Internet DRAFT - draft-ietf-uta-mta-sts
draft-ietf-uta-mta-sts
Using TLS in Applications D. Margolis
Internet-Draft M. Risher
Intended status: Standards Track Google, Inc
Expires: December 18, 2018 B. Ramakrishnan
Yahoo!, Inc
A. Brotman
Comcast, Inc
J. Jones
Microsoft, Inc
June 16, 2018
SMTP MTA Strict Transport Security (MTA-STS)
draft-ietf-uta-mta-sts-21
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.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 18, 2018.
Copyright Notice
Copyright (c) 2018 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
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Related Technologies . . . . . . . . . . . . . . . . . . . . 4
3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4
3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4
3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 6
3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 9
3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 10
4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 10
4.1. MX Host Validation . . . . . . . . . . . . . . . . . . . 11
4.2. Recipient MTA Certificate Validation . . . . . . . . . . 11
5. Policy Application . . . . . . . . . . . . . . . . . . . . . 11
5.1. Policy Application Control Flow . . . . . . . . . . . . . 12
6. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 12
7. Interoperability Considerations . . . . . . . . . . . . . . . 13
7.1. SNI Support . . . . . . . . . . . . . . . . . . . . . . . 13
7.2. Minimum TLS Version Support . . . . . . . . . . . . . . . 13
8. Operational Considerations . . . . . . . . . . . . . . . . . 14
8.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 14
8.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 14
8.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 15
8.4. Preserving MX Candidate Traversal . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 16
9.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 16
9.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 17
10. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 17
10.2. Preventing Policy Discovery . . . . . . . . . . . . . . 18
10.3. Denial of Service . . . . . . . . . . . . . . . . . . . 18
10.4. Weak Policy Constraints . . . . . . . . . . . . . . . . 19
10.5. Compromise of the Web PKI System . . . . . . . . . . . . 19
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. MTA-STS example record & policy . . . . . . . . . . 23
Appendix B. Message delivery pseudocode . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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1. 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:
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
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:
o MTA-STS Policy: A commitment by the Policy Domain to support PKIX
[RFC5280] 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
"alice@example.com" this would ordinarily be "example.com", but
this may be overridden by explicit routing rules (as described in
Section 3.4, "Policy Selection for Smart Hosts and Subdomains").
o Policy Host: The HTTPS host which serves the MTA-STS Policy for a
Policy Domain. Rules for constructing the hostname are described
in Section 3.2, "MTA-STS Policies".
o Sender: The SMTP Mail Transfer Agent sending an email message.
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o ABNF: Augmented Backus-Naur Form, a syntax for formally specifying
syntax, defined in [RFC5234] and [RFC7405].
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 10, "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.
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 "example.com", this record would
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be "_mta-sts.example.com". 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:
_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 Section 5.1, "Policy Application Control Flow".) If the
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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 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:
o "version": Currently only "STSv1" is supported.
o "mode": One of "enforce", "testing", or "none", indicating the
expected behavior of a sending MTA in the case of a policy
validation failure. See Section 5, "Policy Application." for more
details about the three modes.
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
greater.
o "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
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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
Section 4.1, "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"
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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-mx-label = sts-policy-alphanum /
sts-policy-alphanum *(sts-policy-alphanum / "-")
sts-policy-alphanum
sts-policy-mx-toplabel = ALPHA / ALPHA *(sts-policy-alphanum / "-")
sts-policy-alphanum
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]>
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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.
3.3. 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:
o Matching is performed only against the DNS-ID identifiers.
o 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.
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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 Section 10, 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".
3.4. 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 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".
4. 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 Section 4.1, "MX Host Validation".
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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 Section 4.2, "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 Section 5, "Policy
Application" for a description of sending MTA behavior when policy
validation fails.
4.1. 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".
4.2. 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.
5. 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).
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3. "none": In this mode, sending MTAs should treat the policy domain
as though it does not have any active policy; see Section 8.3,
"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.
5.1. 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. 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
Section 4, "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).
6. 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:
o HTTPS policy fetch failures when a valid TXT record is present.
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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
"none".
7. Interoperability Considerations
7.1. 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.
7.2. 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.
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8. Operational Considerations
8.1. 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
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.
8.2. 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.
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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.
8.3. 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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8.4. 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.
9. IANA Considerations
9.1. Well-Known URIs Registry
A new "well-known" URI as described in Section 3 will be registered
in the Well-Known URIs registry as described below:
URI Suffix: mta-sts.txt Change Controller: IETF
9.2. 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"
policy.
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9.3. 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"
policy.
10. 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:
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.
10.1. 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
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compromising a certificate authority) are thus able to circumvent STS
authentication.
10.2. 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 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 Section 8.3.)
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.
10.3. 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
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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.
10.4. 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 "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.
10.5. 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
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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.
11. 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
12. References
12.1. Normative References
[I-D.ietf-uta-smtp-tlsrpt]
Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J.,
and M. Risher, "SMTP TLS Reporting", draft-ietf-uta-smtp-
tlsrpt-22 (work in progress), May 2018.
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[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-
editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000, <https://www.rfc-
editor.org/info/rfc2818>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<https://www.rfc-editor.org/info/rfc3492>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, <https://www.rfc-
editor.org/info/rfc5234>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, <https://www.rfc-
editor.org/info/rfc5246>.
[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,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, <https://www.rfc-
editor.org/info/rfc5321>.
[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-
editor.org/info/rfc5785>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011, <https://www.rfc-
editor.org/info/rfc6066>.
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[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, <https://www.rfc-editor.org/info/rfc6125>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
12.2. Informative References
[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,
<https://www.rfc-editor.org/info/rfc4033>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008, <https://www.rfc-
editor.org/info/rfc5322>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010, <https://www.rfc-
editor.org/info/rfc5891>.
[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, <https://www.rfc-editor.org/info/rfc6818>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
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[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,
<https://www.rfc-editor.org/info/rfc7234>.
[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-
editor.org/info/rfc7672>.
Appendix A. 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
Appendix B. 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".
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}
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".
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}
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 {
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return tryWithPolicy(message, domain, policy)
}
// Try to deliver the message normally (i.e., without MTA-STS).
}
Authors' Addresses
Daniel Margolis
Google, Inc
Email: dmargolis@google.com
Mark Risher
Google, Inc
Email: risher@google.com
Binu Ramakrishnan
Yahoo!, Inc
Email: rbinu@yahoo-inc.com
Alexander Brotman
Comcast, Inc
Email: alex_brotman@comcast.com
Janet Jones
Microsoft, Inc
Email: janet.jones@microsoft.com
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