Using TLS in Applications D. Margolis
Internet-Draft Google, Inc
Intended status: Standards Track A. Brotman
Expires: December 01, 2017 Comcast, Inc
B. Ramakrishnan
Yahoo!, Inc
J. Jones
Microsoft, Inc
M. Risher
Google, Inc
May 31, 2017
SMTP TLS Reporting
draft-ietf-uta-smtp-tlsrpt-06
Abstract
A number of protocols exist for establishing encrypted channels
between SMTP Mail Transfer Agents, including STARTTLS [RFC3207], DANE
[RFC6698], and MTA-STS (TODO: Add ref). These protocols can fail due
to misconfiguration or active attack, leading to undelivered messages
or delivery over unencrypted or unauthenticated channels. This
document describes a reporting mechanism and format by which sending
systems can share statistics and specific information about potential
failures with recipient domains. Recipient domains can then use this
information to both detect potential attackers and diagnose
unintentional misconfigurations.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 26, 2017.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Related Technologies . . . . . . . . . . . . . . . . . . . . 4
3. Reporting Policy . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Example Reporting Policy . . . . . . . . . . . . . . . . 5
3.1.1. Report using MAILTO . . . . . . . . . . . . . . . . . 6
3.1.2. Report using HTTPS . . . . . . . . . . . . . . . . . 6
4. Reporting Schema . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Report Time-frame . . . . . . . . . . . . . . . . . . . . 7
4.2. Delivery Summary . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Success Count . . . . . . . . . . . . . . . . . . . . 7
4.2.2. Failure Count . . . . . . . . . . . . . . . . . . . . 7
4.3. Result Types . . . . . . . . . . . . . . . . . . . . . . 7
4.3.1. Negotiation Failures . . . . . . . . . . . . . . . . 7
4.3.2. Policy Failures . . . . . . . . . . . . . . . . . . . 8
4.3.3. General Failures . . . . . . . . . . . . . . . . . . 8
4.3.4. Transient Failures . . . . . . . . . . . . . . . . . 9
4.4. JSON Report Schema . . . . . . . . . . . . . . . . . . . 9
5. Report Delivery . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Report Filename . . . . . . . . . . . . . . . . . . . . . 11
5.2. Compression . . . . . . . . . . . . . . . . . . . . . . . 12
5.3. Email Transport . . . . . . . . . . . . . . . . . . . . . 12
5.3.1. Example Report . . . . . . . . . . . . . . . . . . . 13
5.4. HTTPS Transport . . . . . . . . . . . . . . . . . . . . . 14
5.5. Delivery Retry . . . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
6.1. Message headers . . . . . . . . . . . . . . . . . . . . . 15
6.2. Report Type . . . . . . . . . . . . . . . . . . . . . . . 15
6.3. application/tlsrpt+* Media Types . . . . . . . . . . . . 15
6.4. STARTTLS Validation Result Types . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
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8. Appendix 1: Example Reporting Policy . . . . . . . . . . . . 18
8.1. Report using MAILTO . . . . . . . . . . . . . . . . . . . 18
8.2. Report using HTTPS . . . . . . . . . . . . . . . . . . . 18
9. Appendix 2: Example JSON Report . . . . . . . . . . . . . . . 18
10. Normative References . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
hosts to establish secure SMTP sessions over TLS. The protocol
design is based on "Opportunistic Security" (OS) [RFC7435], which
maintains interoperability with clients that do not support STARTTLS
but means that any attacker who can delete parts of the SMTP session
(such as the "250 STARTTLS" response) or redirect the entire SMTP
session (perhaps by overwriting the resolved MX record of the
delivery domain) can perform a downgrade or interception attack.
Because such "downgrade attacks" are not necessarily apparent to the
receiving MTA, this document defines a mechanism for sending domains
to report on failures at multiple stages of the MTA-to-MTA
conversation.
Recipient domains may also use the mechanisms defined by MTA-STS
(TODO: Add ref) or DANE [RFC6698] to publish additional encryption
and authentication requirements; this document defines a mechanism
for sending domains that are compatible with MTA-STS or DANE to share
success and failure statistics with recipient domains.
Specifically, this document defines a reporting schema that covers
failures in routing, STARTTLS negotiation, and both DANE [RFC6698]
and MTA-STS (TODO: Add ref) policy validation errors, and a standard
TXT record that recipient domains can use to indicate where reports
in this format should be sent.
This document is intended as a companion to the specification for
SMTP MTA Strict Transport Security (MTA-STS, TODO: Add ref).
1.1. Terminology
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
We also define the following terms for further use in this document:
o MTA-STS Policy: A definition of the expected TLS availability,
behavior, and desired actions for a given domain when a sending
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MTA encounters problems in negotiating a secure channel. MTA-STS
is defined in [TODO]
o DANE Policy: A mechanism by which administrators can supply a
record that can be used to validate the certificate presented by
an MTA. DANE is defined in [RFC6698].
o TLSRPT Policy: A policy specifying the endpoint to which sending
MTAs should deliver reports.
o Policy Domain: The domain against which an MTA-STS or DANE Policy
is defined.
o Sending MTA: The MTA initiating the delivery of an email message.
2. Related Technologies
o This document is intended as a companion to the specification for
SMTP MTA Strict Transport Security (MTA-STS, TODO: Add RFC ref).
o SMTP-TLSRPT defines a mechanism for sending domains that are
compatible with MTA-STS or DANE to share success and failure
statistics with recipient domains. DANE is defined in [RFC6698]
and MTA-STS is defined in [TODO : Add RFC ref]
3. Reporting Policy
A domain publishes a record to its DNS indicating that it wishes to
receive reports. These SMTP TLSRPT policies are distributed via DNS
from the Policy Domain's zone, as TXT records (similar to DMARC
policies) under the name "_smtp-tlsrpt". For example, for the Policy
Domain "example.com", the recipient's TLSRPT policy can be retrieved
from "_smtp-tlsrpt.example.com".
Policies consist of the following directives:
o "v": This value MUST be equal to "TLSRPTv1".
o "rua": A URI specifying the endpoint to which aggregate
information about policy failures should be sent (see Section 4,
"Reporting Schema", for more information). Two URI schemes are
supported: "mailto" and "https".
o In the case of "https", reports should be submitted via POST
([RFC2818]) to the specified URI.
o In the case of "mailto", reports should be submitted to the
specified email address ([RFC6068]). When sending failure reports
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via SMTP, sending MTAs MUST deliver reports despite any TLS-
related failures. This may mean that the reports are delivered in
the clear.
The formal definition of the "_smtp-tlsrpt" TXT record, defined using
[RFC5234], is as follows:
tlsrpt-record = tlsrpt-version *WSP field-delim *WSP tlsrpt-rua
[field-delim [tlsrpt-extensions]]
field-delim = %x3B ; ";"
tlsrpt-version = %x76 *WSP "=" *WSP %x54 %x4C %x53 %x52
%x50 %x54 %x76 %x31 ; "v=TSRPTv1"
tlsrpt-rua = %x72 %x75 %x61 *WSP "=" *WSP tlsrpt-uri ; "rua=..."
tlsrpt-uri = URI
; "URI" is imported from [@!RFC3986]; commas (ASCII
; 0x2C) and exclamation points (ASCII 0x21)
; MUST be encoded; the numeric portion MUST fit
; within an unsigned 64-bit integer
tlsrpt-extensions = tlsrpt-extension *(field-delim tlsrpt-extension)
[field-delim]
; extension fields
tlsrpt-extension = tlsrpt-ext-name *WSP "=" *WSP tlsrpt-ext-value
tlsrpt-ext-name = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".")
tlsrpt-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding
; "=", ";", SP, and
; control chars
If multiple TXT records for "_smtp-tlsrpt" are returned by the
resolver, records which do not begin with "v=TLSRPTv1;" are
discarded. If the number of resulting records is not one, senders
MUST assume the recipient domain does not implement TLSRPT. Parsers
MUST accept TXT records which are syntactically valid (i.e. valid
key-value pairs seprated by semi-colons) and implementing a superset
of this specification, in which case unknown fields SHALL be ignored.
3.1. Example Reporting Policy
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3.1.1. Report using MAILTO
_smtp-tlsrpt.example.com. IN TXT \
"v=TLSRPTv1;rua=mailto:reports@example.com"
3.1.2. Report using HTTPS
_smtp-tlsrpt.example.com. IN TXT \
"v=TLSRPTv1; \
rua=https://reporting.example.com/v1/tlsrpt"
4. Reporting Schema
The report is composed as a plain text file encoded in the JSON
format ([RFC7159]).
Aggregate reports contain the following fields:
o Report metadata:
* The organization responsible for the report
* Contact information for one or more responsible parties for the
contents of the report
* A unique identifier for the report
* The reporting date range for the report
o Policy, consisting of:
* One of the following policy types: (1) The MTA-STS policy
applied (as a string) (2) The DANE TLSA record applied (as a
string, with each RR entry of the RRset listed and separated by
a semicolon) (3) The literal string "no-policy-found", if
neither a TLSA nor MTA-STS policy could be found.
* The domain for which the policy is applied
* The MX host
* An identifier for the policy (where applicable)
o Aggregate counts, comprising result type, sending MTA IP,
receiving MTA hostname, session count, and an optional additional
information field containing a URI for recipients to review
further information on a failure type.
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Note that the failure types are non-exclusive; an aggregate report
may contain overlapping "counts" of failure types when a single send
attempt encountered multiple errors.
4.1. Report Time-frame
The report SHOULD cover a full day, from 0000-2400 UTC. This should
allow for easier correlation of failure events.
4.2. Delivery Summary
4.2.1. Success Count
o "success-count": This indicates that the sending MTA was able to
successfully negotiate a policy-compliant TLS connection, and
serves to provide a "heartbeat" to receiving domains that
reporting is functional and tabulating correctly. This field
contains an aggregate count of successful connections for the
reporting system.
4.2.2. Failure Count
o "failure-count": This indicates that the sending MTA was unable to
successfully establish a connection with the receiving platform.
Section 4.3, "Result Types", will elaborate on the failed
negotiation attempts. This field contains an aggregate count of
failed connections.
4.3. Result Types
The list of result types will start with the minimal set below, and
is expected to grow over time based on real-world experience. The
initial set is:
4.3.1. Negotiation Failures
o "starttls-not-supported": This indicates that the recipient MX did
not support STARTTLS.
o "certificate-host-mismatch": This indicates that the certificate
presented did not adhere to the constraints specified in the MTA-
STS or DANE policy, e.g. if the MX does not match any identities
listed in the Subject Alternate Name (SAN) [RFC5280].
o "certificate-expired": This indicates that the certificate has
expired.
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o "certificate-not-trusted": This a label that covers multiple
certificate related failures that include, but not limited to
errors such as untrusted/unknown CAs, certificate name
constraints, certificate chain errors etc. When using this
declaration, the reporting MTA SHOULD utilize the "failure-reason"
to provide more information to the receiving entity.
o "validation-failure": This indicates a general failure for a
reason not matching a category above. When using this
declaration, the reporting MTA SHOULD utilize the "failure-reason"
to provide more information to the receiving entity.
4.3.2. Policy Failures
4.3.2.1. DANE-specific Policy Failures
o "tlsa-invalid": This indicates a validation error in the TLSA
record associated with a DANE policy. None of the records in the
RRset were found to be valid.
o "dnssec-invalid": This would indicate that no valid records were
returned from the recursive resolver. The request returned with
SERVFAIL for the requested TLSA record.
4.3.2.2. MTA-STS-specific Policy Failures
o "sts-policy-invalid": This indicates a validation error for the
overall MTA-STS policy.
o "sts-webpki-invalid": This indicates that the MTA-STS policy could
not be authenticated using PKIX validation.
4.3.3. General Failures
When a negotiation failure can not be categorized into one of the
"Negotiation Failures" stated above, the reporter SHOULD use the
"validation-failure" category. As TLS grows and becomes more
complex, new mechanisms may not be easily categorized. This allows
for a generic feedback category. When this category is used, the
reporter SHOULD also use the "failure-reason-code" to give some
feedback to the receiving entity. This is intended to be a short
text field, and the contents of the field should be an error code or
error text, such as "X509_V_ERR_UNHANDLED_CRITICAL_CRL_EXTENSION".
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4.3.4. Transient Failures
Transient errors due to too-busy network, TCP timeouts, etc. are not
required to be reported.
4.4. JSON Report Schema
The JSON schema is derived from the HPKP JSON schema [RFC7469] (cf.
Section 3)
{
"organization-name": organization-name,
"date-range": {
"start-datetime": date-time,
"end-datetime": date-time
},
"contact-info": email-address,
"report-id": report-id,
"policy": {
"policy-type": policy-type,
"policy-string": policy-string,
"policy-domain": domain,
"mx-host": mx-host-pattern
},
"summary": {
"success-aggregate": total-successful-session-count,
"failure-aggregate:" total-failure-session-count
}
"failure-details": [
{
"result-type": result-type,
"sending-mta-ip": ip-address,
"receiving-mx-hostname": receiving-mx-hostname,
"receiving-mx-helo": receiving-mx-helo,
"session-count": failed-session-count,
"additional-information": additional-info-uri,
"failure-reason-code": "Text body"
}
]
}
JSON Report Format
o "organization-name": The name of the organization responsible for
the report. It is provided as a string.
o "date-time": The date-time indicates the start- and end-times for
the report range. It is provided as a string formatted according
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to Section 5.6, "Internet Date/Time Format", of [RFC3339]. The
report should be for a full UTC day, 0000-2400.
o "email-address": The contact information for a responsible party
of the report. It is provided as a string formatted according to
Section 3.4.1, "Addr-Spec", of [RFC5322].
o "report-id": A unique identifier for the report. Report authors
may use whatever scheme they prefer to generate a unique
identifier. It is provided as a string.
o "policy-type": The type of policy that was applied by the sending
domain. Presently, the only three valid choices are "tlsa",
"sts", and the literal string "no-policy-found". It is provided
as a string.
o "policy-string": The JSON string serialization ([RFC7159] section
7) of the policy, whether TLSA record ([RFC6698] section 2.3) or
MTA-STS policy.
o "domain": The Policy Domain is the domain against which the MTA-
STS or DANE policy is defined.
o "mx-host-pattern": The pattern of MX hostnames from the applied
policy. It is provided as a string, and is interpreted in the
same manner as the "Checking of Wildcard Certificates" rules in
Section 6.4.3 of [RFC6125].
o "result-type": A value from Section 4.3, "Result Types", above.
o "ip-address": The IP address of the sending MTA that attempted the
STARTTLS connection. It is provided as a string representation of
an IPv4 or IPv6 address in dot-decimal or colon-hexadecimal
notation.
o "receiving-mx-hostname": The hostname of the receiving MTA MX
record with which the sending MTA attempted to negotiate a
STARTTLS connection.
o "receiving-mx-helo": (optional) The HELO or EHLO string from the
banner announced during the reported session.
o "success-aggregate": The aggregate number (integer) of
successfully negotiated TLS-enabled connections to the receiving
site.
o "failure-aggregate": The aggregate number (integer) of failures to
negotiate an TLS-enabled connection to the receiving site.
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o "session-count": The number of (attempted) sessions that match the
relevant "result-type" for this section.
o "additional-info-uri": An optional URI pointing to additional
information around the relevant "result-type". For example, this
URI might host the complete certificate chain presented during an
attempted STARTTLS session.
o "failure-reason-code": A text field to include an TLS-related
error code or error message.
5. Report Delivery
Reports can be delivered either as an email message via SMTP or via
HTTP POST.
5.1. Report Filename
The filename is typically constructed using the following ABNF:
filename = sender "!" policy-domain "!" begin-timestamp
"!" end-timestamp [ "!" unique-id ] "." extension
unique-id = 1*(ALPHA / DIGIT)
sender = domain ; imported from [@!RFC5322]
policy-domain = domain
begin-timestamp = 1*DIGIT
; seconds since 00:00:00 UTC January 1, 1970
; indicating start of the time range contained
; in the report
end-timestamp = 1*DIGIT
; seconds since 00:00:00 UTC January 1, 1970
; indicating end of the time range contained
; in the report
extension = "json" / "json.gz"
The extension MUST be "json" for a plain JSON file, or "json.gz" for
a JSON file compressed using GZIP.
"unique-id" allows an optional unique ID generated by the Sending MTA
to distinguish among multiple reports generated simultaneously by
different sources within the same Policy Domain. For example, this
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is a possible filename for the gzip file of a report to the Policy
Domain "example.net" from the Sending MTA "mail.sender.example.com":
`mail.sender.example.com!example.net!1470013207!1470186007!001.json.gz`
5.2. Compression
The report SHOULD be subjected to GZIP compression for both email and
HTTPS transport. Declining to apply compression can cause the report
to be too large for a receiver to process (a commonly observed
receiver limit is ten megabytes); compressing the file increases the
chances of acceptance of the report at some compute cost.
5.3. Email Transport
The report MAY be delivered by email. To make the reports machine-
parsable for the receivers, we define a top-level media type
"multipart/report" with a new parameter "report-type="tlsrpt"".
Inside it, there are two parts: The first part is human readable,
typically "text/plain", and the second part is machine readable with
a new media type defined called "application/tlsrpt+json". If
compressed, the report should use the media type "application/
tlsrpt+gzip".
In addition, the following two new top level message header fields
are defined:
TLS-Report-Domain: Receiver-Domain
TLS-Report-Submitter: Sender-Domain
These message headers would allow for easy searching for all reports
submitted by a report domain or a particular submitter, for example
in IMAP:
"s SEARCH HEADER "TLS-Report-Domain" "example.com""
It is presumed that the aggregate reporting address will be equipped
to process new message header fields and extract MIME parts with the
prescribed media type and filename, and ignore the rest.
The [RFC5322].Subject field for individual report submissions SHOULD
conform to the following ABNF:
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tlsrpt-subject = %x52.65.70.6f.72.74 1*FWS ; "Report"
%x44.6f.6d.61.69.6e.3a 1*FWS ; "Domain:"
domain-name 1*FWS ; from RFC 6376
%x53.75.62.6d.69.74.74.65.72.3a ; "Submitter:"
1*FWS domain-name 1*FWS
%x52.65.70.6f.72.74.2d.49.44.3a ; "Report-ID:"
msg-id ; from RFC 5322
The first domain-name indicates the DNS domain name about which the
report was generated. The second domain-name indicates the DNS
domain name representing the Sending MTA generating the report. The
purpose of the Report-ID: portion of the field is to enable the
Policy Domain to identify and ignore duplicate reports that might be
sent by a Sending MTA.
For instance, this is a possible Subject field for a report to the
Policy Domain "example.net" from the Sending MTA
"mail.sender.example.com". It is line-wrapped as allowed by
[RFC5322]:
Subject: Report Domain: example.net
Submitter: mail.sender.example.com
Report-ID: <735ff.e317+bf22029@mailexample.net>
5.3.1. Example Report
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From: tlsrpt@mail.sender.example.com
Date: Fri, May 09 2017 16:54:30 -0800
To: mts-sts-tlsrpt@example.net
Subject: Report Domain: example.net
Submitter: mail.sender.example.com
Report-ID: <735ff.e317+bf22029@example.net>
TLS-Report-Domain: example.net
TLS-Report-Submitter: mail.sender.example.com
MIME-Version: 1.0
Content-Type: multipart/report; report-type="tlsrpt";
boundary="----=_NextPart_000_024E_01CC9B0A.AFE54C00"
Content-Language: en-us
This is a multipart message in MIME format.
------=_NextPart_000_024E_01CC9B0A.AFE54C00
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
This is an aggregate TLS report from mail.sender.example.com
------=_NextPart_000_024E_01CC9B0A.AFE54C00
Content-Type: application/tlsrpt+gzip
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="mail.sender.example!example.com!
1013662812!1013749130.gz"
<gzipped content of report>
------=_NextPart_000_024E_01CC9B0A.AFE54C00--
...
Note that, when sending failure reports via SMTP, sending MTAs MUST
NOT honor MTA-STS or DANE TLSA failures.
5.4. HTTPS Transport
The report MAY be delivered by POST to HTTPS. If compressed, the
report should use the media type "application/tlsrpt+gzip", and
"application/tlsrpt+json" otherwise (see section Section 6, "IANA
Considerations").
5.5. Delivery Retry
In the event of a delivery failure, regardless of the delivery
method, a sender SHOULD attempt redelivery for up to 24hrs after the
initial attempt. As previously stated the reports are optional, so
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while it is ideal to attempt redelivery, it is not required. If
multiple retries are attempted, they should be on a logarithmic
scale.
6. IANA Considerations
The following are the IANA considerations discussed in this document.
6.1. Message headers
Below is the Internet Assigned Numbers Authority (IANA) Permanent
Message Header Field registration information per [RFC3864].
Header field name: TLS-Report-Domain
Applicable protocol: smtp
Status: standard
Author/Change controller: IETF
Specification document(s): this one
Header field name: TLS-Report-Submitter
Applicable protocol: smtp
Status: standard
Author/Change controller: IETF
Specification document(s): this one
6.2. Report Type
This document registers a new parameter "report-type="tlsrpt"" under
"multipart/report" top-level media type for use with [RFC6522].
The media type suitable for use as a report-type is defined in the
following section.
6.3. application/tlsrpt+* Media Types
This document registers multiple media types, listed in Table 1
below.
+-------------+----------------+-------------+-------------------+
| Type | Subtype | File extn | Specification |
+-------------+----------------+-------------+-------------------+
| application | tlsrpt+json | .json | Section 5.3 |
| application | tlsrpt+gzip | .gz | Section 5.3 |
+-------------+----------------+-------------+-------------------+
Table 1: SMTP TLS Reporting Media Types
Type name: application
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Subtype name: This documents registers multiple subtypes, as listed
in Table 1.
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: Encoding considerations are identical to
those specified for the "application/json" media type. See
[RFC7159].
Security considerations: Security considerations relating to SMTP TLS
Reporting are discussed in Section 7.
Interoperability considerations: This document specifies format of
conforming messages and the interpretation thereof.
Published specification: This document is the specification for these
media types; see Table 1 for the section documenting each media type.
Applications that use this media type: Mail User Agents (MUA) and
Mail Transfer Agents.
Additional information:
Magic number(s): n/a
File extension(s): As listed in Table 1.
Macintosh file type code(s): n/a
Person & email address to contact for further information: See
Authors' Addresses section.
Intended usage: COMMON
Restrictions on usage: n/a
Author: See Authors' Addresses section.
Change controller: Internet Engineering Task Force
(mailto:iesg@ietf.org).
6.4. STARTTLS Validation Result Types
This document creates a new registry, "STARTTLS Validation Result
Types". The initial entries in the registry are:
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+-------------------------------+
| Result Type |
+-------------------------------+
| "starttls-not-supported" |
| "certificate-host-mismatch" |
| "certificate-expired" |
| "tlsa-invalid" |
| "dnssec-invalid" |
| "sts-policy-invalid" |
| "sts-webpki-invalid" |
| "validation-failure" |
+-------------------------------+
The above entries are described in section Section 4.3, "Result
Types." New result types can be added to this registry without the
need to update this document.
7. Security Considerations
SMTP TLS Reporting provides transparency into misconfigurations or
attempts to intercept or tamper with mail between hosts who support
STARTTLS. There are several security risks presented by the
existence of this reporting channel:
o Flooding of the Aggregate report URI (rua) endpoint: An attacker
could flood the endpoint with excessive reporting traffic and
prevent the receiving domain from accepting additional reports.
This type of Denial-of-Service attack would limit visibility into
STARTTLS failures, leaving the receiving domain blind to an
ongoing attack.
o Untrusted content: An attacker could inject malicious code into
the report, opening a vulnerability in the receiving domain.
Implementers are advised to take precautions against evaluating
the contents of the report.
o Report snooping: An attacker could create a bogus TLSRPT record to
receive statistics about a domain the attacker does not own.
Since an attacker able to poison DNS is already able to receive
counts of SMTP connections (and, absent DANE or MTA-STS policies,
actual SMTP message payloads), this does not present a significant
new vulnerability.
o Reports as DDoS: TLSRPT allows specifying destinations for the
reports that are outside the authority of the Policy Domain, which
allows domains to delegate processing of reports to a partner
organization. However, an attacker who controls the Policy Domain
DNS could also use this mechanism to direct the reports to an
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unwitting victim, flooding that victim with excessive reports.
DMARC [RFC7489] defines a solution for verifying delegation to
avoid such attacks; the need for this is greater with DMARC,
however, because DMARC allows an attacker to trigger reports to a
target from an innocent third party by sending that third party
mail (which triggers a report from the third party to the target).
In the case of TLSRPT, the attacker would have to induce the third
party to send the attacker mail in order to trigger reports from
the third party to the victim; this reduces the risk of such an
attack and the need for a verification mechanism.
8. Appendix 1: Example Reporting Policy
8.1. Report using MAILTO
_smtp-tlsrpt.mail.example.com. IN TXT \
"v=TLSRPTv1;rua=mailto:reports@example.com"
8.2. Report using HTTPS
_smtp-tlsrpt.mail.example.com. IN TXT \
"v=TLSRPTv1; \
rua=https://reporting.example.com/v1/tlsrpt"
9. Appendix 2: Example JSON Report
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{
"organization-name": "Company-X",
"date-range": {
"start-datetime": "2016-04-01T00:00:00Z",
"end-datetime": "2016-04-01T23:59:59Z"
},
"contact-info": "sts-reporting@company-x.com",
"report-id": "5065427c-23d3-47ca-b6e0-946ea0e8c4be",
"policy": {
"policy-type": "sts",
"policy-string": "{ \"version\": \"STSv1\",\"mode\": \"report\", \"mx\": [\"*.mail.company-y.com\"], \"max_age\": 86400 }",
"policy-domain": "company-y.com",
"mx-host": "*.mail.company-y.com"
},
"summary": {
"success-aggregate": 5326,
"failure-aggregate": 303
}
"failure-details": [{
"result-type": "certificate-expired",
"sending-mta-ip": "98.136.216.25",
"receiving-mx-hostname": "mx1.mail.company-y.com",
"session-count": 100
}, {
"result-type": "starttls-not-supported",
"sending-mta-ip": "98.22.33.99",
"receiving-mx-hostname": "mx2.mail.company-y.com",
"session-count": 200,
"additional-information": "hxxps://reports.company-x.com/
report_info?id=5065427c-23d3#StarttlsNotSupported"
}, {
"result-type: "validation-failure",
"sending-mta-ip": "47.97.15.2",
"receiving-mx-hostname: "mx-backup.mail.company-y.com",
"session-count": 3,
"failure-error-code": "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED"
}]
}
Figure: Example JSON report for a messages from Company-X to
Company-Y, where 100 sessions were attempted to Company Y servers
with an expired certificate and 200 sessions were attempted to
Company Y servers that did not successfully respond to the "STARTTLS"
command. Additionally 3 sessions failed due to
"X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED".
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10. 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>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/
RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<http://www.rfc-editor.org/info/rfc3339>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<http://www.rfc-editor.org/info/rfc3864>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI
10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>.
[RFC6068] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,
<http://www.rfc-editor.org/info/rfc6068>.
[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>.
[RFC6522] Kucherawy, M., Ed., "The Multipart/Report Media Type for
the Reporting of Mail System Administrative Messages", STD
73, RFC 6522, DOI 10.17487/RFC6522, January 2012,
<http://www.rfc-editor.org/info/rfc6522>.
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[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <http://www.rfc-editor.org/info/rfc7435>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<http://www.rfc-editor.org/info/rfc7489>.
Authors' Addresses
Daniel Margolis
Google, Inc
Email: dmargolis (at) google.com
Alexander Brotman
Comcast, Inc
Email: alex_brotman (at) comcast.com
Binu Ramakrishnan
Yahoo!, Inc
Email: rbinu (at) yahoo-inc (dot com)
Janet Jones
Microsoft, Inc
Email: janet.jones (at) microsoft (dot com)
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Mark Risher
Google, Inc
Email: risher (at) google (dot com)
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