SMTP MTA Strict Transport Security (MTA-STS)

Using TLS in Applications                                    D. Margolis
Internet-Draft                                                 M. Risher
Intended status: Standards Track                             Google, Inc
Expires: March 9, 2018                                   B. Ramakrishnan
                                                             Yahoo!, Inc
                                                              A. Brotman
                                                            Comcast, Inc
                                                                J. Jones
                                                          Microsoft, Inc
                                                       September 5, 2017

              SMTP MTA Strict Transport Security (MTA-STS)


   SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a
   mechanism enabling mail service providers to declare their ability to
   receive Transport Layer Security (TLS) secure SMTP connections, and
   to specify whether sending SMTP servers should refuse to deliver to
   MX hosts that do not offer TLS with a trusted server certificate.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 9, 2018.

Copyright Notice

   Copyright (c) 2017 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
   ( in effect on the date of

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Related Technologies  . . . . . . . . . . . . . . . . . . . .   3
   3.  Policy Discovery  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  MTA-STS TXT Records . . . . . . . . . . . . . . . . . . .   4
     3.2.  MTA-STS Policies  . . . . . . . . . . . . . . . . . . . .   5
     3.3.  HTTPS Policy Fetching . . . . . . . . . . . . . . . . . .   8
     3.4.  Policy Selection for Smart Hosts and Subdomains . . . . .   9
     3.5.  MX Certificate Validation . . . . . . . . . . . . . . . .   9
   4.  Policy Application  . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Policy Application Control Flow . . . . . . . . . . . . .  11
   5.  Reporting Failures  . . . . . . . . . . . . . . . . . . . . .  11
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .  12
     6.1.  Policy Updates  . . . . . . . . . . . . . . . . . . . . .  12
     6.2.  Removing MTA-STS  . . . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Well-Known URIs Registry  . . . . . . . . . . . . . . . .  13
     7.2.  MTA-STS TXT Record Fields . . . . . . . . . . . . . . . .  13
     7.3.  MTA-STS Policy Fields . . . . . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     8.1.  Obtaining a Signed Certificate  . . . . . . . . . . . . .  14
     8.2.  Preventing Policy Discovery . . . . . . . . . . . . . . .  14
     8.3.  Denial of Service . . . . . . . . . . . . . . . . . . . .  15
     8.4.  Weak Policy Constraints . . . . . . . . . . . . . . . . .  16
     8.5.  Compromise of the Web PKI System  . . . . . . . . . . . .  16
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  16
   10. Appendix 1: MTA-STS example record & policy . . . . . . . . .  17
   11. Appendix 2: Message delivery pseudocode . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     12.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
   hosts to negotiate the use of a TLS channel for encrypted mail

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   While this opportunistic encryption protocol by itself provides a
   high barrier against passive man-in-the-middle traffic interception,
   any attacker who can delete parts of the SMTP session (such as the
   "250 STARTTLS" response) or who can redirect the entire SMTP session
   (perhaps by overwriting the resolved MX record of the delivery
   domain) can perform downgrade or interception attacks.

   This document defines a mechanism for recipient domains to publish
   policies specifying:

   o  whether MTAs sending mail to this domain can expect PKIX-
      authenticated TLS support

   o  what a conforming client should do with messages when TLS cannot
      be successfully negotiated

1.1.  Terminology

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

   We also define the following terms for further use in this document:

   o  MTA-STS Policy: A commitment by the Policy Domain to support PKIX
      authenticated TLS for the specified MX hosts.

   o  Policy Domain: The domain for which an MTA-STS Policy is defined.
      This is the next-hop domain; when sending mail to
      "" this would ordinarily be "", but
      this may be overridden by explicit routing rules (as described in
      Section 3.4, "Policy Selection for Smart Hosts and Subdomains").

2.  Related Technologies

   The DANE TLSA record [RFC7672] is similar, in that DANE is also
   designed to upgrade unauthenticated encryption or plaintext
   transmission into authenticated, downgrade-resistant encrypted
   transmission.  DANE requires DNSSEC [RFC4033] for authentication; the
   mechanism described here instead relies on certificate authorities
   (CAs) and does not require DNSSEC, at a cost of risking malicious
   downgrades.  For a thorough discussion of this trade-off, see
   Section 8, "Security Considerations".

   In addition, MTA-STS provides an optional report-only mode, enabling
   soft deployments to detect policy failures; partial deployments can
   be achieved in DANE by deploying TLSA records only for some of a

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   domain's MXs, but such a mechanism is not possible for the per-domain
   policies used by MTA-STS.

   The primary motivation of MTA-STS is to provide a mechanism for
   domains to upgrade their transport security even when deploying
   DNSSEC is undesirable or impractical.  However, MTA-STS is designed
   not to interfere with DANE deployments when the two overlap; in
   particular, senders who implement MTA-STS validation MUST NOT allow a
   "valid" or "report-only" MTA-STS validation to override a failing
   DANE validation.

3.  Policy Discovery

   MTA-STS policies are distributed via HTTPS from a "well-known"
   [RFC5785] path served within the Policy Domain, and their presence
   and current version are indicated by a TXT record at the Policy
   Domain.  These TXT records additionally contain a policy "id" field,
   allowing sending MTAs to check the currency of a cached policy
   without performing an HTTPS request.

   To discover if a recipient domain implements MTA-STS, a sender need
   only resolve a single TXT record.  To see if an updated policy is
   available for a domain for which the sender has a previously cached
   policy, the sender need only check the TXT record's version "id"
   against the cached value.

3.1.  MTA-STS TXT Records

   The MTA-STS TXT record is a TXT record with the name "_mta-sts" at
   the Policy Domain.  For the domain "", this record would
   be "".  MTA-STS TXT records MUST be US-ASCII,
   semicolon-separated key/value pairs containing the following fields:

   o  "v": (plain-text, required).  Currently only "STSv1" is supported.

   o  "id": (plain-text, required).  A short string used to track policy
      updates.  This string MUST uniquely identify a given instance of a
      policy, such that senders can determine when the policy has been
      updated by comparing to the "id" of a previously seen policy.
      There is no implied ordering of "id" fields between revisions.

   An example TXT record is as below:

   "  IN TXT "v=STSv1; id=20160831085700Z;""

   The formal definition of the "_mta-sts" TXT record, defined using
   [RFC7405], is as follows:

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sts-text-record = sts-version 1*(field-delim sts-field) [field-delim]

sts-field       = sts-id /                        ; Note that sts-id record
                  sts-extension                   ; is required.

field-delim     = *WSP ";" *WSP

sts-version     = %s"v=STSv1"

sts-id          = %s"id=" 1*32(ALPHA / DIGIT)     ; id=...

sts-extension   = sts-ext-name "=" sts-ext-value  ; name=value

sts-ext-name    = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".")

sts-ext-value   = 1*(%x21-3A / %x3C / %x3E-7E)    ; chars excluding "=",
                                                  ; ";", SP, and control
                                                  ; chars

   If multiple TXT records for "_mta-sts" are returned by the resolver,
   records which do not begin with "v=STSv1;" are discarded.  If the
   number of resulting records is not one, senders MUST assume the
   recipient domain does not implement MTA-STS and skip the remaining
   steps of policy discovery.  If the resulting TXT record contains
   multiple strings, then the record MUST be treated as if those strings
   are concatenated together without adding spaces.

3.2.  MTA-STS Policies

   The policy itself is a set of key/value pairs (similar to [RFC5322]
   header fields) served via the HTTPS GET method from the fixed
   [RFC5785] "well-known" path of ".well-known/mta-sts.txt" served by
   the "mta-sts" host at the Policy Domain.  Thus for "" the
   path is "".

   The [RFC7231] "Content-Type" media type for this resource MUST be
   "text/plain".  When fetching a policy, senders SHOULD validate that
   the media type is "text/plain" to guard against cases where
   webservers allow untrusted users to host non-text content (typically,
   HTML or images) at a user-defined path.  Additional "Content-Type"
   parameters are ignored.

   This resource contains the following line-separated key/value pairs:

   o  "version": (plain-text).  Currently only "STSv1" is supported.

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   o  "mode": (plain-text).  One of "enforce", "report", or "none",
      indicating the expected behavior of a sending MTA in the case of a
      policy validation failure.

   o  "max_age": Max lifetime of the policy (plain-text non-negative
      integer seconds, maximum value of 31557600).  Well-behaved clients
      SHOULD cache a policy for up to this value from last policy fetch
      time.  To mitigate the risks of attacks at policy refresh time, it
      is expected that this value typically be in the range of weeks or

   o  "mx": MX identity patterns (list of plain-text strings).  One or
      more patterns matching a Common Name ([RFC6125]) or Subject
      Alternative Name ([RFC5280]) DNS-ID present in the X.509
      certificate presented by any MX receiving mail for this domain.
      For example: "mx: mx:" indicates
      that mail for this domain might be handled by any MX with a
      certificate valid for a host at "" or
      "".  Valid patterns can be either fully specified names
      ("") or suffixes ("") matching the right-
      hand parts of a server's identity; the latter case are
      distinguished by a leading period.  If there are more than one MX
      specified by the policy, they MUST be on separate lines within the
      policy file.  In the case of Internationalized Domain Names
      ([RFC5891]), the MX MUST specify the Punycode-encoded A-label
      [RFC3492] and not the Unicode-encoded U-label.  The full semantics
      of certificate validation are described in Section 3.5, "MX
      Certificate Validation."

   An example policy is as below:

                         version: STSv1
                         mode: enforce
                         max_age: 123456

   The formal definition of the policy resource, defined using
   [RFC7405], is as follows:

sts-policy-record        = *WSP sts-policy-field *WSP
                           *(CRLF *WSP sts-policy-field *WSP)

sts-policy-field         = sts-policy-version /           ; required once
                           sts-policy-mode    /           ; required once
                           sts-policy-max-age /           ; required once

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                           0*(sts-policy-mx *WSP CRLF) /  ; required at
                                                          ; least once
                                                          ; except when mode
                                                          ; is "none"
                           sts-policy-extension           ; other fields

field-delim              = ":" *WSP

sts-policy-version       = sts-policy-version-field field-delim

sts-policy-version-field = %s"version"

sts-policy-version-value = %s"STSv1"

sts-policy-mode          = sts-policy-mode-field field-delim

sts-policy-mode-field    = %s"mode"

sts-policy-model-value   =  %s"report" / %s"enforce" / %s"none"

sts-policy-mx            = sts-policy-mx-field field-delim

sts-policy-mx-field      = %s"mx"

sts-policy-mx-value      = 1*(ALPHA / DIGIT / "_" / "-" / ".")

sts-policy-max-age       = sts-policy-max-age-field field-delim

sts-policy-max-age-field = %s"max_age"

sts-policy-max-age-value = 1*10(DIGIT)

sts-policy-extension     = sts-policy-ext-name field-delim ; additional
                           sts-policy-ext-value            ; extension
                                                           ; fields

sts-policy-ext-name      = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".")

sts-policy-ext-value     = 1*(%x21-3A / %x3C / %x3E-7E)    ; chars excluding
                                                           ; "=", ";", SP,
                                                           ; and control
                                                           ; chars

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   Parsers MUST accept TXT records and policy files which are
   syntactically valid (i.e. valid key/value pairs separated by semi-
   colons for TXT records) and but containing additional key/value pairs
   not specified in this document, in which case unknown fields SHALL be
   ignored.  If any non-repeated field--i.e. all fields excepting "mx"--
   is duplicated, all entries except for the first SHALL be ignored.  If
   any field is not specified, the policy SHALL be treated as invalid.

3.3.  HTTPS Policy Fetching

   When fetching a new policy or updating a policy, the HTTPS endpoint
   MUST present a X.509 certificate which is valid for the "mta-sts"
   host (e.g.  "") as described below, chain to a
   root CA that is trusted by the sending MTA, and be non-expired.  It
   is expected that sending MTAs use a set of trusted CAs similar to
   those in widely deployed Web browsers and operating systems.

   The certificate is valid for the "mta-sts" host with respect to the
   rules described in [RFC6125], with the following application-specific

   o  Matching is performed only against the DNS-ID and CN-ID

   o  DNS domain names in server certificates MAY contain the wildcard
      character '*' as the complete left-most label within the

   The certificate MAY be checked for revocation via the Online
   Certificate Status Protocol (OCSP) [RFC2560], certificate revocation
   lists (CRLs), or some other mechanism.

   Policies fetched via HTTPS are only valid if the HTTP response code
   is 200 (OK).  HTTP 3xx redirects MUST NOT be followed, and HTTP
   caching (as specified in [RFC7234]) MUST NOT be used.

   Senders may wish to rate-limit the frequency of attempts to fetch the
   HTTPS endpoint even if a valid TXT record for the recipient domain
   exists.  In the case that the HTTPS GET fails, we suggest
   implementions may limit further attempts to a period of five minutes
   or longer per version ID, to avoid overwhelming resource-constrained
   recipients with cascading failures.

   Senders MAY impose a timeout on the HTTPS GET and/or a limit on the
   maximum size of the response body to avoid long delays or resource
   exhaustion during attempted policy updates.  A suggested timeout is
   one minute, and a suggested maximum policy size 64 kilobytes; policy

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   hosts SHOULD respond to requests with a complete policy body within
   that timeout and size limit.

   If a valid TXT record is found but no policy can be fetched via HTTPS
   (for any reason), and there is no valid (non-expired) previously-
   cached policy, senders MUST continue with delivery as though the
   domain has not implemented MTA-STS.

   Conversely, if no "live" policy can be discovered via DNS or fetched
   via HTTPS, but a valid (non-expired) policy exists in the sender's
   cache, the sender MUST apply that cached policy.

3.4.  Policy Selection for Smart Hosts and Subdomains

   When sending mail via a "smart host"--an intermediate SMTP relay
   rather than the message recipient's server--compliant senders MUST
   treat the smart host domain as the policy domain for the purposes of
   policy discovery and application.

   When sending mail to a mailbox at a subdomain, compliant senders MUST
   NOT attempt to fetch a policy from the parent zone.  Thus for mail
   sent to "", the policy can be fetched only from
   "", not "".

   #Policy Validation

   When sending to an MX at a domain for which the sender has a valid
   and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST

   1.  That the recipient MX supports STARTTLS and offers a valid PKIX-
       based TLS certificate.

   2.  That at least one of the policy's "mx" patterns matches at least
       one of the identities presented in the MX's X.509 certificate, as
       described in "MX Certificate Validation".

   This section does not dictate the behavior of sending MTAs when
   policies fail to validate; in particular, validation failures of
   policies which specify mode values of "report" or "none" MUST NOT be
   interpreted as delivery failures, as described in Section 4, "Policy

3.5.  MX Certificate Validation

   The certificate presented by the receiving MX MUST chain to a root CA
   that is trusted by the sending MTA and be non-expired.  The
   certificate MUST have a CN-ID ([RFC6125]) or subject alternative name

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   (SAN, [RFC5280]) with a DNS-ID matching the "mx" pattern.  The MX's
   certificate MAY also be checked for revocation via OCSP [RFC2560],
   CRLs [RFC6818], or some other mechanism.

   Because the "mx" patterns are not hostnames, however, matching is not
   identical to other common cases of X.509 certificate authentication
   (as described, for example, in [RFC6125]).  Consider the example
   policy given above, with an "mx" pattern containing "".
   In this case, if the MX server's X.509 certificate contains a SAN
   matching "*", we are required to implement "wildcard-to-
   wildcard" matching.

   To simplify this case, we impose the following constraints on
   wildcard certificates, identical to those in [RFC7672] section 3.2.3
   and [@?RFC6125 section 6.4.3: wildcards are valid in DNS-IDs or CN-
   IDs, but must be the entire first label of the identifier (that is,
   "*", not "mail*").  Senders who are comparing
   a "suffix" MX pattern with a wildcard identifier should thus strip
   the wildcard and ensure that the two sides match label-by-label,
   until all labels of the shorter side (if unequal length) are

   Note that a wildcard _must_ match a label; an "mx" pattern of
   "" thus does not match a SAN of "", nor does a
   SAN of "*" match an "mx" of "".

   A simple pseudocode implementation of this algorithm is presented in
   the Appendix.

4.  Policy Application

   When sending to an MX at a domain for which the sender has a valid,
   non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies
   the result of a policy validation failure one of two ways, depending
   on the value of the policy "mode" field:

   1.  "enforce": In this mode, sending MTAs MUST NOT deliver the
       message to hosts which fail MX matching or certificate

   2.  "report": In this mode, sending MTAs which also implement the
       TLSRPT specification (TODO: add ref) merely send a report
       indicating policy application failures (so long as TLSRPT is also
       implemented by the recipient domain).

   3.  "none": In this mode, sending MTAs should treat the policy domain
       as though it does not have any active policy; see Section 6.2,
       "Removing MTA-STS", for use of this mode value.

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   When a message fails to deliver due to an "enforce" policy, a
   compliant MTA MUST NOT permanently fail to deliver messages before
   checking for the presence of an updated policy at the Policy Domain.
   (In all cases, MTAs SHOULD treat such failures as transient errors
   and retry delivery later.)  This allows implementing domains to
   update long-lived policies on the fly.

4.1.  Policy Application Control Flow

   An example control flow for a compliant sender consists of the
   following steps:

   1.  Check for a cached policy whose time-since-fetch has not exceeded
       its "max_age".  If none exists, attempt to fetch a new policy
       (perhaps asynchronously, so as not to block message delivery).
       Optionally, sending MTAs may unconditionally check for a new
       policy at this step.

   2.  For each candidate MX, in order of MX priority, attempt to
       deliver the message, enforcing STARTTLS and, assuming a policy is
       present, PKIX certificate validation as described in Section 3.5,
       "MX Certificate Validation."

   3.  A message delivery MUST NOT be permanently failed until the
       sender has first checked for the presence of a new policy (as
       indicated by the "id" field in the "_mta-sts" TXT record).  If a
       new policy is not found, existing rules for the case of temporary
       message delivery failures apply (as discussed in [RFC5321]

5.  Reporting Failures

   MTA-STS is intended to be used along with TLSRPT (TODO: add ref) in
   order to ensure implementing domains can detect cases of both benign
   and malicious failures, and to ensure that failures that indicate an
   active attack are discoverable.  As such, senders who also implement
   TLSRPT SHOULD treat the following events as reportable failures:

   o  HTTPS policy fetch failures when a valid TXT record is present.

   o  Policy fetch failures of any kind when a valid policy exists in
      the policy cache, except if that policy's mode is "none".

   o  Delivery attempts in which a contacted MX does not support
      STARTTLS or does not present a certificate which validates
      according to the applied policy, except if that policy's mode is

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6.  Operational Considerations

6.1.  Policy Updates

   Updating the policy requires that the owner make changes in two
   places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and
   at the corresponding HTTPS endpoint.  As a result, recipients should
   expect a policy will continue to be used by senders until both the
   HTTPS and TXT endpoints are updated and the TXT record's TTL has

   In other words, a sender who is unable to successfully deliver a
   message while applying a cache of the recipient's now-outdated policy
   may be unable to discover that a new policy exists until the DNS TTL
   has passed.  Recipients should therefore ensure that old policies
   continue to work for message delivery during this period of time, or
   risk message delays.

   Recipients should also prefer to update the HTTPS policy body before
   updating the TXT record; this ordering avoids the risk that senders,
   seeing a new TXT record, mistakenly cache the old policy from HTTPS.

6.2.  Removing MTA-STS

   In order to facilitate clean opt-out of MTA-STS by implementing
   policy domains, and to distinguish clearly between failures which
   indicate attacks and those which indicate such opt-outs, MTA-STS
   implements the "none" mode, which allows validated policies to
   indicate authoritatively that the policy domain wishes to no longer
   implement MTA-STS and may, in the future, remove the MTA-STS TXT and
   policy endpoints entirely.

   A suggested workflow to implement such an opt out is as follows:

   1.  Publish a new policy with "mode" equal to "none" and a small
       "max_age" (e.g.  one day).

   2.  Publish a new TXT record to trigger fetching of the new policy.

   3.  When all previously served policies have expired--normally this
       is the time the previously published policy was last served plus
       that policy's "max_age", but note that older policies may have
       been served with a greater "max_age", allowing overlapping policy
       caches--safely remove the TXT record and HTTPS endpoint.

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7.  IANA Considerations

7.1.  Well-Known URIs Registry

   A new .well-known URI will be registered in the Well-Known URIs
   registry as described below:

   URI Suffix: mta-sts.txt Change Controller: IETF

7.2.  MTA-STS TXT Record Fields

   IANA is requested to create a new registry titled "MTA-STS TXT Record
   Fields".  The initial entries in the registry are:

       | Field Name |    Description     |       Reference        |
       |     v      |   Record version   | Section 3.1 of RFC XXX |
       |     id     | Policy instance ID | Section 3.1 of RFC XXX |

   New fields are added to this registry using IANA's "Expert Review"

7.3.  MTA-STS Policy Fields

   IANA is requested to create a new registry titled "MTA-STS Policy
   Fields".  The initial entries in the registry are:

      | Field Name |     Description      |       Reference        |
      |  version   |    Policy version    | Section 3.2 of RFC XXX |
      |    mode    | Enforcement behavior | Section 3.2 of RFC XXX |
      |  max_age   |   Policy lifetime    | Section 3.2 of RFC XXX |
      |     mx     |    MX identities     | Section 3.2 of RFC XXX |

   New fields are added to this registry using IANA's "Expert Review"

8.  Security Considerations

   SMTP MTA Strict Transport Security attempts to protect against an
   active attacker who wishes to intercept or tamper with mail between
   hosts who support STARTTLS.  There are two classes of attacks

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   o  Foiling TLS negotiation, for example by deleting the "250
      STARTTLS" response from a server or altering TLS session
      negotiation.  This would result in the SMTP session occurring over
      plaintext, despite both parties supporting TLS.

   o  Impersonating the destination mail server, whereby the sender
      might deliver the message to an impostor, who could then monitor
      and/or modify messages despite opportunistic TLS.  This
      impersonation could be accomplished by spoofing the DNS MX record
      for the recipient domain, or by redirecting client connections
      intended for the legitimate recipient server (for example, by
      altering BGP routing tables).

   MTA-STS can thwart such attacks only if the sender is able to
   previously obtain and cache a policy for the recipient domain, and
   only if the attacker is unable to obtain a valid certificate that
   complies with that policy.  Below, we consider specific attacks on
   this model.

8.1.  Obtaining a Signed Certificate

   SMTP MTA-STS relies on certificate validation via PKIX based TLS
   identity checking [RFC6125].  Attackers who are able to obtain a
   valid certificate for the targeted recipient mail service (e.g. by
   compromising a certificate authority) are thus able to circumvent STS

8.2.  Preventing Policy Discovery

   Since MTA-STS uses DNS TXT records for policy discovery, an attacker
   who is able to block DNS responses can suppress the discovery of an
   MTA-STS Policy, making the Policy Domain appear not to have an MTA-
   STS Policy.  The sender policy cache is designed to resist this
   attack by decreasing the frequency of policy discovery and thus
   reducing the window of vulnerability; it is nonetheless a risk that
   attackers who can predict or induce policy discovery--for example, by
   inducing a victim sending domain to send mail to a never-before-
   contacted recipient while carrying out a man-in-the-middle attack--
   may be able to foil policy discovery and effectively downgrade the
   security of the message delivery.

   Since this attack depends upon intercepting initial policy discovery,
   we strongly recommend implementers to prefer policy "max_age" values
   to be as long as is practical.

   Because this attack is also possible upon refresh of a cached policy,
   we suggest implementers do not wait until a cached policy has expired
   before checking for an update; if senders attempt to refresh the

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   cache regularly (for instance, by checking their cached version
   string against the TXT record on each successful send, or in a
   background task that runs daily or weekly), an attacker would have to
   foil policy discovery consistently over the lifetime of a cached
   policy to prevent a successful refresh.

   Additionally, MTAs should alert administrators to repeated policy
   refresh failures long before cached policies expire (through warning
   logs or similar applicable mechanisms), allowing administrators to
   detect such a persistent attack on policy refresh.  (However, they
   should not implement such alerts if the cached policy has a "none"
   mode, to allow clean MTA-STS removal, as described in Section 6.2.)

   Resistance to downgrade attacks of this nature--due to the ability to
   authoritatively determine "lack of a record" even for non-
   participating recipients--is a feature of DANE, due to its use of
   DNSSEC for policy discovery.

8.3.  Denial of Service

   We additionally consider the Denial of Service risk posed by an
   attacker who can modify the DNS records for a victim domain.  Absent
   MTA-STS, such an attacker can cause a sending MTA to cache invalid MX
   records, but only for however long the sending resolver caches those
   records.  With MTA-STS, the attacker can additionally advertise a
   new, long-"max_age" MTA-STS policy with "mx" constraints that
   validate the malicious MX record, causing senders to cache the policy
   and refuse to deliver messages once the victim has resecured the MX

   This attack is mitigated in part by the ability of a victim domain to
   (at any time) publish a new policy updating the cached, malicious
   policy, though this does require the victim domain to both obtain a
   valid CA-signed certificate and to understand and properly configure

   Similarly, we consider the possibility of domains that deliberately
   allow untrusted users to serve untrusted content on user-specified
   subdomains.  In some cases (e.g. the service this takes
   the form of providing HTTPS hosting of user-registered subdomains; in
   other cases (e.g. dynamic DNS providers) this takes the form of
   allowing untrusted users to register custom DNS records at the
   provider's domain.

   In these cases, there is a risk that untrusted users would be able to
   serve custom content at the "mta-sts" host, including serving an
   illegitimate MTA-STS policy.  We believe this attack is rendered more
   difficult by the need for the attacker to also serve the "_mta-sts"

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   TXT record on the same domain--something not, to our knowledge,
   widely provided to untrusted users.  This attack is additionally
   mitigated by the aforementioned ability for a victim domain to update
   an invalid policy at any future date.

8.4.  Weak Policy Constraints

   Even if an attacker cannot modify a served policy, the potential
   exists for configurations that allow attackers on the same domain to
   receive mail for that domain.  For example, an easy configuration
   option when authoring an MTA-STS Policy for "" is to set
   the "mx" equal to ""; recipient domains must consider in
   this case the risk that any user possessing a valid hostname and CA-
   signed certificate (for example, "") will, from
   the perspective of MTA-STS Policy validation, be a valid MX host for
   that domain.

8.5.  Compromise of the Web PKI System

   A host of risks apply to the PKI system used for certificate
   authentication, both of the "mta-sts" HTTPS host's certificate and
   the SMTP servers' certificates.  These risks are broadly applicable
   within the Web PKI ecosystem and are not specific to MTA-STS;
   nonetheless, they deserve some consideration in this context.

   Broadly speaking, attackers may compromise the system by obtaining
   certificates under fraudulent circumstances (i.e. by impersonating
   the legitimate owner of the victim domain), by compromising a
   Certificate Authority or Delegate Authority's private keys, by
   obtaining a legitimate certificate issued to the victim domain, and

   One approach commonly employed by Web browsers to help mitigate
   against some of these attacks is to allow for revocation of
   compromised or fraudulent certificates via OCSP [RFC2560] or CRLs
   [RFC6818].  Such mechanisms themselves represent tradeoffs and are
   not universally implemented; we nonetheless recommend implementors of
   MTA-STS to implement revocation mechanisms which are most applicable
   to their implementations.

9.  Contributors

   Nicolas Lidzborski Google, Inc nlidz (at) google (dot com)

   Wei Chuang Google, Inc weihaw (at) google (dot com)

   Brandon Long Google, Inc blong (at) google (dot com)

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   Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com)

   Klaus Umbach 1&1 Mail & Media Development & Technology GmbH
   klaus.umbach (at) 1und1 (dot de)

   Markus Laber 1&1 Mail & Media Development & Technology GmbH
   markus.laber (at) 1und1 (dot de)

10.  Appendix 1: MTA-STS example record & policy

   The owner of "" wishes to begin using MTA-STS with a
   policy that will solicit reports from senders without affecting how
   the messages are processed, in order to verify the identity of MXs
   that handle mail for "", confirm that TLS is correctly
   used, and ensure that certificates presented by the recipient MX

   MTA-STS policy indicator TXT RR:  IN TXT "v=STSv1; id=20160831085700Z;"

   MTA-STS Policy file served as the response body at <https://mta->

                         version: STSv1
                         mode: report
                         max_age: 12345678

11.  Appendix 2: Message delivery pseudocode

   Below is pseudocode demonstrating the logic of a compliant sending

   While this pseudocode implementation suggests synchronous policy
   retrieval in the delivery path, in a working implementation that may
   be undesirable, and we expect some implementers to instead prefer a
   background fetch that does not block delivery if no cached policy is

func isEnforce(policy) {
  // Return true if the policy mode is "enforce".

func isNonExpired(policy) {

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  // Return true if the policy is not expired.

func tryStartTls(connection) {
  // Attempt to open an SMTP connection with STARTTLS with the MX.

func isWildcardMatch(pat, host) {
  // Literal matches are true.
  if pat == host {
    return true
  // Leading '.' matches a wildcard against the first part, i.e.
  // matches but not
  if pat[0] == '.' {
    parts = SplitN(host, '.', 2)  // Split on the first '.'.
    if len(parts) > 1 && parts[1] == pat[1:] {
      return true
  return false

func certMatches(connection, policy) {
  // Assume a handy function to return CN and DNS-ID SANs.
  for san in getDnsIdSansAndCnFromCert(connection) {
    for mx in {
      // Return if the server certificate from "connection" matches the "mx" host.
      if san[0] == '*' {
        // Invalid wildcard!
        if san[1] != '.' continue
        san = san[1:]
      if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) {
        return true
  return false

func tryDeliverMail(connection, message) {
  // Attempt to deliver "message" via "connection".

func tryGetNewPolicy(domain) {
  // Check for an MTA-STS TXT record for "domain" in DNS, and return the
  // indicated policy.

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func cachePolicy(domain, policy) {
  // Store "policy" as the cached policy for "domain".

func tryGetCachedPolicy(domain) {
  // Return a cached policy for "domain".

func reportError(error) {
  // Report an error via TLSRPT.

func tryMxAccordingTo(message, mx, policy) {
  connection := connect(mx)
  if !connection {
    return false  // Can't connect to the MX so it's not an MTA-STS error.
  secure := true
  if !tryStartTls(connection) {
    secure = false
  } else if !certMatches(connection, policy) {
    secure = false
  if secure || !isEnforce(policy) {
    return tryDeliverMail(connection, message)
  return false

func tryWithPolicy(message, domain, policy) {
  mxes := getMxForDomain(domain)
  for mx in mxes {
    if tryMxAccordingTo(message, mx, policy) {
      return true
  return false

func handleMessage(message) {
  domain := ... // domain part after '@' from recipient
  policy := tryGetNewPolicy(domain)
  if policy {
    cachePolicy(domain, policy)

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  } else {
    policy = tryGetCachedPolicy(domain)
  if policy {
    return tryWithPolicy(message, domain, policy)
  // Try to deliver the message normally (i.e. without MTA-STS).

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008, <https://www.rfc-

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010, <https://www.rfc-

   [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, <>.

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   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014, <https://www.rfc-

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,

12.2.  Informative References

   [RFC2560]  Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
              Adams, "X.509 Internet Public Key Infrastructure Online
              Certificate Status Protocol - OCSP", RFC 2560,
              DOI 10.17487/RFC2560, June 1999, <https://www.rfc-

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <>.

   [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,

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008, <https://www.rfc-

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010, <https://www.rfc-

   [RFC6818]  Yee, P., "Updates to the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January
              2013, <>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,

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   [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, <https://www.rfc-

Authors' Addresses

   Daniel Margolis
   Google, Inc

   Email: dmargolis (at)

   Mark Risher
   Google, Inc

   Email: risher (at) google (dot com)

   Binu Ramakrishnan
   Yahoo!, Inc

   Email: rbinu (at) yahoo-inc (dot com)

   Alexander Brotman
   Comcast, Inc

   Email: alex_brotman (at)

   Janet Jones
   Microsoft, Inc

   Email: janet.jones (at) microsoft (dot com)

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