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Title = "SMTP MTA Strict Transport Security (MTA-STS)" abbrev = "MTA-STS" category = "std" docName = "draft-ietf-uta-mta-sts-21" ipr = "trust200902" area = "Applications" workgroup = "Using TLS in Applications" keyword = [""]

date = 2018-06-16T00:00:00Z

[[author]] initials="D." surname="Margolis" fullname="Daniel Margolis" organization="Google, Inc" [author.address] email="dmargolis@google.com" [[author]] initials="M." surname="Risher" fullname="Mark Risher" organization="Google, Inc" [author.address] email="risher@google.com" [[author]] initials="B." surname="Ramakrishnan" fullname="Binu Ramakrishnan" organization="Yahoo!, Inc" [author.address] email="rbinu@yahoo-inc.com" [[author]] initials="A." surname="Brotman" fullname="Alexander Brotman" organization="Comcast, Inc" [author.address] email="alex_brotman@comcast.com" [[author]] initials="J." surname="Jones" fullname="Janet Jones" organization="Microsoft, Inc" [author.address] email="janet.jones@microsoft.com"

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.# Abstract

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.

{mainmatter}

Introduction

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

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, via a combination of DNS and HTTPS, specifying:

  • whether MTAs sending mail to this domain can expect PKIX-authenticated TLS support
  • what a conforming client should do with messages when TLS cannot be successfully negotiated

Terminology

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.

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

  • MTA-STS Policy: A commitment by the Policy Domain to support PKIX [@?RFC5280] authenticated TLS for the specified MX hosts.
  • Policy Domain: The domain for which an MTA-STS Policy is defined. This is the next-hop domain; when sending mail to "alice@example.com" this would ordinarily be "example.com", but this may be overridden by explicit routing rules (as described in (#policy-selection-for-smart-hosts-and-subdomains), "Policy Selection for Smart Hosts and Subdomains").
  • Policy Host: The HTTPS host which serves the MTA-STS Policy for a Policy Domain. Rules for constructing the hostname are described in (#mtasts-policies), "MTA-STS Policies".
  • Sender: The SMTP Mail Transfer Agent sending an email message.
  • ABNF: Augmented Backus-Naur Form, a syntax for formally specifying syntax, defined in [@!RFC5234] and [@!RFC7405].

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 (#security-considerations), "Security Considerations".

In addition, MTA-STS provides an optional testing-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 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 ensure 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 "testing"-only MTA-STS validation to override a failing DANE validation.

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.

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 example.com, this record would be _mta-sts.example.com. MTA-STS TXT records MUST be US-ASCII, semicolon-separated key/value pairs containing the following fields:

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

_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"

The formal definition of the _mta-sts TXT record, defined using ABNF ([@!RFC7405]), is as follows:

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

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

sts-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 CTLs

The TXT record MUST begin with sts-version field, and the order of other fields is not significant. 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, or if the resulting record is syntactically invalid, senders MUST assume the recipient domain does not have an available MTA-STS policy and skip the remaining steps of policy discovery. (Note that absence of a usable TXT record is not by itself sufficient to remove a sender's previously cached policy for the Policy Domain, as discussed in (#policy-application-control-flow), "Policy Application Control Flow".) If the resulting TXT record contains multiple strings, then the record MUST be treated as if those strings are concatenated together without adding spaces.

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 Policy Host. The Policy Host DNS name is constructed by prepending mta-sts to the Policy Domain.

Thus for a Policy Domain of example.com the full URL is https://mta-sts.example.com/.well-known/mta-sts.txt.

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. All parameters other than charset=utf-8 or charset=us-ascii are ignored. Additional "Content-Type" parameters are also ignored.

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

  • version: Currently only "STSv1" is supported.

  • mode: One of "enforce", "testing", or "none", indicating the expected behavior of a sending MTA in the case of a policy validation failure. See (#policy-application), "Policy Application." for more details about the three modes.

  • 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 greater.

  • mx: Allowed MX patterns. One or more patterns matching allowed MX hosts for the Policy Domain. As an example,

     mx: mail.example.com <CRLF>
     mx: *.example.net
    

    indicates that mail for this domain might be handled by MX mail.example.com or any MX at example.net. Valid patterns can be either fully specified names (example.com) or suffixes prefixed by a wildcard (*.example.net). If a policy specifies more than one MX, each MX MUST have its own mx: key, and each MX key/value pair MUST be on its own line in the policy file. In the case of Internationalized Domain Names ([@?RFC5891]), the mx value MUST specify the Punycode-encoded A-label [@!RFC3492] to match against, and not the Unicode-encoded U-label. The full semantics of certificate validation (including the use of wildcard patterns) are described in (#mx-host-validation), "MX Host Validation."

An example policy is as below:

version: STSv1
mode: enforce
mx: mail.example.com
mx: *.example.net
mx: backupmx.example.com
max_age: 604800

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

sts-policy-record        = sts-policy-field *WSP
                           *(sts-policy-term sts-policy-field *WSP)
                           [sts-policy-term]

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

                           sts-policy-term /
                           ; required at least once, except when
                           ; mode is "none"

                           sts-policy-extension      ; other fields

sts-policy-field-delim   = ":" *WSP

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

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

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

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

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

sts-policy-mode-value    =  %s"testing" / %s"enforce" / %s"none"

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

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

sts-policy-mx-value      = ["*."] Domain

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

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

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

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

sts-policy-ext-name      = (sts-policy-alphanum)
                           *31(sta-policy-alphanum / "_" / "-" / ".")
                           
sts-policy-term          = LF / CRLF

sts-policy-ext-value     = sts-policy-vchar
                           [*(%x20 / sts-policy-vchar)
                           sts-policy-vchar]
                           ; chars, including UTF-8 [@!RFC3629],
                           ; excluding CTLs and no 
                           ; leading/trailing spaces

sts-policy-alphanum     = ALPHA / DIGIT

sts-policy-vchar        = %x21-7E / UTF8-2 / UTF8-3 / UTF8-4

UTF8-2          =   <Defined in Section 4 of [@!RFC3629]>

UTF8-3          =   <Defined in Section 4 of [@!RFC3629]>

UTF8-4          =   <Defined in Section 4 of [@!RFC3629]>

Domain          =   <see RFC 5321 4.1.2>

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), possibly 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.

HTTPS Policy Fetching

Policy bodies are, as described above, retrieved by sending MTAs via HTTPS [@!RFC2818]. During the TLS handshake initiated to fetch a new or updated policy from the Policy Host, the Policy Host HTTPS server MUST present a X.509 certificate which is valid for the mta-sts DNS-ID ([@?RFC6125]) (e.g., mta-sts.example.com) 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. See [@?RFC5280] for more details about certificate verification.

The certificate is valid for the Policy Host (i.e., mta-sts prepended to the Policy Domain) with respect to the rules described in [@!RFC6125], with the following application-specific considerations:

  • Matching is performed only against the DNS-ID identifiers.

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

The certificate MAY be checked for revocation via the Online Certificate Status Protocol (OCSP) [@?RFC6960], 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, implementers SHOULD 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 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.

Finally, to mitigate the risk of persistent interference with policy refresh, as discussed in-depth in (#security-considerations), MTAs SHOULD proactively refresh cached policies before they expire; a suggested refresh frequency is once per day. To enable administrators to discover problems with policy refresh, MTAs SHOULD alert administrators (through the use of logs or similar) when such attempts fail, unless the cached policy mode is none.

Policy Selection for Smart Hosts and Subdomains

When sending mail via a "smart host"--an administratively configured intermediate SMTP relay, which is different from the message recipient's server as determined from DNS --compliant senders MUST treat the smart host domain as the policy domain for the purposes of policy discovery and application. This specification does not provide a means of associating policies with email addresses that employ Address Literals [@!RFC5321].

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 "user@mail.example.com", the policy can be fetched only from "mail.example.com", not "example.com".

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 check whether:

  1. At least one of the policy's "mx" patterns matches the selected MX host, as described in (#mx-host-validation), "MX Host Validation".

  2. The recipient mail server supports STARTTLS and offers a PKIX-based TLS certificate, during TLS handshake, which is valid for that host, as described in (#recipient-mta-certificate-validation), "Recipient MTA Certificate Validation".

When these conditions are not met, a policy is said to fail to validate. This section does not dictate the behavior of sending MTAs when the above conditions are not met; see (#policy-application), "Policy Application" for a description of sending MTA behavior when policy validation fails.

MX Host Validation

A receiving candidate MX host is valid according to an applied MTA-STS policy if the MX record name matches one or more of the mx fields in the applied policy. Matching is identical to the rules given in [@!RFC6125], with restriction that the wildcard character * may only be used to match the entire left-most label in the presented identifier. Thus the mx pattern *.example.com matches mail.example.com but not example.com or foo.bar.example.com.

Recipient MTA Certificate Validation

The certificate presented by the receiving MTA MUST not be expired, and MUST chain to a root CA that is trusted by the sending MTA. The certificate MUST have a subject alternative name (SAN, [@!RFC5280]) with a DNS-ID ([@?RFC6125]) matching the host name, per the rules given in [@!RFC6125]. The MX's certificate MAY also be checked for revocation via OCSP [@?RFC6960], CRLs [@?RFC6818], or some other mechanism.

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 validation, or do not support STARTTLS.

  2. testing: In this mode, sending MTAs which also implement the TLSRPT specification [@!I-D.ietf-uta-smtp-tlsrpt] 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 (#removing-mtasts), "Removing MTA-STS", for use of this mode value.

When a message fails to deliver due to an enforce policy, a compliant MTA MUST NOT permanently fail to deliver messages before checking, via DNS, 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.

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. If a policy is present with an enforce mode, when attempting to deliver to each candidate MX, ensure STARTTLS support and host identity validity as described in (#policy-validation), "Policy Validation". If a candidate fails validation, continue to the next candidate (if there is one).
  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] section 4.5.4.1).

Reporting Failures

MTA-STS is intended to be used along with TLSRPT [@!I-D.ietf-uta-smtp-tlsrpt] 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:

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

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

  • 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 none.

Interoperability Considerations

SNI Support

To ensure that the server sends the right certificate chain, the SMTP client MUST have support for the TLS SNI extension [@!RFC6066]. When connecting to a HTTP server to retrieve the MTA-STS policy, the SNI extension MUST contain the name of the policy host (e.g., mta-sts.example.com). When connecting to an SMTP server, the SNI extension MUST contain the MX hostname.

HTTP servers used to deliver MTA-STS policies MAY rely on SNI to determine which certificate chain to present to the client. HTTP servers MUST respond with a certificate chain that matches the policy hostname or abort the TLS handshake if unable to do so. Clients that do not send SNI information may not see the expected certificate chain.

SMTP servers MAY rely on SNI to determine which certificate chain to present to the client. However servers that have one identity and a single matching certificate do not require SNI support. Servers MUST NOT enforce the use of SNI by clients, as the client may be using unauthenticated opportunistic TLS and may not expect any particular certificate from the server. If the client sends no SNI extension or sends an SNI extension for an unsupported server name, the server MUST simply send a fallback certificate chain of its choice. The reason for not enforcing strict matching of the requested SNI hostname is that MTA-STS TLS clients may be typically willing to accept multiple server names but can only send one name in the SNI extension. The server's fallback certificate may match a different name that is acceptable to the client, e.g., the original next-hop domain.

Minimum TLS Version Support

MTAs supporting MTA-STS MUST have support for TLS version 1.2 [@!RFC5246] or higher. The general TLS usage guidance in [@!RFC7525] SHOULD be followed.

Operational Considerations

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 passed.

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 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.

Policy Delegation

Domain owners commonly delegate SMTP hosting to a different organization, such as an ISP or a Web host. In such a case, they may wish to also delegate the MTA-STS policy to the same organization which can be accomplished with two changes.

First, the Policy Domain must point the _mta-sts record, via CNAME, to the _mta-sts record maintained by the hosting organization. This allows the hosting organization to control update signaling.

Second, the Policy Domain must point the "well-known" policy location to the hosting organization. This can be done either by setting the mta-sts record to an IP address or CNAME specified by the hosting organization and by giving the hosting organization a TLS certificate which is valid for that host, or by setting up a "reverse proxy" (also known as a "gateway") server that serves as the Policy Domain's policy the policy currently served by the hosting organization.

For example, given a user domain user.example hosted by a mail provider provider.example, the following configuration would allow policy delegation:

DNS:

_mta-sts.user.example.  IN CNAME _mta-sts.provider.example.

Policy:

> GET /.well-known/mta-sts.txt Host: mta-sts.user.example
< HTTP/1.1 200 OK  # Response proxies content from
                   # https://mta-sts.provider.example

Note that in all such cases, the policy endpoint (https://mta-sts.user.example/.well-known/mta-sts.txt in this example) must still present a certificate valid for the Policy Host (mta-sts.user.example), and not for that host at the provider's domain (mta-sts.provider.example).

Note that while sending MTAs MUST NOT use HTTP caching when fetching policies via HTTPS, such caching may nonetheless be useful to a reverse proxy configured as described in this section. An HTTPS policy endpoint expecting to be proxied for multiple hosted domains--as with a large mail hosting provider or similar--may wish to indicate an HTTP Cache-Control max-age response directive (as specified in [@?RFC7234]) of 60 seconds as a reasonable value to save reverse proxies an unnecessarily high-rate of proxied policy fetching.

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.

Preserving MX Candidate Traversal

Implementors of send-time MTA-STS validation in mail transfer agents should take note of the risks of modifying the logic of traversing MX candidate lists. Because an MTA-STS policy can be used to prefilter invalid MX candidates from the MX candidate list, it is tempting to implement a "two-pass" model, where MX candidates are first filtered for possible validity according to the MTA-STS policy, and then the remaining candidates attempted in order as without an MTA-STS policy. This may lead to incorrect implementations, such a message loops; implementors are instead recommended to traverse the MX candidate list as usual, and treat invalid candidates as though they were unreachable (i.e., as though there were some transient error when trying to deliver to that candidate).

One consequence of validating MX hosts in order of ordinary candidate traversal is that, in the event that a higher-priority MX is MTA-STS valid and a lower-priority MX is not, senders may never encounter the lower-priority MX, leading to a risk that policy misconfigurations that apply only to "backup" MXes may only be discovered in the case of primary MX failure.

IANA Considerations

Well-Known URIs Registry

A new "well-known" URI as described in (#policy-discovery) will be registered in the Well-Known URIs registry as described below:

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

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 (#mtasts-txt-records) of RFC XXX
id Policy instance ID (#mtasts-txt-records) of RFC XXX

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

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 (#mtasts-policies) of RFC XXX
mode Enforcement behavior (#mtasts-policies) of RFC XXX
max_age Policy lifetime (#mtasts-policies) of RFC XXX
mx MX identities (#mtasts-policies) of RFC XXX

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

Security Considerations

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

  • 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.

  • 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.

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 authentication.

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 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, implementers SHOULD prefer policy max_age values to be as long as is practical.

Because this attack is also possible upon refresh of a cached policy, implementors SHOULD NOT wait until a cached policy has expired before checking for an update; if senders attempt to refresh the cache regularly (for example, by fetching currently live policy in a background task that runs daily or weekly, regardless of the state of the _mta-sts TXT record, and updating their cache's "max age" accordingly), 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 (#removing-mtasts).)

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.

Denial of Service

We additionally consider the Denial of Service risk posed by an attacker who can modify the DNS records for a recipient 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 records.

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 MTA-STS.

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 Tumblr.com) 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 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.

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 example.com is to set the mx equal to *.example.com; recipient domains must consider in this case the risk that any user possessing a valid hostname and CA-signed certificate (for example, dhcp-123.example.com) will, from the perspective of MTA-STS Policy validation, be a valid MX host for that domain.

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 similar.

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 [@?RFC6960] 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.

Contributors

Wei Chuang Google, Inc weihaw@google.com

Viktor Dukhovni ietf-dane@dukhovni.de

Markus Laber 1&1 Mail & Media Development & Technology GmbH markus.laber@1und1.de

Nicolas Lidzborski Google, Inc nlidz@google.com

Brandon Long Google, Inc blong@google.com

Franck Martin LinkedIn, Inc fmartin@linkedin.com

Klaus Umbach 1&1 Mail & Media Development & Technology GmbH klaus.umbach@1und1.de

{backmatter}

MTA-STS example record & policy

The owner of example.com 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 example.com, confirm that TLS is correctly used, and ensure that certificates presented by the recipient MX validate.

MTA-STS policy indicator TXT RR:

_mta-sts.example.com.  IN TXT "v=STSv1; id=20160831085700Z;"

MTA-STS Policy file served as the response body at https://mta-sts.example.com/.well-known/mta-sts.txt:

version: STSv1
mode: testing
mx: mx1.example.com
mx: mx2.example.com
mx: mx.backup-example.com
max_age: 1296000

Message delivery pseudocode

Below is pseudocode demonstrating the logic of a compliant sending MTA.

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 present.


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

func isNonExpired(policy) {
  // Return true if the policy is not expired.
}

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

func certMatches(connection, host) {
  // Assume a handy function to return check if the server certificate presented
  // in "connection" is valid for "host".
}

func policyMatches(candidate, policy) {
  for mx in policy.mx {
    // Literal match.
    if mx == candidate {
      return true
    }
    // Wildcard matches only the leftmost label.
    // Wildcards must always be followed by a '.'.
    if mx[0] == '*' {
      parts = SplitN(candidate, '.', 2)  // Split on the first '.'.
      if len(parts) > 1 && parts[1] == mx[2:] {
        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.
}

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 !policyMatches(mx, policy) {
    secure = false
    reportError(E_HOST_MISMATCH)
  } else if !tryStartTls(connection) {
    secure = false
    reportError(E_NO_VALID_TLS)
  } else if !certMatches(connection, policy) {
    secure = false
    reportError(E_CERT_MISMATCH)
  }
  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)
  } else {
    policy = tryGetCachedPolicy(domain)
  }
  if policy {
    return tryWithPolicy(message, domain, policy)
  }
  // Try to deliver the message normally (i.e., without MTA-STS).
}