| Internet Engineering Task Force | J. Fenton |
| Internet-Draft | Altmode Networks |
| Intended status: Standards Track | September 5, 2017 |
| Expires: March 9, 2018 |
SMTP Require TLS Option
draft-ietf-uta-smtp-require-tls-00
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 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 policy mechanisms such as SMTP STS and DANE be ignored when relaying a high priority message.
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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 fails to authenticate it.
Policy mechanisms such as DANE and SMTP 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 urgent 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 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 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, or conversely that delivery should be prioritized over ability to negotiate TLS. For messages requiring TLS negotiation, 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 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.
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. With the exception of REQUIRETLS=NO (described below), 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.
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 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.
When sending a message tagged with REQUIRETLS other than the REQUIRETLS=NO option, the sending (client) MTA MUST:
If any of the above steps fail, the client SHOULD 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. 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 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.
Messages tagged REQUIRETLS=NO are handled differently from other REQUIRETLS messages, as follows. When sending a message tagged with REQUIRETLS=NO, the sending (client) MTA MUST:
Some SMTP servers that are configured to expect STARTTLS connections as a matter of policy may not accept messages in the absence of STARTTLS. This MUST be expected, and a non-delivery notification returned to the sender.
Messages tagged with the REQUIRETLS=NO option will be sent without the option to SMTP servers not supporting REQUIRETLS. REQUIRETLS=NO MAY therefore not persist through multiple email relay hops.
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.
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 require that retrieval of messages requiring transport encryption take place over authenticated, encrypted channels.
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.
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 (other than REQUIRETLS=NO) 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 specifying REQUIRETLS (other than REQUIRETLS=NO) 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 extend to mailing lists. 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.
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 STARTTLS other than the STARTTLS=NO option, since messages specifying that option are specifying less concern 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.