| Internet Engineering Task Force | J. Fenton |
| Internet-Draft | Altmode Networks |
| Intended status: Standards Track | November 20, 2018 |
| Expires: May 24, 2019 |
SMTP Require TLS Option
draft-ietf-uta-smtp-require-tls-05
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, by default, prioritized over security. This document describes an SMTP service extension, REQUIRETLS, and message header field, RequireTLS. If the REQUIRETLS option or RequireTLS message header field is used when sending a message, it asserts a request on the part of the message sender to override the default negotiation of TLS, either by requiring that TLS be negotiated when the message is relayed, or by requesting that recipient-side policy mechanisms such as MTA-STS and DANE be ignored when relaying a message for which security is unimportant.
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The SMTP STARTTLS service extension 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. By default, TLS is used only upon mutual agreement (successful negotiation) of STARTTLS between the client and server; if this is not possible, the message is sent without transport encryption. Furthermore, it is common practice for the client to negotiate TLS even if the SMTP server's certificate is invalid.
Policy mechanisms such as DANE and MTA-STS may impose requirements for the use of TLS for email destined for some domains. However, such policies do not allow the sender to specify which messages are more sensitive and require transport-level encryption, and which ones are less sensitive and ought to be relayed even if TLS cannot be negotiated successfully.
The default 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 accompanied by eavesdropping), and insertion of a man-in-the-middle attacker exploiting the lack of server authentication by the client. Attacks are described in more detail in the Security Considerations section of this document.
REQUIRETLS consists of two mechanisms: an SMTP service extension and a message header field. The service extension is used 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 supports REQUIRETLS, in effect promising that it will honor the requirement to enforce TLS transmission and REQUIRETLS support for onward transmission of those messages.
The RequireTLS message header field is used to convey a request to ignore recipient-side policy mechanisms such as MTA-STS and DANE, thereby prioritizing delivery over ability to negotiate TLS. Unlike the service extension, the RequireTLS header field allows the message to transit through one or more MTAs that do not support REQUIRETLS.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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:
One new message header field, RequireTLS, is defined by this specification. It is used for messages requesting that recipient TLS policy (including MTA-STS and DANE) be ignored.
The RequireTLS header field has a single REQUIRED parameter:
More than one instance of the RequireTLS header field MUST NOT appear in a given message.
Upon receipt of the REQUIRETLS option on a MAIL FROM command during the receipt of a message, an SMTP server MUST tag that message as needing REQUIRETLS handling.
Upon receipt of a message not specifying the REQUIRETLS option on its MAIL FROM command but containing the RequireTLS header field in its message header, an SMTP server implementing this specification MUST tag that message with the option specified in the RequireTLS header field. If the REQUIRETLS MAIL FROM parameter is specified, the RequireTLS header field MUST be ignored but MAY be included in onward relay of the message.
The manner in which the above 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.
When sending a message tagged as requiring TLS, the sending (client) MTA MUST:
If any of the above steps fail, the client MUST issue a QUIT to the server and repeat steps 2-5 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. The client MAY send other, unprotected, messages to that server if it has any prior to issuing the QUIT. If there are no more MX hosts, the client MUST NOT transmit the message to the domain.
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 SHOULD be used:
Refer to Section 5 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.
Messages tagged RequireTLS: NO are handled as follows. When sending such a message, the sending (client) MTA MUST:
Some SMTP servers may be configured to require STARTTLS connections as a matter of policy and not accept messages in the absence of STARTTLS. A non-delivery notification MUST be returned to the sender if message relay fails due to an inability to negotiate STARTTLS when required by the server.
Since messages tagged with RequireTLS: NO will sometimes be sent to SMTP servers not supporting REQUIRETLS, that option will not be uniformly observed by all SMTP relay hops.
An MUA or other agent making the initial introduction of a message has authority to decide whether to require TLS. When TLS is to be required, it MUST do so by negotiating STARTTLS and REQUIRETLS and include the REQUIRETLS option on the MAIL FROM command, as is done for message relay.
When TLS is not to be required, the sender MUST include the RequireTLS header field in the message. SMTP servers implementing this specification MUST interpret this header field as described in Section 4.1.
In either case, the decision whether to specify REQUIRETLS MAY be done based on a user interface selection or based on a ruleset or other policy. The manner in which the decision to require TLS is made is implementation-dependent and is beyond the scope of this specification.
Messages are usually retrieved by end users using protocols other than SMTP such as IMAP, POP, or web mail systems. Mail delivery agents supporting the REQUIRETLS SMTP option SHOULD observe the guidelines in [RFC8314].
Non-delivery ("bounce") messages usually contain important metadata about the message to which they refer, including the original message header. They therefore MUST be protected in the same manner as the original message. All non-delivery messages resulting from messages with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS error or some other, MUST also specify the REQUIRETLS SMTP option unless redacted as described below.
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 requiring TLS are advised to make sure that they are capable of receiving mail using REQUIRETLS as well. Otherwise, such non-delivery messages will be lost.
If a REQUIRETLS message is bounced, the server MUST behave as if RET=HDRS was present as described in [RFC3461]. If both RET=FULL and REQUIRETLS are present, the RET=FULL MUST be disregarded and MAY be transformed to RET=HDRS on relay. The SMTP client for a REQUIRETLS bounce message MUST use an empty MAIL FROM return-path as required by [RFC5321]. When the MAIL FROM return-path is empty, the REQUIRETLS parameter SHOULD NOT cause a bounce message to be discarded even if the next-hop relay does not advertise REQUIRETLS.
Senders of messages requiring TLS are advised to consider the possibility that bounce messages will be lost as a result of REQUIRETLS return path failure, and that some information could be leaked if a bounce message is not able to be transmitted with REQUIRETLS.
Mailing lists, upon receipt of a message, originate new messages to list addresses. This is distinct from an aliasing operation that redirects the original message, in some cases to multiple recipients. The requirement to preserve the REQUIRETLS tag therefore does not necessarily extend to mailing lists, although the inclusion of the RequireTLS header field MAY cause messages sent to mailing lists to inherit this characteristic. REQUIRETLS users SHOULD be made aware of this limitation so that they use caution when sending to mailing lists and do not assume that REQUIRETLS applies to messages from the list operator to list members.
Mailing list operators MAY 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.
If published as an RFC, this draft requests the addition of the following keyword to the SMTP Service Extensions Registry:
Textual name: RequireTLS
EHLO keyword value: REQUIRETLS
Syntax and parameters: (no parameters)
Additional SMTP verbs: none
MAIL and RCPT parameters: REQUIRETLS parameter on MAIL
Behavior: Use of the REQUIRETLS parameter on the
MAIL verb causes that message to require
the use of TLS and tagging with
REQUIRETLS for all onward relay.
Command length increment: 11 characters
If published as an RFC, this draft requests the addition of an entry to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry:
Code: 5.7.YYY
Sample Text: REQUIRETLS support required
Associated basic status code: 530
Description: This indicates that the message was not
able to be forwarded because it was
received with a REQUIRETLS requirement
and none of the SMTP servers to which
the message should be forwarded provide
this support.
Reference: (this document)
Submitter: J. Fenton
Change controller: IESG
If published as an RFC, this draft requests the addition of an entry to the Permanent Message Header Field Names Registry:
Header field name: RequireTLS
Applicable protocol: mail
Status: provisional
Author/change controller: IETF UTA Working Group
Specification document: (this document)
This section is to be updated for publication by the RFC Editor.
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 requested security takes precedence over delivery and domain-level policy.
The following considerations apply to the REQUIRETLS service extension but not the RequireTLS header field, since messages specifying the header field are less concerned with transport security.
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.
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. REQUIRETLS requires successful certificate validation before sending the message.
Another active attack involves the spoofing of DNS MX records of the recipient domain. An attacker having this capability could potentially cause the message to be redirected to a mail server under the attacker's own control, which would presumably have a valid certificate. REQUIRETLS requires that the recipient domain's MX record lookup be validated either using DNSSEC or via a published MTA-STS policy that specifies the acceptable SMTP server hostname(s) for the recipient domain.
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 or S/MIME.
The author would like to acknowledge many helpful suggestions on the ietf-smtp and uta mailing lists, in particular those of Viktor Dukhovni, Chris Newman, Tony Finch, Jeremy Harris, Arvel Hathcock, John Klensin, John Levine, Rolf Sonneveld, and Per Thorsheim.
To be removed by RFC Editor upon publication as an RFC.
Require validation of SMTP server hostname via DNSSEC or MTA-STS policy when TLS is required.
Working Group Last Call changes, including:
| [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. |
| [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. |
| [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. |