| Internet Engineering Task Force | J. Fenton |
| Internet-Draft | Altmode Networks |
| Intended status: Standards Track | January 16, 2018 |
| Expires: July 20, 2018 |
SMTP Require TLS Option
draft-ietf-uta-smtp-require-tls-01
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, Require-TLS. If the REQUIRETLS option or Require-TLS 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 Require-TLS 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 Require-TLS 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:
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.
One new message header field, Require-TLS, is defined by this specification. It is used for messages requesting that recipient TLS policy (MTA-STS or DANE) be ignored.
The Require-TLS header field has a single required parameter:
More than one instance of the Require-TLS 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 with the option(s) specified in the REQUIRETLS parameter.
Upon receipt of a message not specifying the REQUIRETLS option on its MAIL FROM command but containing the Require-TLS header field in its message header, an SMTP server implementing this specification MUST tag that message with the option specified in the Require-TLS header field. If the REQUIRETLS MAIL FROM parameter is specified, the Require-TLS 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 2-4 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 or if the MX record lookup is not DNSSEC-protected and DNSSEC verification is required, 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. This MUST be expected, and a non-delivery notification returned to the sender.
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, and if so, using what authentication method(s). 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 Require-TLS header field in the message. SMTP servers implementing this specification will interpret this header field as described in Section 4.1.
In either case, the decision whether to specify REQUIRETLS, and with what option(s), 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 REQUIRETLS SHOULD observe the guidelines in [I-D.ietf-uta-email-deep].
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, 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.
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 at the same authentication method(s) as messages they send. Otherwise, such non-delivery messages will be lost.
If unable to send a bounce message due to a REQUIRETLS failure (the return path not supporting the TLS requirements in the original message), the MTA sending the bounce message MAY send a redacted non-delivery message to the postmaster of the domain identified in the envelope-From address identifying the message only by Message-ID and indicating the type of failure. The original From, Return-path, To, Sender, Cc, and related header fields MUST NOT be included in this message.
Senders of messages requiring TLS are advised to consider the increased likelihood that bounce messages will be lost as a result of REQUIRETLS return path failure.
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 necessarily extend to mailing lists, although the inclusion of the Require-TLS 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 keyword REQUIRETLS to the SMTP Service Extensions Registry.
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, DNSSEC, and NO 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 in the 5.7.YYY range to indicate lack of REQUIRETLS support by an SMTP server to which a message is being routed.
If published as an RFC, this draft requests the addition of the header field name Require-TLS to the Permanent Message Header Field Names Registry.
This section is to be removed during conversion into an RFC 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. 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 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.
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, 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.