Internet DRAFT - draft-newman-email-deep
draft-newman-email-deep
Network Working Group K. Moore
Internet-Draft Network Heretics
Updates: 1939, 3464, 3501, 5068, C. Newman
6186 (if approved) Oracle
Intended status: Standards Track August 16, 2014
Expires: February 17, 2015
Deployable Enhanced Email Privacy (DEEP)
draft-newman-email-deep-02.txt
Abstract
This specification defines a set of requirements and facilities
designed to improve email privacy. This provides mechanisms intended
to increase use of already deployed Transport Layer Security (TLS)
technology, provide a model for mail user agents privacy assurance,
and enable mail service providers to advertise improved TLS privacy
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 17, 2015.
Copyright Notice
Copyright (c) 2014 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
to this document. Code Components extracted from this document must
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Terminology Used in This Document . . . . . . 4
3. Mail Account Privacy Assurance Level . . . . . . . . . . . . . 5
3.1. High Privacy Assurance . . . . . . . . . . . . . . . . . 5
3.2. Certificate Pinning . . . . . . . . . . . . . . . . . . . 6
3.3. Low Privacy Assurance . . . . . . . . . . . . . . . . . . 6
3.4. Other Privacy Assurance Levels . . . . . . . . . . . . . 7
4. Implicit TLS . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Implicit TLS for POP . . . . . . . . . . . . . . . . . . 7
4.2. Implicit TLS for IMAP . . . . . . . . . . . . . . . . . . 8
4.3. Implicit TLS for SMTP Submission . . . . . . . . . . . . 8
4.4. Implicit TLS Connection Closure for POP, IMAP and SMTP . 8
5. Email Security Upgrading Using Security Latches . . . . . . . 9
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 . . . . . . . . . . . 12
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. Use of SRV records in Establishing Configuration . . . . . . . 16
9. Implementation Requirements . . . . . . . . . . . . . . . . . 17
9.1. All Implementations (Client and Server) . . . . . . . . . 17
9.1.1. Client Certificate Authentication . . . . . . . . . . 18
9.2. Mail Server Implementation Requirements . . . . . . . . . 18
9.3. Mail User Agent Implementation Requirements . . . . . . . 19
9.4. Non-configurable MUAs and nonstandard access protocols . 20
9.5. DEEP Compliance for Anti-Virus/Anti-Spam Software and
Services . . . . . . . . . . . . . . . . . . . . . . . . 20
10. Mail Service Provider Requirements . . . . . . . . . . . . . . 20
10.1. Server Requirements . . . . . . . . . . . . . . . . . . . 20
10.2. MSPs MUST provide Submission Servers . . . . . . . . . . 20
10.3. TLS Server Certificate Requirements . . . . . . . . . . . 21
10.4. Recommended DNS records for mail protocol servers . . . . 21
10.4.1. MX records . . . . . . . . . . . . . . . . . . . . . . 21
10.4.2. SRV records . . . . . . . . . . . . . . . . . . . . . 21
10.4.3. TLSA records . . . . . . . . . . . . . . . . . . . . . 22
10.4.4. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . . 22
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10.5. MSP Server Monitoring . . . . . . . . . . . . . . . . . . 22
10.6. Advertisement of DEEP status . . . . . . . . . . . . . . 22
10.7. Require TLS . . . . . . . . . . . . . . . . . . . . . . . 22
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
11.1. Security Tag Registry . . . . . . . . . . . . . . . . . . 22
11.2. Initial Set of Security Tags . . . . . . . . . . . . . . 23
11.3. POP3S Port Registration Update . . . . . . . . . . . . . 25
11.4. IMAPS Port Registration Update . . . . . . . . . . . . . 25
11.5. Submissions Port Registration . . . . . . . . . . . . . . 26
11.6. DEEP IMAP Capability . . . . . . . . . . . . . . . . . . 27
11.7. DEEP POP3 Capability . . . . . . . . . . . . . . . . . . 27
11.8. DEEP SMTP EHLO Keyword . . . . . . . . . . . . . . . . . 27
11.9. SMTP Enhanced Status Code . . . . . . . . . . . . . . . . 27
11.10. MAIL Parameters Additional-registered-clauses
Sub-Registry . . . . . . . . . . . . . . . . . . . . . . 28
12. Security Considerations . . . . . . . . . . . . . . . . . . . 28
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
13.1. Normative References . . . . . . . . . . . . . . . . . . 29
13.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Design Considerations . . . . . . . . . . . . . . . . 31
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . . 32
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 34
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
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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) [RFC5321] usually has
Transport Layer Security (TLS) [RFC5246] support but often does not
use it in a way that maximizes end-user privacy. 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 privacy assurance level with each mail account,
and the default privacy 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;
o MUAs and mail protocol servers cooperate (via mechanisms defined
in this specification) to upgrade security/privacy feature use and
record/indicate that usage appropriately.
Improved use of TLS with SMTP for message relaying is described in a
separate document [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
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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 Privacy Assurance Level
The configuration necessary for a mail account includes an email
address, connection information and authentication credentials for at
least one mail access server (IMAP or POP) and at least one SMTP
submission server. A mail user agent (MUA) typically supports one or
more mail account configurations. MUAs compliant with this
specification MUST associate a privacy assurance level with each mail
account. MUAs MUST implement a high privacy level as described in
the next section.
MUAs SHOULD continuously indicate to the user the privacy for an
account's connections (e.g., via a lock icon, background colors and
indications similar to those commonly used in web browsers for this
purpose). Note that this 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 privacy level provided by the least private
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 privacy assurance level as the default for newly configured
accounts.
3.1. High Privacy Assurance
A mail account has a high privacy 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.
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o MUAs MUST implement [I-D.melnikov-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 privacy
if he/she chooses. Reduction of expected account privacy MUST NOT
be done on a click-through basis.
The end user is part of the system that protects the user's privacy
and security. As a result, it's critical not to present the end user
with a simple action that reduces their privacy in response to
certificate validation failure. An MUA which offers a user actions
such as "connect anyway", "trust certificate for future connections"
or "lower privacy assurance for this account" in response to
certificate validation failure is not providing a high privacy
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. Certificate Pinning
MUAs MAY implement certificate pinning as part of account setup, but
MUST NOT offer this as an option in response to a failed certificate
validation for an existing account. Certificate pinning occurs when
the user agent saves a server certificate with the account settings
and trusts that certificate for subsequent connections to that
server. An MUA that allows certificate pinning MUST NOT 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 privacy assurance and an MUA MUST NOT indicate a
high privacy level for an account or connection using a pinned
certificate.
3.3. Low Privacy Assurance
MUAs MAY implement a low privacy 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
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negotiation fails for reasons other than 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 privacy indication for the
account or connection.
3.4. Other Privacy Assurance Levels
This specification is not intended to limit experimentation and
innovation with respect to user privacy. As a result more privacy
assurance levels are permitted. However, levels below the "low
privacy assurance" described in the previous section are discouraged
and implementers are cautioned that end users may be confused by too
many privacy 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 deployed, an alternate
mechanism where TLS is negotiated immediately at connection start on
a separate port (referred to in this document as "Implicit TLS") has
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.melnikov-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.
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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.melnikov-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.melnikov-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
layered on top of Implicit TLS consists of the name of the service as
used without TLS, with an "s" appended.)
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.
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5. Email Security Upgrading Using Security Latches
Once an improved email security or privacy 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.
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 fallback to TLS
1.0 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 privacy assurance levels and
security latches, then both the end-user and the service provider
independently have the ability to improve the end-user's privacy.
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. If these tags are still advertised by the server after
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negotiation, 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 privacy
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.
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.
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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.
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 privacy and offer to
resolve a previously-recorded DEEP status https URL if one is
available. An MUA might suggest deleting the account and re-creating
it as a cumbersome mechanism to reset the latches. MUAs are
discouraged from offering a lightweight option to reset or ignore
latches as this defeats the privacy 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 privacy or security. The TLS
cipher suite provides additional information about the level of
privacy or 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_DHE_RSA_WITH_AES_128_CBC_SHA) 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
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7. Extensions for DEEP Status and Reporting
This memo defines optional mechanisms for use by MUAs to communicate
DEEP status to servers. 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
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 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
privacy level 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 privacy 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.
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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.
<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/privacy-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/privacy-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 privacy 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_RSA_WITH_AES_128_CBC_SHA")
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/privacy-support.html>
S: .
Example
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 privacy 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 tls10 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
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. 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. 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 privacy assurance level,
unless the user has explicitly requested reduced privacy. This will
have the effect of causing the MUA to ignore advertised
configurations which do not support TLS, even when those advertised
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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.
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.
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o All implementations MUST be configurable to support implicit TLS
using the TLS 1.2 protocol or later [RFC5246] including support
for the mandatory-to-implement TLS 1.2 cipher suite
TLS_RSA_WITH_AES_128_CBC_SHA.
o IMAP implementations MUST support the IMAP4rev1 mandatory-to-
implement cipher suite TLS_RSA_WITH_RC4_128_MD5 for any
connections made or received via IMAP although this MAY be
disabled by default.
o All implementations MUST be configurable to require TLS before
performing any operation other than capability discovery and
STARTTLS.
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.
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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.
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 at configuration time, the expected
level of privacy 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.
o MUAs SHOULD detect when STARTTLS and/or implicit TLS becomes
available for a protocol and set the tls10 latch if the server
advertises that latch.
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.
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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.
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 to the real MTA. Deploying AVAS services in
this way can cause many problems [RFC2979] including direct
interference with DEEP and privacy 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 all
security latches that the actual MTA advertises.
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.
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MSPs MAY also support submission of messages via one or more
designated SMTP servers to facilitate compatibility with legacy MUAs.
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
certificates 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. 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.
10.4.4. 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.
10.5. 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 privacy. 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.
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:
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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 or privacy
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
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.
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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)
Submitter: Authors of this document
Change Controller: IESG
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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)
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)
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
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Reference: RFC XXXX (this document once published)
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)
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)
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)
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
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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
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
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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 privacy 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]:
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 privacy 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 privacy, or
know which clients to use in a test deployment prior to upgrading a
server to have higher security requirements.
13. References
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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.
[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.
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[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.
[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.melnikov-email-tls-certs]
Melnikov, A., "Updated TLS Server Identity Check Procedure
for Email Related Protocols",
draft-melnikov-email-tls-certs-01 (work in progress),
October 2013.
[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.
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.
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[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.
[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
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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
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
privacy 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:
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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.
o Discussion of pinning certificates is new and may be inadequate.
Suggestions to improve the text are welcome.
o This document should reference draft-ietf-uta-tls-bcp and possibly
other guidance documents. Suggested text on where/how to
reference this and possibly other TLS guidance (e.g., must
staple). would be welcome.
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 three involved authors are willing to merge
draft-melnikov-email-tls-certs into this document. However, this
will take time so we are only willing to do so if there is rough
consensus on the decision (so it's a one time action) and doing so
will not significantly delay publication.
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. As this document is now focusing on MUA security/privacy,
discussing how to do SRV record account setup or account refresh
securely, probably using DANE, would be in scope for this
document. It has been suggested that we add this.
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o This document does not cover use of TLS with SMTP relay.
Appendix C. Change Log
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.
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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
Many 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 Dan Newman and Alexey Melnikov for
review feedback. Thanks to Paul Hoffman for interesting feedback in
initial conversations about this idea.
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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