Internet Engineering Task Force J. Fenton
Internet-Draft August 16, 2016
Intended status: Standards Track
Expires: February 17, 2017

SMTP Require TLS Option


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 transmission 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.

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This Internet-Draft will expire on February 17, 2017.

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Table of Contents

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 eavesdropping), and insertion of a man-in-the-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:

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 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:

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. If the message is being locally aliased and redistributed to multiple addresses, all instances of the message MUST be tagged in the same manner.

3.2. REQUIRETLS Sender Requirements

When sending a message tagged with a TLS requirement, the sending (client) MTA MUST:

If any of the above steps fail, the client SHOULD issue a QUIT to the server and repeat the above process with each host on the recipient domain's list of MX hosts in an attempt to find a mail path that meets the sender's requirements. If there are no more MX hosts or if the MX record lookup is not DNSSEC-protected and DNSSEC verification is required, the client MUST NOT transmit the message and MUST issue an SMTP QUIT command to the server. The client MAY send other, unprotected, messages to that server prior to issuing the QUIT if it has any.

Following such a failure, the SMTP client MUST send a non-delivery notification to the reverse-path of the failed message as described in section 3.6 of [RFC5321]. The following status codes [RFC5248] SHOULD be used:

Refer to Section 4. for further requirements regarding non-delivery messages.

If all REQUIRETLS requirements have been met, 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 retrieved by 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 retrieval take place over authenticated, encrypted channels.

4. Non-delivery message handling

Non-delivery ("bounce") messages contain important metadata, and therefore MUST be protected in the same manner as the original message. All non-delivery messages, whether resulting from a REQUIRETLS error or some other, MUST employ REQUIRETLS using 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 non-delivery messages will be lost.

5. Mailing list considerations

Mailing lists, upon receipt of a message, originate new messages to list addresses, as distinct from an aliasing operation that redirects the original message, in some cases to multiple recipients. The requirement to preserve the REQUIRETLS tag and options therefore does not extend to mailing lists. REQUIRETLS users SHOULD use caution when sending to mailing lists and SHOULD NOT assume that REQUIRETLS applies to messages from the list operator to list members.

Mailing list operators MAY, of course, apply REQUIRETLS requirements in incoming messages to the resulting messages they originate. If this is done, they SHOULD also apply these requirements to administrative traffic, such as messages to moderators requesting approval of messages.

6. 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.

7. 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.

7.1. 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.

7.2. 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 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.

7.3. 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].

8. Acknowledgements

The author would like to acknowledge many helpful suggestions on the ietf-smtp and uta mailing lists, in particular those of Viktor Dukhovni, Tony Finch, Jeremy Harris, Arvel Hathcock, John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim.

9. Revision History

To be removed by RFC Editor upon publication as an RFC.

9.1. Changes Since -01 Draft

9.2. Changes Since -00 Draft

10. References

10.1. Normative References

[MailParams] Internet Assigned Numbers Authority (IANA), "IANA Mail Parameters", 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, February 2002.
[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.
[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.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008.
[SMTPStatusCodes] Internet Assigned Numbers Authority (IANA), "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry", 2008.

10.2. Informative References

[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003.
[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.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D. and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007.
[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.
[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, DOI 10.17487/RFC6125, March 2011.
[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.

Author's Address

Jim Fenton 704 Benvenue Avenue Los Altos, California 94024 USA EMail: