%%%
Title = "SMTP TLS Reporting" abbrev = "SMTP-TLSRPT" category = "std" docName = "draft-ietf-uta-smtp-tlsrpt-23" ipr = "trust200902" area = "Applications" workgroup = "Using TLS in Applications" keyword = [""]
date = 2018-06-14T00:00:00Z
[[author]] initials="D." surname="Margolis" fullname="Daniel Margolis" organization="Google, Inc" [author.address] email="[email protected]" [[author]] initials="A." surname="Brotman" fullname="Alexander Brotman" organization="Comcast, Inc" [author.address] email="[email protected]" [[author]] initials="B." surname="Ramakrishnan" fullname="Binu Ramakrishnan" organization="Yahoo!, Inc" [author.address] email="[email protected]" [[author]] initials="J." surname="Jones" fullname="Janet Jones" organization="Microsoft, Inc" [author.address] email="[email protected]" [[author]] initials="M." surname="Risher" fullname="Mark Risher" organization="Google, Inc" [author.address] email="[email protected]"
%%%
.# Abstract
A number of protocols exist for establishing encrypted channels between SMTP Mail Transfer Agents, including STARTTLS, DANE TLSA, and MTA-STS. 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 attacks and diagnose unintentional misconfigurations.
{mainmatter}
The STARTTLS extension to SMTP [@?RFC3207] allows SMTP clients and hosts to establish secure SMTP sessions over TLS. The protocol design uses an approach that has come to be known as "Opportunistic Security" (OS) [@?RFC7435]. This method maintains interoperability with clients that do not support STARTTLS, but means that any attacker could potentially eavesdrop on a session. An attacker could perform a downgrade or interception attack by deleting 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).
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 [@!I-D.ietf-uta-mta-sts] 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, DNS resolution, STARTTLS negotiation, and both DANE [@!RFC6698] and MTA-STS [@!I-D.ietf-uta-mta-sts] policy validation errors, and a standard TXT record that recipient domains can use to indicate where reports in this format should be sent. The report can also serve as a heartbeat that systems are successfully negotiating TLS during sessions as expected.
This document is intended as a companion to the specification for SMTP MTA Strict Transport Security [@!I-D.ietf-uta-mta-sts], as well as adds reporting abilities for those implementing DANE [@!RFC7672].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [BCP 14] [@!RFC2119] [@!RFC8174] when, and only when, they appear in all capitals, as shown here.
We also define the following terms for further use in this document:
- MTA-STS Policy: A mechanism by which administrators can specify the expected TLS availability, presented identity, and desired actions for a given email recipient domain. MTA-STS is defined in [@!I-D.ietf-uta-mta-sts].
- DANE Policy: A mechanism by which administrators can use DNSSEC to commit an MTA to support STARTTLS and to publish criteria to be used to validate its presented certificates. DANE for SMTP is defined in [RFC7672], with the base specification in [RFC6698] (updated in [RFC7671].
- TLSRPT Policy: A policy specifying the endpoint to which sending MTAs should deliver reports.
- Policy Domain: The domain against which an MTA-STS or DANE Policy is defined. For MTA-STS this is typically the same as the envelope recipient domain [RFC5321], but when mail is routed to a "smarthost" gateway by local policy, the "smarthost" domain name is used instead. For DANE the Policy Domain is the "TLSA base domain" of the receiving SMTP server as described in [RFC7672] (Section 2.2.3) and [RFC6698] (Section 3).
- Sending MTA: The MTA initiating the relay of an email message.
- Aggregate Report URI (rua): A comma-separated list of locations where the report is to be submitted.
- This document is intended as a companion to the specification for SMTP MTA Strict Transport Security [@!I-D.ietf-uta-mta-sts].
- 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 [@!I-D.ietf-uta-mta-sts].
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._tls. For example, for the Policy Domain
example.com, the recipient's TLSRPT policy can be retrieved from
_smtp._tls.example.com.
Policies consist of the following directives:
v: This document defines version 1 of TLSRPT, for which this value MUST be equal toTLSRPTv1. Other versions may be defined in later documents.rua: A URI specifying the endpoint to which aggregate information about policy validation results should be sent (see (#reporting-schema), "Reporting Schema", for more information). Two URI schemes are supported:mailtoandhttps. As with DMARC [@?RFC7489], the policy domain can specify a comma-separated list of URIs.- In the case of
https, reports should be submitted via POST ([@!RFC7231]) to the specified URI. Report submitters MAY ignore certificate validation errors when submitting reports via https. - In the case of
mailto, reports should be submitted to the specified email address ([@!RFC6068]). When sending failure reports via SMTP, sending MTAs MUST deliver reports despite any TLS-related failures and SHOULD NOT include this SMTP session in the next report. When sending failure reports via HTTPS, sending MTAs MAY deliver reports despite any TLS-related faliures. This may mean that the reports are delivered in the clear. Reports sent via SMTP MUST contain a valid DKIM [@!RFC6376] signature by the reporting domain. Reports lacking such a signature MUST be ignored by the recipient. DKIM signatures must not use the "l=" attribute to limit the body length used in the signature. The DKIM TXT record must contain the appropriate service type declaration,s=tlsrpt, and if not present the receiving system SHOULD ignore reports signed using this record.
The formal definition of the _smtp._tls TXT record, defined using
[@!RFC5234] & [@!RFC7405], is as follows:
tlsrpt-record = tlsrpt-version 1*(field-delim tlsrpt-field)
[field-delim]
field-delim = *WSP ";" *WSP
tlsrpt-field = tlsrpt-rua / ; Note that the
tlsrpt-extension ; tlsrpt-rua record is
; required.
tlsrpt-version = %s"v=TLSRPTv1"
tlsrpt-rua = %s"rua="
tlsrpt-uri *(*WSP "," *WSP tlsrpt-uri)
tlsrpt-uri = URI
; "URI" is imported from [RFC3986];
; commas (ASCII 0x2C), exclamation
; points (ASCII 0x21), and semicolons
; (ASCII 0x3B) MUST be encoded
tlsrpt-extension = tlsrpt-ext-name "=" 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._tls 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. If the resulting TXT record
contains multiple strings (as described in Section 3.1.3 of [@!RFC4408]),
then the record MUST be treated as if those strings are concatenated
together without adding spaces.
The record supports the abillity to declare more than one rua, and if there exists more than one, the reporter MAY attempt to deliver to each of the supported rua destinations. A receiver MAY opt to only attempt delivery to one of the endpoints, however the report SHOULD NOT be considered successfully delivered until one of the endpoints accepts delivery of the report.
Parsers MUST accept TXT records which are syntactically valid (i.e. valid key-value pairs separated by semi-colons) and implementing a superset of this specification, in which case unknown fields SHALL be ignored.
_smtp._tls.example.com. IN TXT \
"v=TLSRPTv1;rua=mailto:[email protected]"
_smtp._tls.example.com. IN TXT \
"v=TLSRPTv1; \
rua=https://reporting.example.com/v1/tlsrpt"
The report is composed as a plain text file encoded in the I-JSON format ([@!RFC7493]).
Aggregate reports contain the following fields:
- 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
- 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 DANE nor MTA-STS policy could be found. - The domain for which the policy is applied
- The MX host
- 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
- 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.
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. Reporters may report multiple applied
policies (for example, an MTA-STS policy and a DANE TLSA record for the
same domain and MX). Because of this, even in the case where only a single
policy was applied, the "policies" field of the report body MUST be an array and
not a singular value.
In the case of multiple failure types, the failure-details array
would contain multiple entries. Each entry would have its own set of
infomation pertaining to that failure type.
The report SHOULD cover a full day, from 0000-2400 UTC. This should allow for easier correlation of failure events. To avoid a Denial of Service against the system processing the reports, the reports should be delivered after some delay, perhaps several hours.
As an example, a sending site might want to introduce a random delay of up to four hours:
func generate_sleep_delay() {
min_delay = 1
max_delay = 14400
rand = random(min_delay,max_delay)
return rand
}
func generate_report(policy_domain) {
do_rpt_work(policy_domain)
send_rpt(policy_domain)
}
func generate_tlsrpt() {
sleep(generate_sleep_delay())
for policy_domain in list_of_tlsrpt_enabled_domains {
generate_report(policy_domain)
}
}
A sending site might wish to introduce a random delay per destination site, up to four hours:
func generate_sleep_delay() {
min_delay = 1
max_delay = 14400
rand = random(min_delay,max_delay)
return rand
}
func generate_report(policy_domain) {
sleep(generate_sleep_delay())
do_rpt_work(policy_domain)
send_rpt(policy_domain)
}
func generate_tlsrpt() {
for policy_domain in list_of_tlsrpt_enabled_domains {
generate_report(policy_domain)
}
}
total-successful-session-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.
total-failure-session-count: This indicates that the sending MTA was unable to successfully establish a connection with the receiving platform. (#result-types), "Result Types", will elaborate on the failed negotiation attempts. This field contains an aggregate count of failed connections.
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:
starttls-not-supported: This indicates that the recipient MX did not support STARTTLS.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 hostname does not match any identities listed in the Subject Alternate Name (SAN) [@!RFC5280].certificate-expired: This indicates that the certificate has expired.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 thefailure-reason-codeto provide more information to the receiving entity.validation-failure: This indicates a general failure for a reason not matching a category above. When using this declaration, the reporting MTA SHOULD utilize thefailure-reason-codeto provide more information to the receiving entity.
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.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.dane-required: This indicates that the sending system is configured to require DANE TLSA records for all the MX hosts of the destination domain, but no DNSSEC-validated TLSA records were present for the MX host that is the subject of the report. Mandatory DANE for SMTP is described in section 6 of [@?RFC7672]. Such policies may be created by mutual agreement between two organizations that frequently exchange sensitive content via email.
sts-policy-invalid: This indicates a validation error for the overall MTA-STS policy.sts-webpki-invalid: This indicates that the MTA-STS policy could not be authenticated using PKIX validation.
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".
Transient errors due to too-busy network, TCP timeouts, etc. are not required to be reported.
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,
"policies": [{
"policy": {
"policy-type": policy-type,
"policy-string": policy-string,
"policy-domain": domain,
"mx-host": mx-host-pattern
},
"summary": {
"total-successful-session-count": total-successful-session-count,
"total-failure-session-count": 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,
"receiving-ip": receiving-ip,
"failed-session-count": failed-session-count,
"additional-information": additional-info-uri,
"failure-reason-code": failure-reason-code
}
]
}
]
}
Figure: JSON Report Format
organization-name: The name of the organization responsible for the report. It is provided as a string.date-time: The date-time indicates the start- and end-times for the report range. It is provided as a string formatted according to Section 5.6, "Internet Date/Time Format", of [@!RFC3339]. The report should be for a full UTC day, 0000-2400.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 [@!RFC5321].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.policy-type: The type of policy that was applied by the sending domain. Presently, the only three valid choices aretlsa,sts, and the literal stringno-policy-found. It is provided as a string.policy-string: An encoding of the applied policy as a JSON array of strings, whether TLSA record ([@!RFC6698] section 2.3) or MTA-STS policy. Examples follow in the next section.domain: The Policy Domain is the domain against which the MTA-STS or DANE policy is defined. In the case of Internationalized Domain Names ([@?RFC5891]), the domain MUST consist of the Punycode-encoded A-labels ([@!RFC3492]) and not the U-labels.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]. In the case of Internationalized Domain Names ([@!RFC5891]), the domain MUST consist of the Punycode-encoded A-labels ([@!RFC3492]) and not the U-labels.result-type: A value from (#result-types), "Result Types", above.ip-address: The IP address of the sending MTA that attempted the STARTTLS connection. It is provided as a string representation of an IPv4 (see below) or IPv6 ([@!RFC5952]) address in dot-decimal or colon-hexadecimal notation.receiving-mx-hostname: The hostname of the receiving MTA MX record with which the sending MTA attempted to negotiate a STARTTLS connection.receiving-mx-helo: (optional) The HELO or EHLO string from the banner announced during the reported session.receiving-ip: The destination IP address that was using when creating the outbound session. It is provided as a string representation of an IPv4 (see below) or IPv6 ([@!RFC5952]) address in dot-decimal or colon-hexadecimal notation.total-successful-session-count: The aggregate count (integer, encoded as a JSON number) of successfully negotiated TLS-enabled connections to the receiving site.total-failure-session-count: The aggregate count (integer, encoded as a JSON number) of failures to negotiate a TLS-enabled connection to the receiving site.failed-session-count: The number of (attempted) sessions that match the relevantresult-typefor this section (integer, encoded as a JSON number).additional-info-uri: An optional URI [@!RFC3986] pointing to additional information around the relevantresult-type. For example, this URI might host the complete certificate chain presented during an attempted STARTTLS session.failure-reason-code: A text field to include a TLS-related error code or error message.
For report purposes, an IPv4 Address is defined via the following ABNF:
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
Part of the report body includes the policy that is applied when attemping relay to the destination.
For DANE TLSA policies, this is a JSON array of strings each representing the RDATA of a single TLSA resource record as a space-separated list of its four TLSA fields; the fields are in presentation format (defined in [@!RFC6698] Section 2.2) with no internal spaces or grouping parentheses:
[
"3 0 1 1F850A337E6DB9C609C522D136A475638CC43E1ED424F8EEC8513D747D1D085D",
"3 0 1 12350A337E6DB9C6123522D136A475638CC43E1ED424F8EEC8513D747D1D1234"
]
For MTA-STS policies, this is an array of JSON strings that represents the policy that is declared by the receiving site, including any errors that may be present. Note that where there are multiple "mx" values, they must be listed as separate "mx" elements in the policy array, rather as a single nested "mx" sub-array.
[
"version: STSv1",
"mode: testing",
"mx: mx1.example.com",
"mx: mx2.example.com",
"mx: mx.backup-example.com",
"max_age: 604800"
]
Reports can be delivered either as an email message via SMTP or via HTTP POST.
The filename is RECOMMENDED to be constructed using the following ABNF:
filename = sender "!" policy-domain "!" begin-timestamp
"!" end-timestamp [ "!" unique-id ] "." extension
unique-id = 1*(ALPHA / DIGIT)
sender = domain ; From the [RFC5321] that is used
; as the domain for the `contact-info`
; address in the report body.
; In the case of Internationalized Domain
; Names ([@?RFC5891]), the domain MUST consist of
; the Punycode-encoded A-labels ([@!RFC3492]) and
; not the U-labels.
policy-domain = domain
; In the case of Internationalized Domain
; Names ([@?RFC5891]), the domain MUST consist of
; the Punycode-encoded A-labels ([@!RFC3492]) and
; not the U-labels.
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 is a possible filename for a compressed report to the Policy Domain "example.net" from the Sending MTA "mail.sndr.example.com":
mail.sndr.example.com!example.net!1470013207!1470186007!001.json.gz
The report SHOULD be subjected to GZIP [@!RFC1952] 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.
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
The TLS-Report-Submitter value MUST match the value found in the
[@!RFC5321] domain from the contact-info from the report body. These
message headers MUST be included and should allow for easy searching
for all reports submitted by a report domain or a particular submitter,
for example in IMAP [@?RFC3501]:
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. These additional headers SHOULD be included in the DKIM [@!RFC6376] signature for the message.
The [@!RFC5322].Subject field for report submissions SHOULD conform to the following ABNF:
tlsrpt-subject = %s"Report" FWS ; "Report"
%s"Domain:" FWS ; "Domain:"
domain-name FWS ; per [RFC6376]
%s"Submitter:" FWS ; "Submitter:"
domain-name FWS ; per [RFC6376]
%s"Report-ID:" FWS ; "Report-ID:
"<" id-left "@" id-right ">" ; per [RFC5322]
[CFWS] ; per [RFC5322]
; (as with FWS)
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: <[email protected]>
From: [email protected]
Date: Fri, May 09 2017 16:54:30 -0800
To: [email protected]
Subject: Report Domain: example.net
Submitter: mail.sender.example.com
Report-ID: <[email protected]>
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.json.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.
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 (#iana-considerations),
"IANA Considerations").
The receiving system MUST return a "successful" response from its HTTPS server, typically a 200 or 201 HTTP code [@?RFC7321]. Other codes could indicate a delivery failure, and may be retried as per local sender policy. The receiving system is not expected to process reports at receipt time, and MAY store them for processing at a later time.
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 while it is ideal to attempt redelivery, it is not required. If multiple retries are attempted, ideally they SHOULD be done with exponential backoff.
As stated above, there are a variable number of ways to declare information about the data therein. If any of items declared via subject or filename disagree with the report, the report MUST be considered the authoritative source.
The following are the IANA considerations discussed in this document.
Below is the Internet Assigned Numbers Authority (IANA) Permanent Message Header Field registration information per [@?RFC3864].
Header field name: TLS-Report-Domain
Applicable protocol: mail
Status: standard
Author/Change controller: IETF
Specification document(s): this one
Header field name: TLS-Report-Submitter
Applicable protocol: mail
Status: standard
Author/Change controller: IETF
Specification document(s): this one
This document creates a new registry for "report-type" parameter to the Content-Type header field for the "multipart/report" top-level media type defined in [@!RFC6522].
The registry name is "Report Type Registry", and the procedure for updating the registry will be "Specification Required".
An entry in this registry should contain:
-
the report-type being registered
-
one or more registered media-types that can be used with this report-type
-
the document containing the registration action
-
an optional comment
The initial entries are:
Report-Type: tlsrpt Media Type: application/tlsrpt+gzip, application/tlsrpt+json Registered By: [RFCXXXX] Comment: Media types suitable for use with this report-type are defined in Sections 6.4 and 6.5 of [RFCXXXX]
Report-Type: disposition-notification Media Type: message/disposition-notification Registered By: [@?RFC8098] Section 10
Report-Type: disposition-notification Media Type: message/global-disposition-notification Registered By: [@?RFC6533] Section 6
Report-Type: delivery-status Media Type: message/delivery-status Registered By: [@?RFC3464] Appendix D
Report-Type: delivery-status Media Type: message/global-delivery-status Registered By: [@?RFC6533] Section 6
This document registers a new media type suffix "+gzip". The GZIP format is a public domain, cross-platform, interoperable file storage and transfer format, specified in [@!RFC1952]; it supports compression and is used as the underlying representation by a variety of file formats. The media type "application/gzip" has been registered for such files. The suffix "+gzip" MAY be used with any media type whose representation follows that established for "application/gzip". The media type structured syntax suffix registration form follows:
Type name: GZIP file storage and transfer format
+suffix: +gzip
References: [@!RFC1952][@!RFC6713]
Encoding considerations: GZIP is a binary encoding.
Fragment identifier considerations: The syntax and semantics of fragment identifiers specified for +gzip SHOULD be as specified for "application/gzip". (At publication of this document, there is no fragment identification syntax defined for "application/gzip".) The syntax and semantics for fragment identifiers for a specific "xxx/yyy+gzip" SHOULD be processed as follows:
For cases defined in +gzip, where the fragment identifier
resolves per the +gzip rules, then process as specified in
+gzip.
For cases defined in +gzip, where the fragment identifier does
not resolve per the +gzip rules, then process as specified in
"xxx/yyy+gzip".
For cases not defined in +gzip, then process as specified in
"xxx/yyy+gzip".
Interoperability considerations: n/a
Security considerations: GZIP format doesn't provide confidentiality protection. Integrity protection is provided by an Adler-32 checksum, which is not cryptographically strong. See also security considerations of [@?RFC6713]. Each individual media type registered with a +gzip suffix can have additional security considerations. Additionally, GZIP objects can contain multiple files and associated paths. File paths must be validated when the files are extracted; a malicious file path could otherwise cause the extractor to overwrite application or system files.
Contact: [email protected]
Author/Change controller: Internet Engineering Task Force (mailto:[email protected]).
This document registers multiple media types, beginning with Table 1 below.
+-------------+----------------+-------------+-------------------+
| Type | Subtype | File extn | Specification |
+-------------+----------------+-------------+-------------------+
| application | tlsrpt+json | .json | Section 5.3 |
+-------------+----------------+-------------+-------------------+
Table 1: SMTP TLS Reporting Media Type
Type name: application
Subtype name: tlsrpt+json
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: Encoding considerations are identical to
those specified for the application/json media type. See
[@!RFC7493].
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: Section 5.3 of this document.
Applications that use this media type: Mail User Agents (MUA) and Mail Transfer Agents.
Additional information:
Magic number(s): n/a
File extension(s): ".json"
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:[email protected]).
+-------------+----------------+-------------+-------------------+
| Type | Subtype | File extn | Specification |
+-------------+----------------+-------------+-------------------+
| application | tlsrpt+gzip | .gz | Section 5.3 |
+-------------+----------------+-------------+-------------------+
Table 2: SMTP TLS Reporting Media Type
Type name: application
Subtype name: tlsrpt+gzip
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: Binary
Security considerations: Security considerations relating to SMTP TLS Reporting are discussed in Section 7. Security considerations related to gzip compression are discussed in [RFC6713].
Interoperability considerations: This document specifies format of conforming messages and the interpretation thereof.
Published specification: Section 5.3 of this document.
Applications that use this media type: Mail User Agents (MUA) and Mail Transfer Agents.
Additional information:
Magic number(s): The first two bytes are 0x1f, 0x8b.
File extension(s): ".gz"
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:[email protected]).
This document creates a new registry, "STARTTLS Validation Result Types". The initial entries in the registry are:
+-------------------------------+-----------+
| Result Type | Desc |
+-------------------------------+-----------+
| "starttls-not-supported" | 4.3 |
| "certificate-host-mismatch" | 4.3 |
| "certificate-expired" | 4.3 |
| "tlsa-invalid" | 4.3 |
| "dnssec-invalid" | 4.3 |
| "dane-required" | 4.3 |
| "certificate-not-trusted" | 4.3 |
| "sts-policy-invalid" | 4.3 |
| "sts-webpki-invalid" | 4.3 |
| "validation-failure" | 4.3 |
+-------------------------------+-----------+
The above entries are described in section (#result-types), "Result Types." New result types can be added to this registry using "Expert Review" IANA registration policy.
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:
-
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.
-
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.
-
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.
-
Ignoring HTTPS validation when submitting reports: When reporting benign misconfigurations, it is likely that a misconfigured SMTP server may also mean a misconfigured HTTPS server; as a result, reporters who required HTTPS validity on the reporting endpoint may fail to alert administrators about such misconfigurations. Conversely, in the event of an actual attack, an attacker who wished to create a gap in reporting and could intercept HTTPS reports could, just as easily, simply thwart the resolution of the TLSRPT TXT record or establishment of the TCP session to the HTTPS endpoint. Furthermore, such a man-in-the-middle attacker could discover most or all of the metadata exposed in a report merely through passive observation. As a result, we consider the risks of failure to deliver reports on misconfigurations to outweigh those of attackers intercepting reports.
-
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 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.
Finally, because TLSRPT is intended to help administrators discover man-in-the-middle attacks against transport-layer encryption, including attacks designed to thwart negotiation of encrypted connections (by downgrading opportunistic encryption or, in the case of MTA-STS, preventing discovery of a new MTA-STS policy), we must also consider the risk that an adversary who can induce such a downgrade attack can also prevent discovery of the TLSRPT TXT record (and thus prevent discovery of the successful downgrade attack). Administrators are thus encouraged to deploy TLSRPT TXT records with a large TTL (reducing the window for successful application of transient attacks against DNS resolution of the record) or to deploy DNSSEC on the deploying zone.
MTAs are generally considered public knowledge, however, the internals of how those MTAs are configured and the users of those MTAs may not be as public. It should be noted that when providing a receiving site with information, it may reveal information about the sender's configuration, or even information about the senders themselves. Consider that by sending a report, it might disclose your SSL library version as the inability to negotiate a session may be a known incompatbility between two library versions, or perhaps commonly used in a operating system release that is centered in a certain region. The risk may be minimal, but should be considered.
{backmatter}
_smtp._tls.mail.example.com. IN TXT \
"v=TLSRPTv1;rua=mailto:[email protected]"
_smtp._tls.mail.example.com. IN TXT \
"v=TLSRPTv1; \
rua=https://reporting.example.com/v1/tlsrpt"
Below is an example JSON report for 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".
{
"organization-name": "Company-X",
"date-range": {
"start-datetime": "2016-04-01T00:00:00Z",
"end-datetime": "2016-04-01T23:59:59Z"
},
"contact-info": "[email protected]",
"report-id": "5065427c-23d3-47ca-b6e0-946ea0e8c4be",
"policies": [{
"policy": {
"policy-type": "sts",
"policy-string": ["version: STSv1","mode: testing",
"mx: *.mail.company-y.example","max_age: 86400"],
"policy-domain": "company-y.example",
"mx-host": "*.mail.company-y.example"
},
"summary": {
"total-successful-session-count": 5326,
"total-failure-session-count": 303
},
"failure-details": [{
"result-type": "certificate-expired",
"sending-mta-ip": "2001:db8:abcd:0012::1",
"receiving-mx-hostname": "mx1.mail.company-y.example",
"failed-session-count": 100
}, {
"result-type": "starttls-not-supported",
"sending-mta-ip": "2001:db8:abcd:0013::1",
"receiving-mx-hostname": "mx2.mail.company-y.example",
"receiving-ip": "203.0.113.56",
"failed-session-count": 200,
"additional-information": "https://reports.company-x.example/
report_info ? id = 5065427 c - 23 d3# StarttlsNotSupported "
}, {
"result-type": "validation-failure",
"sending-mta-ip": "198.51.100.62",
"receiving-ip": "203.0.113.58",
"receiving-mx-hostname": "mx-backup.mail.company-y.example",
"failed-session-count": 3,
"failure-reason-code": "X509_V_ERR_PROXY_PATH_LENGTH_EXCEEDED"
}]
}]
}