draft-ietf-uta-mta-sts-17.txt   draft-ietf-uta-mta-sts-18.txt 
Using TLS in Applications D. Margolis Using TLS in Applications D. Margolis
Internet-Draft M. Risher Internet-Draft M. Risher
Intended status: Standards Track Google, Inc Intended status: Standards Track Google, Inc
Expires: November 4, 2018 B. Ramakrishnan Expires: November 21, 2018 B. Ramakrishnan
Yahoo!, Inc Yahoo!, Inc
A. Brotman A. Brotman
Comcast, Inc Comcast, Inc
J. Jones J. Jones
Microsoft, Inc Microsoft, Inc
May 3, 2018 May 20, 2018
SMTP MTA Strict Transport Security (MTA-STS) SMTP MTA Strict Transport Security (MTA-STS)
draft-ietf-uta-mta-sts-17 draft-ietf-uta-mta-sts-18
Abstract Abstract
SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a
mechanism enabling mail service providers to declare their ability to mechanism enabling mail service providers to declare their ability to
receive Transport Layer Security (TLS) secure SMTP connections, and receive Transport Layer Security (TLS) secure SMTP connections, and
to specify whether sending SMTP servers should refuse to deliver to to specify whether sending SMTP servers should refuse to deliver to
MX hosts that do not offer TLS with a trusted server certificate. MX hosts that do not offer TLS with a trusted server certificate.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 3, 2018. This Internet-Draft will expire on November 21, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Related Technologies . . . . . . . . . . . . . . . . . . . . 3 2. Related Technologies . . . . . . . . . . . . . . . . . . . . 4
3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4 3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4
3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4 3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4
3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 5 3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 6
3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 8 3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 9
3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 9 3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 10
4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 10 4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 10
4.1. MX Certificate Validation . . . . . . . . . . . . . . . . 10 4.1. MX Host Validation . . . . . . . . . . . . . . . . . . . 11
4.2. Recipient MTA Certificate Validation . . . . . . . . . . 11
5. Policy Application . . . . . . . . . . . . . . . . . . . . . 11 5. Policy Application . . . . . . . . . . . . . . . . . . . . . 11
5.1. Policy Application Control Flow . . . . . . . . . . . . . 11 5.1. Policy Application Control Flow . . . . . . . . . . . . . 12
6. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 12 6. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 12
7. Interoperability Considerations . . . . . . . . . . . . . . . 12 7. Interoperability Considerations . . . . . . . . . . . . . . . 13
7.1. SNI Support . . . . . . . . . . . . . . . . . . . . . . . 12 7.1. SNI Support . . . . . . . . . . . . . . . . . . . . . . . 13
7.2. Minimum TLS Version Support . . . . . . . . . . . . . . . 13 7.2. Minimum TLS Version Support . . . . . . . . . . . . . . . 13
8. Operational Considerations . . . . . . . . . . . . . . . . . 13 8. Operational Considerations . . . . . . . . . . . . . . . . . 13
8.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 13 8.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 13
8.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 13 8.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 14
8.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 14 8.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 8.4. Preserving MX Candidate Traversal . . . . . . . . . . . . 15
9.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 15 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 15 9.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 16
9.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 15 9.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16 9.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 16
10.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 16 10. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10.2. Preventing Policy Discovery . . . . . . . . . . . . . . 16 10.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 17
10.3. Denial of Service . . . . . . . . . . . . . . . . . . . 17 10.2. Preventing Policy Discovery . . . . . . . . . . . . . . 18
10.4. Weak Policy Constraints . . . . . . . . . . . . . . . . 18 10.3. Denial of Service . . . . . . . . . . . . . . . . . . . 18
10.5. Compromise of the Web PKI System . . . . . . . . . . . . 18 10.4. Weak Policy Constraints . . . . . . . . . . . . . . . . 19
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 19 10.5. Compromise of the Web PKI System . . . . . . . . . . . . 19
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . 19 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . 20 12.1. Normative References . . . . . . . . . . . . . . . . . . 20
Appendix A. MTA-STS example record & policy . . . . . . . . . . 21 12.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix B. Message delivery pseudocode . . . . . . . . . . . . 22 Appendix A. MTA-STS example record & policy . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 Appendix B. Message delivery pseudocode . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction 1. Introduction
The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
hosts to negotiate the use of a TLS channel for encrypted mail hosts to negotiate the use of a TLS channel for encrypted mail
transmission. transmission.
While this opportunistic encryption protocol by itself provides a While this opportunistic encryption protocol by itself provides a
high barrier against passive man-in-the-middle traffic interception, high barrier against passive man-in-the-middle traffic interception,
any attacker who can delete parts of the SMTP session (such as the any attacker who can delete parts of the SMTP session (such as the
"250 STARTTLS" response) or who can redirect the entire SMTP session "250 STARTTLS" response) or who can redirect the entire SMTP session
(perhaps by overwriting the resolved MX record of the delivery (perhaps by overwriting the resolved MX record of the delivery
domain) can perform downgrade or interception attacks. domain) can perform downgrade or interception attacks.
This document defines a mechanism for recipient domains to publish This document defines a mechanism for recipient domains to publish
policies specifying: policies, via a combination of DNS and HTTPS, specifying:
o whether MTAs sending mail to this domain can expect PKIX- o whether MTAs sending mail to this domain can expect PKIX-
authenticated TLS support authenticated TLS support
o what a conforming client should do with messages when TLS cannot o what a conforming client should do with messages when TLS cannot
be successfully negotiated be successfully negotiated
1.1. Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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o MTA-STS Policy: A commitment by the Policy Domain to support PKIX o MTA-STS Policy: A commitment by the Policy Domain to support PKIX
[RFC5280] authenticated TLS for the specified MX hosts. [RFC5280] authenticated TLS for the specified MX hosts.
o Policy Domain: The domain for which an MTA-STS Policy is defined. o Policy Domain: The domain for which an MTA-STS Policy is defined.
This is the next-hop domain; when sending mail to This is the next-hop domain; when sending mail to
"alice@example.com" this would ordinarily be "example.com", but "alice@example.com" this would ordinarily be "example.com", but
this may be overridden by explicit routing rules (as described in this may be overridden by explicit routing rules (as described in
Section 3.4, "Policy Selection for Smart Hosts and Subdomains"). 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.
2. Related Technologies 2. Related Technologies
The DANE TLSA record [RFC7672] is similar, in that DANE is also The DANE TLSA record [RFC7672] is similar, in that DANE is also
designed to upgrade unauthenticated encryption or plaintext designed to upgrade unauthenticated encryption or plaintext
transmission into authenticated, downgrade-resistant encrypted transmission into authenticated, downgrade-resistant encrypted
transmission. DANE requires DNSSEC [RFC4033] for authentication; the transmission. DANE requires DNSSEC [RFC4033] for authentication; the
mechanism described here instead relies on certificate authorities mechanism described here instead relies on certificate authorities
(CAs) and does not require DNSSEC, at a cost of risking malicious (CAs) and does not require DNSSEC, at a cost of risking malicious
downgrades. For a thorough discussion of this trade-off, see downgrades. For a thorough discussion of this trade-off, see
Section 10, "Security Considerations". Section 10, "Security Considerations".
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sts-ext-name = (ALPHA / DIGIT) sts-ext-name = (ALPHA / DIGIT)
*31(ALPHA / DIGIT / "_" / "-" / ".") *31(ALPHA / DIGIT / "_" / "-" / ".")
sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E)
; chars excluding "=", ";", and control chars ; chars excluding "=", ";", and control chars
The TXT record MUST begin with sts-version field, and the order of The TXT record MUST begin with sts-version field, and the order of
other fields is not significant. If multiple TXT records for "_mta- other fields is not significant. If multiple TXT records for "_mta-
sts" are returned by the resolver, records which do not begin with sts" are returned by the resolver, records which do not begin with
"v=STSv1;" are discarded. If the number of resulting records is not "v=STSv1;" are discarded. If the number of resulting records is not
one, senders MUST assume the recipient domain does not implement MTA- one, senders MUST assume the recipient domain does not have an
STS and skip the remaining steps of policy discovery. If the available MTA-STS policy and skip the remaining steps of policy
resulting TXT record contains multiple strings, then the record MUST discovery. (Note that lack of an available policy does not signal
be treated as if those strings are concatenated together without opting out of MTA-STS altogether if the sender has a previously
adding spaces. cached policy for the recipient domain, as discussed in Section 5.1,
"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.
3.2. MTA-STS Policies 3.2. MTA-STS Policies
The policy itself is a set of key/value pairs (similar to [RFC5322] The policy itself is a set of key/value pairs (similar to [RFC5322]
header fields) served via the HTTPS GET method from the fixed header fields) served via the HTTPS GET method from the fixed
[RFC5785] "well-known" path of ".well-known/mta-sts.txt" served by [RFC5785] "well-known" path of ".well-known/mta-sts.txt" served by
the "mta-sts" host at the Policy Domain. Thus for "example.com" the the Policy Host. The Policy Host DNS name is constructed by
path is "https://mta-sts.example.com/.well-known/mta-sts.txt". prepending "mta-sts" to the Policy Domain.
Thus for a Policy Domain of "example.com" the path is "https://mta-
sts.example.com/.well-known/mta-sts.txt".
When fetching a policy, senders SHOULD validate that the media type When fetching a policy, senders SHOULD validate that the media type
is "text/plain" to guard against cases where webservers allow is "text/plain" to guard against cases where webservers allow
untrusted users to host non-text content (typically, HTML or images) untrusted users to host non-text content (typically, HTML or images)
at a user-defined path. All parameters other charset=utf-8 or at a user-defined path. All parameters other than charset=utf-8 or
charset=us-ascii are ignored. Additional "Content-Type" parameters charset=us-ascii are ignored. Additional "Content-Type" parameters
are also ignored. are also ignored.
This resource contains the following CRLF-separated key/value pairs: This resource contains the following CRLF-separated key/value pairs:
o "version": (plain-text). Currently only "STSv1" is supported. o "version": Currently only "STSv1" is supported.
o "mode": (plain-text). One of "enforce", "testing", or "none", o "mode": One of "enforce", "testing", or "none", indicating the
indicating the expected behavior of a sending MTA in the case of a expected behavior of a sending MTA in the case of a policy
policy validation failure. See Section 5, "Policy Application." validation failure. See Section 5, "Policy Application." for more
for more details about the three modes. details about the three modes.
o "max_age": Max lifetime of the policy (plain-text non-negative o "max_age": Max lifetime of the policy (plain-text non-negative
integer seconds, maximum value of 31557600). Well-behaved clients integer seconds, maximum value of 31557600). Well-behaved clients
SHOULD cache a policy for up to this value from last policy fetch 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 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 is expected that this value typically be in the range of weeks or
greater. greater.
o "mx": MX identity patterns (list of plain-text strings). One or o "mx": Allowed MX patterns. One or more patterns matching allowed
more patterns matching a Common Name or Subject Alternative Name MX hosts for the Policy Domain. As an example,
([RFC5280]) DNS-ID ([RFC6125]) present in the X.509 certificate
presented by any MX receiving mail for this domain. For example:
mx: mail.example.com <CRLF> mx: mail.example.com <CRLF>
mx: .example.net mx: *.example.net
indicates that mail for this domain might be handled by any MX with a indicates that mail for this domain might be handled by MX
certificate valid for a host at "mail.example.com" or "example.net". "mail.example.com" or any MX at "example.net". Valid patterns can be
Valid patterns can be either fully specified names ("example.com") or either fully specified names ("example.com") or suffixes prefixed by
suffixes (".example.net") matching the right-hand parts of a server's a wildcard ("*.example.net"). If a policy specifies more than one
identity; the latter case are distinguished by a leading period. If MX, each MX MUST have its own "mx:" key, and each MX key/value pair
there are more than one MX specified by the policy, they MUST be on MUST be on its own line in the policy file. In the case of
separate lines within the policy file. In the case of Internationalized Domain Names ([RFC5891]), the "mx" value MUST
Internationalized Domain Names ([RFC5891]), the MX MUST specify the specify the Punycode-encoded A-label [RFC3492] to match against, and
Punycode-encoded A-label [RFC3492] and not the Unicode-encoded not the Unicode-encoded U-label. The full semantics of certificate
U-label. The full semantics of certificate validation are described validation (including the use of wildcard patterns) are described in
in Section 4.1, "MX Certificate Validation." Section 4.1, "MX Host Validation."
An example policy is as below: An example policy is as below:
version: STSv1 version: STSv1
mode: enforce mode: enforce
mx: mail.example.com mx: mail.example.com
mx: .example.net mx: *.example.net
mx: backupmx.example.com mx: backupmx.example.com
max_age: 123456 max_age: 604800
The formal definition of the policy resource, defined using The formal definition of the policy resource, defined using
[RFC7405], is as follows: [RFC7405], is as follows:
sts-policy-record = sts-policy-field *WSP sts-policy-record = sts-policy-field *WSP
*(sts-policy-term sts-policy-field *WSP) *(CRLF sts-policy-field *WSP)
[sts-policy-term] [CRLF]
sts-policy-field = sts-policy-version / ; required once sts-policy-field = sts-policy-version / ; required once
sts-policy-mode / ; required once sts-policy-mode / ; required once
sts-policy-max-age / ; required once sts-policy-max-age / ; required once
sts-policy-mx / 0*(sts-policy-mx *WSP CRLF) /
; required at least once, except when ; required at least once, except when
; mode is "none" ; mode is "none"
sts-policy-extension ; other fields sts-policy-extension ; other fields
field-delim = ":" *WSP field-delim = ":" *WSP
sts-policy-version = sts-policy-version-field field-delim sts-policy-version = sts-policy-version-field field-delim
sts-policy-version-value sts-policy-version-value
sts-policy-version-field = %s"version" sts-policy-version-field = %s"version"
sts-policy-version-value = %s"STSv1" sts-policy-version-value = %s"STSv1"
sts-policy-mode = sts-policy-mode-field field-delim sts-policy-mode = sts-policy-mode-field field-delim
sts-policy-mode-value sts-policy-mode-value
sts-policy-mode-field = %s"mode" sts-policy-mode-field = %s"mode"
sts-policy-mode-value = %s"testing" / %s"enforce" / %s"none" sts-policy-mode-value = %s"testing" / %s"enforce" / %s"none"
sts-policy-mx = sts-policy-mx-field field-delim sts-policy-mx = sts-policy-mx-field field-delim
sts-policy-mx-value sts-policy-mx-value
sts-policy-mx-field = %s"mx" sts-policy-mx-field = %s"mx"
sts-policy-mx-value = 1*(ALPHA / DIGIT / "_" / "-" / ".") sts-policy-mx-value = ["*."] *(sts-policy-mx-label ".")
sts-policy-mx-toplabel
sts-policy-max-age = sts-policy-max-age-field field-delim sts-policy-mx-label = sts-policy-alphanum |
sts-policy-max-age-value sts-policy-alphanum *(sts-policy-alphanum | "-")
sts-policy-alphanum
sts-policy-max-age-field = %s"max_age" sts-policy-mx-toplabel = ALPHA | ALPHA *(sts-policy-alphanum | "-")
sts-policy-alphanum
sts-policy-max-age-value = 1*10(DIGIT) sts-policy-max-age = sts-policy-max-age-field field-delim
sts-policy-max-age-value
sts-policy-extension = sts-policy-ext-name ; additional sts-policy-max-age-field = %s"max_age"
field-delim ; extension
sts-policy-ext-value ; fields
sts-policy-ext-name = (ALPHA / DIGIT) sts-policy-max-age-value = 1*10(DIGIT)
*31(ALPHA / DIGIT / "_" / "-" / ".")
sts-policy-term = CRLF / LF sts-policy-extension = sts-policy-ext-name ; additional
field-delim ; extension
sts-policy-ext-value ; fields
sts-policy-ext-value = sts-policy-vchar sts-policy-ext-name = (sts-policy-alphanum)
[*(%x20 / sts-policy-vchar) *31(sta-policy-alphanum / "_" / "-" / ".")
sts-policy-vchar]
; chars, including UTF-8 [RFC3629],
; excluding CTLs and no
; leading/trailing spaces
sts-policy-vchar = %x21-7E / UTF8-2 / UTF8-3 / UTF8-4 sts-policy-term = CRLF / LF
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
Parsers MUST accept TXT records and policy files which are Parsers MUST accept TXT records and policy files which are
syntactically valid (i.e. valid key/value pairs separated by semi- syntactically valid (i.e., valid key/value pairs separated by semi-
colons for TXT records) and but containing additional key/value pairs colons for TXT records) and but containing additional key/value pairs
not specified in this document, in which case unknown fields SHALL be not specified in this document, in which case unknown fields SHALL be
ignored. If any non-repeated field--i.e. all fields excepting "mx"-- ignored. If any non-repeated field--i.e., all fields excepting "mx"
is duplicated, all entries except for the first SHALL be ignored. If --is duplicated, all entries except for the first SHALL be ignored.
any field is not specified, the policy SHALL be treated as invalid. If any field is not specified, the policy SHALL be treated as
invalid.
3.3. HTTPS Policy Fetching 3.3. HTTPS Policy Fetching
When fetching a new policy or updating a policy, the HTTPS endpoint 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" MUST present a X.509 certificate which is valid for the "mta-sts"
host (e.g. "mta-sts.example.com") as described below, chain to a DNS-ID ([RFC6125]) (e.g., "mta-sts.example.com") as described below,
root CA that is trusted by the sending MTA, and be non-expired. It chain to a root CA that is trusted by the sending MTA, and be non-
is expected that sending MTAs use a set of trusted CAs similar to expired. It is expected that sending MTAs use a set of trusted CAs
those in widely deployed Web browsers and operating systems. See similar to those in widely deployed Web browsers and operating
[RFC5280] for more details about certificate verification. systems. See [RFC5280] for more details about certificate
verification.
The certificate is valid for the "mta-sts" host with respect to the The certificate is valid for the Policy Host (i.e., "mta-sts"
rules described in [RFC6125], with the following application-specific prepended to the Policy Domain) with respect to the rules described
considerations: in [RFC6125], with the following application-specific considerations:
o Matching is performed only against the DNS-ID identifiers. o Matching is performed only against the DNS-ID identifiers.
o DNS domain names in server certificates MAY contain the wildcard o DNS domain names in server certificates MAY contain the wildcard
character '*' as the complete left-most label within the character '*' as the complete left-most label within the
identifier. identifier.
The certificate MAY be checked for revocation via the Online The certificate MAY be checked for revocation via the Online
Certificate Status Protocol (OCSP) [RFC6960], certificate revocation Certificate Status Protocol (OCSP) [RFC6960], certificate revocation
lists (CRLs), or some other mechanism. lists (CRLs), or some other mechanism.
Policies fetched via HTTPS are only valid if the HTTP response code Policies fetched via HTTPS are only valid if the HTTP response code
is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP
caching (as specified in [RFC7234]) MUST NOT be used. caching (as specified in [RFC7234]) MUST NOT be used.
Senders may wish to rate-limit the frequency of attempts to fetch the 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 HTTPS endpoint even if a valid TXT record for the recipient domain
exists. In the case that the HTTPS GET fails, we suggest exists. In the case that the HTTPS GET fails, we implementions
implementions may limit further attempts to a period of five minutes SHOULD limit further attempts to a period of five minutes or longer
or longer per version ID, to avoid overwhelming resource-constrained per version ID, to avoid overwhelming resource-constrained recipients
recipients with cascading failures. with cascading failures.
Senders MAY impose a timeout on the HTTPS GET and/or a limit on the 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 maximum size of the response body to avoid long delays or resource
exhaustion during attempted policy updates. A suggested timeout is exhaustion during attempted policy updates. A suggested timeout is
one minute, and a suggested maximum policy size 64 kilobytes; policy one minute, and a suggested maximum policy size 64 kilobytes; policy
hosts SHOULD respond to requests with a complete policy body within hosts SHOULD respond to requests with a complete policy body within
that timeout and size limit. that timeout and size limit.
If a valid TXT record is found but no policy can be fetched via HTTPS 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- (for any reason), and there is no valid (non-expired) previously-
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When sending mail to a mailbox at a subdomain, compliant senders MUST 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 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 sent to "user@mail.example.com", the policy can be fetched only from
"mail.example.com", not "example.com". "mail.example.com", not "example.com".
4. Policy Validation 4. Policy Validation
When sending to an MX at a domain for which the sender has a valid 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 and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST
validate: check whether:
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 1. At least one of the policy's "mx" patterns matches the selected
policies fail to validate; see Section 5, "Policy Application" for a MX host, as described in Section 4.1, "MX Host Validation".
description of sending MTA behavior when policy validation fails.
4.1. MX Certificate 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 Section 4.2, "Recipient MTA
Certificate Validation".
The certificate presented by the receiving MX MUST chain to a root CA When these conditions are not met, a policy is said to fail to
that is trusted by the sending MTA and be non-expired. The validate. This section does not dictate the behavior of sending MTAs
certificate MUST have a subject alternative name (SAN, [RFC5280]) when the above conditions are not met; see Section 5, "Policy
with a DNS-ID ([RFC6125]) matching the "mx" pattern. The MX's Application" for a description of sending MTA behavior when policy
certificate MAY also be checked for revocation via OCSP [RFC6960], validation fails.
CRLs [RFC6818], or some other mechanism.
Because the "mx" patterns are not hostnames, however, matching is not 4.1. MX Host Validation
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 ".example.com".
In this case, if the MX server's X.509 certificate contains a SAN
matching "*.example.com", we are required to implement "wildcard-to-
wildcard" matching.
To simplify this case, we impose the following constraints on A receiving candidate MX host is valid according to an applied MTA-
wildcard certificates, identical to those in [RFC7672] section 3.2.3 STS policy if the MX record name matches one or more of the "mx"
and [RFC6125] section 6.4.3: wildcards are valid in DNS-IDs, but must fields in the applied policy. Matching is identical to the rules
be the entire first label of the identifier (that is, given in [RFC6125], with restriction that the wildcard character "*"
"*.example.com", not "mail*.example.com"). Senders who are comparing may only be used to match the entire left-most label in the presented
a "suffix" MX pattern with a wildcard identifier should thus strip identifier. Thus the mx pattern "*.example.com" matches
the wildcard and ensure that the two sides match label-by-label, "mail.example.com" but not "example.com" or "foo.bar.example.com".
until all labels of the shorter side (if unequal length) are
consumed.
Note that a wildcard must match a label; an "mx" pattern of 4.2. Recipient MTA Certificate Validation
".example.com" thus does not match a SAN of "example.com", nor does a
SAN of "*.example.com" match an "mx" of "example.com".
A simple pseudocode implementation of this algorithm is presented in The certificate presented by the receiving MTA MUST chain to a root
Appendix B. CA that is trusted by the sending MTA and be non-expired. 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 5. Policy Application
When sending to an MX at a domain for which the sender has a valid, 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 non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies
the result of a policy validation failure one of two ways, depending the result of a policy validation failure one of two ways, depending
on the value of the policy "mode" field: on the value of the policy "mode" field:
1. "enforce": In this mode, sending MTAs MUST NOT deliver the 1. "enforce": In this mode, sending MTAs MUST NOT deliver the
message to hosts which fail MX matching or certificate message to hosts which fail MX matching or certificate
validation. validation, or do not support STARTTLS.
2. "testing": In this mode, sending MTAs which also implement the 2. "testing": In this mode, sending MTAs which also implement the
TLSRPT specification [I-D.ietf-uta-smtp-tlsrpt] merely send a TLSRPT specification [I-D.ietf-uta-smtp-tlsrpt] merely send a
report indicating policy application failures (so long as TLSRPT report indicating policy application failures (so long as TLSRPT
is also implemented by the recipient domain). is also implemented by the recipient domain).
3. "none": In this mode, sending MTAs should treat the policy domain 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, as though it does not have any active policy; see Section 8.3,
"Removing MTA-STS", for use of this mode value. "Removing MTA-STS", for use of this mode value.
When a message fails to deliver due to an "enforce" policy, a When a message fails to deliver due to an "enforce" policy, a
compliant MTA MUST NOT permanently fail to deliver messages before compliant MTA MUST NOT permanently fail to deliver messages before
checking for the presence of an updated policy at the Policy Domain. checking, via DNS, for the presence of an updated policy at the
(In all cases, MTAs SHOULD treat such failures as transient errors Policy Domain. (In all cases, MTAs SHOULD treat such failures as
and retry delivery later.) This allows implementing domains to transient errors and retry delivery later.) This allows implementing
update long-lived policies on the fly. domains to update long-lived policies on the fly.
5.1. Policy Application Control Flow 5.1. Policy Application Control Flow
An example control flow for a compliant sender consists of the An example control flow for a compliant sender consists of the
following steps: following steps:
1. Check for a cached policy whose time-since-fetch has not exceeded 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 its "max_age". If none exists, attempt to fetch a new policy
(perhaps asynchronously, so as not to block message delivery). (perhaps asynchronously, so as not to block message delivery).
Optionally, sending MTAs may unconditionally check for a new Optionally, sending MTAs may unconditionally check for a new
policy at this step. policy at this step.
2. For each candidate MX, in order of MX priority, attempt to 2. For each candidate MX, in order of MX priority, attempt to
deliver the message, enforcing STARTTLS and, assuming a policy is deliver the message. If a policy is present with an "enforce"
present, PKIX certificate validation as described in Section 4.1, mode, when attempting to deliver to each candidate MX, ensure
"MX Certificate Validation." 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 3. A message delivery MUST NOT be permanently failed until the
sender has first checked for the presence of a new policy (as 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 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 new policy is not found, existing rules for the case of temporary
message delivery failures apply (as discussed in [RFC5321] message delivery failures apply (as discussed in [RFC5321]
section 4.5.4.1). section 4.5.4.1).
6. Reporting Failures 6. Reporting Failures
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according to the applied policy, except if that policy's mode is according to the applied policy, except if that policy's mode is
"none". "none".
7. Interoperability Considerations 7. Interoperability Considerations
7.1. SNI Support 7.1. SNI Support
To ensure that the server sends the right certificate chain, the SMTP To ensure that the server sends the right certificate chain, the SMTP
client MUST have support for the TLS SNI extension [RFC6066]. When client MUST have support for the TLS SNI extension [RFC6066]. When
connecting to a HTTP server to retrieve the MTA-STS policy, the SNI 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- extension MUST contain the name of the policy host (e.g., "mta-
sts.example.com"). When connecting to an SMTP server, the SNI sts.example.com"). When connecting to an SMTP server, the SNI
extension MUST contain the MX hostname. extension MUST contain the MX hostname.
HTTP servers used to deliver MTA-STS policies MAY rely on SNI to HTTP servers used to deliver MTA-STS policies MAY rely on SNI to
determine which certificate chain to present to the client. HTTP determine which certificate chain to present to the client. HTTP
servers MUST respond with a certificate chain that matches the policy servers MUST respond with a certificate chain that matches the policy
hostname or abort the TLS handshake if unable to do so. Clients that hostname or abort the TLS handshake if unable to do so. Clients that
do not send SNI information may not see the expected certificate do not send SNI information may not see the expected certificate
chain. chain.
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Updating the policy requires that the owner make changes in two 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 places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and
at the corresponding HTTPS endpoint. As a result, recipients should at the corresponding HTTPS endpoint. As a result, recipients should
expect a policy will continue to be used by senders until both the 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 HTTPS and TXT endpoints are updated and the TXT record's TTL has
passed. passed.
In other words, a sender who is unable to successfully deliver a In other words, a sender who is unable to successfully deliver a
message while applying a cache of the recipient's now-outdated policy 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 may be unable to discover that a new policy exists until the DNS TTL
has passed. Recipients should therefore ensure that old policies has passed. Recipients SHOULD therefore ensure that old policies
continue to work for message delivery during this period of time, or continue to work for message delivery during this period of time, or
risk message delays. risk message delays.
Recipients should also prefer to update the HTTPS policy body before Recipients SHOULD also update the HTTPS policy body before updating
updating the TXT record; this ordering avoids the risk that senders, the TXT record; this ordering avoids the risk that senders, seeing a
seeing a new TXT record, mistakenly cache the old policy from HTTPS. new TXT record, mistakenly cache the old policy from HTTPS.
8.2. Policy Delegation 8.2. Policy Delegation
Domain owners commonly delegate SMTP hosting to a different Domain owners commonly delegate SMTP hosting to a different
organization, such as an ISP or a Web host. In such a case, they may 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 wish to also delegate the MTA-STS policy to the same organization
which can be accomplished with two changes. which can be accomplished with two changes.
First, the Policy Domain must point the "_mta-sts" record, via CNAME, First, the Policy Domain must point the "_mta-sts" record, via CNAME,
to the "_mta-sts" record maintained by the hosting organization. to the "_mta-sts" record maintained by the hosting organization.
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For example, given a user domain "user.example" hosted by a mail For example, given a user domain "user.example" hosted by a mail
provider "provider.example", the following configuration would allow provider "provider.example", the following configuration would allow
policy delegation: policy delegation:
DNS: DNS:
_mta-sts.user.example. IN CNAME _mta-sts.provider.example. _mta-sts.user.example. IN CNAME _mta-sts.provider.example.
Policy: Policy:
> GET /.well-known/mta-sts.txt > GET /.well-known/mta-sts.txt Host: mta-sts.user.example
> Host: mta-sts.user.example < HTTP/1.1 200 OK # Response proxies content from
< HTTP/1.1 200 OK # Response proxies content from # https://mta-sts.provider.example
# 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 Domain ("user.example"),
and not for that of the provider ("provider.example").
Note that while sending MTAs MUST NOT use HTTP caching when fetching Note that while sending MTAs MUST NOT use HTTP caching when fetching
policies via HTTPS, such caching may nonetheless be useful to a policies via HTTPS, such caching may nonetheless be useful to a
reverse proxy configured as described in this section. An HTTPS reverse proxy configured as described in this section. An HTTPS
policy endpoint expecting to be proxied for multiple hosted domains-- policy endpoint expecting to be proxied for multiple hosted domains--
as with a large mail hosting provider or similar--may wish to as with a large mail hosting provider or similar--may wish to
indicate an HTTP Cache-Control "max-age" response directive (as indicate an HTTP Cache-Control "max-age" response directive (as
specified in [RFC7234]) of 60 seconds as a reasonable value to save specified in [RFC7234]) of 60 seconds as a reasonable value to save
reverse proxies an unnecessarily high-rate of proxied policy reverse proxies an unnecessarily high-rate of proxied policy
fetching. fetching.
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policy domains, and to distinguish clearly between failures which policy domains, and to distinguish clearly between failures which
indicate attacks and those which indicate such opt-outs, MTA-STS indicate attacks and those which indicate such opt-outs, MTA-STS
implements the "none" mode, which allows validated policies to implements the "none" mode, which allows validated policies to
indicate authoritatively that the policy domain wishes to no longer indicate authoritatively that the policy domain wishes to no longer
implement MTA-STS and may, in the future, remove the MTA-STS TXT and implement MTA-STS and may, in the future, remove the MTA-STS TXT and
policy endpoints entirely. policy endpoints entirely.
A suggested workflow to implement such an opt out is as follows: 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 1. Publish a new policy with "mode" equal to "none" and a small
"max_age" (e.g. one day). "max_age" (e.g., one day).
2. Publish a new TXT record to trigger fetching of the new policy. 2. Publish a new TXT record to trigger fetching of the new policy.
3. When all previously served policies have expired--normally this 3. When all previously served policies have expired--normally this
is the time the previously published policy was last served plus is the time the previously published policy was last served plus
that policy's "max_age", but note that older policies may have that policy's "max_age", but note that older policies may have
been served with a greater "max_age", allowing overlapping policy been served with a greater "max_age", allowing overlapping policy
caches--safely remove the TXT record and HTTPS endpoint. caches--safely remove the TXT record and HTTPS endpoint.
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. IANA Considerations
9.1. Well-Known URIs Registry 9.1. Well-Known URIs Registry
A new "well-known" URI as described in Section 3 will be registered A new "well-known" URI as described in Section 3 will be registered
in the Well-Known URIs registry as described below: in the Well-Known URIs registry as described below:
URI Suffix: mta-sts.txt Change Controller: IETF URI Suffix: mta-sts.txt Change Controller: IETF
9.2. MTA-STS TXT Record Fields 9.2. MTA-STS TXT Record Fields
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| max_age | Policy lifetime | 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 | | mx | MX identities | Section 3.2 of RFC XXX |
+------------+----------------------+------------------------+ +------------+----------------------+------------------------+
New fields are added to this registry using IANA's "Expert Review" New fields are added to this registry using IANA's "Expert Review"
policy. policy.
10. Security Considerations 10. Security Considerations
SMTP MTA Strict Transport Security attempts to protect against an SMTP MTA Strict Transport Security attempts to protect against an
active attacker who wishes to intercept or tamper with mail between active attacker trying to intercept or tamper with mail between hosts
hosts who support STARTTLS. There are two classes of attacks that support STARTTLS. There are two classes of attacks considered:
considered:
o Foiling TLS negotiation, for example by deleting the "250 o Foiling TLS negotiation, for example by deleting the "250
STARTTLS" response from a server or altering TLS session STARTTLS" response from a server or altering TLS session
negotiation. This would result in the SMTP session occurring over negotiation. This would result in the SMTP session occurring over
plaintext, despite both parties supporting TLS. plaintext, despite both parties supporting TLS.
o Impersonating the destination mail server, whereby the sender o Impersonating the destination mail server, whereby the sender
might deliver the message to an impostor, who could then monitor might deliver the message to an impostor, who could then monitor
and/or modify messages despite opportunistic TLS. This and/or modify messages despite opportunistic TLS. This
impersonation could be accomplished by spoofing the DNS MX record impersonation could be accomplished by spoofing the DNS MX record
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MTA-STS can thwart such attacks only if the sender is able to MTA-STS can thwart such attacks only if the sender is able to
previously obtain and cache a policy for the recipient domain, and previously obtain and cache a policy for the recipient domain, and
only if the attacker is unable to obtain a valid certificate that only if the attacker is unable to obtain a valid certificate that
complies with that policy. Below, we consider specific attacks on complies with that policy. Below, we consider specific attacks on
this model. this model.
10.1. Obtaining a Signed Certificate 10.1. Obtaining a Signed Certificate
SMTP MTA-STS relies on certificate validation via PKIX based TLS SMTP MTA-STS relies on certificate validation via PKIX based TLS
identity checking [RFC6125]. Attackers who are able to obtain a identity checking [RFC6125]. Attackers who are able to obtain a
valid certificate for the targeted recipient mail service (e.g. by valid certificate for the targeted recipient mail service (e.g., by
compromising a certificate authority) are thus able to circumvent STS compromising a certificate authority) are thus able to circumvent STS
authentication. authentication.
10.2. Preventing Policy Discovery 10.2. Preventing Policy Discovery
Since MTA-STS uses DNS TXT records for policy discovery, an attacker 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 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- MTA-STS Policy, making the Policy Domain appear not to have an MTA-
STS Policy. The sender policy cache is designed to resist this STS Policy. The sender policy cache is designed to resist this
attack by decreasing the frequency of policy discovery and thus attack by decreasing the frequency of policy discovery and thus
reducing the window of vulnerability; it is nonetheless a risk that reducing the window of vulnerability; it is nonetheless a risk that
attackers who can predict or induce policy discovery--for example, by attackers who can predict or induce policy discovery--for example, by
inducing a sending domain to send mail to a never-before-contacted 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 recipient while carrying out a man-in-the-middle attack--may be able
to foil policy discovery and effectively downgrade the security of to foil policy discovery and effectively downgrade the security of
the message delivery. the message delivery.
Since this attack depends upon intercepting initial policy discovery, Since this attack depends upon intercepting initial policy discovery,
we strongly recommend implementers to prefer policy "max_age" values implementers SHOULD prefer policy "max_age" values to be as long as
to be as long as is practical. is practical.
Because this attack is also possible upon refresh of a cached policy, Because this attack is also possible upon refresh of a cached policy,
we suggest implementers do not wait until a cached policy has expired implementors SHOULD NOT wait until a cached policy has expired before
before checking for an update; if senders attempt to refresh the checking for an update; if senders attempt to refresh the cache
cache regularly (for instance, by checking their cached version regularly (for example, by fetching currently live policy in a
string against the TXT record on each successful send, or in a background task that runs daily or weekly, regardless of the state of
background task that runs daily or weekly), an attacker would have to the "_mta_sts" TXT record, and updating their cache's "max age"
foil policy discovery consistently over the lifetime of a cached accordingly), an attacker would have to foil policy discovery
policy to prevent a successful refresh. consistently over the lifetime of a cached policy to prevent a
successful refresh.
Additionally, MTAs should alert administrators to repeated policy Additionally, MTAs SHOULD alert administrators to repeated policy
refresh failures long before cached policies expire (through warning refresh failures long before cached policies expire (through warning
logs or similar applicable mechanisms), allowing administrators to logs or similar applicable mechanisms), allowing administrators to
detect such a persistent attack on policy refresh. (However, they detect such a persistent attack on policy refresh. (However, they
should not implement such alerts if the cached policy has a "none" 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.) 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 Resistance to downgrade attacks of this nature--due to the ability to
authoritatively determine "lack of a record" even for non- authoritatively determine "lack of a record" even for non-
participating recipients--is a feature of DANE, due to its use of participating recipients--is a feature of DANE, due to its use of
DNSSEC for policy discovery. DNSSEC for policy discovery.
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the MX records. the MX records.
This attack is mitigated in part by the ability of a victim domain to 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 (at any time) publish a new policy updating the cached, malicious
policy, though this does require the victim domain to both obtain a policy, though this does require the victim domain to both obtain a
valid CA-signed certificate and to understand and properly configure valid CA-signed certificate and to understand and properly configure
MTA-STS. MTA-STS.
Similarly, we consider the possibility of domains that deliberately Similarly, we consider the possibility of domains that deliberately
allow untrusted users to serve untrusted content on user-specified allow untrusted users to serve untrusted content on user-specified
subdomains. In some cases (e.g. the service Tumblr.com) this takes subdomains. In some cases (e.g., the service Tumblr.com) this takes
the form of providing HTTPS hosting of user-registered subdomains; in the form of providing HTTPS hosting of user-registered subdomains; in
other cases (e.g. dynamic DNS providers) this takes the form of other cases (e.g. dynamic DNS providers) this takes the form of
allowing untrusted users to register custom DNS records at the allowing untrusted users to register custom DNS records at the
provider's domain. provider's domain.
In these cases, there is a risk that untrusted users would be able to 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 serve custom content at the "mta-sts" host, including serving an
illegitimate MTA-STS policy. We believe this attack is rendered more illegitimate MTA-STS policy. We believe this attack is rendered more
difficult by the need for the attacker to also serve the "_mta-sts" difficult by the need for the attacker to also serve the "_mta-sts"
TXT record on the same domain--something not, to our knowledge, TXT record on the same domain--something not, to our knowledge,
widely provided to untrusted users. This attack is additionally widely provided to untrusted users. This attack is additionally
mitigated by the aforementioned ability for a victim domain to update mitigated by the aforementioned ability for a victim domain to update
an invalid policy at any future date. an invalid policy at any future date.
10.4. Weak Policy Constraints 10.4. Weak Policy Constraints
Even if an attacker cannot modify a served policy, the potential Even if an attacker cannot modify a served policy, the potential
exists for configurations that allow attackers on the same domain to exists for configurations that allow attackers on the same domain to
receive mail for that domain. For example, an easy configuration receive mail for that domain. For example, an easy configuration
option when authoring an MTA-STS Policy for "example.com" is to set option when authoring an MTA-STS Policy for "example.com" is to set
the "mx" equal to ".example.com"; recipient domains must consider in the "mx" equal to "*.example.com"; recipient domains must consider in
this case the risk that any user possessing a valid hostname and CA- this case the risk that any user possessing a valid hostname and CA-
signed certificate (for example, "dhcp-123.example.com") will, from signed certificate (for example, "dhcp-123.example.com") will, from
the perspective of MTA-STS Policy validation, be a valid MX host for the perspective of MTA-STS Policy validation, be a valid MX host for
that domain. that domain.
10.5. Compromise of the Web PKI System 10.5. Compromise of the Web PKI System
A host of risks apply to the PKI system used for certificate A host of risks apply to the PKI system used for certificate
authentication, both of the "mta-sts" HTTPS host's certificate and authentication, both of the "mta-sts" HTTPS host's certificate and
the SMTP servers' certificates. These risks are broadly applicable the SMTP servers' certificates. These risks are broadly applicable
within the Web PKI ecosystem and are not specific to MTA-STS; within the Web PKI ecosystem and are not specific to MTA-STS;
nonetheless, they deserve some consideration in this context. nonetheless, they deserve some consideration in this context.
Broadly speaking, attackers may compromise the system by obtaining Broadly speaking, attackers may compromise the system by obtaining
certificates under fraudulent circumstances (i.e. by impersonating certificates under fraudulent circumstances (i.e., by impersonating
the legitimate owner of the victim domain), by compromising a the legitimate owner of the victim domain), by compromising a
Certificate Authority or Delegate Authority's private keys, by Certificate Authority or Delegate Authority's private keys, by
obtaining a legitimate certificate issued to the victim domain, and obtaining a legitimate certificate issued to the victim domain, and
similar. similar.
One approach commonly employed by Web browsers to help mitigate One approach commonly employed by Web browsers to help mitigate
against some of these attacks is to allow for revocation of against some of these attacks is to allow for revocation of
compromised or fraudulent certificates via OCSP [RFC6960] or CRLs compromised or fraudulent certificates via OCSP [RFC6960] or CRLs
[RFC6818]. Such mechanisms themselves represent tradeoffs and are [RFC6818]. Such mechanisms themselves represent tradeoffs and are
not universally implemented; we nonetheless recommend implementors of not universally implemented; we nonetheless recommend implementors of
MTA-STS to implement revocation mechanisms which are most applicable MTA-STS to implement revocation mechanisms which are most applicable
to their implementations. to their implementations.
11. Contributors 11. Contributors
Wei Chuang Google, Inc weihaw (at) google (dot com) Wei Chuang Google, Inc weihaw@google.com
Viktor Dukhovni ietf-dane (at) dukhovni (dot org) Viktor Dukhovni ietf-dane@dukhovni.de
Markus Laber 1&1 Mail & Media Development & Technology GmbH Markus Laber 1&1 Mail & Media Development & Technology GmbH
markus.laber (at) 1und1 (dot de) markus.laber@1und1.de
Nicolas Lidzborski Google, Inc nlidz (at) google (dot com) Nicolas Lidzborski Google, Inc nlidz@google.com
Brandon Long Google, Inc blong (at) google (dot com) Brandon Long Google, Inc blong@google.com
Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com) Franck Martin LinkedIn, Inc fmartin@linkedin.com
Klaus Umbach 1&1 Mail & Media Development & Technology GmbH Klaus Umbach 1&1 Mail & Media Development & Technology GmbH
klaus.umbach (at) 1und1 (dot de) klaus.umbach@1und1.de
12. References 12. References
12.1. Normative References 12.1. Normative References
[I-D.ietf-uta-smtp-tlsrpt] [I-D.ietf-uta-smtp-tlsrpt]
Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J., Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J.,
and M. Risher, "SMTP TLS Reporting", draft-ietf-uta-smtp- and M. Risher, "SMTP TLS Reporting", draft-ietf-uta-smtp-
tlsrpt-18 (work in progress), April 2018. tlsrpt-20 (work in progress), May 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc- DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>. 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 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>. February 2002, <https://www.rfc-editor.org/info/rfc3207>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<https://www.rfc-editor.org/info/rfc3492>. <https://www.rfc-editor.org/info/rfc3492>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
skipping to change at page 22, line 10 skipping to change at page 23, line 26
_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;" _mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"
MTA-STS Policy file served as the response body at "https://mta- MTA-STS Policy file served as the response body at "https://mta-
sts.example.com/.well-known/mta-sts.txt": sts.example.com/.well-known/mta-sts.txt":
version: STSv1 version: STSv1
mode: testing mode: testing
mx: mx1.example.com mx: mx1.example.com
mx: mx2.example.com mx: mx2.example.com
mx: mx.backup-example.com mx: mx.backup-example.com
max_age: 12345678 max_age: 1296000
Appendix B. Message delivery pseudocode Appendix B. Message delivery pseudocode
Below is pseudocode demonstrating the logic of a compliant sending Below is pseudocode demonstrating the logic of a compliant sending
MTA. MTA.
While this pseudocode implementation suggests synchronous policy While this pseudocode implementation suggests synchronous policy
retrieval in the delivery path, in a working implementation that may retrieval in the delivery path, in a working implementation that may
be undesirable, and we expect some implementers to instead prefer a be undesirable, and we expect some implementers to instead prefer a
background fetch that does not block delivery if no cached policy is background fetch that does not block delivery if no cached policy is
skipping to change at page 22, line 35 skipping to change at page 23, line 51
} }
func isNonExpired(policy) { func isNonExpired(policy) {
// Return true if the policy is not expired. // Return true if the policy is not expired.
} }
func tryStartTls(connection) { func tryStartTls(connection) {
// Attempt to open an SMTP connection with STARTTLS with the MX. // Attempt to open an SMTP connection with STARTTLS with the MX.
} }
func isWildcardMatch(pat, host) { func certMatches(connection, host) {
// Literal matches are true. // Assume a handy function to return check if the server certificate presented
if pat == host { // in "connection" is valid for "host".
return true
}
// Leading '.' matches a wildcard against the first part, i.e.
// .example.com matches x.example.com but not x.y.example.com.
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) { func policyMatches(candidate, policy) {
// Assume a handy function to return DNS-ID SANs. for mx in policy.mx {
for san in getDnsIdSansFromCert(connection) { // Literal match.
for mx in policy.mx { if mx == candidate {
// Return if the server certificate from "connection" matches the return true
// "mx" host. }
if san[0] == '*' { // Wildcard matches only the leftmost label.
// Invalid wildcard! // Wildcards must always be followed by a '.'.
if san[1] != '.' continue if mx[0] == '*' {
san = san[1:] parts = SplitN(candidate, '.', 2) // Split on the first '.'.
} if len(parts) > 1 && parts[1] == mx[2:] {
if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) {
return true return true
} }
} }
} }
return false return false
} }
func tryDeliverMail(connection, message) { func tryDeliverMail(connection, message) {
// Attempt to deliver "message" via "connection". // Attempt to deliver "message" via "connection".
} }
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func reportError(error) { func reportError(error) {
// Report an error via TLSRPT. // Report an error via TLSRPT.
} }
func tryMxAccordingTo(message, mx, policy) { func tryMxAccordingTo(message, mx, policy) {
connection := connect(mx) connection := connect(mx)
if !connection { if !connection {
return false // Can't connect to the MX so it's not an MTA-STS return false // Can't connect to the MX so it's not an MTA-STS
// error. // error.
} }
secure := true secure := true
if !tryStartTls(connection) { if !policyMatches(mx, policy) {
secure = false
reportError(E_HOST_MISMATCH)
} else if !tryStartTls(connection) {
secure = false secure = false
reportError(E_NO_VALID_TLS) reportError(E_NO_VALID_TLS)
} else if !certMatches(connection, policy) { } else if !certMatches(connection, policy) {
secure = false secure = false
reportError(E_CERT_MISMATCH) reportError(E_CERT_MISMATCH)
} }
if secure || !isEnforce(policy) { if secure || !isEnforce(policy) {
return tryDeliverMail(connection, message) return tryDeliverMail(connection, message)
} }
return false return false
skipping to change at page 24, line 36 skipping to change at page 25, line 44
domain := ... // domain part after '@' from recipient domain := ... // domain part after '@' from recipient
policy := tryGetNewPolicy(domain) policy := tryGetNewPolicy(domain)
if policy { if policy {
cachePolicy(domain, policy) cachePolicy(domain, policy)
} else { } else {
policy = tryGetCachedPolicy(domain) policy = tryGetCachedPolicy(domain)
} }
if policy { if policy {
return tryWithPolicy(message, domain, policy) return tryWithPolicy(message, domain, policy)
} }
// Try to deliver the message normally (i.e. without MTA-STS). // Try to deliver the message normally (i.e., without MTA-STS).
} }
Authors' Addresses Authors' Addresses
Daniel Margolis Daniel Margolis
Google, Inc Google, Inc
Email: dmargolis (at) google (dot com) Email: dmargolis@google.com
Mark Risher Mark Risher
Google, Inc Google, Inc
Email: risher (at) google (dot com) Email: risher@google.com
Binu Ramakrishnan Binu Ramakrishnan
Yahoo!, Inc Yahoo!, Inc
Email: rbinu (at) yahoo-inc (dot com) Email: rbinu@yahoo-inc.com
Alexander Brotman Alexander Brotman
Comcast, Inc Comcast, Inc
Email: alex_brotman@comcast.com Email: alex_brotman@comcast.com
Janet Jones Janet Jones
Microsoft, Inc Microsoft, Inc
Email: janet.jones (at) microsoft (dot com) Email: janet.jones@microsoft.com
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