,
if a client certificate was supplied during the TLS handshake. even if a client certificate was supplied during the TLS handshake.
See Section 5.5 and Section 4.2 for additional information on client See Sections 5.5 and 4.2 for additional information on client
certificate authentication. See Section 7.1 for port registration certificate authentication. See Section 7.1 for port registration
information. information.
3.2. Implicit TLS for IMAP 3.2. Implicit TLS for IMAP
When a TCP connection is established for the "imaps" service (default When a TCP connection is established for the "imaps" service (default
port 993), a TLS handshake begins immediately. Clients MUST port 993), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in implement the certificate validation mechanism described in
[RFC7817]. Once the TLS session is established, IMAP [RFC3501] [RFC7817]. Once the TLS session is established, IMAP [RFC3501]
protocol messages are exchanged as TLS application data for the protocol messages are exchanged as TLS application data for the
remainder of the TCP connection. If a client certificate was remainder of the TCP connection. If a client certificate was
provided during the TLS handshake that the server finds acceptable, provided during the TLS handshake that the server finds acceptable,
the server MAY issue a PREAUTH greeting in which case both the server the server MAY issue a PREAUTH greeting, in which case both the
and client enter AUTHENTICATED state. If the server issues an OK server and the client enter the AUTHENTICATED state. If the server
greeting then both server and client enter NOT AUTHENTICATED state. issues an OK greeting, then both the server and the client enter the
NOT AUTHENTICATED state.
See Section 5.5 and Section 4.2 for additional information on client See Sections 5.5 and 4.2 for additional information on client
certificate authentication. See Section 7.1 and Section 7.2 for port certificate authentication. See Section 7.2 for port registration
registration information. information.
3.3. Implicit TLS for SMTP Submission 3.3. Implicit TLS for SMTP Submission
When a TCP connection is established for the "submissions" service When a TCP connection is established for the "submissions" service
(default port 465), a TLS handshake begins immediately. Clients MUST (default port 465), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in implement the certificate validation mechanism described in
[RFC7817]. Once a TLS session is established, message submission [RFC7817]. Once the TLS session is established, Message Submission
protocol data [RFC6409] is exchanged as TLS application data for the protocol data [RFC6409] is exchanged as TLS application data for the
remainder of the TCP connection. (Note: the "submissions" service remainder of the TCP connection. (Note: The "submissions" service
name is defined in section 10.3 of this document, and follows the name is defined in Section 7.3 of this document and follows the usual
usual convention that the name of a service layered on top of convention that the name of a service layered on top of Implicit TLS
Implicit TLS consists of the name of the service as used without TLS, consists of the name of the service as used without TLS, with an "s"
with an "s" appended.) appended.)
The STARTTLS mechanism on port 587 is relatively widely deployed due The STARTTLS mechanism on port 587 is relatively widely deployed due
to the situation with port 465 (discussed in Section 7.3). This to the situation with port 465 (discussed in Section 7.3). This
differs from IMAP and POP services where Implicit TLS is more widely differs from IMAP and POP services where Implicit TLS is more widely
deployed on servers than STARTTLS. It is desirable to migrate core deployed on servers than STARTTLS. It is desirable to migrate core
protocols used by MUA software to Implicit TLS over time for protocols used by MUA software to Implicit TLS over time, for
consistency as well as the additional reasons discussed in consistency as well as for the additional reasons discussed in
Appendix A. However, to maximize use of encryption for submission it Appendix A. However, to maximize the use of encryption for
is desirable to support both mechanisms for Message Submission over submission, it is desirable to support both mechanisms for Message
TLS for a transition period of several years. As a result, clients Submission over TLS for a transition period of several years. As a
and servers SHOULD implement both STARTTLS on port 587 and Implicit result, clients and servers SHOULD implement both STARTTLS on
TLS on port 465 for this transition period. Note that there is no port 587 and Implicit TLS on port 465 for this transition period.
significant difference between the security properties of STARTTLS on Note that there is no significant difference between the security
port 587 and Implicit TLS on port 465 if the implementations are properties of STARTTLS on port 587 and Implicit TLS on port 465 if
correct and both client and server are configured to require the implementations are correct and if both the client and the server
successful negotiation of TLS prior to message submission. are configured to require successful negotiation of TLS prior to
Message Submission.
Note that the "submissions" port provides access to a Mail Submission Note that the "submissions" port provides access to a Message
Agent (MSA) as defined in [RFC6409] so requirements and Submission Agent (MSA) as defined in [RFC6409], so requirements and
recommendations for MSAs in that document apply to the submissions recommendations for MSAs in that document, including the requirement
port, including the requirement to implement SMTP AUTH [RFC4954]. to implement SMTP AUTH [RFC4954] and the requirements of Email
Submission Operations [RFC5068], also apply to the submissions port.
See Section 5.5 and Section 4.2 for additional information on client See Sections 5.5 and 4.2 for additional information on client
certificate authentication. See Section 7.3 for port registration certificate authentication. See Section 7.3 for port registration
information. information.
3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP Submission 3.4. Implicit TLS Connection Closure for POP, IMAP, and SMTP Submission
When a client or server wishes to close the connection, it SHOULD When a client or server wishes to close the connection, it SHOULD
initiate the exchange of TLS close alerts before TCP connection initiate the exchange of TLS close alerts before TCP connection
termination. The client MAY, after sending a TLS close alert, termination. The client MAY, after sending a TLS close alert,
gracefully close the TCP connection (e.g. call the close() function gracefully close the TCP connection (e.g., call the close() function
on the TCP socket or otherwise issue a TCP CLOSE ([RFC0793] section on the TCP socket or otherwise issue a TCP CLOSE ([RFC793],
3.5) without waiting for a TLS response from the server. Section 3.5)) without waiting for a TLS response from the server.
4. Use of TLS by Mail Access Services and Message Submission Services 4. Use of TLS by Mail Access Servers and Message Submission Servers
The following requirements and recommendations apply to Mail Access The following requirements and recommendations apply to Mail Access
Services and Mail Submission Services: Servers and Mail Submission Servers, or, if indicated, to MSPs:
o Mail Service Providers (MSPs) that support POP, IMAP, and/or o MSPs that support POP, IMAP, and/or Message Submission MUST
Message Submission, MUST support TLS access for those services. support TLS access for those protocol servers.
o Other services than POP, IMAP and/or Message Submission provided o Servers provided by MSPs other than POP, IMAP, and/or Message
by MSPs SHOULD support TLS access, and MUST support TLS access for Submission SHOULD support TLS access and MUST support TLS access
those services which support authentication via username and for those servers that support authentication via username and
password. password.
o MSPs that support POP, IMAP, and/or Message Submission, SHOULD o MSPs that support POP, IMAP, and/or Message Submission SHOULD
provide and support instances of those services which use Implicit provide and support instances of those services that use Implicit
TLS. (See Section 3.) TLS. (See Section 3.)
o For compatibility with existing MUAs and existing MUA o For compatibility with existing MUAs and existing MUA
configurations, MSPs SHOULD also, in the near term, provide configurations, MSPs SHOULD also, in the near term, provide
instances of these services which support STARTTLS. This will instances of these services that support STARTTLS. This will
permit legacy MUAs to discover new availability of TLS capability permit legacy MUAs to discover new availability of TLS capability
on servers, and may increase use of TLS by such MUAs. However, on servers and may increase the use of TLS by such MUAs. However,
servers SHOULD NOT advertise STARTTLS if use of the STARTTLS servers SHOULD NOT advertise STARTTLS if the use of the STARTTLS
command by a client is likely to fail (for example, if the server command by a client is likely to fail (for example, if the server
has no server certificate configured.) has no server certificate configured).
o MSPs SHOULD advertise their Mail Access Services and Mail o MSPs SHOULD advertise their Mail Access Servers and Mail
Submission Services using DNS SRV records according to [RFC6186]. Submission Servers, using DNS SRV records according to [RFC6186].
(In addition to making correct configuration easier for MUAs, this (In addition to making correct configuration easier for MUAs, this
provides a way by which MUAs can discover when an MSP begins to provides a way by which MUAs can discover when an MSP begins to
offer TLS-based services.) Services supporting TLS SHOULD be offer TLS-based services.) Servers supporting TLS SHOULD be
advertised in preference to cleartext services (if offered). In advertised in preference to cleartext servers (if offered). In
addition, services using Implicit TLS SHOULD be advertised in addition, servers using Implicit TLS SHOULD be advertised in
preference to services supporting STARTTLS (if offered). (See preference to servers supporting STARTTLS (if offered). (See also
also Section 4.5.) Section 4.5.)
o MSPs SHOULD deprecate use of cleartext Mail Access Services and o MSPs SHOULD deprecate the use of cleartext Mail Access Servers and
Mail Submission Services as soon as practicable. (See Mail Submission Servers as soon as practicable. (See
Section 4.1.) Section 4.1.)
o MSPs currently supporting such use of cleartext SMTP (on port 25) o MSPs currently supporting such use of cleartext SMTP (on port 25)
as a means of message submission by their users (whether or not as a means of Message Submission by their users (whether or not
requiring authentication) SHOULD transition their users to using requiring authentication) SHOULD transition their users to using
TLS (either Implicit TLS or STARTTLS) as soon as practicable. TLS (either Implicit TLS or STARTTLS) as soon as practicable.
o Mail services MUST support TLS 1.2 or later. o Mail Access Servers and Mail Submission Servers MUST support
TLS 1.2 or later.
o All Mail services SHOULD implement the recommended TLS cipher o All Mail Access Servers and Mail Submission Servers SHOULD
suites described in [RFC7525] or a future BCP or standards track implement the recommended TLS ciphersuites described in [RFC7525]
revision of that document. or a future BCP or Standards Track revision of that document.
o Mail services currently supporting SSL 2.x, SSL 3.0, or TLS 1.0 o As soon as practicable, MSPs currently supporting Secure Sockets
SHOULD transition their users to later versions of TLS, and Layer (SSL) 2.x, SSL 3.0, or TLS 1.0 SHOULD transition their users
discontinue support for those versions of SSL and TLS, as soon as to TLS 1.1 or later and discontinue support for those earlier
practicable. versions of SSL and TLS.
o Mail Submission Servers accepting mail using TLS SHOULD include o Mail Submission Servers accepting mail using TLS SHOULD include in
the TLS ciphersuite of the session in which the mail was received, the Received field of the outgoing message the TLS ciphersuite of
in the Received field of the outgoing message. (See Section 4.3.) the session in which the mail was received. (See Section 4.3.)
o All Mail services implementing TLS SHOULD log TLS cipher o All Mail Access Servers and Mail Submission Servers implementing
information along with any connection or authentication logs that TLS SHOULD log TLS cipher information along with any connection or
they maintain. authentication logs that they maintain.
Additional considerations and details appear below. Additional considerations and details appear below.
4.1. Deprecation of Services Using Cleartext and TLS Versions < 1.1 4.1. Deprecation of Services Using Cleartext and TLS Versions
Less Than 1.1
The specific means employed for deprecation of cleartext Mail Access The specific means employed for deprecation of cleartext Mail Access
Services and Mail Submission Services MAY vary from one MSP to the Servers and Mail Submission Servers MAY vary from one MSP to the next
next in light of their user communities' needs and constraints. For in light of their user communities' needs and constraints. For
example, an MSP MAY implement a gradual transition in which, over example, an MSP MAY implement a gradual transition in which, over
time, more and more users are forbidden to authenticate to cleartext time, more and more users are forbidden to authenticate to cleartext
instances of these services, thus encouraging those users to migrate instances of these servers, thus encouraging those users to migrate
to Implicit TLS. Access to cleartext services should eventually be to Implicit TLS. Access to cleartext servers should eventually be
either disabled, or limited strictly for use by legacy systems which either (a) disabled or (b) limited strictly for use by legacy systems
cannot be upgraded. that cannot be upgraded.
After a user's ability to authenticate to a service using cleartext After a user's ability to authenticate to a server using cleartext is
is revoked, the server denying such access MUST NOT provide any revoked, the server denying such access MUST NOT provide any
indication over a cleartext channel of whether the user's indication over a cleartext channel of whether the user's
authentication credentials were valid. An attempt to authenticate as authentication credentials were valid. An attempt to authenticate as
such a user using either invalid credentials or valid credentials such a user using either invalid credentials or valid credentials
MUST both result in the same indication of access being denied. MUST both result in the same indication of access being denied.
Also, users previously authenticating with passwords sent as Also, users previously authenticating with passwords sent as
cleartext SHOULD be required to change those passwords when migrating cleartext SHOULD be required to change those passwords when migrating
to TLS, if the old passwords were likely to have been compromised. to TLS, if the old passwords were likely to have been compromised.
(For any large community of users using public Internet to access (For any large community of users using the public Internet to access
mail without encryption, compromise of at least some of those mail without encryption, the compromise of at least some of those
passwords should be assumed.) passwords should be assumed.)
Transition of users from SSL or TLS 1.0 to later versions of TLS MAY 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 be accomplished by a means similar to that described above. There
are multiple ways to accomplish this. One way is for the server to are multiple ways to accomplish this. One way is for the server to
refuse a ClientHello message from any client sending a refuse a ClientHello message from any client sending a
ClientHello.version field corresponding to any version of SSL or TLS ClientHello.version field corresponding to any version of SSL or
1.0. Another way is for the server to accept ClientHello messages TLS 1.0. Another way is for the server to accept ClientHello
from some client versions that it does not wish to support, but later messages from some client versions that it does not wish to support
refuse to allow the user to authenticate. The latter method may but later refuse to allow the user to authenticate. The latter
provide a better indication to the user of the reason for the failure method may provide a better indication to the user of the reason for
but (depending on the protocol and method of authentication used) may the failure but (depending on the protocol and method of
also risk exposure of the user's password over an channel which is authentication used) may also risk exposure of the user's password
known to not provide adequate confidentiality. 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 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 or greater from the start. However, an MSP may find it necessary to
make exceptions to accommodate some legacy systems which support only make exceptions to accommodate some legacy systems that support only
earlier versions of TLS, or only cleartext. earlier versions of TLS or only cleartext.
4.2. Mail Server Use of Client Certificate Authentication 4.2. Mail Server Use of Client Certificate Authentication
Mail servers MAY implement client certificate authentication on the Mail Submission Servers and Mail Access Servers MAY implement client
Implicit TLS port. Servers MUST NOT request a client certificate certificate authentication on the Implicit TLS port. Such servers
during the TLS handshake unless the server is configured to accept MUST NOT request a client certificate during the TLS handshake unless
some client certificates as sufficient for authentication and the the server is configured to accept some client certificates as
server has the ability to determine a mail server authorization sufficient for authentication and the server has the ability to
identity matching such certificates. How to make this determination determine a mail server authorization identity matching such
is presently implementation specific. certificates. How to make this determination is presently
implementation specific.
If the server accepts the client's certificate as sufficient for If the server accepts the client's certificate as sufficient for
authorization, it MUST enable the SASL EXTERNAL [RFC4422] mechanism. authorization, it MUST enable the Simple Authentication and Security
An IMAPS server MAY issue a PREAUTH greeting instead of enabling SASL Layer (SASL) EXTERNAL mechanism [RFC4422]. An IMAPS server MAY issue
EXTERNAL. a PREAUTH greeting instead of enabling SASL EXTERNAL.
4.3. Recording TLS Cipher Suite in Received Header 4.3. Recording TLS Ciphersuite in "Received" Header Field
The ESMTPS transmission type [RFC3848] provides trace information The ESMTPS transmission type [RFC3848] provides trace information
that can indicate TLS was used when transferring mail. However, TLS that can indicate that TLS was used when transferring mail. However,
usage by itself is not a guarantee of confidentiality or security. TLS usage by itself is not a guarantee of confidentiality or
The TLS cipher suite provides additional information about the level security. The TLS ciphersuite provides additional information about
of security made available for a connection. This defines a new SMTP the level of security made available for a connection. This section
"tls" Received header additional-registered-clause that is used to defines a new SMTP "tls" Received header additional-registered-clause
record the TLS cipher suite that was negotiated for the connection. that is used to record the TLS ciphersuite that was negotiated for
This clause SHOULD be included whenever a Submission server generates the connection. This clause SHOULD be included whenever a Submission
a Received header field for a message received via TLS. The value server generates a Received header field for a message received via
included in this additional clause SHOULD be the registered cipher TLS. The value included in this additional clause SHOULD be the
suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) included in registered ciphersuite name (e.g.,
the TLS cipher suite registry. In the event the implementation does TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) included in the "TLS Cipher
not know the name of the cipher suite (a situation that should be Suite Registry". In the event that the implementation does not know
remedied promptly), a four-digit hexadecimal cipher suite identifier the name of the ciphersuite (a situation that should be remedied
MAY be used. In addition, the Diffie-Hellman group name associated promptly), a four-digit hexadecimal ciphersuite identifier MAY be
with the ciphersuite MAY be included (when applicable and known) used. In addition, the Diffie-Hellman group name associated with the
following the ciphersuite name. The ABNF for the field follows: 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 "group" FWS dh-group ] tls-cipher-clause = CFWS "tls" FWS tls-cipher
[ CFWS tls-dh-group-clause ]
tls-cipher = tls-cipher-name / tls-cipher-hex tls-cipher = tls-cipher-name / tls-cipher-hex
tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_") tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_")
; as registered in IANA cipher suite registry ; as registered in the IANA "TLS Cipher Suite Registry"
; <https://www.iana.org/assignments/tls-parameters>
tls-cipher-hex = "0x" 4HEXDIG 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>
dh-group = ALPHA *(ALPHA / DIGIT / "_" / "-")
; as registered in IANA TLS Supported Groups Registry
4.4. TLS Server Certificate Requirements 4.4. TLS Server Certificate Requirements
MSPs MUST maintain valid server certificates for all servers. See MSPs MUST maintain valid server certificates for all servers. See
[RFC7817] for the recommendations and requirements necessary to [RFC7817] for the recommendations and requirements necessary to
achieve this. achieve this.
If a protocol server provides service for more than one mail domain, 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 it MAY use a separate IP address for each domain and/or a server
certificate that advertises multiple domains. This will generally be certificate that advertises multiple domains. This will generally be
necessary unless and until it is acceptable to impose the constraint necessary unless and until it is acceptable to impose the constraint
that the server and all clients support the Server Name Indication that the server and all clients support the Server Name Indication
extension to TLS [RFC6066]. Mail servers supporting SNI need to (SNI) extension to TLS [RFC6066]. Mail servers supporting the SNI
support the post-SRV hostname to interoperate with MUAs that have not need to support the post-SRV hostname to interoperate with MUAs that
implemented RFC 6186. For more discussion of this problem, see have not implemented [RFC6186]. For more discussion of this problem,
section 5.1 of [RFC7817]. see Section 5.1 of [RFC7817].
4.5. Recommended DNS records for mail protocol servers 4.5. Recommended DNS Records for Mail Protocol Servers
This section discusses not only the DNS records that are recommended, This section discusses not only the DNS records that are recommended
but also implications of DNS records for server configuration and TLS but also implications of DNS records for server configuration and TLS
server certificates. server certificates.
4.5.1. MX records 4.5.1. MX Records
It is recommended that MSPs advertise MX records for handling of It is recommended that MSPs advertise MX records for the handling of
inbound mail (instead of relying entirely on A or AAAA records), and inbound mail (instead of relying entirely on A or AAAA records) and
that those MX records be signed using DNSSEC [RFC4033]. This is that those MX records be signed using DNSSEC [RFC4033]. This is
mentioned here only for completeness, as handling of inbound mail is mentioned here only for completeness, as the handling of inbound mail
out of scope for this document. is out of scope for this document.
4.5.2. SRV records 4.5.2. SRV Records
MSPs SHOULD advertise SRV records to aid MUAs in determination of MSPs SHOULD advertise SRV records to aid MUAs in determining the
proper configuration of servers, per the instructions in [RFC6186]. proper configuration of servers, per the instructions in [RFC6186].
MSPs SHOULD advertise servers that support Implicit TLS in preference MSPs SHOULD advertise servers that support Implicit TLS in preference
to those which support cleartext and/or STARTTLS operation. to servers that support cleartext and/or STARTTLS operation.
4.5.3. DNSSEC 4.5.3. DNSSEC
All DNS records advertised by an MSP as a means of aiding clients in 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 communicating with the MSP's servers SHOULD be signed using DNSSEC if
if and when the parent DNS zone supports doing so. and when the parent DNS zone supports doing so.
4.5.4. TLSA records 4.5.4. TLSA Records
MSPs SHOULD advertise TLSA records to provide an additional trust MSPs SHOULD advertise TLSA records to provide an additional trust
anchor for public keys used in TLS server certificates. However, anchor for public keys used in TLS server certificates. However,
TLSA records MUST NOT be advertised unless they are signed using TLSA records MUST NOT be advertised unless they are signed using
DNSSEC. DNSSEC.
4.6. Changes to Internet Facing Servers 4.6. Changes to Internet-Facing Servers
When an MSP changes the Internet Facing Servers providing mail access When an MSP changes the Internet-facing Mail Access Servers and Mail
and mail submission services, including SMTP-based spam/virus Submission Servers, including SMTP-based spam/virus filters, it is
filters, it is generally necessary to support the same and/or a newer generally necessary to support the same and/or a newer version of TLS
version of TLS and the same security directives that were previously than the one previously used.
advertised.
5. Use of TLS by Mail User Agents 5. Use of TLS by Mail User Agents
The following requirements and recommendations apply to Mail User The following requirements and recommendations apply to MUAs:
Agents:
o MUAs SHOULD be capable of using DNS SRV records to discover Mail o MUAs SHOULD be capable of using DNS SRV records to discover Mail
Access Services and Mail Submission Services that are advertised Access Servers and Mail Submission Servers that are advertised by
by a MSP for an account being configured. Other means of an MSP for an account being configured. Other means of
discovering server configuration information (e.g. a database discovering server configuration information (e.g., a database
maintained by the MUA vendor) MAY also be supported. (See maintained by the MUA vendor) MAY also be supported. (See
Section 5.1 for more information.) Section 5.1 for more information.)
o MUAs SHOULD be configurable to require a minimum level of o MUAs SHOULD be configurable to require a minimum level of
confidentiality for any particular Mail Account, and refuse to confidentiality for any particular Mail Account and refuse to
exchange information via any service associated with that Mail exchange information via any service associated with that Mail
Account if the session does not provide that minimum level of Account if the session does not provide that minimum level of
confidentiality. (See Section 5.2.) confidentiality. (See Section 5.2.)
o MUAs MUST NOT treat a session as meeting a minimum level of o MUAs MUST NOT treat a session as meeting a minimum level of
confidentiality if the server's TLS certificate cannot be confidentiality if the server's TLS certificate cannot be
validated. (See Section 5.3.) validated. (See Section 5.3.)
o MUAs MAY impose other minimum confidentiality requirements in the o MUAs MAY impose other minimum confidentiality requirements in the
future, e.g. in order to discourage use of TLS versions or future, e.g., in order to discourage the use of TLS versions or
cryptographic algorithms in which weaknesses have been discovered. cryptographic algorithms in which weaknesses have been discovered.
o MUAs SHOULD provide a prominent indication of the level of o MUAs SHOULD provide a prominent indication of the level of
confidentiality associated with an account configuration that is confidentiality associated with an account configuration that is
appropriate for the user interface (for example, a "lock" icon or appropriate for the user interface (for example, a "lock" icon or
changed background color for a visual interface, or some sort of changed background color for a visual interface, or some sort of
audible indication for an audio user interface), at appropriate audible indication for an audio user interface), at appropriate
times and/or locations in order to inform the user of the times and/or locations, in order to inform the user of the
confidentiality of the communications associated with that confidentiality of the communications associated with that
account. For example, this might be done whenever (a) prompting account. For example, this might be done whenever (a) the user is
the user for authentication credentials, (b) the user is composing prompted for authentication credentials, (b) the user is composing
mail that will be sent to a particular submission server, (c) a mail that will be sent to a particular submission server, (c) a
list of accounts is displayed (particularly if the user can select list of accounts is displayed (particularly if the user can select
from that list to read mail), or (d) the user is requesting to from that list to read mail), or (d) the user is asking to view or
view or update any configuration data that will be stored on a update any configuration data that will be stored on a remote
remote server. If, however, an MUA provides such an indication, server. If, however, an MUA provides such an indication, it
it MUST NOT indicate confidentiality for any connection that does MUST NOT indicate confidentiality for any connection that does not
not at least use TLS 1.1 with certificate verification and also at least use TLS 1.1 with certificate verification and also meet
meet the minimum confidentiality requirements associated with that the minimum confidentiality requirements associated with that
account. account.
o MUAs MUST implement TLS 1.2 [RFC5246] or later. Earlier TLS and 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 SSL versions MAY also be supported, so long as the MUA requires at
least TLS 1.1 [RFC4346] when accessing accounts that are least TLS 1.1 [RFC4346] when accessing accounts that are
configured to impose minimum confidentiality requirements. configured to impose minimum confidentiality requirements.
o All MUAs SHOULD implement the recommended TLS cipher suites o All MUAs SHOULD implement the recommended TLS ciphersuites
described in [RFC7525] or a future BCP or standards track revision described in [RFC7525] or a future BCP or Standards Track revision
of that document. of that document.
o MUAs that are configured to not require minimum confidentiality o MUAs that are configured to not require minimum confidentiality
for one or more accounts SHOULD detect when TLS becomes available for one or more accounts SHOULD detect when TLS becomes available
on those accounts (using [RFC6186] or other means), and offer to on those accounts (using [RFC6186] or other means) and offer to
upgrade the account to require TLS. upgrade the account to require TLS.
Additional considerations and details appear below. Additional considerations and details appear below.
5.1. Use of SRV records in Establishing Configuration 5.1. Use of SRV Records in Establishing Configuration
This section updates [RFC6186] by changing the preference rules and This document updates [RFC6186] by changing the preference rules and
adding a new SRV service label _submissions._tcp to refer to Message adding a new SRV service label _submissions._tcp to refer to Message
Submission with Implicit TLS. Submission with Implicit TLS.
User-configurable MUAs SHOULD support use of [RFC6186] for account User-configurable MUAs SHOULD support the use of [RFC6186] for
setup. However, when using configuration information obtained by account setup. However, when using configuration information
this method, MUAs SHOULD ignore advertised services that do not obtained via this method, MUAs SHOULD ignore advertised services that
satisfy minimum confidentiality requirements, unless the user has do not satisfy minimum confidentiality requirements, unless the user
explicitly requested reduced confidentiality. This will have the has explicitly requested reduced confidentiality. This will have the
effect of causing the MUA to default to ignoring advertised effect of causing the MUA to default to ignoring advertised
configurations that do not support TLS, even when those advertised configurations that do not support TLS, even when those advertised
configurations have a higher priority than other advertised configurations have a higher priority than other advertised
configurations. configurations.
When using [RFC6186] configuration information, Mail User Agents When using configuration information per [RFC6186], MUAs SHOULD NOT
SHOULD NOT automatically establish new configurations that do not automatically establish new configurations that do not require TLS
require TLS for all servers, unless there are no advertised for all servers, unless there are no advertised configurations using
configurations using TLS. If such a configuration is chosen, prior TLS. If such a configuration is chosen, prior to attempting to
to attempting to authenticate to the server or use the server for authenticate to the server or use the server for Message Submission,
message submission, the MUA SHOULD warn the user that traffic to that the MUA SHOULD warn the user that traffic to that server will not be
server will not be encrypted and that it will therefore likely be encrypted and that it will therefore likely be intercepted by
intercepted by unauthorized parties. The specific wording is to be unauthorized parties. The specific wording is to be determined by
determined by the implementation, but it should adequately capture the implementation, but it should adequately capture the sense of
the sense of risk given the widespread incidence of mass surveillance risk, given the widespread incidence of mass surveillance of email
of email traffic. traffic.
Similarly, a MUA MUST NOT attempt to "test" a particular mail account Similarly, an MUA MUST NOT attempt to "test" a particular Mail
configuration by submitting the user's authentication credentials to Account configuration by submitting the user's authentication
a server, unless a TLS session meeting minimum confidentiality levels credentials to a server, unless a TLS session meeting minimum
has been established with that server. If minimum confidentiality confidentiality levels has been established with that server. If
requirements have not been satisfied, the MUA must explicitly warn minimum confidentiality requirements have not been satisfied, the MUA
the user that his password may be exposed to attackers before testing must explicitly warn that the user's password may be exposed to
the new configuration. attackers before testing the new configuration.
When establishing a new configuration for connecting to an IMAP, POP, When establishing a new configuration for connecting to an IMAP, POP,
or SMTP submission server, based on SRV records, an MUA SHOULD either or SMTP submission server, based on SRV records, an MUA SHOULD verify
verify that the SRV records are signed using DNSSEC, or that the that either (a) the SRV records are signed using DNSSEC or (b) the
target FQDN of the SRV record matches the original server FQDN for target Fully Qualified Domain Name (FQDN) of the SRV record matches
which the SRV queries were made. If the target FQDN is not in the the original server FQDN for which the SRV queries were made. If the
queried domain, the MUA SHOULD verify with the user that the SRV target FQDN is not in the queried domain, the MUA SHOULD verify with
target FQDN is suitable for use, before executing any connections to the user that the SRV target FQDN is suitable for use, before
the host. (See [RFC6186] section 6). executing any connections to the host. (See Section 6 of [RFC6186].)
An MUA MUST NOT consult SRV records to determine which servers to use An MUA MUST NOT consult SRV records to determine which servers to use
on every connection attempt, unless those SRV records are signed by on every connection attempt, unless those SRV records are signed by
DNSSEC and have a valid signature. However, an MUA MAY consult SRV DNSSEC and have a valid signature. However, an MUA MAY consult SRV
records from time to time to determine if an MSP's server records from time to time to determine if an MSP's server
configuration has changed, and alert the user if it appears that this 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 has happened. This can also serve as a means to encourage users to
upgrade their configurations to require TLS if and when their MSPs upgrade their configurations to require TLS if and when their MSPs
support it. support it.
5.2. Minimum Confidentiality Level 5.2. Minimum Confidentiality Level
MUAs SHOULD, by default, require a minimum level of confidentiality MUAs SHOULD, by default, require a minimum level of confidentiality
for services accessed by each account. For MUAs supporting the for services accessed by each account. For MUAs supporting the
ability to access multiple mail accounts, this requirement SHOULD be ability to access multiple Mail Accounts, this requirement SHOULD be
configurable on a per-account basis. configurable on a per-account basis.
The default minimum expected level of confidentiality for all new The default minimum expected level of confidentiality for all new
accounts MUST require successful validation of the server's accounts MUST require successful validation of the server's
certificate and SHOULD require negotiation of TLS version 1.1 or certificate and SHOULD require negotiation of TLS version 1.1 or
greater. (Future revisions to this specification may raise these greater. (Future revisions to this specification may raise these
requirements or impose additional requirements to address newly- requirements or impose additional requirements to address newly
discovered weaknesses in protocols or cryptographic algorithms.) discovered weaknesses in protocols or cryptographic algorithms.)
MUAs MAY permit the user to disable this minimum confidentiality MUAs MAY permit the user to disable this minimum confidentiality
requirement during initial account configuration, or subsequently requirement during initial account configuration or when subsequently
editing an account configuration, but MUST warn users that such a editing an account configuration but MUST warn users that such a
configuration will not assure privacy for either passwords or configuration will not assure privacy for either passwords or
messages. messages.
An MUA which is configured to require a minimum level of An MUA that is configured to require a minimum level of
confidentiality for a mail account MUST NOT attempt to perform any confidentiality for a Mail Account MUST NOT attempt to perform any
operation other than capability discovery, or STARTTLS for servers operation other than capability discovery, or STARTTLS for servers
not using Implicit TLS, unless the minimum level of confidentiality not using Implicit TLS, unless the minimum level of confidentiality
is provided by that connection. is provided by that connection.
MUAs SHOULD NOT allow users to easily access or send mail via an MUAs SHOULD NOT allow users to easily access or send mail via a
connection, or authenticate to any service using a password, if that connection, or authenticate to any service using a password, if that
account is configured to impose minimum confidentiality requirements account is configured to impose minimum confidentiality requirements
and that connection does not meet all of those requirements. An and that connection does not meet all of those requirements. An
example of "easily access" would be to display a dialog informing the example of "easy access" would be to display a dialog informing the
user that the security requirements of the account were not met by user that the security requirements of the account were not met by
the connection, but allowing the user to "click through" to send mail the connection but allowing the user to "click through" to send mail
or access the service anyway. Experience indicates that users or access the service anyway. Experience indicates that users
presented with such an option often "click through" without presented with such an option often "click through" without
understanding the risks that they're accepting by doing so. understanding the risks that they're accepting by doing so.
Furthermore, users who frequently find the need to "click through" to Furthermore, users who frequently find the need to "click through" to
use an insecure connection may become conditioned to do so as a use an insecure connection may become conditioned to do so as a
matter of habit, before considering whether the risks are reasonable matter of habit, before considering whether the risks are reasonable
in each specific instance. in each specific instance.
An MUA which is not configured to require a minimum level of An MUA that is not configured to require a minimum level of
confidentiality for a mail account SHOULD still attempt to connect to confidentiality for a Mail Account SHOULD still attempt to connect to
the services associated with that account using the most secure means the services associated with that account using the most secure means
available, e.g. by using Implicit TLS or STARTTLS. available, e.g., by using Implicit TLS or STARTTLS.
5.3. Certificiate Validation 5.3. Certificate Validation
MUAs MUST validate TLS server certificates according to [RFC7817] and MUAs MUST validate TLS server certificates according to [RFC7817] and
PKIX [RFC5280]. PKIX [RFC5280].
MUAs MAY also support DANE [RFC6698] as a means of validating server MUAs MAY also support DNS-Based Authentication of Named Entities
certificates in order to meet minimum confidentiality requirements. (DANE) [RFC6698] as a means of validating server certificates in
order to meet minimum confidentiality requirements.
MUAs MAY support use of certificate pinning but MUST NOT consider a MUAs MAY support the use of certificate pinning but MUST NOT consider
connection in which the server's authenticity relies on certificate a connection in which the server's authenticity relies on certificate
pinning, as providing the minimum level of confidentiality. (See pinning as providing the minimum level of confidentiality. (See
Section 5.4.) Section 5.4.)
5.4. Certificate Pinning 5.4. Certificate Pinning
During account setup, the MUA will identify servers that provide During account setup, the MUA will identify servers that provide
account services such as mail access and mail submission (the account services such as mail access and mail submission (Section 5.1
previous section describes one way to do this). The certificates for describes one way to do this). The certificates for these servers
these servers are verified using the rules described in [RFC7817] and are verified using the rules described in [RFC7817] and PKIX
PKIX [RFC5280]. In the event the certificate does not validate due [RFC5280]. In the event that the certificate does not validate due
to an expired certificate, lack of appropriate chain of trust, or to an expired certificate, a lack of an appropriate chain of trust,
lack of identifier match, the MUA MAY offer to create a persistent or a lack of an identifier match, the MUA MAY offer to create a
binding between that certificate and the saved host name for the persistent binding between that certificate and the saved hostname
server, for use when accessing that account's servers. This is for the server, for use when accessing that account's servers. This
called certificate pinning. is called "certificate pinning".
(Note: This use of the term "certificate pinning" means something (Note: This use of the term "certificate pinning" means something
subtly different than "HTTP Public Key Pinning" [RFC7469]. The dual subtly different than HTTP Public Key Pinning as described in
use of the same term is confusing, but unfortunately both uses are [RFC7469]. The dual use of the same term is confusing, but
well-established.) unfortunately both uses are well established.)
Certificate pinning is only appropriate during Mail Account setup and
Certificate pinning is only appropriate during mail account setup and
MUST NOT be offered as an option in response to a failed certificate MUST NOT be offered as an option in response to a failed certificate
validation for an existing mail account. An MUA that allows validation for an existing Mail Account. An MUA that allows
certificate pinning MUST NOT allow a certificate pinned for one certificate pinning MUST NOT allow a certificate pinned for one
account to validate connections for other accounts. An MUA that account to validate connections for other accounts. An MUA that
allows certificate pinning MUST also allow a user to undo the allows certificate pinning MUST also allow a user to undo the
pinning, i.e. to revoke trust in a certificate that has previously pinning, i.e., to revoke trust in a certificate that has previously
been pinned. been pinned.
A pinned certificate is subject to a man-in-the-middle attack at A pinned certificate is subject to a man-in-the-middle attack at
account setup time, and typically lacks a mechanism to automatically account setup time and typically lacks a mechanism to automatically
revoke or securely refresh the certificate. Note also that a man-in- revoke or securely refresh the certificate. Note also that a man-in-
the-middle attack at account setup time will expose the user's the-middle attack at account setup time will expose the user's
password to the attacker (if a password is used). Therefore use of a password to the attacker (if a password is used). Therefore, the use
pinned certificate does not meet the requirement for a minimum of a pinned certificate does not meet the requirement for a minimum
confidentiality level, and an MUA MUST NOT indicate to the user that confidentiality level, and an MUA MUST NOT indicate to the user that
the such confidentiality is provided. Additional advice on such confidentiality is provided. Additional advice on certificate
certificate pinning is present in [RFC6125]. pinning is presented in [RFC6125].
5.5. Client Certificate Authentication 5.5. Client Certificate Authentication
MUAs MAY implement client certificate authentication on the Implicit MUAs MAY implement client certificate authentication on the Implicit
TLS port. An MUA MUST NOT provide a client certificate during the TLS port. An MUA MUST NOT provide a client certificate during the
TLS handshake unless the server requests one and the MUA has been TLS handshake unless the server requests one and the MUA has been
authorized to use that client certificate with that account. Having authorized to use that client certificate with that account. Having
the end-user explicitly configure a client certificate for use with a the end user explicitly configure a client certificate for use with a
given account is sufficient to meet this requirement. However, given account is sufficient to meet this requirement. However,
installing a client certificate for use with one account MUST NOT installing a client certificate for use with one account MUST NOT
automatically authorize use of that certificate with other accounts. automatically authorize the use of that certificate with other
This is not intended to prohibit site-specific authorization accounts. This is not intended to prohibit site-specific
mechanisms, such as a site-administrator-controlled mechanism to authorization mechanisms, such as (a) a site-administrator-controlled
authorize use of a client certificate with a given account, or a mechanism to authorize the use of a client certificate with a given
domain-name matching mechanism. account or (b) a domain-name-matching mechanism.
Note: The requirement that the server request a certificate is just a Note: The requirement that the server request a certificate is just a
restatement of the TLS protocol rules, e.g. [RFC5246] section 7.4.6. restatement of the TLS protocol rules, e.g., Section 7.4.6 of
The requirement that the client not send a certificate not known to [RFC5246]. The requirement that the client not send a certificate
be acceptable to the server is pragmatic in multiple ways: the not known to be acceptable to the server is pragmatic in multiple
current TLS protocol provides no way for the client to know which of ways: the current TLS protocol provides no way for the client to know
potentially multiple certificates it should use; also, when the which of the potentially multiple certificates it should use; also,
client sends a certificate it is potentially disclosing its identity when the client sends a certificate, it is potentially disclosing its
(or its user's identity) to both the server and to any party with identity (or its user's identity) to both the server and any party
access to the transmission medium, perhaps unnecessarily and for no with access to the transmission medium, perhaps unnecessarily and for
useful purpose. no useful purpose.
A client supporting client certificate authentication with Implicit A client supporting client certificate authentication with Implicit
TLS MUST implement the SASL EXTERNAL [RFC4422] mechanism using the TLS MUST implement the SASL EXTERNAL mechanism [RFC4422], using the
appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for
SMTP Submission [RFC4954], AUTHENTICATE for IMAP [RFC3501]). SMTP Submission [RFC4954], or AUTHENTICATE for IMAP [RFC3501]).
6. Considerations related to Anti-Virus/Anti-Spam Software and Services 6. Considerations Related to Antivirus/Antispam Software and Services
There are multiple ways to connect an Anti-Virus and/or Anti-Spam There are multiple ways to connect an AVAS service (e.g., "Antivirus
(AVAS) service to a mail server. Some mechanisms, such as the de- & Antispam") to a mail server. Some mechanisms, such as the de facto
facto milter protocol, are out of scope for this specification. "milter" protocol, are out of scope for this specification. However,
However, some services use an SMTP relay proxy that intercepts mail some services use an SMTP relay proxy that intercepts mail at the
at the application layer to perform a scan and proxy or forward to application layer to perform a scan and proxy or forward to another
another MTA. Deploying AVAS services in this way can cause many Mail Transfer Agent (MTA). Deploying AVAS services in this way can
problems [RFC2979] including direct interference with this cause many problems [RFC2979], including direct interference with
specification, and other forms of confidentiality or security this specification, and other forms of confidentiality or security
reduction. An AVAS product or service is considered compatible with reduction. An AVAS product or service is considered compatible with
this specification if all IMAP, POP and SMTP-related software this specification if all IMAP, POP, and SMTP-related software
(including proxies) it includes are compliant with this (including proxies) it includes are compliant with this
specification. specification.
Note that end-to-end email encryption prevents AVAS software and Note that end-to-end email encryption prevents AVAS software and
services from using email content as part of a spam or virus services from using email content as part of a spam or virus
assessment. Furthermore, while a minimum confidentiality level can assessment. Furthermore, although a minimum confidentiality level
prevent a man-in-the-middle from introducing spam or virus content can prevent a man-in-the-middle from introducing spam or virus
between the MUA and Submission server, it does not prevent other content between the MUA and Submission server, it does not prevent
forms of client or account compromise. Use of AVAS services for other forms of client or account compromise. The use of AVAS
submitted email therefore remains necessary. services for submitted email therefore remains necessary.
7. IANA Considerations 7. IANA Considerations
7.1. POP3S Port Registration Update 7.1. POP3S Port Registration Update
IANA is asked to update the registration of the TCP well-known port IANA has updated the registration of the TCP well-known port 995
995 using the following template ([RFC6335]): using the following template [RFC6335]:
Service Name: pop3s Service Name: pop3s
Transport Protocol: TCP Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org> Assignee: IESG <iesg@ietf.org>
Contact: IESG <iesg@ietf.org> Contact: IETF Chair <chair@ietf.org>
Description: POP3 over TLS protocol Description: POP3 over TLS protocol
Reference: RFC XXXX (this document once published) Reference: RFC 8314
Port Number: 995 Port Number: 995
7.2. IMAPS Port Registration Update 7.2. IMAPS Port Registration Update
IANA is asked to update the registration of the TCP well-known port IANA has updated the registration of the TCP well-known port 993
993 using the following templates ([RFC6335]): using the following template [RFC6335]:
Service Name: imaps Service Name: imaps
Transport Protocol: TCP Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org> Assignee: IESG <iesg@ietf.org>
Contact: IESG <iesg@ietf.org> Contact: IETF Chair <chair@ietf.org>
Description: IMAP over TLS protocol Description: IMAP over TLS protocol
Reference: RFC XXXX (this document once published) Reference: RFC 8314
Port Number: 993 Port Number: 993
No changes to existing UDP port assignments for pop3s or imaps are No changes to existing UDP port assignments for pop3s or imaps are
being requested. being requested.
7.3. Submissions Port Registration 7.3. Submissions Port Registration
IANA is asked to assign an alternate usage of TCP port 465 in IANA has assigned an alternate usage of TCP port 465 in addition to
addition to the current assignment using the following template the current assignment using the following template [RFC6335]:
([RFC6335]):
Service Name: submissions Service Name: submissions
Transport Protocol: TCP Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org> Assignee: IESG <iesg@ietf.org>
Contact: IESG <iesg@ietf.org> Contact: IETF Chair <chair@ietf.org>
Description: Message Submission over TLS protocol Description: Message Submission over TLS protocol
Reference: RFC XXXX (this document once published) Reference: RFC 8314
Port Number: 465 Port Number: 465
This is a one-time procedural exception to the rules in RFC 6335. This is a one-time procedural exception to the rules in [RFC6335].
This requires explicit IESG approval and does not set a precedent. This requires explicit IESG approval and does not set a precedent.
Note: Since the purpose of this alternate usage assignment is to Note: Since the purpose of this alternate usage assignment is to
align with widespread existing practice, and there is no known usage align with widespread existing practice and there is no known usage
of UDP port 465 for message submission over TLS, IANA is not being of UDP port 465 for Message Submission over TLS, IANA has not
asked to assign an alternate usage of UDP port 465. assigned an alternate usage of UDP port 465.
Historically, port 465 was briefly registered as the "smtps" port. Historically, port 465 was briefly registered as the "smtps" port.
This registration made no sense as the SMTP transport MX This registration made no sense, as the SMTP transport MX
infrastructure has no way to specify a port, so port 25 is always infrastructure has no way to specify a port, so port 25 is always
used. As a result, the registration was revoked and was subsequently used. As a result, the registration was revoked and was subsequently
reassigned to a different service. In hindsight, the "smtps" reassigned to a different service. In hindsight, the "smtps"
registration should have been renamed or reserved rather than registration should have been renamed or reserved rather than
revoked. Unfortunately, some widely deployed mail software revoked. Unfortunately, some widely deployed mail software
interpreted "smtps" as "submissions" [RFC6409] and used that port for interpreted "smtps" as "submissions" [RFC6409] and used that port for
email submission by default when an end-user requests security during email submission by default when an end user requested security
account setup. If a new port is assigned for the submissions during account setup. If a new port is assigned for the submissions
service, email software will either continue with unregistered use of service, either (a) email software will continue with unregistered
port 465 (leaving the port registry inaccurate relative to de-facto use of port 465 (leaving the port registry inaccurate relative to
practice and wasting a well-known port), or confusion between the de- de facto practice and wasting a well-known port) or (b) confusion
facto and registered ports will cause harmful interoperability between the de facto and registered ports will cause harmful
problems that will deter use of TLS for message submission. The interoperability problems that will deter the use of TLS for Message
authors believe both of these outcomes are less desirable than a wart Submission. The authors of this document believe that both of these
in the registry documenting real-world usage of a port for two outcomes are less desirable than a "wart" in the registry documenting
purposes. Although STARTTLS-on-port-587 has deployed, it has not real-world usage of a port for two purposes. Although STARTTLS on
replaced deployed use of Implicit TLS submission on port 465. 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 7.4. Additional Registered Clauses for "Received" Fields
Per the provisions in [RFC5321], IANA is requested to add two Per the provisions in [RFC5321], IANA has added two additional-
additional-registered-clauses for Received fields as defined in registered-clauses for Received fields as defined in Section 4.3 of
Section 4.3 of this document: this document:
o "tls" indicating the TLS cipher used (if applicable), and o "tls": Indicates the TLS cipher used (if applicable)
o "group" indicating the Diffie-Hellman group used with the TLS o "group": Indicates the Diffie-Hellman group used with the TLS
cipher (if applicable) cipher (if applicable)
The descriptions and syntax of these additional clauses are in The descriptions and syntax of these additional clauses are provided
Section 4.3 of this document. in Section 4.3 of this document.
8. Security Considerations 8. Security Considerations
This entire document is about security considerations. In general, This entire document is about security considerations. In general,
this is targeted to improve mail confidentiality and to mitigate this is targeted to improve mail confidentiality and to mitigate
threats external to the email system such as network-level snooping threats external to the email system such as network-level snooping
or interception; this is not intended to mitigate active attackers or interception; this is not intended to mitigate active attackers
who have compromised service provider systems. who have compromised service provider systems.
Implementers should be aware that use of client certificates with TLS Implementers should be aware that the use of client certificates with
1.2 reveals the user's identity to any party with ability to read TLS 1.2 reveals the user's identity to any party with the ability to
packets from the transmission medium, and therefore may compromise read packets from the transmission medium and therefore may
the user's privacy. There seems to be no easy fix with TLS 1.2 or compromise the user's privacy. There seems to be no easy fix with
earlier versions other than to avoid presenting client certificates TLS 1.2 or earlier versions, other than to avoid presenting client
except when there is explicit authorization to do so. TLS 1.3 certificates except when there is explicit authorization to do so.
[I-D.ietf-tls-tls13] appears to reduce the privacy risk somewhat. TLS 1.3 [TLS-1.3] appears to reduce this privacy risk somewhat.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981, RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>. <https://www.rfc-editor.org/info/rfc793>.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<https://www.rfc-editor.org/info/rfc1939>. <https://www.rfc-editor.org/info/rfc1939>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>. February 2002, <https://www.rfc-editor.org/info/rfc3207>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501,
<https://www.rfc-editor.org/info/rfc3501>. March 2003, <https://www.rfc-editor.org/info/rfc3501>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005, RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>. <https://www.rfc-editor.org/info/rfc4033>.
[RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol [RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
(POP3) Simple Authentication and Security Layer (SASL) (POP3) Simple Authentication and Security Layer (SASL)
Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034, Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034,
July 2007, <https://www.rfc-editor.org/info/rfc5034>. July 2007, <https://www.rfc-editor.org/info/rfc5034>.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T. [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Finch, "Email Submission Operations: Access and Syntax Specifications: ABNF", STD 68, RFC 5234,
Accountability Requirements", BCP 134, RFC 5068,
DOI 10.17487/RFC5068, November 2007,
<https://www.rfc-editor.org/info/rfc5068>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>. <https://www.rfc-editor.org/info/rfc5234>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, (TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>. <https://www.rfc-editor.org/info/rfc5246>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
skipping to change at page 20, line 36 skipping to change at page 21, line 26
Submission/Access Services", RFC 6186, Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011, DOI 10.17487/RFC6186, March 2011,
<https://www.rfc-editor.org/info/rfc6186>. <https://www.rfc-editor.org/info/rfc6186>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<https://www.rfc-editor.org/info/rfc6409>. <https://www.rfc-editor.org/info/rfc6409>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS) of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
2012, <https://www.rfc-editor.org/info/rfc6698>. August 2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer "Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525,
2015, <https://www.rfc-editor.org/info/rfc7525>. May 2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672, (DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015, DOI 10.17487/RFC7672, October 2015,
<https://www.rfc-editor.org/info/rfc7672>. <https://www.rfc-editor.org/info/rfc7672>.
[RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS) [RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS)
Server Identity Check Procedure for Email-Related Server Identity Check Procedure for Email-Related
Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016, Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016,
<https://www.rfc-editor.org/info/rfc7817>. <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>.
9.2. Informative References 9.2. Informative References
[I-D.ietf-tls-tls13] [CERT-555316]
Rescorla, E., "The Transport Layer Security (TLS) Protocol CERT, "Vulnerability Note VU#555316: STARTTLS plaintext
Version 1.3", draft-ietf-tls-tls13-21 (work in progress), command injection vulnerability", Carnegie Mellon
July 2017. University Software Engineering Institute, September 2011,
<https://www.kb.cert.org/vuls/id/555316>.
[I-D.ietf-uta-mta-sts] [Email-TLS]
Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A., Moore, K., "Recommendations for use of TLS by Electronic
and J. Jones, "SMTP MTA Strict Transport Security (MTA- Mail Access Protocols", Work in Progress, draft-moore-
STS)", draft-ietf-uta-mta-sts-09 (work in progress), email-tls-00, October 2013.
September 2017.
[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", [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999, RFC 2595, DOI 10.17487/RFC2595, June 1999,
<https://www.rfc-editor.org/info/rfc2595>. <https://www.rfc-editor.org/info/rfc2595>.
[RFC2979] Freed, N., "Behavior of and Requirements for Internet [RFC2979] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000,
<https://www.rfc-editor.org/info/rfc2979>. <https://www.rfc-editor.org/info/rfc2979>.
[RFC3848] Newman, C., "ESMTP and LMTP Transmission Types [RFC3848] Newman, C., "ESMTP and LMTP Transmission Types
Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004,
<https://www.rfc-editor.org/info/rfc3848>. <https://www.rfc-editor.org/info/rfc3848>.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, (TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006, DOI 10.17487/RFC4346, April 2006,
<https://www.rfc-editor.org/info/rfc4346>. <https://www.rfc-editor.org/info/rfc4346>.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple [RFC4422] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422, Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006, DOI 10.17487/RFC4422, June 2006,
<https://www.rfc-editor.org/info/rfc4422>. <https://www.rfc-editor.org/info/rfc4422>.
[RFC4954] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service [RFC4954] Siemborski, R., Ed., and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954, Extension for Authentication", RFC 4954,
DOI 10.17487/RFC4954, July 2007, DOI 10.17487/RFC4954, July 2007,
<https://www.rfc-editor.org/info/rfc4954>. <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, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>. <https://www.rfc-editor.org/info/rfc5321>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011, DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>. <https://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509 within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
2011, <https://www.rfc-editor.org/info/rfc6125>. March 2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA) Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165, Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011, RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>. <https://www.rfc-editor.org/info/rfc6335>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469,
2015, <https://www.rfc-editor.org/info/rfc7469>. April 2015, <https://www.rfc-editor.org/info/rfc7469>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [TLS-1.3] Rescorla, E., "The Transport Layer Security (TLS) Protocol
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, Version 1.3", Work in Progress, draft-ietf-tls-tls13-23,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. January 2018.
Appendix A. Design Considerations Appendix A. Design Considerations
This section is not normative. This section is not normative.
The first version of this was written independently from draft-moore- The first version of this document was written independently from the
email-tls-00.txt; subsequent versions merge ideas from both drafts. 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 that 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 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 author was perhaps the first to propose that idea. In hindsight,
authors now believe that that approach was a mistake. At this point both authors now believe that that approach was a mistake. At this
the authors believe that while anything that makes it easier to point, the authors believe that while anything that makes it easier
deploy TLS is good, the desirable end state is that these protocols to deploy TLS is good, the desirable end state is that these
always use TLS, leaving no need for a separate port for cleartext protocols always use TLS, leaving no need for a separate port for
operation except to support legacy clients while they continue to be cleartext operation except to support legacy clients while they
used. The separate port model for TLS is inherently simpler to continue to be used. The separate-port model for TLS is inherently
implement, debug and deploy. It also enables a "generic TLS load- simpler to implement, debug, and deploy. It also enables a "generic
balancer" that accepts secure client connections for arbitrary foo- TLS load-balancer" that accepts secure client connections for
over-TLS protocols and forwards them to a server that may or may not arbitrary foo-over-TLS protocols and forwards them to a server that
support TLS. Such load-balancers cause many problems because they may or may not support TLS. Such load-balancers cause many problems
violate the end-to-end principle and the server loses the ability to because they violate the end-to-end principle and the server loses
log security-relevant information about the client unless the the ability to log security-relevant information about the client
protocol is designed to forward that information (as this unless the protocol is designed to forward that information (as this
specification does for the cipher suite). However, they can result specification does for the ciphersuite). However, they can result in
in TLS deployment where it would not otherwise happen which is a TLS deployment where it would not otherwise happen, which is a
sufficiently important goal that it overrides the problems. sufficiently important goal that it overrides any problems.
Although STARTTLS appears only slightly more complex than separate- Although STARTTLS appears only slightly more complex than
port TLS, we again learned the lesson that complexity is the enemy of separate-port TLS, we again learned the lesson that complexity is the
security in the form of the STARTTLS command injection vulnerability enemy of security in the form of the STARTTLS command injection
(CERT vulnerability ID #555316). Although there's nothing inherently vulnerability (Computer Emergency Readiness Team (CERT) vulnerability
wrong with STARTTLS, the fact it resulted in a common implementation ID #555316 [CERT-555316]). Although there's nothing inherently wrong
error (made independently by multiple implementers) suggests it is a with STARTTLS, the fact that it resulted in a common implementation
less secure architecture than Implicit TLS. 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. Section 7 of RFC 2595 critiques the separate-port approach to TLS.
The first bullet was a correct critique. There are proposals in the The first bullet was a correct critique. There are proposals in the
http community to address that, and use of SRV records as described HTTP community to address that, and the use of SRV records as
in RFC 6186 resolves that critique for email. The second bullet is described in RFC 6186 resolves that critique for email. The second
correct as well, but not very important because useful deployment of bullet is correct as well but is not very important because useful
security layers other than TLS in email is small enough to be deployment of security layers other than TLS in email is small enough
effectively irrelevant. (Also it's less correct than it used to be to be effectively irrelevant. (Also, it's less correct than it used
because "export" ciphersuites are no longer supported in modern to be because "export" ciphersuites are no longer supported in modern
versions of TLS.) The third bullet is incorrect because it misses versions of TLS.) The third bullet is incorrect because it misses
the desirable option of "use and latch-on TLS if available". The the desirable option of "use TLS for all subsequent connections to
fourth bullet may be correct, but is not a problem yet with current this server once TLS is successfully negotiated". The fourth bullet
port consumption rates. The fundamental error was prioritizing a may be correct, but it is not a problem yet with current port
perceived better design based on a mostly valid critique over real- consumption rates. The fundamental error was prioritizing a
world deployability. But getting security and confidentiality perceived better design based on a mostly valid critique over
facilities actually deployed is so important it should trump design real-world deployability. But getting security and confidentiality
purity considerations. 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 Port 465 is presently used for two purposes: for submissions by a
large number of clients and service providers and for the "urd" large number of clients and service providers and for the "urd"
protocol by one vendor. Actually documenting this current state is protocol by one vendor. Actually documenting this current state is
controversial as discussed in the IANA considerations section. controversial, as discussed in the IANA Considerations section.
However, there is no good alternative. Registering a new port for However, there is no good alternative. Registering a new port for
submissions when port 465 is widely used for that purpose already submissions when port 465 is already widely used for that purpose
will just create interoperability problems. Registering a port will just create interoperability problems. Registering a port
that's only used if advertised by an SRV record (RFC 6186) would not that's only used if advertised by an SRV record (RFC 6186) would not
create interoperability problems but would require all client and create interoperability problems but would require all client
server deployments and software to change significantly which is deployments, server deployments, and software to change
contrary to the goal of promoting more TLS use. Encouraging use of significantly, which is contrary to the goal of promoting the
STARTTLS on port 587 would not create interoperability problems, but increased use of TLS. Encouraging the use of STARTTLS on port 587
is unlikely to have impact on current undocumented use of port 465 would not create interoperability problems, but it is unlikely to
and makes the guidance in this document less consistent. The have any impact on the current undocumented use of port 465 and makes
remaining option is to document the current state of the world and the guidance in this document less consistent. The remaining option
support future use of port 465 for submission as this increases is to document the current state of the world and support future use
consistency and ease-of-deployment for TLS email submission. of port 465 for submission, as this increases consistency and ease of
deployment for TLS email submission.
Appendix B. Change Log
Changes since draft-ietf-uta-email-deep-07:
o After discussion with the WG in Prague, removed BCP language and
once again made unambiguous that this is intended as a standards-
track document.
o Server implementations now MUST implement TLS 1.2, consistent with
RFC 7525. MUAs may still consider a TLS 1.1 session as meeting
minimum confidentiality requirements.
o MSPs now MUST support TLS for POP, IMAP, Submission, and any other
services that use username/password authentication.
o Added text to clarify the purpose of recommending that MSPs use
DNS SRV records to advertise services.
o Changed text about MUAs not blindly trusting unsigned SRV records,
to instead restate RFC 6186 requirements.
Changes since draft-ietf-uta-email-deep-06:
o On the recommendation of one of the co-chairs and some working
group members, rewrote document with the intended status of BCP.
This involved removing a great deal of text that consisted
essentially of new protocol specification, especially the STS
features, on the theory that a BCP should base its recommendations
on current practice, and that new protocol features should be
subject to the interoperability test requirements associated with
normal standards-track documents.
Changes since draft-ietf-uta-email-deep-05:
o Clarify throughout that the confidentiality assurance level
associated with a mail account is a minimum level; attempt to
distinguish this from the current confidentiality level provided
by a connection between client and server.
o Change naming for confidentiality assurance levels: instead of
"high" or "no" confidence, assign numbers 1 and 0 to them
respectively. This because it seems likely that in the not-too-
distant future, what was defined in -05 as "high" confidence will
be considered insufficient, and calling that "high" confidence
will become misleading. For example, relying entirely on a list
of trusted CAs to validate server certificates from arbitrary
parties, appears to be less and less reliable in practice at
thwarting MITM attacks.
o Clarify that if some services associated with a mail account don't
meet the minimum confidentiality assurance level assigned to that
account, other services that do meet that minimum confidentiality
assurance level may continue to be used.
o Clarify that successful negotiation of at least TLS version 1.1 is
required as a condition of meeting confidentiality assurance level
1.
o Clarify that validation of a server certificate using either DANE
or PKIX is sufficient to meet the certificate validation
requirement of confidentiality assurance level 1.
o Clarify that minimum confidentiality assurance levels are separate
from security directives, and that the requirements of both
mechanisms must be met.
o Explicitly cite an example that a security directive of tls-
version=1.2 won't be saved if the currently negotiated tls-version
is 1.1. (This example already appeared a bit later in the text,
but for author KM it seemed to make the mechanism clearer to use
this example earlier.)
o Clarify some protocol examples as to whether PKIX or DANE was used
to verify a server's certificate.
o Remove most references to DEEP as the conversion from DEEP to MUA-
STS seemed incomplete, but kept the DEEP command for use in POP3
on the assumption that author CN wanted it that way.
o Removed most references to "latch" and derivative words.
o Added pkix+dane as a value for the tls-cert directive, to indicate
(from a server) that both PKIX and DANE validation will be
supported, or (from a client) that both PKIX and DANE were used to
validate a certificate. Also clarified what each of any, pkix,
dane, and pkix+dane mean when advertised by a server and in
particular that tls-cert=any provides no assurance of future PKIX
verifiability in contrast to tls-cert=pkix or tls-cert=pkix+dane.
It seemed important to support the ability to evolve to using
multiple trust anchors for certificate validation, but also to
allow servers to have the option to migrate from PKIX to DANE if
that made sense for them. This change seemed less disruptive than
either defining additional directives, or allowing multiple
instances of the same directive with different values to appear in
the same advertisement.
o Clarify interaction of this specification with anti-virus / anti-
spam mechanisms.
Changes since draft-ietf-uta-email-deep-04:
o Swap sections 5.1 and 5.3 ("Email Security Tags" and "Server DEEP
Status") as that order may aid understanding of the model. Also
rewrote parts of these two sections to try to make the model
clearer.
o Add text about versioning of security tags to make the model
clearer.
o Add example of security tag upgrade.
o Convert remaining mention of TLS 1.0 to TLS 1.1.
o Change document title from DEEP to MUA STS to align with SMTP
relay STS.
* Slight updates to abstract and introductions.
* Rename security latches/tags to security directives.
* Rename server DEEP status to STS policy.
* Change syntax to use directive-style HSTS syntax.
o Make HSTS reference normative.
o Remove SMTP DSN header as that belongs in SMTP relay STS document.
Changes since draft-ietf-uta-email-deep-03:
o Add more references to ietf-uta-email-tls-certs in implementation
requirements section.
o Replace primary reference to RFC 6125 with ietf-uta-email-tls-
certs, so move RFC 6125 to informative list for this
specification.
Changes since draft-ietf-uta-email-deep-02:
o Make reference to design considerations explicit rather than
"elsewhere in this document".
o Change provider requirement so SMTP submission services are
separate from SMTP MTA services as opposed to the previous
phrasing that required the servers be separate (which is too
restrictive).
o Update DANE SMTP reference
Changes since draft-ietf-uta-email-deep-01:
o Change text in tls11 and tls12 registrations to clarify
certificate rules, including additional PKIX and DANE references.
o Change from tls10 to tls11 (including reference) as the minimum.
o Fix typo in example 5.
o Remove open issues section; enough time has passed so not worth
waiting for more input.
Changes since draft-ietf-uta-email-deep-00:
o Update and clarify abstract
o use term confidentiality instead of privacy in most cases.
o update open issues to request input for missing text.
o move certificate pinning sub-section to account setup section and
attempt to define it more precisely.
o Add note about end-to-end encryption in AVAS section.
o swap order of DNSSEC and TLSA sub-sections.
o change meaning of 'tls10' and 'tls12' latches to require
certificate validation.
o Replace cipher suite advice with reference to RFC 7525. Change
examples to use TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as cipher
suite.
o Add text to update IMAP, POP3 and Message Submission standards
with newer TLS advice.
o Add clearer text in introduction that this does not cover SMTP
relay.
o Update references to uta-tls-certs.
o Add paragraph to Implicit TLS for SMTP Submission section
recommending that STARTTLS also be implemented.
Changes since draft-newman-email-deep-02:
o Changed "privacy assurance" to "confidentiality assurance"
o Changed "low privacy assurance" to "no confidentiality assurance"
o Attempt to improve definition of confidentiality assurance level.
o Add SHOULD indicate when MUA is showing list of mail accounts.
o Add SHOULD NOT latch tls10, tls12 tags until TLS negotiated.
o Removed sentence about deleting and re-creating the account in
latch failure section.
o Remove use of word "fallback" with respect to TLS version
negotiation.
o Added bullet about changes to Internet facing servers to MSP
section.
o minor wording improvements based on feedback
Changes since -01:
o Updated abstract, introduction and document structure to focus
more on mail user agent privacy assurance.
o Added email account privacy section, also moving section on
account setup using SRV records to that section.
o Finished writing IANA considerations section
o Remove provisional concept and instead have server explicitly list
security tags clients should latch.
o Added note that rules for the submissions port follow the same
rules as those for the submit port.
o Reference and update advice in [RFC5068].
o Fixed typo in Client Certificate Authentication section.
o Removed tls-pfs security latch and all mention of perfect forward
secrecy as it was controversial.
o Added reference to HSTS.
Changes since -00:
o Rewrote introduction to merge ideas from draft-moore-email-tls-00.
o Added Implicit TLS section, Account configuration section and IANA
port registration updates based on draft-moore-email-tls-00.
o Add protocol details necessary to standardize implicit TLS for
POP/IMAP/submission, using ideas from draft-melnikov-pop3-over-
tls.
o Reduce initial set of security tags based on feedback.
o Add deep status concept to allow a window for software updates to
be backed out before latches make that problematic, as well as to
provide service providers with a mechanism they can use to assist
customers in the event of a privacy failure.
o Add DNS SRV section from draft-moore-email-tls-00.
o Write most of the missing IANA considerations section.
o Rewrite most of implementation requirements section based more on
draft-moore-email-tls-00. Remove new cipher requirements for now
because those may be dealt with elsewhere.
Appendix C. Acknowledgements Acknowledgements
Thanks to Ned Freed for discussion of the initial latch concepts in Thanks to Ned Freed for discussion of the initial concepts in this
this document. Thanks to Alexey Melnikov for draft-melnikov-pop3- document. Thanks to Alexey Melnikov for [POP3-over-TLS], which was
over-tls-02, which was the basis of the POP3 Implicit TLS text. the basis of the POP3 Implicit TLS text. Thanks to Russ Housley,
Thanks to Russ Housley, Alexey Melnikov and Dan Newman for review Alexey Melnikov, and Dan Newman for review feedback. Thanks to
feedback. Thanks to Paul Hoffman for interesting feedback in initial Paul Hoffman for interesting feedback in initial conversations about
conversations about this idea. this idea.
Authors' Addresses Authors' Addresses
Keith Moore Keith Moore
Windrock, Inc. Windrock, Inc.
PO Box 1934 PO Box 1934
Knoxville, TN 37901 Knoxville, TN 37901
US United States of America
Email: moore@network-heretics.com Email: moore@network-heretics.com
Chris Newman Chris Newman
Oracle Oracle
440 E. Huntington Dr., Suite 400 440 E. Huntington Dr., Suite 400
Arcadia, CA 91006 Arcadia, CA 91006
US United States of America
Email: chris.newman@oracle.com Email: chris.newman@oracle.com
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