Network Working Group K. Moore
Internet-Draft Network Heretics
Updates: 1939, 2595, 3464, 3501, 5068, C. Newman
6186, 6409 (if approved) Oracle
Intended status: Standards Track July 6, 2015
Expires: January 7, 2016
Deployable Enhanced Email Privacy (DEEP)
draft-ietf-uta-email-deep-01.txt
Abstract
This specification defines a set of requirements and facilities
designed to improve email confidentiality between a mail user agent
(MUA) and a mail submission or mail access server. This provides
mechanisms intended to increase use of already deployed Transport
Layer Security (TLS) technology, provide a model for mail user
agent's confidentiality assurance, and enable mail service providers
to advertise improved TLS confidentiality facilities.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 7, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology Used in This Document . . . . . . 4
3. Mail Account Confidentiality Assurance Level . . . . . . . . 4
3.1. High Confidentiality Assurance . . . . . . . . . . . . . 5
3.2. No Confidentiality Assurance . . . . . . . . . . . . . . 6
3.3. Other Confidentiality Assurance Levels . . . . . . . . . 6
4. Implicit TLS . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Implicit TLS for POP . . . . . . . . . . . . . . . . . . 7
4.2. Implicit TLS for IMAP . . . . . . . . . . . . . . . . . . 7
4.3. Implicit TLS for SMTP Submission . . . . . . . . . . . . 7
4.4. Implicit TLS Connection Closure for POP, IMAP and SMTP . 8
5. Email Security Upgrading Using Security Latches . . . . . . . 8
5.1. Email Security Tags . . . . . . . . . . . . . . . . . . . 9
5.2. Initial Set of Email Security Tags . . . . . . . . . . . 10
5.3. Server DEEP Status . . . . . . . . . . . . . . . . . . . 10
5.4. Email Security Tag Latch Failures . . . . . . . . . . . . 11
6. Recording TLS Cipher Suite in Received Header . . . . . . . . 11
7. Extensions for DEEP Status and Reporting . . . . . . . . . . 11
7.1. IMAP DEEP Extension . . . . . . . . . . . . . . . . . . . 12
7.2. POP DEEP Extension . . . . . . . . . . . . . . . . . . . 14
7.3. SMTP DEEP Extension . . . . . . . . . . . . . . . . . . . 15
7.4. SMTP Error Extension . . . . . . . . . . . . . . . . . . 16
8. Account Setup Considerations . . . . . . . . . . . . . . . . 16
8.1. Use of SRV records in Establishing Configuration . . . . 16
8.2. Certificate Pinning . . . . . . . . . . . . . . . . . . . 17
9. Implementation Requirements . . . . . . . . . . . . . . . . . 18
9.1. All Implementations (Client and Server) . . . . . . . . . 18
9.1.1. Client Certificate Authentication . . . . . . . . . . 19
9.2. Mail Server Implementation Requirements . . . . . . . . . 19
9.3. Mail User Agent Implementation Requirements . . . . . . . 20
9.4. Non-configurable MUAs and nonstandard access protocols . 20
9.5. DEEP Compliance for Anti-Virus/Anti-Spam Software and
Services . . . . . . . . . . . . . . . . . . . . . . . . 21
10. Mail Service Provider Requirements . . . . . . . . . . . . . 21
10.1. Server Requirements . . . . . . . . . . . . . . . . . . 21
10.2. MSPs MUST provide Submission Servers . . . . . . . . . . 21
10.3. TLS Server Certificate Requirements . . . . . . . . . . 22
10.4. Recommended DNS records for mail protocol servers . . . 22
10.4.1. MX records . . . . . . . . . . . . . . . . . . . . . 22
10.4.2. SRV records . . . . . . . . . . . . . . . . . . . . 22
10.4.3. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 23
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10.4.4. TLSA records . . . . . . . . . . . . . . . . . . . . 23
10.5. MSP Server Monitoring . . . . . . . . . . . . . . . . . 23
10.6. Advertisement of DEEP status . . . . . . . . . . . . . . 23
10.7. Require TLS . . . . . . . . . . . . . . . . . . . . . . 23
10.8. Changes to Internet Facing Servers . . . . . . . . . . . 23
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
11.1. Security Tag Registry . . . . . . . . . . . . . . . . . 24
11.2. Initial Set of Security Tags . . . . . . . . . . . . . . 24
11.3. POP3S Port Registration Update . . . . . . . . . . . . . 26
11.4. IMAPS Port Registration Update . . . . . . . . . . . . . 26
11.5. Submissions Port Registration . . . . . . . . . . . . . 27
11.6. DEEP IMAP Capability . . . . . . . . . . . . . . . . . . 27
11.7. DEEP POP3 Capability . . . . . . . . . . . . . . . . . . 27
11.8. DEEP SMTP EHLO Keyword . . . . . . . . . . . . . . . . . 28
11.9. SMTP Enhanced Status Code . . . . . . . . . . . . . . . 28
11.10. MAIL Parameters Additional-registered-clauses Sub-
Registry . . . . . . . . . . . . . . . . . . . . . . . . 28
12. Security Considerations . . . . . . . . . . . . . . . . . . . 29
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
13.1. Normative References . . . . . . . . . . . . . . . . . . 29
13.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. Design Considerations . . . . . . . . . . . . . . . 32
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . 33
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 34
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
Software that provides email service via Internet Message Access
Protocol (IMAP) [RFC3501], Post Office Protocol (POP) [RFC1939] and/
or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409] usually
has Transport Layer Security (TLS) [RFC5246] support but often does
not use it in a way that maximizes end-user confidentiality. This
specification proposes changes to email software and deployments
intended to increase the use of TLS and record when that use occurs.
In brief, this memo now recommends that:
o MUAs associate a confidentiality assurance level with each mail
account, and the default level requires use of TLS with
certificate validation for all TCP connections;
o TLS on a well-known port ("Implicit TLS") be supported for IMAP,
POP, and SMTP Submission [RFC6409] for all electronic mail user
agents (MUAs), servers, and service providers;
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o MUAs and mail protocol servers cooperate (via mechanisms defined
in this specification) to upgrade security feature use and record/
indicate that usage appropriately.
This does not address use of TLS with SMTP for message relay (where
Message Submission [RFC6409] does not apply). Improved use of TLS
with SMTP for message relay requires a different approach. Future
work may address that topic and one approach is described in
[I-D.ietf-dane-smtp-with-dane].
The recommendations in this memo do not replace the functionality of,
and are not intended as a substitute for, end-to-end encryption of
electronic mail.
This draft is subject to change. Implementation of this proposal is
not recommended at this time. Please discuss this proposal on the
ietf-uta mailing list.
2. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification expresses syntax using the Augmented Backus-Naur
Form (ABNF) as described in [RFC5234], including the core rules in
Appendix B and rules from [RFC5322].
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. If a single "C:" or "S:" label applies to
multiple lines, then the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange.
3. Mail Account Confidentiality Assurance Level
A "mail account" refers to the network services an end user uses to
read, submit and manage email communications on the Internet. This
typically involves at least one mail access server (IMAP or POP) and
at least one SMTP submission server. An end users uses a mail user
agent (MUA) to access a mail account and most MUAs support one or
more mail accounts. This document uses the term "confidentiality
assurance level" to indicate the degree to which the network
connections between an MUA and a mail account have confidentiality
protection from both passive and active attackers on the network.
The configuration necessary for a mail account includes an email
address, connection information and authentication credentials for
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network services. MUAs compliant with this specification MUST also
associate a confidentiality assurance level with each mail account.
MUAs MUST implement a high confidentiality assurance level as
described in the next section.
MUAs SHOULD continuously indicate to the user the confidentiality
assurance level of the account currently in use when reading,
submitting and managing mail (e.g., via a lock icon, background
colors and indications similar to those commonly used in web browsers
for a similar purpose) and SHOULD indicate the confidentiality
assurance level for each account whenever displaying a list of mail
accounts. Note that the displayed confidentiality assurance level
could be higher than the level set at account configuration but never
lower. If multiple active connections are associated with an account
or view, the indication should match the level provided by the least
confidential connection.
Account configuration occurs when an MUA is first used to access a
particular service, when a user wishes to access or submit mail
through servers in addition to those specified or found during first
use, or when a user explicitly requests to change account
configuration parameters such as server names, user names, passwords,
client certificates, etc. Account configuration can be entirely
manual (entering server names explicitly) or partially automated via
a mechanism such as DNS SRV records [RFC6186]. MUAs SHOULD use the
high confidentiality assurance level as the default for newly
configured accounts.
3.1. High Confidentiality Assurance
A mail account has a high confidentiality assurance when the
following conditions are met on all TCP server connections associated
with an account. This includes connections to POP, IMAP and SMTP
submission servers as well as any other associated protocols defined
now or in the future. Examples of protocols associated with a mail
account include managesieve [RFC5804] and MTQP [RFC3887].
o TCP connections MUST attempt to negotiate TLS via either Implicit
TLS Section 4 or STARTTLS.
o MUAs MUST implement [I-D.ietf-uta-email-tls-certs] and PKIX
[RFC5280].
o MUAs MAY implement DANE [RFC6698].
o User agents MUST abort a TLS session if the TLS negotiation fails
or the server's certificate or identity fails to verify. A user
may reconfigure the account to lower the expected level of
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confidentiality if he/she chooses. Reduction of expected account
confidentiality MUST NOT be done on a click-through basis.
The end user is part of the system that protects the user's
confidentiality and security. As a result, it's critical not to
present the end user with a simple action that reduces their
confidentiality in response to certificate validation failure. An
MUA which offers a user actions such as "connect anyway", "trust
certificate for future connections" or "lower confidentiality
assurance for this account" in response to certificate validation
failure is not providing a high confidentiality assurance as defined
in this section and thus does not comply with this document.
Examples of acceptable actions to offer would be "work offline", "try
again later", and "open service provider status web page".
3.2. No Confidentiality Assurance
MUAs MAY implement a no confidentiality assurance level for accounts.
At this level, the MUA MUST attempt to negotiate TLS, but MAY ignore
server certificate validation failures. MUAs MAY support use of
connections without TLS, but if they do they SHOULD attempt TLS first
if available and MUST implement code to reconnect without TLS if TLS
negotiation fails for reasons other than server certificate validity.
Note that if the TLS certificate is not successfully validated as
described in Section 3.1 or a version of SSL/TLS prior to TLS 1.0 is
used, the client MUST NOT present a high confidentiality indication
for the account or connection.
3.3. Other Confidentiality Assurance Levels
This specification is not intended to limit experimentation and
innovation with respect to user confidentiality. As a result more
confidentiality assurance levels are permitted. However, levels
below "no confidentiality assurance" described in the previous
section are discouraged and implementers are cautioned that end users
may be confused by too many confidentiality assurance levels.
4. Implicit TLS
Previous standards for use of email protocols with TLS used the
STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501]. With
STARTTLS, the client establishes a clear text application session and
determines whether to issue a STARTTLS command based on server
capabilities and client configuration. If the client issues a
STARTTLS command, a TLS handshake follows that can upgrade the
connection. While this mechanism has been deployed, an alternate
mechanism where TLS is negotiated immediately at connection start on
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a separate port (referred to in this document as "Implicit TLS") has
been deployed more successfully. To increase use of TLS, this
specification recommends use of implicit TLS by new POP, IMAP and
SMTP Submission software.
4.1. Implicit TLS for POP
When a TCP connection is established for the "pop3s" service (default
port 995), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in
[I-D.ietf-uta-email-tls-certs]. Once the TLS session is established,
POP3 [RFC1939] protocol messages are exchanged as TLS application
data for the remainder of the TCP connection. After the server sends
a +OK greeting, the server and client MUST enter AUTHORIZATION state,
even if client credentials were supplied during the TLS handshake.
See Section 9.1.1 for additional information on client certificate
authentication. See Section 11.3 for port registration information.
4.2. Implicit TLS for IMAP
When a TCP connection is established for the "imaps" service (default
port 993), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in [RFC3501]
and SHOULD implement the certificate validation mechanism described
in [I-D.ietf-uta-email-tls-certs]. Once the TLS session is
established, IMAP [RFC3501] protocol messages are exchanged as TLS
application data for the remainder of the TCP connection. If client
credentials were provided during the TLS handshake that the server
finds acceptable, the server MAY issue a PREAUTH greeting in which
case both the server and client enter AUTHENTICATED state. If the
server issues an OK greeting then both server and client enter NOT
AUTHENTICATED state.
See Section 9.1.1 for additional information on client certificate
authentication. See Section 11.4 for port registration information.
4.3. Implicit TLS for SMTP Submission
When a TCP connection is established for the "submissions" service
(default port 465), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in
[I-D.ietf-uta-email-tls-certs]. Once a TLS session is established,
message submission protocol data [RFC6409] is exchanged as TLS
application data for the remainder of the TCP connection. (Note: the
"submissions" service name is defined in section 10.3 of this
document, and follows the usual convention that the name of a service
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layered on top of Implicit TLS consists of the name of the service as
used without TLS, with an "s" appended.)
The STARTTLS mechanism on port 587 is relatively widely deployed due
to the situation with port 465 (discussed in Section 11.5). This
differs from IMAP and POP services where implicit TLS is more widely
deployed on servers than STARTTLS. It is desirable to migrate MUA
software to implicit TLS over time for consistency as well as the
reasons discussed elsewhere in this document. However, to maximize
use of encryption for submission it is desirable to support both
mechanisms for Message Submission over TLS for a transition period of
several years. As a result, clients and servers SHOULD implement
both STARTTLS on port 587 and implicit TLS on port 465 for this
transition period. Note that there is no significant difference
between the security properties of STARTTLS on port 587 and implicit
TLS on port 465 if the implementations are correct and both client
and server are configured to require successful negotiation of TLS
prior to message submission (as required in Section 9.1).
Note that the submissions port provides access to a Mail Submission
Agent (MSA) as defined in [RFC6409] so requirements and
recommendations for MSAs in that document apply to the submissions
port, including the requirement to implement SMTP AUTH [RFC4954].
See Section 9.1.1 for additional information on client certificate
authentication. See Section 11.5 for port registration information.
4.4. Implicit TLS Connection Closure for POP, IMAP and SMTP
When a client or server wishes to close the connection, it SHOULD
initiate the exchange of TLS close alerts before TCP connection
termination. The client MAY, after sending a TLS close alert,
gracefully close the TCP connection without waiting for a TLS
response from the server.
5. Email Security Upgrading Using Security Latches
Once an improved email security mechanism is deployed and ready for
general use, it is desirable to continue using it for all future
email service. For example, TLS is widely deployed in email
software, but use of TLS is often not required. At the time this is
written, deployed mail user agents (MUAs) [RFC5598] usually make a
determination if TLS is available when an account is first configured
and may require use of TLS with that account if and only if it was
initially available. If the service provider makes TLS available
after initial client configuration, many MUAs will not notice the
change.
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Alternatively, a security feature may be purely opportunistic and
thus subject to downgrade attacks. For example, at the time this was
written, most TLS stacks that support TLS 1.2 will use an older TLS
version if the peer does not support TLS 1.2 and some do so without
alerting the client of the reduced security. Thus a variety of
active attacks could cause the loss of TLS 1.2 benefits. Only if
client policy is upgraded to require TLS 1.2 can the client prevent
all downgrade attacks. However, this sort of security policy upgrade
will be ignored by most users unless it is automated.
This section describes a mechanism, called "security latches", which
is designed to permit an MUA to recognize when a service provider has
committed to provide certain server security features, and that it's
safe for the client to change its configuration for that account to
require that such features be present in future sessions with that
server. When an MUA implements both confidentiality assurance levels
and security latches, then both the end-user and the service provider
independently have the ability to improve the end-user's
confidentiality.
Note that security latches are a mechanism similar to HTTP Strict
Transport Security (HSTS) [RFC6797] but are extensible.
5.1. Email Security Tags
Each security latch is given a name known as an email security tag.
An email security tag is a short alphanumeric token that represents a
security facility that can be used by an IMAP, POP or SMTP Submission
session. When a server advertises a security tag it is making a
commitment to support that security facility indefinitely and
recommending that the client save that security tag with the account
configuration and require that security feature for future
connections to that server. When a security tag is saved by the
client in this way, it is then considered latched. For the "tls10"
and/or "tls12" tags, the client SHOULD refuse to connect to the
server unless the appropriate level of TLS is successfully
negotiated. The client SHOULD NOT latch these two tags until TLS has
been successfully negotiated as described in the tag definition. If
the tags are advertised within an appropriate TLS-protected
connection, the client SHOULD latch these tags. Other security tags
are latched if they are advertised by the server, TLS is active and
the client successfully authenticates the server with the TLS
session. Once a security tag is latched, all subsequent connections
to that host require that security feature. For this confidentiality
protection to work as desired clients MUST NOT offer a click-through-
to-connect action when unable to achieve connection security matching
the latched security tags.
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An identifier for a security tag has the following formal syntax:
security-tag = ALPHA *63(ALPHA / DIGIT / "-" / "_")
5.2. Initial Set of Email Security Tags
This section describes an initial set of email security tags. The
IANA Considerations Section 11 defines a registry so that more tags
can be defined in the future. The initial set of tags are defined in
Section 11.2 and include tls10, tls12, tls-cert and tls-dane-tlsa.
5.3. Server DEEP Status
Servers supporting this extension MUST advertise a DEEP status. This
status includes a list of security-tags the server administrator has
explicitly configured as recommended for use by end-users (the list
MAY be empty), an optional https Uniform Resource Locator (URL)
[RFC2818] that the client can save and subsequently resolve for the
user in the event of a security connection problem, and the DEEP
status can be extended by future updates to this specification. DEEP
status has the following formal syntax:
EXTCHAR = 0x20-21 / 0x23-2E / 0x30-3B / 0x3D-40
/ 0x5B-60 / 0x7B-7E
; printable characters excluding " \ < and ALPHA
deep-extend = EXTCHAR *(EXTCHAR / ALPHA / "<")
; clients MUST ignore, for future extensibility
deep-status = [deep-tag *(SP deep-tag)]
deep-tag = deep-https / security-tag / deep-extend
deep-https = "<" <URI from RFC 3986 with https scheme> ">"
The syntax for a Uniform Resource Identifier (URI) is defined in
[RFC3986]. Protocol extensions to advertise DEEP status are defined
in Section 7.
If the client successfully negotiates TLS and authenticates the
server (e.g., via tls-cert, tls-dane-tlsa or SCRAM-SHA1-PLUS with
channel bindings [RFC5802]), then the client SHOULD record the
server's DEEP status information in the account configuration with
the server's hostname. Otherwise, the client SHOULD ignore the
server-provided DEEP status except for the "tls10" and "tls12"
security tags.
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5.4. Email Security Tag Latch Failures
When a security tag latch has been set for connections from a client
to a server and the property identified by that tag is no longer
available, this results in a connection failure. An MUA SHOULD
inform the user of a potential threat to their confidentiality and
offer to resolve a previously-recorded DEEP status https URL if one
is available. MUAs are discouraged from offering a lightweight
option to reset or ignore latches as this defeats the benefit they
provide to end users.
6. Recording TLS Cipher Suite in Received Header
The ESMTPS transmission type [RFC3848] provides trace information
that can indicate TLS was used when transferring mail. However, TLS
usage by itself is not a guarantee of confidentiality or security.
The TLS cipher suite provides additional information about the level
of security made available for a connection. This defines a new SMTP
"tls" Received header additional-registered-clause that is used to
record the TLS cipher suite that was negotiated for the connection.
The value included in this additional clause SHOULD be the registered
cipher suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256)
included in the TLS cipher suite registry. In the event the
implementation does not know the name of the cipher suite (a
situation that should be remedied promptly), a four-digit hexadecimal
cipher suite identifier MAY be used. The ABNF for the field follows:
tls-cipher-clause = CFWS "tls" FWS tls-cipher
tls-cipher = tls-cipher-suite-name / tls-cipher-suite-hex
tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_")
; as registered in IANA cipher suite registry
tls-cipher-hex = "0x" 4HEXDIG
7. Extensions for DEEP Status and Reporting
This memo defines optional mechanisms for use by MUAs to communicate
DEEP status to servers and for servers to advertise available
latches. One purpose of such mechanisms is to permit servers to
determine which and how many clients have latched security
facilities, and thus, to permit operators to be aware of potential
impact to their users should support for such facilities be changed.
For IMAP, the existing ID command is extended to provide this
capability. For SMTP Submission, a new CLIENT command is defined.
No similar mechanism is defined for POP in this version of the memo
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to keep POP simpler, but one may be added in the future if deemed
necessary.
In addition, for each of IMAP, POP, and SMTP, a new DEEP capability
is defined so the client can access the server's DEEP status.
7.1. IMAP DEEP Extension
When an IMAP server advertises the DEEP capability, that indicates
the IMAP server implements IMAP4 ID [RFC2971] with additional field
values defined here. This is grouped with the ID command because
that is the existing IMAP mechanism for clients to report data for
server logging, and provides a way for the server to report the DEEP
status.
deep From server to client, the argument to this ID field is the
server DEEP status. Servers MUST provide this information in
response to an ID command.
latch From client to server, this is a space-separated list of
security tags the client has latched for this server. Servers MAY
record this information so administrators know the expected latch-
related security properties of the client and can thus act to
avoid security latch failures (e.g., by renewing server
certificates on time, etc).
latch-fail From client to server, a space-separated list including
one or more security tag the client has latched that the client
was unable to achieve. This allows clients to report errors to
the server prior to terminating the connection to the server in
the event an acceptable security level is unavailable.
security-tags From client to server, this is a space-separated list
of security tags the client supports that are not latched.
tls Server-side IMAP proxies that accept TLS connections from
clients and connect in-the-clear over a fully private secure
network to the server SHOULD use this field to report the tls-
cipher (syntax as defined in Section 6) to the server.
IMAP clients SHOULD use the IMAP ID command to report latch failures
and determine the server DEEP status. Clients MAY use the ID command
to report other latch or security tag information. IMAP servers MUST
implement the ID command at least to report DEEP status to clients.
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<client connected to port 993 and negotiated TLS successfully>
S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN
AUTH=SCRAM-SHA-1] hello
C: a001 ID ("name" "Demo Mail" "version" "1.5" "latch"
"tls10 tls-cert" "security-tags" "tls12")
S: * ID ("name" "Demo Server" "version" "1.7" "deep-status"
"<https://www.example.com/security-support.html>")
S: a001 OK ID completed
Example 1
This example shows a client that successfully negotiated TLS version
1.0 or later and verified the server's certificate as required by
IMAP. The client supports TLS 1.2. However, even if the client
successfully negotiated TLS 1.2, it will not latch that security tag
automatically because the server did not advertise that tag. If the
client successfully validated the server certificate, it will latch
the provided URL.
<client connected to port 993 and negotiated TLS successfully>
S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN
AUTH=SCRAM-SHA-1] hello
C: a001 ID ("name" "Demo Mail" "version" "1.5" "latch-failure"
"tls-cert")
S: * ID ("name" "Demo Server" "version" "1.7" "deep-status"
"tls10 <https://www.example.com/security-support.html>")
S: a001 OK ID completed
C: a002 LOGOUT
Example 2
This example shows a client that negotiated TLS, but was unable to
verify the server's certificate. The latch-failure informs the
server of this problem, at which point the client can disconnect. If
the client had previously latched a URI for security problems from
this server, it could offer to resolve that URI. However, the deep-
status in this exchange is ignored due to the latch failure.
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<IMAP Proxy connected over private network on port 143, there is
a client connected to the proxy on port 993 that negotiated TLS>
S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN
AUTH=SCRAM-SHA-1] hello
C: a001 ID ("name" "Demo Mail" "version" "1.5" "latch"
"tls10 tls-cert" "security-tags" "tls12"
"tls" "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256")
S: * ID ("name" "Demo Server" "version" "1.7" "deep-status"
"tls10 tls-cert <https://www.example.com/support.html>")
S: a001 OK ID completed
Example 3
This example shows the connection from an IMAP proxy to a back-end
server. The client connected to the proxy and sent the ID command
shown in example 1, and the proxy has added the "tls" item to the ID
command so the back-end server can log the cipher suite that was used
on the connection from the client.
7.2. POP DEEP Extension
POP servers supporting this specification MUST implement the POP3
extension mechanism [RFC2449]. POP servers MUST advertise the DEEP
capability with an argument indicating the server's DEEP status.
<client connected to port 995 and negotiated TLS successfully>
S: +OK POP server ready
C: CAPA
S: +OK Capability list follows
S: TOP
S: SASL PLAIN SCRAM-SHA-1
S: RESP-CODES
S: PIPELINING
S: UIDL
S: DEEP tls10 tls12 <https://www.example.com/security-support.html>
S: .
Example 4
After verifying the TLS server certificate and issuing CAPA, the
client can latch any or all of the DEEP status. If the client
connects to this same server later and has a security failure, the
client can direct the user's browser to the previously-latched URI
where the service provider may provide advice to the end user.
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7.3. SMTP DEEP Extension
SMTP Submission servers supporting this specification MUST implement
the DEEP SMTP extension. The name of this extension is DEEP. The
EHLO keyword value is DEEP and the deep-status ABNF is the syntax of
the EHLO keyword parameters. This does not add parameters to the
MAIL FROM or RCPT TO commands. This also adds a CLIENT command to
SMTP which is used to report client information to the server. The
formal syntax for the command follows:
deep-cmd = "CLIENT" 1*(SP deep-parameter)
deep-parameter = name / version / latch / latch-fail
/ security-tags / tls / future-extension
name = "name=" esmtp-value
version = "version=" esmtp-value
latch = "latch=" security-tag *("," security-tag)
latch-fail = "latch-fail=" security-tag
*("," security-tag)
security-tags = "security-tags=" security-tag
*("," security-tag)
tls = "tls=" tls-cipher
future-extension = esmtp-param
esmtp-param = <as defined in RFC 5321>
esmtp-value = <as defined in RFC 5321>
The CLIENT command parameters listed here have the same meaning as
the parameters used in the IMAP DEEP extension (Section 7.1). The
server responds to the CLIENT command with a "250" if the command has
correct syntax and a "501" if the command has incorrect syntax.
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<client connected to port 465 and negotiated TLS successfully>
S: 220 example.com Demo SMTP Submission Server
C: EHLO client.example.com
S: 250-example.com
S: 250-8BITMIME
S: 250-PIPELINING
S: 250-DSN
S: 250-AUTH PLAIN LOGIN
S: 250-DEEP g tls-cert <https://www.example.com/status.html>
S: 250-BURL imap
S: 250 SIZE 0
C: CLIENT name=demo_submit version=1.5 latch=tls10,tls-cert
security-tags=tls12
S: 250 OK
Example 5
7.4. SMTP Error Extension
Although this document focuses on SMTP Submission, it is possible to
use security latches for SMTP transport as well. When MTA transport
fails due to a security latch, the MTA MUST use the SMTP enhanced
status code X.7.TBD (RFC Editor note: update this TBD). The SMTP
notary response [RFC3464] for a security latch failure MUST include
an additional "SMTP-Security-Latch" recipient-specific header field
that includes a space-delimited list including one or more security
latch that failed. The ABNF for this new field follows:
CFWS = <defined in RFC 5322>
FWS = <defined in RFC 5322>
smtp-security-latch = "SMTP-Security-Latch:" CFWS
security-tag *(FWS security-tag)
8. Account Setup Considerations
8.1. Use of SRV records in Establishing Configuration
This section updates [RFC6186] by changing the preference rules and
adding a new SRV service label _submissions._tcp to refer to Message
Submission with implicit TLS.
User-configurable MUAs SHOULD support use of [RFC6186] for account
setup. However, when using configuration information obtained by
this method, MUAs SHOULD default to a high confidentiality assurance
level, unless the user has explicitly requested reduced
confidentiality. This will have the effect of causing the MUA to
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ignore advertised configurations that do not support TLS, even when
those advertised configurations have a higher priority than other
advertised configurations.
When using [RFC6186] configuration information, Mail User Agents
SHOULD NOT automatically establish new configurations that do not
require TLS for all servers, unless there are no advertised
configurations using TLS. If such a configuration is chosen, prior
to attempting to authenticate to the server or use the server for
message submission, the MUA SHOULD warn the user that traffic to that
server will not be encrypted and that it will therefore likely be
intercepted by unauthorized parties. The specific wording is to be
determined by the implementation, but it should adequately capture
the sense of risk given the widespread incidence of mass surveillance
of email traffic.
When establishing a new configuration for connecting to an IMAP, POP,
or SMTP Submission server, an MUA SHOULD NOT blindly trust SRV
records unless they are signed by DNSSEC and have a valid signature.
Instead, the MUA SHOULD warn the user that the DNS-advertised
mechanism for connecting to the server is not authenticated, and
request the user to manually verify the connection details by
reference to his or her mail service provider's documentation.
Similarly, an MUA MUST NOT consult SRV records to determine which
servers to use on every connection attempt, unless those SRV records
are signed by DNSSEC and have a valid signature. However, an MUA MAY
consult SRV records from time to time to determine if an MSP's server
configuration has changed, and alert the user if it appears that this
has happened. This can also serve as a means to encourage users to
upgrade their configurations to require TLS if and when their MSPs
support it.
8.2. Certificate Pinning
During account setup, the MUA will identify servers that provide
account services such as mail access and mail submission (the
previous section describes one way to do this). The certificates for
these servers are verified using the rules described in
[I-D.ietf-uta-email-tls-certs] and PKIX [RFC5280]. In the event the
certificate does not validate due to an expired certificate, lack of
appropriate chain of trust or lack of identifier match, the MUA MAY
create a persistent binding between that certificate and the saved
host name for the server. This is called certificate pinning.
Certificate pinning is only appropriate during account setup and MUST
NOT be offered in response to a failed certificate validation for an
existing account. An MUA that allows certificate pinning MUST NOT
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allow a certificate pinned for one account to validate connections
for other accounts.
A pinned certificate is subject to a man-in-the-middle attack at
account setup time, and lacks a mechanism to revoke or securely
refresh the certificate. Therefore use of a pinned certificate does
not provide a high confidentiality assurance and an MUA MUST NOT
indicate a high level for an account or connection using a pinned
certificate. Additional advice on certificate pinning is present in
[RFC6125].
9. Implementation Requirements
This section details requirements for implementations of electronic
mail protocol clients and servers. A requirement for a client or
server implementation to support a particular feature is not the same
thing as a requirement that a client or server running a conforming
implementation be configured to use that feature. Requirements for
Mail Service Providers (MSPs) are distinct from requirements for
protocol implementations, and are listed in a separate section.
9.1. All Implementations (Client and Server)
These requirements apply to MUAs as well as POP, IMAP and SMTP
Submission servers.
o All implementations MUST be configurable to support implicit TLS
using the TLS 1.2 protocol or later [RFC5246].
o All implementations MUST implement the recommended cipher suites
described in [RFC7525] or a future BCP or standards track revision
of that document.
o All implementations MUST be configurable to require TLS before
performing any operation other than capability discovery and
STARTTLS.
o The IMAP specification [RFC3501] is hereby modified to revoke the
second paragraph of section 11.1 and replace it with the text from
the first three bullet items in this list.
o The standard for use of TLS with IMAP, POP3 and ACAP [RFC2595] is
modified to revoke section 2.1 and replace it with the text from
the first three bullet items in this list.
o The standard for Message Submission [RFC6409] is updated to add
the first three bullet items above to section 4.3 as well as to
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require implementation of the TLS server identity check as
described in [I-D.ietf-uta-email-tls-certs] and PKIX [RFC5280].
9.1.1. Client Certificate Authentication
MUAs and mail servers MAY implement client certificate authentication
on the implicit TLS port. Servers MUST NOT request a client
certificate during the TLS handshake unless the server is configured
to accept some client certificates as sufficient for authentication
and the server has the ability to determine a mail server
authorization identity matching such certificates. How to make this
determination is presently implementation specific. Clients MUST NOT
provide a client certificate during the TLS handshake unless the
server requests one and the client has determined the certificate can
be safely used with that specific server, OR the client has been
explicitly configured by the user to use that particular certificate
with that server. How to make this determination is presently
implementation specific. If the server accepts the client's
certificate as sufficient for authorization, it MUST enable the SASL
EXTERNAL [RFC4422] mechanism. An IMAPS server MAY issue a PREAUTH
greeting instead of enabling SASL EXTERNAL. A client supporting
client certificate authentication with implicit TLS MUST implement
the SASL EXTERNAL [RFC4422] mechanism using the appropriate
authentication command (AUTH for POP3 [RFC5034], AUTH for SMTP
Submission [RFC4954], AUTHENTICATE for IMAP [RFC3501]).
9.2. Mail Server Implementation Requirements
These requirements apply to servers that implement POP, IMAP or SMTP
Submission.
o Servers MUST implement the DEEP extension described in Section 7
o IMAP and SMTP submission servers SHOULD implement and be
configurable to support STARTTLS. This enables discovery of new
TLS availability, and can increase usage of TLS by legacy clients.
o Servers MUST NOT advertise STARTTLS if it is unlikely to succeed
based on server configuration (e.g., there is no server
certificate installed).
o SMTP message submission servers that have negotiated TLS SHOULD
add a Received header field to the message including the tls
clause described in Section 6.
o Servers MUST be configurable to include the TLS cipher information
in any connection or user logging or auditing facility they
provide.
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9.3. Mail User Agent Implementation Requirements
This section describes requirements on Mail User Agents (MUAs) using
IMAP, POP, and/or Submission protocols. Note: Requirements
pertaining to use of Submission servers are also applicable to use of
SMTP servers (e.g., port 25) for mail submission.
o User agents SHOULD indicate to users at configuration time, the
expected level of confidentiality based on appropriate security
inputs such as which security latches are pre-set, the number of
trust anchors, certificate validity, use of an extended validation
certificate, TLS version supported, and TLS cipher suites
supported by both server and client. This indication SHOULD also
be present when editing or viewing account configuration.
o MUAs SHOULD detect when STARTTLS and/or implicit TLS becomes
available for a protocol and set the tls10 latch if the server
advertises the tls10 security tag after a successful TLS
negotiation.
o Whenever requested to establish any configuration that does not
require both TLS and server certificate verification to talk to a
server or account, an MUA SHOULD warn its user that his or her
mail traffic (including password, if applicable) will be exposed
to attackers, and give the user an opportunity to abort the
connection prior to transmission of any such password or traffic.
o MUAs SHOULD implement the "tls12" security latch (the TLS library
has to provide an API that controls permissible TLS versions and
communicates the negotiated TLS protocol version to the
application for this to be possible).
o See Section 3 for additional requirements.
9.4. Non-configurable MUAs and nonstandard access protocols
MUAs which are not configurable to use user-specified servers MUST
implement TLS or similarly other strong encryption mechanism when
communicating with their mail servers. This generally applies to
MUAs that are pre-configured to operate with one or more specific
services, whether or not supplied by the vendor of those services.
MUAs using protocols other than IMAP, POP, and Submission to
communicate with mail servers, MUST implement TLS or other similarly
robust encryption mechanism in conjunction with those protocols.
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9.5. DEEP Compliance for Anti-Virus/Anti-Spam Software and Services
There are multiple ways to connect an Anti-Virus and/or Anti-Spam
(AVAS) service to a mail server. Some mechanisms, such as the de-
facto milter protocol do not impact DEEP. However, some services use
an SMTP relay proxy that intercepts mail at the application layer to
perform a scan and proxy or forward to another MTA. Deploying AVAS
services in this way can cause many problems [RFC2979] including
direct interference with DEEP and confidentiality or security
reduction. An AVAS product or service is considered DEEP compliant
if all IMAP, POP and SMTP-related software it includes is DEEP
compliant and it advertises and supports all security latches that
the actual servers advertise.
Note that end-to-end email encryption prevents AVAS software and
services from using email content as part of a spam or virus
assessment. Furthermore, while DEEP high confidentiality assurance
can prevent a man-in-the-middle from introducing spam or virus
content between the MUA and Submission server, it does not prevent
other forms of client or account compromise so use of AVAS services
for submitted email remains necessary.
10. Mail Service Provider Requirements
This section details requirements for providers of IMAP, POP, and/or
SMTP submission services, for providers who claim to conform to this
specification.
10.1. Server Requirements
Mail Service Providers MUST use server implementations that conform
to this specification.
10.2. MSPs MUST provide Submission Servers
This document updates the advice in [RFC5068] by making Implicit TLS
on port 465 the preferred submission port.
Mail Service Providers that accept mail submissions from end-users
using the Internet Protocol MUST provide one or more SMTP Submission
servers for this purpose, separate from the SMTP servers used to
process incoming mail. Those submission servers MUST be configured
to support Implicit TLS on port 465 and SHOULD support STARTTLS if
port 587 is used.
MSPs MAY also support submission of messages via one or more
designated SMTP servers to facilitate compatibility with legacy MUAs.
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Discussion: SMTP servers used to accept incoming mail or to relay
mail are expected to accept mail in cleartext. This is incompatible
with the purpose of this memo which is to encourage encryption of
traffic between mail servers. There is no such requirement for mail
submission servers to accept mail in cleartext or without
authentication. For other reasons, use of separate SMTP submission
servers has been best practice for many years.
10.3. TLS Server Certificate Requirements
MSPs MUST maintain valid server certificates for all servers. Those
server certificates SHOULD present DNS-IDs and SRV-IDs conforming to
[RFC6125] and which will be recognized by MUAs meeting the
requirements of that specification. In addition, those server
certificates MAY provide other DNS-IDs, SRV-IDs, or CN-IDs needed for
compatibility with existing MUAs.
If a protocol server provides service for more than one mail domain,
it MAY use a separate IP address for each domain and/or a server
certificate that advertises multiple domains. This will generally be
necessary unless and until it is acceptable to impose the constraint
that the server and all clients support the Server Name Indication
extension to TLS [RFC6066].
10.4. Recommended DNS records for mail protocol servers
This section discusses not only the DNS records that are recommended,
but also implications of DNS records for server configuration and TLS
server certificates.
10.4.1. MX records
It is recommended that MSPs advertise MX records for handling of
inbound mail (instead of relying entirely on A or AAAA records), and
that those MX records be signed using DNSSEC. This is mentioned here
only for completeness, as handling of inbound mail is out of scope
for this document.
10.4.2. SRV records
MSPs SHOULD advertise SRV records to aid MUAs in determination of
proper configuration of servers, per the instructions in [RFC6186].
MSPs SHOULD advertise servers that support Implicit TLS in preference
to those which support cleartext and/or STARTTLS operation.
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10.4.3. DNSSEC
All DNS records advertised by an MSP as a means of aiding clients in
communicating with the MSP's servers, SHOULD be signed using DNSSEC.
10.4.4. TLSA records
MSPs SHOULD advertise TLSA records to provide an additional trust
anchor for public keys used in TLS server certificates. However,
TLSA records MUST NOT be advertised unless they are signed using
DNSSEC.
10.5. MSP Server Monitoring
MSPs SHOULD regularly and frequently monitor their various servers to
make sure that: TLS server certificates remain valid and are not
about to expire, TLSA records match the public keys advertised in
server certificates, are signed using DNSSEC, server configurations
are consistent with SRV advertisements, and DNSSEC signatures are
valid and verifiable. Failure to detect expired certificates and DNS
configuration errors in a timely fashion can result in significant
loss of service for an MSP's users and a significant support burden
for the MSP.
10.6. Advertisement of DEEP status
MSPs SHOULD advertise a DEEP status that includes tls10, tls-cert and
an HTTPS URL that can be used to inform clients of service outages or
problems impacting client confidentiality. Note that advertising
tls-cert is a commitment to maintain and renew server certificates.
10.7. Require TLS
New servers and services SHOULD be configured to require TLS unless
it's necessary to support legacy clients or existing client
configurations.
10.8. Changes to Internet Facing Servers
When an MSP changes the Internet Facing Servers providing mail access
and mail submission services, including SMTP-based spam/virus
filters, it is generally necessary to support the same and/or a newer
version of TLS and the same security tags that were previously
advertised.
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11. IANA Considerations
11.1. Security Tag Registry
IANA shall create (has created) the registry "Email Security Tags".
This registry is a single table and will use an expert review process
[RFC5226]. Each registration will contain the following fields:
Name: The name of the security tag. This follows the security-tag
ABNF.
Description: This describes the meaning of the security tag and the
conditions under which the tag is latched.
Intended Usage: One of COMMON, LIMITED USE or OBSOLETE.
Reference: Optional reference to specification.
Submitter: The identify of the submitter or submitters.
Change Controller: The identity of the change controller for the
registration. This will be "IESG" in case of registrations in
IETF-produced documents.
The expert reviewer will verify the tag name follows the ABNF, and
that the description field is clear, unambiguous, does not overlap
existing deployed technology, does not create security problems and
appropriately considers interoperability issues. Email security tags
intended for LIMITED USE have a lower review bar (interoperability
and overlap issues are less of a concern). The reviewer may approve
a registration, reject for a stated reason or recommend the proposal
have standards track review due to importance or difficult
subtleties.
Standards-track registrations may be updated if the relevant
standards are updated as a consequence of that action. Non-
standards-track entries may be updated by the listed change
controller. The entry's name and submitter may not be changed. In
exceptional cases, any aspect of any registered entity may be updated
at the direction of the IESG (for example, to correct a conflict).
11.2. Initial Set of Security Tags
This document defines four initial security tags for the security tag
registry as follows:
Name: tls10
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Description: This indicates TLS version 1.0 [RFC2246] or later was
negotiated successfully including negotiation of a strong
encryption layer with a symmetric key of at least 128 bits. This
tag does not indicate the server certificate was valid. This tag
is latched if the client sees this tag in the advertised server
DEEP status provided after successfully negotiating TLS version
1.0 or later.
Intended Usage: COMMON
Reference: RFC XXXX (this document once published)
Submitter: Authors of this document
Change Controller: IESG
Name: tls12
Description: This indicates TLS version 1.2 [RFC5246] or later was
negotiated successfully including negotiation of a strong
encryption layer with a symmetric key of at least 128 bits. This
tag does not indicate the server certificate was valid. This tag
is latched if the client sees this tag in the advertised server
DEEP status provided after successfully negotiating TLS version
1.2 or later.
Intended Usage: COMMON
Reference: RFC XXXX (this document once published)
Submitter: Authors of this document
Change Controller: IESG
Name: tls-cert
Description: This tag indicates that TLS was successfully negotiated
and the server certificate was successfully verified by the client
using PKIX [RFC5280] and the server certificate identity was
verified using the algorithm appropriate for the protocol (see
Section 4). This tag is latched if the client sees this tag in
the advertised server DEEP status after successfully negotiating
TLS and verifying the certificate and server identity.
Intended Usage: COMMON
Reference: RFC XXXX (this document once published)
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Submitter: Authors of this document
Change Controller: IESG
Name: tls-dane-tlsa
Description: This tag indicates that TLS was successfully negotiated
and the server certificate was successfully verified by the client
using the procedures described in [RFC6698] and the server
certificate identity was verified using the algorithm appropriate
for the protocol (see Section 4). This tag is latched if the
client sees this tag in the advertised server DEEP status after
successfully negotiating TLS and verifying the certificate and
server identity.
Intended Usage: COMMON
Reference: RFC XXXX (this document once published)
Submitter: Authors of this document
Change Controller: IESG
11.3. POP3S Port Registration Update
IANA is asked to update the registration of the TCP well-known port
995 using the following template ([RFC6335]):
Service Name: pop3s
Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org>
Description: POP3 over TLS protocol
Reference: RFC XXXX (this document once published)
Port Number: 995
11.4. IMAPS Port Registration Update
IANA is asked to update the registration of the TCP well-known port
993 using the following template ([RFC6335]):
Service Name: imaps
Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org>
Description: IMAP over TLS protocol
Reference: RFC XXXX (this document once published)
Port Number: 993
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11.5. Submissions Port Registration
IANA is asked to assign an alternate usage of port 465 in addition to
the current assignment using the following template ([RFC6335]):
Service Name: submissions
Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org>
Description: Message Submission over TLS protocol
Reference: RFC XXXX (this document once published)
Port Number: 465
This is a one time procedural exception to the rules in RFC 6335.
This requires explicit IESG approval and does not set a precedent.
Historically, port 465 was briefly registered as the "smtps" port.
This registration made no sense as the SMTP transport MX
infrastructure has no way to specify a port so port 25 is always
used. As a result, the registration was revoked and was subsequently
reassigned to a different service. In hindsight, the "smtps"
registration should have been renamed or reserved rather than
revoked. Unfortunately, some widely deployed mail software
interpreted "smtps" as "submissions" [RFC6409] and used that port for
email submission by default when an end-user requests security during
account setup. If a new port is assigned for the submissions
service, email software will either continue with unregistered use of
port 465 (leaving the port registry inaccurate relative to de-facto
practice and wasting a well-known port), or confusion between the de-
facto and registered ports will cause harmful interoperability
problems that will deter use of TLS for message submission. The
authors believe both of these outcomes are less desirable than a wart
in the registry documenting real-world usage of a port for two
purposes. Although STARTTLS-on-port-587 has deployed, it has not
replaced deployed use of implicit TLS submission on port 465.
11.6. DEEP IMAP Capability
This document adds the DEEP capability to the IMAP capabilities
registry. This is described in Section 7.1.
11.7. DEEP POP3 Capability
This document adds the DEEP capability to the POP3 capabilities
registry.
CAPA Tag: DEEP
Arguments: deep-status
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Added Commands: none
Standard Commands affected: none
Announced status / possible differences: both / may change after
STLS
Commands Valid in States: N/A
Specification Reference: This document
Discussion: See Section 7.2.
11.8. DEEP SMTP EHLO Keyword
This document adds the DEEP EHLO Keyword to the SMTP Service
Extension registry. This is described in Section 7.3.
11.9. SMTP Enhanced Status Code
This document adds the following entry to the "SMTP Enhanced Status
Codes" registry created by [RFC5248].
Code: X.7.TBD (IANA, please assign the next available number)
Sample Text: Message Transport Failed due to missing required
security.
Associated Basic Status Code: 450, 454, 550, 554
Description This code indicates an SMTP server was unable to forward
a message to the next host necessary for delivery because it
required a higher level of transport security or confidentiality
than was available. The temporary form of this error is preferred
in case the problem is caused by a temporary administrative error
such as an expired server certificate.
Reference This document
Submitter C. Newman
Change Controller IESG
11.10. MAIL Parameters Additional-registered-clauses Sub-Registry
This document adds the following entry to the "Additional-registered-
clauses" sub-registry of the "MAIL Parameters" registry, created by
[RFC5321]:
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Clause Name: tls
Description: Indicates the TLS cipher suite used for a transport
connection.
Syntax Summary: See tls-cipher ABNF Section 6
Reference: This document.
12. Security Considerations
This entire document is about security considerations. In general,
this is targeted to improve mail confidentiality and to mitigate
threats external to the email system such as network-level snooping
or interception; this is not intended to mitigate active attackers
who have compromised service provider systems.
It could be argued that sharing the name and version of the client
software with the server has privacy implications. Although
providing this information is not required, it is encouraged so that
mail service providers can more effectively inform end-users running
old clients that they need to upgrade to protect their security, or
know which clients to use in a test deployment prior to upgrading a
server to have higher security requirements.
13. References
13.1. Normative References
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2449] Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
Mechanism", RFC 2449, November 1998.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC2971] Showalter, T., "IMAP4 ID extension", RFC 2971, October
2000.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
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[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464, January
2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
(POP3) Simple Authentication and Security Layer (SASL)
Authentication Mechanism", RFC 5034, July 2007.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
Finch, "Email Submission Operations: Access and
Accountability Requirements", BCP 134, RFC 5068, November
2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", BCP 138, RFC 5248, June 2008.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
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[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, March 2011.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.
[I-D.ietf-uta-email-tls-certs]
Melnikov, A., "Updated TLS Server Identity Check Procedure
for Email Related Protocols", draft-ietf-uta-email-tls-
certs-03 (work in progress), June 2015.
[I-D.ietf-dane-smtp-with-dane]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-02
(work in progress), October 2013.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, May 2015.
13.2. Informative References
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.
[RFC2979] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, October 2000.
[RFC3848] Newman, C., "ESMTP and LMTP Transmission Types
Registration", RFC 3848, July 2004.
[RFC3887] Hansen, T., "Message Tracking Query Protocol", RFC 3887,
September 2004.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009.
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[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.
[RFC5804] Melnikov, A. and T. Martin, "A Protocol for Remotely
Managing Sieve Scripts", RFC 5804, July 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165, RFC
6335, August 2011.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, November 2012.
Appendix A. Design Considerations
This section is not normative.
The first version of this was written independently from draft-moore-
email-tls-00.txt; subsequent versions merge ideas from both drafts.
One author of this document was also the author of RFC 2595 that
became the standard for TLS usage with POP and IMAP, and the other
author was perhaps the first to propose that idea. In hindsight both
authors now believe that that approach was a mistake. At this point
the authors believe that while anything that makes it easier to
deploy TLS is good, the desirable end state is that these protocols
always use TLS, leaving no need for a separate port for cleartext
operation except to support legacy clients while they continue to be
used. The separate port model for TLS is inherently simpler to
implement, debug and deploy. It also enables a "generic TLS load-
balancer" that accepts secure client connections for arbitrary foo-
over-TLS protocols and forwards them to a server that may or may not
support TLS. Such load-balancers cause many problems because they
violate the end-to-end principle and the server loses the ability to
log security-relevant information about the client unless the
protocol is designed to forward that information (as this
specification does for the cipher suite). However, they can result
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in TLS deployment where it would not otherwise happen which is a
sufficiently important goal that it overrides the problems.
Although STARTTLS appears only slightly more complex than separate-
port TLS, we again learned the lesson that complexity is the enemy of
security in the form of the STARTTLS command injection vulnerability
(CERT vulnerability ID #555316). Although there's nothing inherently
wrong with STARTTLS, the fact it resulted in a common implementation
error (made independently by multiple implementers) suggests it is a
less secure architecture than Implicit TLS.
Section 7 of RFC 2595 critiques the separate-port approach to TLS.
The first bullet was a correct critique. There are proposals in the
http community to address that, and use of SRV records as described
in RFC 6186 resolves that critique for email. The second bullet is
correct as well, but not very important because useful deployment of
security layers other than TLS in email is small enough to be
effectively irrelevant. The third bullet is incorrect because it
misses the desirable option of "use and latch-on TLS if available".
The fourth bullet may be correct, but is not a problem yet with
current port consumption rates. The fundamental error was
prioritizing a perceived better design based on a mostly valid
critique over real-world deployability. But getting security and
confidentiality facilities actually deployed is so important it
should trump design purity considerations.
Appendix B. Open Issues
There are many open issues with this document. Here is an attempt to
enumerate some of them:
o Port 465 is presently used for two purposes: for submissions by a
large number of clients and service providers and for the "urd"
protocol by one vendor. Actually documenting this current state
is controversial as discussed in the IANA considerations section.
However, there is no good alternative. Registering a new port for
submissions when port 465 is widely used for that purpose already
will just create interoperability problems. Registering a port
that's only used if advertised by an SRV record (RFC 6186) would
not create interoperability problems but would require all client
and server deployments and software to change significantly which
is contrary to the goal of promoting more TLS use. Encouraging
use of STARTTLS on port 587 would not create interoperability
problems, but is unlikely to have impact on current undocumented
use of port 465 and makes the guidance in this document less
consistent.
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o One author believes that the security latch model is complementary
with draft-ietf-dane-smtp-with-dane-02 but hasn't thought about
the issues in depth. We welcome feedback on this point.
o The two authors of this document and the author of draft-melnikov-
email-tls-certs are willing to merge these two documents.
However, it is undesirable to delay publication of either document
so this will be done only if the latter document is not yet
through IESG processing when this document is ready for the IESG.
o It might make sense to split this in two or more documents if it's
getting too long to evaluate in one IETF last call. In
particular, it might make sense to put implementation requirements
and service provider requirements in separate documents. The
authors prefer to edit one document for now and defer discussion
of splitting the document until all technical issues are resolved.
o The use of SRV records [RFC6186] for account setup or refresh is
presently not secure from DNS active attacks unless DNSSEC is
used. If someone wishes to provide suggested text describing how
to use DANE in this process, the WG can consider adding that text
to this document. Absent suggested text, the editor intends to
leave this issue alone.
Appendix C. Change Log
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.
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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.
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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 D. Acknowledgements
Thanks to Ned Freed for discussion of the initial latch concepts in
this document. Thanks to Alexey Melnikov for draft-melnikov-pop3-
over-tls-02, which was the basis of the POP3 implicit TLS text.
Thanks to Russ Housley, Alexey Melnikov and Dan Newman for review
feedback. Thanks to Paul Hoffman for interesting feedback in initial
conversations about this idea.
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Authors' Addresses
Keith Moore
Network Heretics
PO Box 1934
Knoxville, TN 37901
US
Email: moore@network-heretics.com
Chris Newman
Oracle
440 E. Huntington Dr., Suite 400
Arcadia, CA 91006
US
Email: chris.newman@oracle.com
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