draft-ietf-uta-mta-sts-11.txt   draft-ietf-uta-mta-sts-12.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: May 12, 2018 B. Ramakrishnan Expires: June 7, 2018 B. Ramakrishnan
Yahoo!, Inc Yahoo!, Inc
A. Brotman A. Brotman
Comcast, Inc Comcast, Inc
J. Jones J. Jones
Microsoft, Inc Microsoft, Inc
November 8, 2017 December 4, 2017
SMTP MTA Strict Transport Security (MTA-STS) SMTP MTA Strict Transport Security (MTA-STS)
draft-ietf-uta-mta-sts-11 draft-ietf-uta-mta-sts-12
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
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
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 May 12, 2018. This Internet-Draft will expire on June 7, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 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 . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . . . . 5
3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 8 3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 8
3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 9 3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 9
3.5. MX Certificate Validation . . . . . . . . . . . . . . . . 10 4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 9
4. Policy Application . . . . . . . . . . . . . . . . . . . . . 10 4.1. MX Certificate Validation . . . . . . . . . . . . . . . . 10
4.1. Policy Application Control Flow . . . . . . . . . . . . . 11 5. Policy Application . . . . . . . . . . . . . . . . . . . . . 10
5. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 11 5.1. Policy Application Control Flow . . . . . . . . . . . . . 11
6. Interoperability Considerations . . . . . . . . . . . . . . . 12 6. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 11
6.1. SNI Support . . . . . . . . . . . . . . . . . . . . . . . 12 7. Interoperability Considerations . . . . . . . . . . . . . . . 12
6.2. Minimum TLS Version Support . . . . . . . . . . . . . . . 12 7.1. SNI Support . . . . . . . . . . . . . . . . . . . . . . . 12
7. Operational Considerations . . . . . . . . . . . . . . . . . 12 7.2. Minimum TLS Version Support . . . . . . . . . . . . . . . 12
7.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 13 8. Operational Considerations . . . . . . . . . . . . . . . . . 13
7.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 13 8.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 13
7.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 14 8.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 8.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 14
8.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 14 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 15 9.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 14
8.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 15 9.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 9.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 15
9.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 16 10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 16 10.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 16
9.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 17 10.2. Preventing Policy Discovery . . . . . . . . . . . . . . 16
9.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 18 10.3. Denial of Service . . . . . . . . . . . . . . . . . . . 17
9.5. Compromise of the Web PKI System . . . . . . . . . . . . 18 10.4. Weak Policy Constraints . . . . . . . . . . . . . . . . 18
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18 10.5. Compromise of the Web PKI System . . . . . . . . . . . . 18
11. Appendix 1: MTA-STS example record & policy . . . . . . . . . 19 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
12. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 19 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 12.1. Normative References . . . . . . . . . . . . . . . . . . 19
13.1. Normative References . . . . . . . . . . . . . . . . . . 22 12.2. Informative References . . . . . . . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . 23 Appendix A. MTA-STS example record & policy . . . . . . . . . . 21
13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Appendix B. Message delivery pseudocode . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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,
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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 9, "Security Considerations". Section 10, "Security Considerations".
In addition, MTA-STS provides an optional report-only mode, enabling In addition, MTA-STS provides an optional report-only mode, enabling
soft deployments to detect policy failures; partial deployments can soft deployments to detect policy failures; partial deployments can
be achieved in DANE by deploying TLSA records only for some of a 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 domain's MXs, but such a mechanism is not possible for the per-domain
policies used by MTA-STS. policies used by MTA-STS.
The primary motivation of MTA-STS is to provide a mechanism for The primary motivation of MTA-STS is to provide a mechanism for
domains to upgrade their transport security even when deploying domains to ensure transport security even when deploying DNSSEC is
DNSSEC is undesirable or impractical. However, MTA-STS is designed undesirable or impractical. However, MTA-STS is designed not to
not to interfere with DANE deployments when the two overlap; in interfere with DANE deployments when the two overlap; in particular,
particular, senders who implement MTA-STS validation MUST NOT allow a senders who implement MTA-STS validation MUST NOT allow a "valid" or
"valid" or "report-only" MTA-STS validation to override a failing "report-only" MTA-STS validation to override a failing DANE
DANE validation. validation.
3. Policy Discovery 3. Policy Discovery
MTA-STS policies are distributed via HTTPS from a "well-known" MTA-STS policies are distributed via HTTPS from a "well-known"
[RFC5785] path served within the Policy Domain, and their presence [RFC5785] path served within the Policy Domain, and their presence
and current version are indicated by a TXT record at the Policy and current version are indicated by a TXT record at the Policy
Domain. These TXT records additionally contain a policy "id" field, Domain. These TXT records additionally contain a policy "id" field,
allowing sending MTAs to check the currency of a cached policy allowing sending MTAs to check the currency of a cached policy
without performing an HTTPS request. without performing an HTTPS request.
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updated by comparing to the "id" of a previously seen policy. updated by comparing to the "id" of a previously seen policy.
There is no implied ordering of "id" fields between revisions. There is no implied ordering of "id" fields between revisions.
An example TXT record is as below: An example TXT record is as below:
"_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;"" "_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;""
The formal definition of the "_mta-sts" TXT record, defined using The formal definition of the "_mta-sts" TXT record, defined using
[RFC7405], is as follows: [RFC7405], is as follows:
sts-text-record = sts-version 1*(field-delim sts-field) [field-delim] sts-text-record = sts-version 1*(field-delim sts-field) [field-delim]
sts-field = sts-id / ; Note that sts-id record sts-field = sts-id / ; Note that sts-id record
sts-extension ; is required. sts-extension ; is required.
field-delim = *WSP ";" *WSP field-delim = *WSP ";" *WSP
sts-version = %s"v=STSv1" sts-version = %s"v=STSv1"
sts-id = %s"id=" 1*32(ALPHA / DIGIT) ; id=... sts-id = %s"id=" 1*32(ALPHA / DIGIT) ; id=...
sts-extension = sts-ext-name "=" sts-ext-value ; name=value sts-extension = sts-ext-name "=" sts-ext-value ; name=value
sts-ext-name = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".") sts-ext-name = (ALPHA / DIGIT)
*31(ALPHA / DIGIT / "_" / "-" / ".")
sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding "=", sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E)
; ";", SP, and control ; chars excluding "=", ";", SP, and control chars
; chars
If multiple TXT records for "_mta-sts" are returned by the resolver, If multiple TXT records for "_mta-sts" are returned by the resolver,
records which do not begin with "v=STSv1;" are discarded. If the records which do not begin with "v=STSv1;" are discarded. If the
number of resulting records is not one, senders MUST assume the number of resulting records is not one, senders MUST assume the
recipient domain does not implement MTA-STS and skip the remaining recipient domain does not implement MTA-STS and skip the remaining
steps of policy discovery. If the resulting TXT record contains steps of policy discovery. If the resulting TXT record contains
multiple strings, then the record MUST be treated as if those strings multiple strings, then the record MUST be treated as if those strings
are concatenated together without adding spaces. are concatenated together without adding spaces.
3.2. MTA-STS Policies 3.2. MTA-STS Policies
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the "mta-sts" host at the Policy Domain. Thus for "example.com" the the "mta-sts" host at the Policy Domain. Thus for "example.com" the
path is "https://mta-sts.example.com/.well-known/mta-sts.txt". path is "https://mta-sts.example.com/.well-known/mta-sts.txt".
The [RFC7231] "Content-Type" media type for this resource MUST be The [RFC7231] "Content-Type" media type for this resource MUST be
"text/plain". When fetching a policy, senders SHOULD validate that "text/plain". When fetching a policy, senders SHOULD validate that
the media type is "text/plain" to guard against cases where the media type is "text/plain" to guard against cases where
webservers allow untrusted users to host non-text content (typically, webservers allow untrusted users to host non-text content (typically,
HTML or images) at a user-defined path. Additional "Content-Type" HTML or images) at a user-defined path. Additional "Content-Type"
parameters are ignored. parameters are ignored.
This resource contains the following line-separated key/value pairs: This resource contains the following newline-separated key/value
pairs:
o "version": (plain-text). Currently only "STSv1" is supported. o "version": (plain-text). Currently only "STSv1" is supported.
o "mode": (plain-text). One of "enforce", "report", or "none", o "mode": (plain-text). One of "enforce", "testing", or "none",
indicating the expected behavior of a sending MTA in the case of a indicating the expected behavior of a sending MTA in the case of a
policy validation failure. policy validation failure.
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.
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that mail for this domain might be handled by any MX with a that mail for this domain might be handled by any MX with a
certificate valid for a host at "mail.example.com" or certificate valid for a host at "mail.example.com" or
"example.net". Valid patterns can be either fully specified names "example.net". Valid patterns can be either fully specified names
("example.com") or suffixes (".example.net") matching the right- ("example.com") or suffixes (".example.net") matching the right-
hand parts of a server's identity; the latter case are hand parts of a server's identity; the latter case are
distinguished by a leading period. If there are more than one MX distinguished by a leading period. If there are more than one MX
specified by the policy, they MUST be on separate lines within the specified by the policy, they MUST be on separate lines within the
policy file. In the case of Internationalized Domain Names policy file. In the case of Internationalized Domain Names
([RFC5891]), the MX MUST specify the Punycode-encoded A-label ([RFC5891]), the MX MUST specify the Punycode-encoded A-label
[RFC3492] and not the Unicode-encoded U-label. The full semantics [RFC3492] and not the Unicode-encoded U-label. The full semantics
of certificate validation are described in Section 3.5, "MX of certificate validation are described in Section 4.1, "MX
Certificate Validation." Certificate 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: 123456
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 = *WSP sts-policy-field *WSP sts-policy-record = *WSP sts-policy-field *WSP
*(CRLF *WSP sts-policy-field *WSP) *(CRLF *WSP sts-policy-field *WSP)
[CRLF]
sts-policy-field = sts-policy-version / ; required once [CRLF]
sts-policy-mode / ; required once
sts-policy-max-age / ; required once
0*(sts-policy-mx *WSP CRLF) / ; required at
; least once
; except when mode
; is "none"
sts-policy-extension ; other fields
field-delim = ":" *WSP sts-policy-field = sts-policy-version / ; required once
sts-policy-mode / ; required once
sts-policy-max-age / ; required once
sts-policy-version = sts-policy-version-field field-delim 0*(sts-policy-mx *WSP CRLF) /
sts-policy-version-value ; required at least once, except when
; mode is "none"
sts-policy-version-field = %s"version" sts-policy-extension ; other fields
sts-policy-version-value = %s"STSv1" field-delim = ":" *WSP
sts-policy-mode = sts-policy-mode-field field-delim sts-policy-version = sts-policy-version-field field-delim
sts-policy-mode-value sts-policy-version-value
sts-policy-mode-field = %s"mode" sts-policy-version-field = %s"version"
sts-policy-model-value = %s"report" / %s"enforce" / %s"none" sts-policy-version-value = %s"STSv1"
sts-policy-mx = sts-policy-mx-field field-delim sts-policy-mode = sts-policy-mode-field field-delim
sts-policy-mx-value sts-policy-mode-value
sts-policy-mx-field = %s"mx" sts-policy-mode-field = %s"mode"
sts-policy-mx-value = 1*(ALPHA / DIGIT / "_" / "-" / ".") sts-policy-model-value = %s"testing" / %s"enforce" / %s"none"
sts-policy-max-age = sts-policy-max-age-field field-delim sts-policy-mx = sts-policy-mx-field field-delim
sts-policy-max-age-value sts-policy-mx-value
sts-policy-max-age-field = %s"max_age" sts-policy-mx-field = %s"mx"
sts-policy-max-age-value = 1*10(DIGIT) sts-policy-mx-value = 1*(ALPHA / DIGIT / "_" / "-" / ".")
sts-policy-extension = sts-policy-ext-name field-delim ; additional sts-policy-max-age = sts-policy-max-age-field field-delim
sts-policy-ext-value ; extension sts-policy-max-age-value
; fields
sts-policy-ext-name = (ALPHA / DIGIT) sts-policy-max-age-field = %s"max_age"
*31(ALPHA / DIGIT / "_" / "-" / ".")
sts-policy-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding sts-policy-max-age-value = 1*10(DIGIT)
; "=", ";", SP,
; and control sts-policy-extension = sts-policy-ext-name ; additional
; chars field-delim ; extension
sts-policy-ext-value ; fields
sts-policy-ext-name = (ALPHA / DIGIT)
*31(ALPHA / DIGIT / "_" / "-" / ".")
sts-policy-ext-value = 1*(%x21-3A / %x3C / %x3E-7E)
; chars, excluding "=", ";", SP, and
; control chars
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. If
any field is not specified, the policy SHALL be treated as invalid. any field is not specified, the policy SHALL be treated as invalid.
3.3. HTTPS Policy Fetching 3.3. HTTPS Policy Fetching
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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-
cached policy, senders MUST continue with delivery as though the cached policy, senders MUST continue with delivery as though the
domain has not implemented MTA-STS. domain has not implemented MTA-STS.
Conversely, if no "live" policy can be discovered via DNS or fetched 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 via HTTPS, but a valid (non-expired) policy exists in the sender's
cache, the sender MUST apply that cached policy. 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
proactivecly 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 3.4. Policy Selection for Smart Hosts and Subdomains
When sending mail via a "smart host"--an intermediate SMTP relay When sending mail via a "smart host"--an intermediate SMTP relay
rather than the message recipient's server--compliant senders MUST rather than the message recipient's server--compliant senders MUST
treat the smart host domain as the policy domain for the purposes of treat the smart host domain as the policy domain for the purposes of
policy discovery and application. policy discovery and application.
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".
#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: validate:
1. That the recipient MX supports STARTTLS and offers a valid PKIX- 1. That the recipient MX supports STARTTLS and offers a valid PKIX-
based TLS certificate. based TLS certificate.
2. That at least one of the policy's "mx" patterns matches at least 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 one of the identities presented in the MX's X.509 certificate, as
described in "MX Certificate Validation". described in "MX Certificate Validation".
This section does not dictate the behavior of sending MTAs when This section does not dictate the behavior of sending MTAs when
policies fail to validate; in particular, validation failures of policies fail to validate; see Section 5, "Policy Application" for a
policies which specify mode values of "report" or "none" MUST NOT be description of sending MTA behavior when policy validation fails.
interpreted as delivery failures, as described in Section 4, "Policy
Application".
3.5. MX Certificate Validation 4.1. MX Certificate Validation
The certificate presented by the receiving MX MUST chain to a root CA The certificate presented by the receiving MX MUST chain to a root CA
that is trusted by the sending MTA and be non-expired. The that is trusted by the sending MTA and be non-expired. The
certificate MUST have a CN-ID ([RFC6125]) or subject alternative name certificate MUST have a CN-ID ([RFC6125]) or subject alternative name
(SAN, [RFC5280]) with a DNS-ID matching the "mx" pattern. The MX's (SAN, [RFC5280]) with a DNS-ID matching the "mx" pattern. The MX's
certificate MAY also be checked for revocation via OCSP [RFC6960], certificate MAY also be checked for revocation via OCSP [RFC6960],
CRLs [RFC6818], or some other mechanism. CRLs [RFC6818], or some other mechanism.
Because the "mx" patterns are not hostnames, however, matching is not Because the "mx" patterns are not hostnames, however, matching is not
identical to other common cases of X.509 certificate authentication identical to other common cases of X.509 certificate authentication
(as described, for example, in [RFC6125]). Consider the example (as described, for example, in [RFC6125]). Consider the example
policy given above, with an "mx" pattern containing ".example.com". policy given above, with an "mx" pattern containing ".example.com".
In this case, if the MX server's X.509 certificate contains a SAN In this case, if the MX server's X.509 certificate contains a SAN
matching "*.example.com", we are required to implement "wildcard-to- matching "*.example.com", we are required to implement "wildcard-to-
wildcard" matching. wildcard" matching.
To simplify this case, we impose the following constraints on To simplify this case, we impose the following constraints on
wildcard certificates, identical to those in [RFC7672] section 3.2.3 wildcard certificates, identical to those in [RFC7672] section 3.2.3
and [@?RFC6125 section 6.4.3: wildcards are valid in DNS-IDs or CN- and [RFC6125] section 6.4.3: wildcards are valid in DNS-IDs or CN-
IDs, but must be the entire first label of the identifier (that is, IDs, but must be the entire first label of the identifier (that is,
"*.example.com", not "mail*.example.com"). Senders who are comparing "*.example.com", not "mail*.example.com"). Senders who are comparing
a "suffix" MX pattern with a wildcard identifier should thus strip a "suffix" MX pattern with a wildcard identifier should thus strip
the wildcard and ensure that the two sides match label-by-label, the wildcard and ensure that the two sides match label-by-label,
until all labels of the shorter side (if unequal length) are until all labels of the shorter side (if unequal length) are
consumed. consumed.
Note that a wildcard _must_ match a label; an "mx" pattern of Note that a wildcard must match a label; an "mx" pattern of
".example.com" thus does not match a SAN of "example.com", nor does a ".example.com" thus does not match a SAN of "example.com", nor does a
SAN of "*.example.com" match an "mx" of "example.com". SAN of "*.example.com" match an "mx" of "example.com".
A simple pseudocode implementation of this algorithm is presented in A simple pseudocode implementation of this algorithm is presented in
the Appendix. Appendix B.
4. 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.
2. "report": In this mode, sending MTAs which also implement the 2. "report": In this mode, sending MTAs which also implement the
TLSRPT specification (TODO: add ref) merely send a report TLSRPT specification [I-D.ietf-uta-smtp-tlsrpt] merely send a
indicating policy application failures (so long as TLSRPT is also report indicating policy application failures (so long as TLSRPT
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 7.3, 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 for the presence of an updated policy at the Policy Domain.
(In all cases, MTAs SHOULD treat such failures as transient errors (In all cases, MTAs SHOULD treat such failures as transient errors
and retry delivery later.) This allows implementing domains to and retry delivery later.) This allows implementing domains to
update long-lived policies on the fly. update long-lived policies on the fly.
4.1. 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, enforcing STARTTLS and, assuming a policy is
present, PKIX certificate validation as described in Section 3.5, present, PKIX certificate validation as described in Section 4.1,
"MX Certificate Validation." "MX Certificate Validation."
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).
5. Reporting Failures 6. Reporting Failures
MTA-STS is intended to be used along with TLSRPT (TODO: add ref) in MTA-STS is intended to be used along with TLSRPT
order to ensure implementing domains can detect cases of both benign [I-D.ietf-uta-smtp-tlsrpt] in order to ensure implementing domains
and malicious failures, and to ensure that failures that indicate an can detect cases of both benign and malicious failures, and to ensure
active attack are discoverable. As such, senders who also implement that failures that indicate an active attack are discoverable. As
TLSRPT SHOULD treat the following events as reportable failures: such, senders who also implement TLSRPT SHOULD treat the following
events as reportable failures:
o HTTPS policy fetch failures when a valid TXT record is present. o HTTPS policy fetch failures when a valid TXT record is present.
o Policy fetch failures of any kind when a valid policy exists in o Policy fetch failures of any kind when a valid policy exists in
the policy cache, except if that policy's mode is "none". the policy cache, except if that policy's mode is "none".
o Delivery attempts in which a contacted MX does not support o Delivery attempts in which a contacted MX does not support
STARTTLS or does not present a certificate which validates STARTTLS or does not present a certificate which validates
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".
6. Interoperability Considerations 7. Interoperability Considerations
6.1. 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 MUST have support for HTTP servers used to deliver MTA-STS policies MUST have support for
the TLS SNI extension and MAY rely on SNI to determine which the TLS SNI extension and MAY rely on SNI to determine which
certificate chain to present to the client. In either case, HTTP certificate chain to present to the client. In either case, 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. hostname or abort the TLS handshake if unable to do so.
SMTP servers MUST have support for the TLS SNI extension and MAY rely SMTP servers MUST have support for the TLS SNI extension and MAY rely
on SNI to determine which certificate chain to present to the client. on SNI to determine which certificate chain to present to the client.
If the client sends no SNI extension or sends an SNI extension for an If the client sends no SNI extension or sends an SNI extension for an
unsupported server name, the server MUST simply send a fallback unsupported server name, the server MUST simply send a fallback
certificate chain of its choice. The reason for not enforcing strict certificate chain of its choice. The reason for not enforcing strict
matching of the requested SNI hostname is that MTA-STS TLS clients matching of the requested SNI hostname is that MTA-STS TLS clients
may be typically willing to accept multiple server names but can only may be typically willing to accept multiple server names but can only
send one name in the SNI extension. The server's fallback send one name in the SNI extension. The server's fallback
certificate may match a different name that is acceptable to the certificate may match a different name that is acceptable to the
client, e.g., the original next-hop domain. client, e.g., the original next-hop domain.
6.2. Minimum TLS Version Support 7.2. Minimum TLS Version Support
MTAs supporting MTA-STS MUST have support for TLS version 1.2 MTAs supporting MTA-STS MUST have support for TLS version 1.2
[RFC5246] or higher. The general TLS usage guidance in [RFC7525] [RFC5246] or higher. The general TLS usage guidance in [RFC7525]
SHOULD be followed. SHOULD be followed.
7. Operational Considerations 8. Operational Considerations
7.1. Policy Updates
8.1. Policy Updates
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 prefer to update the HTTPS policy body before
updating the TXT record; this ordering avoids the risk that senders, updating the TXT record; this ordering avoids the risk that senders,
seeing a new TXT record, mistakenly cache the old policy from HTTPS. seeing a new TXT record, mistakenly cache the old policy from HTTPS.
7.2. 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.
This allows the hosting organization to control update signaling. This allows the hosting organization to control update signaling.
Second, the Policy Domain must point the "well-known" policy location Second, the Policy Domain must point the "well-known" policy location
to the hosting organization. This can be done either by setting the to the hosting organization. This can be done either by setting the
"mta-sts" record to a host or CNAME specified by the hosting "mta-sts" record to an IP address or CNAME specified by the hosting
organization and by giving the hosting organization a TLS certificate organization and by giving the hosting organization a TLS certificate
which is valid for that host, or by setting up a "reverse proxy" 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 (also known as a "gateway") server that serves as the Policy Domain's
policy the policy currently served by the hosting organization. policy the policy currently served by the hosting organization.
For example, given a user domain "user.com" hosted by a mail provider For example, given a user domain "user.example" hosted by a mail
"provider.com", the following configuration would allow policy provider "provider.example", the following configuration would allow
delegation: policy delegation:
DNS: DNS:
_mta-sts.user.com. IN CNAME _mta-sts.provider.com. _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.com > Host: mta-sts.user.example
< HTTP/1.1 200 OK # Response proxies content from https://mta-sts.provider.com < HTTP/1.1 200 OK # Response proxies content from
# https://mta-sts.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.
7.3. Removing MTA-STS 8.3. Removing MTA-STS
In order to facilitate clean opt-out of MTA-STS by implementing In order to facilitate clean opt-out of MTA-STS by implementing
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:
skipping to change at page 14, line 42 skipping to change at page 14, line 45
"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. IANA Considerations 9. IANA Considerations
8.1. Well-Known URIs Registry 9.1. Well-Known URIs Registry
A new .well-known URI will be registered in the Well-Known URIs A new "well-known" URI as described in Section 3 will be registered
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
8.2. MTA-STS TXT Record Fields 9.2. MTA-STS TXT Record Fields
IANA is requested to create a new registry titled "MTA-STS TXT Record IANA is requested to create a new registry titled "MTA-STS TXT Record
Fields". The initial entries in the registry are: Fields". The initial entries in the registry are:
+------------+--------------------+------------------------+ +------------+--------------------+------------------------+
| Field Name | Description | Reference | | Field Name | Description | Reference |
+------------+--------------------+------------------------+ +------------+--------------------+------------------------+
| v | Record version | Section 3.1 of RFC XXX | | v | Record version | Section 3.1 of RFC XXX |
| id | Policy instance ID | 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" New fields are added to this registry using IANA's "Expert Review"
policy. policy.
8.3. MTA-STS Policy Fields 9.3. MTA-STS Policy Fields
IANA is requested to create a new registry titled "MTA-STS Policy IANA is requested to create a new registry titled "MTA-STS Policy
Fields". The initial entries in the registry are: Fields". The initial entries in the registry are:
+------------+----------------------+------------------------+ +------------+----------------------+------------------------+
| Field Name | Description | Reference | | Field Name | Description | Reference |
+------------+----------------------+------------------------+ +------------+----------------------+------------------------+
| version | Policy version | Section 3.2 of RFC XXX | | version | Policy version | Section 3.2 of RFC XXX |
| mode | Enforcement behavior | 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 | | 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.
9. 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 who wishes to intercept or tamper with mail between
hosts who support STARTTLS. There are two classes of attacks hosts who 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.
skipping to change at page 16, line 14 skipping to change at page 16, line 14
for the recipient domain, or by redirecting client connections for the recipient domain, or by redirecting client connections
intended for the legitimate recipient server (for example, by intended for the legitimate recipient server (for example, by
altering BGP routing tables). altering BGP routing tables).
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.
9.1. 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.
9.2. 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 victim sending domain to send mail to a never-before- inducing a sending domain to send mail to a never-before-contacted
contacted recipient while carrying out a man-in-the-middle attack-- recipient while carrying out a man-in-the-middle attack--may be able
may be able to foil policy discovery and effectively downgrade the to foil policy discovery and effectively downgrade the security of
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 we strongly recommend implementers to prefer policy "max_age" values
to be as long as is practical. to be as long as 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 we suggest implementers do not wait until a cached policy has expired
before checking for an update; if senders attempt to refresh the before checking for an update; if senders attempt to refresh the
cache regularly (for instance, by checking their cached version cache regularly (for instance, by checking their cached version
string against the TXT record on each successful send, or in a string against the TXT record on each successful send, or in a
background task that runs daily or weekly), an attacker would have to background task that runs daily or weekly), an attacker would have to
foil policy discovery consistently over the lifetime of a cached foil policy discovery consistently over the lifetime of a cached
policy to prevent a successful refresh. 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 7.3.) 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.
9.3. Denial of Service 10.3. Denial of Service
We additionally consider the Denial of Service risk posed by an We additionally consider the Denial of Service risk posed by an
attacker who can modify the DNS records for a victim domain. Absent attacker who can modify the DNS records for a recipient domain.
MTA-STS, such an attacker can cause a sending MTA to cache invalid MX Absent MTA-STS, such an attacker can cause a sending MTA to cache
records, but only for however long the sending resolver caches those invalid MX records, but only for however long the sending resolver
records. With MTA-STS, the attacker can additionally advertise a caches those records. With MTA-STS, the attacker can additionally
new, long-"max_age" MTA-STS policy with "mx" constraints that advertise a new, long-"max_age" MTA-STS policy with "mx" constraints
validate the malicious MX record, causing senders to cache the policy that validate the malicious MX record, causing senders to cache the
and refuse to deliver messages once the victim has resecured the MX policy and refuse to deliver messages once the victim has resecured
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
skipping to change at page 18, line 5 skipping to change at page 18, line 5
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.
9.4. 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.
9.5. 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
skipping to change at page 18, line 40 skipping to change at page 18, line 40
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.
10. Contributors 11. Contributors
Nicolas Lidzborski Google, Inc nlidz (at) google (dot com) Nicolas Lidzborski Google, Inc nlidz (at) google (dot com)
Wei Chuang Google, Inc weihaw (at) google (dot com) Wei Chuang Google, Inc weihaw (at) google (dot com)
Brandon Long Google, Inc blong (at) google (dot com) Brandon Long Google, Inc blong (at) google (dot com)
Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com) Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot 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 (at) 1und1 (dot 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 (at) 1und1 (dot de)
11. Appendix 1: MTA-STS example record & policy 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-11 (work in progress), November 2017.
[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>.
[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>.
[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>.
[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>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[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>.
12.2. Informative References
[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>.
[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>.
[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 The owner of "example.com" wishes to begin using MTA-STS with a
policy that will solicit reports from senders without affecting how policy that will solicit reports from senders without affecting how
the messages are processed, in order to verify the identity of MXs the messages are processed, in order to verify the identity of MXs
that handle mail for "example.com", confirm that TLS is correctly that handle mail for "example.com", confirm that TLS is correctly
used, and ensure that certificates presented by the recipient MX used, and ensure that certificates presented by the recipient MX
validate. validate.
MTA-STS policy indicator TXT RR: MTA-STS policy indicator TXT RR:
_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 [1] MTA-STS Policy file served as the response body at "https://mta-
sts.example.com/.well-known/mta-sts.txt":
version: STSv1 version: STSv1
mode: report mode: report
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: 12345678
12. Appendix 2: 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
present. present.
skipping to change at page 20, line 24 skipping to change at page 22, line 39
return true return true
} }
} }
return false return false
} }
func certMatches(connection, policy) { func certMatches(connection, policy) {
// Assume a handy function to return CN and DNS-ID SANs. // Assume a handy function to return CN and DNS-ID SANs.
for san in getDnsIdSansAndCnFromCert(connection) { for san in getDnsIdSansAndCnFromCert(connection) {
for mx in policy.mx { for mx in policy.mx {
// Return if the server certificate from "connection" matches the "mx" // Return if the server certificate from "connection" matches the
// host. // "mx" host.
if san[0] == '*' { if san[0] == '*' {
// Invalid wildcard! // Invalid wildcard!
if san[1] != '.' continue if san[1] != '.' continue
san = san[1:] san = san[1:]
} }
if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) { if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) {
return true return true
} }
} }
} }
skipping to change at page 21, line 15 skipping to change at page 23, line 31
// Return a cached policy for "domain". // Return a cached policy for "domain".
} }
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 error. return false // Can't connect to the MX so it's not an MTA-STS
// error.
} }
secure := true secure := true
if !tryStartTls(connection) { 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) {
skipping to change at page 22, line 7 skipping to change at page 24, line 24
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).
} }
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<http://www.rfc-editor.org/info/rfc3492>.
[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,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://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,
<http://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>.
[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, <http://www.rfc-editor.org/info/rfc6125>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI
10.17487/RFC7231, June 2014, <https://www.rfc-editor.org/
info/rfc7231>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC
7405, DOI 10.17487/RFC7405, December 2014,
<http://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>.
13.2. Informative References
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[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,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI
10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/
RFC5891, August 2010,
<http://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>.
[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,
<http://www.rfc-editor.org/info/rfc7672>.
13.3. URIs
[1] https://mta-sts.example.com/.well-known/mta-sts.txt:
Authors' Addresses Authors' Addresses
Daniel Margolis Daniel Margolis
Google, Inc Google, Inc
Email: dmargolis (at) google.com Email: dmargolis (at) google (dot com)
Mark Risher Mark Risher
Google, Inc Google, Inc
Email: risher (at) google (dot com) Email: risher (at) google (dot com)
Binu Ramakrishnan Binu Ramakrishnan
Yahoo!, Inc Yahoo!, Inc
Email: rbinu (at) yahoo-inc (dot com) Email: rbinu (at) yahoo-inc (dot com)
Alexander Brotman Alexander Brotman
Comcast, Inc Comcast, Inc
Email: alex_brotman (at) comcast.com Email: alex_brotman (at) comcast (dot com)
Janet Jones Janet Jones
Microsoft, Inc Microsoft, Inc
Email: janet.jones (at) microsoft (dot com) Email: janet.jones (at) microsoft (dot com)
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