Internet Engineering Task Force J. Fenton
Internet-Draft February 13, 2016
Intended status: Experimental
Expires: August 16, 2016
SMTP Require TLS Option
draft-fenton-smtp-require-tls-01
Abstract
The SMTP STARTTLS option, used in negotiating transport-level
encryption of SMTP connections, is not as useful from a security
standpoint as it might be because of its opportunistic nature;
message delivery is prioritized over security. This document
describes a complementary SMTP service extension, REQUIRETLS. If the
REQUIRETLS option is used when sending a message, it causes message
delivery to fail if a TLS connection with the required security
characteristics cannot be completed with the next hop MTA or if that
MTA does not also advertise that it supports REQUIRETLS. Message
originators may therefore expect transport security to be used for
messages sent with this option.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 16, 2016.
Copyright Notice
Copyright (c) 2016 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. The REQUIRETLS Service Extension . . . . . . . . . . . . . . 3
3. REQUIRETLS Semantics . . . . . . . . . . . . . . . . . . . . 4
3.1. REQUIRETLS Receipt Requirements . . . . . . . . . . . . . 4
3.2. REQUIRETLS Sender Requirements . . . . . . . . . . . . . 4
3.3. REQUIRETLS Submission . . . . . . . . . . . . . . . . . . 5
3.4. Delivery of REQUIRETLS messages . . . . . . . . . . . . . 5
4. Error handling . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6.1. Passive attacks . . . . . . . . . . . . . . . . . . . . . 6
6.2. Active attacks . . . . . . . . . . . . . . . . . . . . . 6
6.3. Bad Actor MTAs . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. Revision History . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Changes Since -00 Draft . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a
means by which an SMTP server and client can establish a Transport
Layer Security (TLS) protected session for the transmission of email
messages. In this application, TLS is used only upon mutual
agreement (successful negotiation) between the client and server; if
this is not possible, the message is sent unencrypted. Even if a TLS
protected session is established, it is uncommon for the client to
abort the SMTP session if certificate validation fails to
authenticate the SMTP server.
The opportunistic nature of SMTP TLS enables several "on the wire"
attacks on SMTP security between MTAs. These include passive
eavesdropping on connections for which TLS is not used, interference
in the SMTP protocol to prevent TLS from being negotiated (presumably
followed by subsequent eavesdropping), and insertion of a man-in-the-
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middle attacker taking advantage of the lack of server authentication
by the client. Attacks are more described in more detail in the
Security Considerations section of this document.
The REQUIRETLS SMTP service extension allows the SMTP client to
specify that a given message sent during a particular session MUST be
sent over a TLS protected session with specified security
characteristics. It also requires that the SMTP server advertise
that it also supports REQUIRETLS, in effect promising that it will
honor the requirement to require STARTTLS and REQUIRETLS for all
onward transmissions of messages specifying that requirement.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. The REQUIRETLS Service Extension
1. The textual name of the extension is "Require TLS".
2. The EHLO keyword value associated with this extension is
"REQUIRETLS".
3. One MAIL FROM option is defined by this extension.
4. Two new SMTP status codes are defined by this extension to convey
error conditions resulting from failure of the client to
negotiate a TLS connection with the required security and as a
result of an attempt to send to a server not also supporting the
REQUIRETLS extension.
In order to specify REQUIRETLS treatment for a given message, the
REQUIRETLS option is specified on the MAIL FROM command when that
message is transmitted. This option MUST only be specified in the
context of an SMTP session meeting the security requirements that
have been specified:
o The session itself MUST employ TLS transmission.
o Any server authentication requirements specified as an option to
the REQUIRETLS option (see below) MUST have been satisfied in
establishing the current session.
An optional parameter to the REQUIRETLS MAIL FROM option specifies
the requirements for server authentication that MUST be used for any
onward transmission of the following message. The parameter takes
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the form of either a single value or comma-separated list, separated
from the REQUIRETLS option by a single "=" (equals-sign) character.
If present, the parameter MUST take one or more of the following
values:
o CHAIN - The certificate presented by the SMTP server MUST verify
successfully in a trust chain leading to a certificate trusted by
the SMTP client. The choice of trusted (root) certificates by the
client is at their own discretion. The client MAY choose to use
the certificate set maintained by the CA/B forum [citation needed]
for this purpose.
o DANE - The certificate presented by the SMTP server MUST verify
succesfully using DANE as specified in RFC 7672 [RFC7672].
o DNSSEC - The server MUST confirm that any MX record or CNAME
lookup used to locate the SMTP server must be DNSSEC [RFC4035]
signed and valid.
The CHAIN and DANE parameters are additive; if both are specified,
either method of certificate validation is acceptable. If neither
CHAIN nor DANE is specified, the certificate presented by the SMTP
server is not required to be verified.
3. REQUIRETLS Semantics
3.1. REQUIRETLS Receipt Requirements
Upon receipt of a REQUIRETLS option on a MAIL FROM command during the
receipt of a message, an SMTP server MUST tag that message as
requiring TLS transmission with the specified option(s). The manner
in which this tagging takes place is implementation-dependent.
3.2. REQUIRETLS Sender Requirements
When sending a message tagged with a TLS requirement, the sending
(client) MTA MUST:
o Look up the SMTP server to which the message is to be sent. If
the DNSSEC option is included in the message tag, all lookups in
this process MUST use DNSSEC verification and the response MUST be
DNSSEC-signed.
o Open an SMTP session with the peer SMTP server using the EHLO
verb. If the server does not advertise the REQUIRETLS capability,
the client MUST bounce the message with a 5.7.xxx REQUIRETLS
Needed status code.
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o Establish a TLS-protected SMTP session with its peer SMTP server
and authenticate the server's certificate with the specified
authentication method. If it is unable to do so, the client MUST
bounce the message with a 5.7.10 Encryption Needed status code
[RFC5248].
o The SMTP client SHOULD also require that meaningfully secure
cipher algorithms and key lengths be negotiated with the server,
and bounce the message as described above if this does not occur.
The choices of key lengths and algorithms change over time, so a
specific requirement is not presented here.
o Transmit the message, issuing the REQUIRETLS option on the MAIL
FROM command with the required option(s), if any.
3.3. REQUIRETLS Submission
An MUA or other agent making the initial introduction of a message to
SMTP has authority to decide whether to require TLS, and if so, using
what authentication method(s). It does so by issuing the REQUIRETLS
option in the MAIL FROM command during message submission. This MAY
be done based on a user interface selection, on a header field
included in the message, or based on policy. The manner in which the
decision to require TLS is made is implementation-dependent and is
beyond the scope of this specification.
3.4. Delivery of REQUIRETLS messages
Messages are usually delivered to end users using protocols other
than SMTP such as IMAP [RFC3501], POP [RFC1939], or web mail systems.
Mail delivery agents supporting REQUIRETLS SHOULD require that
message delivery take place over authenticated, encrypted channels.
4. Error handling
Error ("bounce") messages contain important metadata, and therefore
MUST be protected in the same manner as the original message. All
error handling, whether resulting from a REQUIRETLS error or some
other, MUST employ REQUIRETLS at the same authentication method(s) as
the message that caused the error to occur.
It should be noted that the path from the origination of an error
bounce message back to the MAIL FROM address may not share the same
REQUIRETLS support as the forward path. Therefore, users of
REQUIRETLS are advised to make sure that they are capable of
receiving mail using REQUIRETLS at the same authentication method(s)
as messages they send. Otherwise, such error bounces will be lost.
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5. IANA Considerations
If published as an RFC, this draft requests the addition of the
keyword REQUIRETLS to the SMTP Service Extensions Registry
[MailParams].
If published as an RFC, this draft also requests the creation of a
registry, REQUIRETLS Security Requirements, to be initially populated
with the CHAIN, DANE, and DNSSEC keywords.
If published as an RFC, this draft requests the addition of an entry
to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
Registry [SMTPStatusCodes] in the 5.7.YYY range to indicate lack of
REQUIRETLS support by an SMTP server to which a message is being
routed.
This section is to be removed during conversion into an RFC by the
RFC Editor.
6. Security Considerations
The purpose of REQUIRETLS is to improve communications security for
email by giving the originator of a message an expectation that it
will be transmitted in an encrypted form "over the wire". When used,
REQUIRETLS changes the traditional behavior of email transmission,
which favors delivery over the ability to send email messages using
transport-layer security, to one in which messages are not
transmitted unless the required security is available.
6.1. Passive attacks
REQUIRETLS is generally effective against passive attackers who are
merely trying to eavesdrop on an SMTP exchange between an SMTP client
and server. This assumes, of course, the cryptographic integrity of
the TLS connection being used.
6.2. Active attacks
Active attacks against TLS encrypted SMTP connections can take many
forms. One such attack is to interfere in the negotiation by
changing the STARTTLS command to something illegal such as XXXXXXXX.
This causes TLS negotiation to fail and messages to be sent in the
clear, where they can be intercepted. REQUIRETLS detects the failure
of STARTTLS and declines to send the message rather than send it
insecurely.
A second form of attack is a man-in-the-middle attack where the
attacker terminates the TLS connection rather than the intended SMTP
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server. This is possible when, as is commonly the case, the SMTP
client either does not verify the server's certificate or establishes
the connection even when the verification fails. The REQUIRETLS
CHAIN and DANE options allow the message sender to specify that
successful certificate validation, using either or both of two
different methods, is required before sending the message.
Another active attack involves the spoofing of DNS MX records of the
recipient domain. An attacker having this capability could cause the
message to be redirected to a mail server under the attacker's own
control, which would presumably have a valid certificate. The
REQUIRETLS DNSSEC option allows the message sender to require that
valid DNSSEC [RFC4033] signatures be obtained when locating the
recipient's mail server, in order to address that attack.
In addition to support of the DNSSEC option, domains receiving email
SHOULD deploy DNSSEC and SMTP clients SHOULD deploy DNSSEC
verification.
6.3. Bad Actor MTAs
A bad-actor MTA along the message transmission path could
misrepresent its support of REQUIRETLS and/or actively strip
REQUIRETLS tags from messages it handles. However, since
intermediate MTAs are already trusted with the cleartext of messages
they handle, and are not part of the threat model for transport-layer
security, they are also not part of the threat model for REQUIRETLS.
It should be reemphasized that since SMTP TLS is a transport-layer
security protocol, messages sent using REQUIRETLS are not encrypted
end-to-end and are visible to MTAs that are part of the message
delivery path. Messages containing sensitive information that MTAs
should not have access to MUST be sent using end-to-end content
encryption such as OpenPGP [RFC4880] or S/MIME [RFC5751].
7. Acknowledgements
The author would like to acknowledge many helpful suggestions on the
ietf-smtp mailing list, and in particular those of Tony Finch, John
Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim.
8. Revision History
To be removed by RFC Editor upon publication as an RFC.
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8.1. Changes Since -00 Draft
o Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM
parameter to better associate REQUIRETLS requirements with
transmission of individual messages.
o Addition of an option to require DNSSEC lookup of the remote mail
server, since this affects the common name of the certificate that
is presented.
o Clarified the wording to more clearly state that TLS sessions must
be established and not simply that STARTTLS is negotiated.
o Introduced need for minimum encryption standards (key lengths and
algorithms)
o Substantially rewritten Security Considerations section
9. References
9.1. Normative References
[MailParams]
Internet Assigned Numbers Authority (IANA), "IANA Mail
Parameters", 2007,
<http://www.iana.org/assignments/mail-parameters>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", BCP 138, RFC 5248,
DOI 10.17487/RFC5248, June 2008,
<http://www.rfc-editor.org/info/rfc5248>.
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[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.
[SMTPStatusCodes]
Internet Assigned Numbers Authority (IANA), "Simple Mail
Transfer Protocol (SMTP) Enhanced Status Codes Registry",
2008, <http://www.iana.org/assignments/
smtp-enhanced-status-codes>.
9.2. Informative References
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<http://www.rfc-editor.org/info/rfc1939>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<http://www.rfc-editor.org/info/rfc4880>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <http://www.rfc-editor.org/info/rfc5751>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015,
<http://www.rfc-editor.org/info/rfc7672>.
Author's Address
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Jim Fenton
704 Benvenue Avenue
Los Altos, California 94024
USA
Email: fenton@bluepopcorn.net
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