6591
PROPOSED STANDARD
Authentication Failure Reporting Using the Abuse Reporting Format
Authors: H. Fontana
Date: April 2012
Area: app
Working Group: marf
Stream: IETF
Updated by:
RFC 6692
Abstract
This memo registers an extension report type for the Abuse Reporting Format (ARF), affecting multiple registries, for use in generating receipt-time reports about messages that fail one or more email message authentication checks. [STANDARDS-TRACK]
RFC 6591
PROPOSED STANDARD
Updated by: 6692
Internet Engineering Task Force (IETF) H. Fontana
Request for Comments: 6591 April 2012
Category: Standards Track
ISSN: 2070-1721
<span class="h1">Authentication Failure Reporting Using the Abuse Reporting Format</span>
Abstract
This memo registers an extension report type for the Abuse Reporting
Format (ARF), affecting multiple registries, for use in generating
receipt-time reports about messages that fail one or more email
message authentication checks.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="https://www.rfc-editor.org/info/rfc6591">http://www.rfc-editor.org/info/rfc6591</a>.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
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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.
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Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-2">2</a>
<a href="#section-2">2</a>. Definitions .....................................................<a href="#page-3">3</a>
<a href="#section-2.1">2.1</a>. Key Words ..................................................<a href="#page-3">3</a>
<a href="#section-2.2">2.2</a>. Email Architecture .........................................<a href="#page-3">3</a>
<a href="#section-2.3">2.3</a>. Base64 .....................................................<a href="#page-3">3</a>
<a href="#section-2.4">2.4</a>. Technologies ...............................................<a href="#page-3">3</a>
<a href="#section-3">3</a>. ARF Extension for Authentication Failure Reporting ..............<a href="#page-3">3</a>
<a href="#section-3.1">3.1</a>. New ARF Feedback Type ......................................<a href="#page-4">4</a>
<a href="#section-3.2">3.2</a>. New ARF Header Field Names .................................<a href="#page-5">5</a>
<a href="#section-3.2.1">3.2.1</a>. Required for All Reports ............................<a href="#page-5">5</a>
<a href="#section-3.2.2">3.2.2</a>. Optional for All Reports ............................<a href="#page-5">5</a>
<a href="#section-3.2.3">3.2.3</a>. Required for DKIM Reports ...........................<a href="#page-5">5</a>
<a href="#section-3.2.4">3.2.4</a>. Optional for DKIM Reports ...........................<a href="#page-6">6</a>
<a href="#section-3.2.5">3.2.5</a>. Required for ADSP Reports ...........................<a href="#page-6">6</a>
<a href="#section-3.2.6">3.2.6</a>. Required for SPF Reports ............................<a href="#page-6">6</a>
<a href="#section-3.3">3.3</a>. Authentication Failure Types ...............................<a href="#page-6">6</a>
<a href="#section-4">4</a>. Syntax for Added ARF Header Fields ..............................<a href="#page-7">7</a>
<a href="#section-5">5</a>. IANA Considerations .............................................<a href="#page-8">8</a>
<a href="#section-5.1">5.1</a>. Updates to ARF Feedback Types ..............................<a href="#page-8">8</a>
<a href="#section-5.2">5.2</a>. Updates to ARF Header Field Names ..........................<a href="#page-8">8</a>
<a href="#section-6">6</a>. Security Considerations ........................................<a href="#page-10">10</a>
<a href="#section-6.1">6.1</a>. Inherited Considerations ..................................<a href="#page-10">10</a>
<a href="#section-6.2">6.2</a>. Forgeries .................................................<a href="#page-10">10</a>
<a href="#section-6.3">6.3</a>. Automatic Generation ......................................<a href="#page-11">11</a>
<a href="#section-6.4">6.4</a>. Envelope Sender Selection .................................<a href="#page-11">11</a>
<a href="#section-6.5">6.5</a>. Reporting Multiple Incidents ..............................<a href="#page-11">11</a>
<a href="#section-6.6">6.6</a>. Redaction of Data in DKIM Reports .........................<a href="#page-12">12</a>
<a href="#section-7">7</a>. References .....................................................<a href="#page-12">12</a>
<a href="#section-7.1">7.1</a>. Normative References ......................................<a href="#page-12">12</a>
<a href="#section-7.2">7.2</a>. Informative References ....................................<a href="#page-13">13</a>
<a href="#appendix-A">Appendix A</a>. Acknowledgements ......................................<a href="#page-14">14</a>
<a href="#appendix-B">Appendix B</a>. Example ...............................................<a href="#page-14">14</a>
<a href="#appendix-B.1">B.1</a>. Example Use of ARF Extension Headers .......................<a href="#page-14">14</a>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
The Abuse Reporting Format [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] defines a message format for sending
reports of abuse in the messaging infrastructure, with an eye towards
automating both the generation and consumption of those reports.
There is now also a desire to extend the ARF to include the reporting
of messages that fail to authenticate using known message
authentication methods, such as DomainKeys Identified Mail [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>] and
Sender Policy Framework [<a href="#ref-SPF" title=""Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1"">SPF</a>], as these are sometimes evidence of
abuse that can be detected and reported through automated means. The
same mechanism can be used to convey forensic information about the
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specific reason the authentication method failed. Thus, this memo
presents such extensions to ARF that allow for detailed reporting of
message authentication method failures.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Definitions</span>
<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>. Key Words</span>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [<a href="#ref-KEYWORDS" title=""Key words for use in RFCs to Indicate Requirement Levels"">KEYWORDS</a>].
<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>. Email Architecture</span>
This memo uses some terms whose definitions and descriptions can be
found in [<a href="#ref-EMAIL-ARCH">EMAIL-ARCH</a>].
<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>. Base64</span>
Base64 is defined in Section 4 of [<a href="#ref-BASE64" title=""The Base16, Base32, and Base64 Data Encodings"">BASE64</a>].
The values that are base64 encodings MAY contain folding whitespace
(FWS) for formatting purposes as per the usual header field wrapping
defined in [<a href="#ref-MAIL" title=""Internet Message Format"">MAIL</a>]. During decoding, any characters not in the base64
alphabet are ignored so that such line wrapping does not harm the
value. The ABNF token "FWS" is defined in [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]. No other
extensions to the valid base64 character set are permitted.
<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>. Technologies</span>
There are technologies in email security that provide authentication
services and some that do authorization. These are often conflated.
A discussion that is useful for establishing context can be found in
Section 1.5.2 of [<a href="#ref-AUTH-RESULTS">AUTH-RESULTS</a>].
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. ARF Extension for Authentication Failure Reporting</span>
The current report format defined in [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] lacks some specific
features required to do effective email authentication failure
reporting. This section defines extensions to ARF to accommodate
this requirement.
A single report describes a single email authentication failure.
Multiple reports MAY be used to report multiple failures for a single
message.
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<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. New ARF Feedback Type</span>
A new feedback type, "auth-failure", is defined in this document as
an extension, per Section 7.3 of [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>].
A message that uses this feedback type has the following modified
header field requirements for the second (machine-parseable) [<a href="#ref-MIME" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">MIME</a>]
part of the report:
Authentication-Results: Syntax as specified in [<a href="#ref-AUTH-RESULTS">AUTH-RESULTS</a>].
Furthermore, [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] specifies this field is OPTIONAL and appears at
most once; for this extension, this field MUST be present, but it
MUST reflect only a single authentication method's result.
Original-Envelope-Id: Syntax as specified in [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>]. Furthermore,
[<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] specifies this field is OPTIONAL and appears at most once;
for this extension, this field's inclusion is RECOMMENDED, where
that value is available, to aid in diagnosing the authentication
failure.
Original-Mail-From: Syntax as specified in [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>]. Furthermore,
[<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] specifies this field is OPTIONAL and appears at most once;
for this extension, this field's inclusion is RECOMMENDED, where
that value is available, to aid in diagnosing the authentication
failure.
Source-IP: Syntax as specified in [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>]. Furthermore, [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>]
specifies this field is OPTIONAL and appears at most once; for
this extension, this field's inclusion is RECOMMENDED, where that
value is available, to aid in diagnosing the authentication
failure.
Reported-Domain: Syntax as specified in [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>]. Furthermore, [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>]
specifies this field is OPTIONAL and appears at most once; for
this extension, this field MUST be present if such a value is
available.
Delivery-Result: As specified in <a href="#section-3.2.2">Section 3.2.2</a>. This field is
OPTIONAL, but it MUST NOT appear more than once. If present, it
SHOULD indicate the outcome of the message in some meaningful way,
but it MAY be set to "other" for local policy reasons.
The third MIME part of the message is either of type "message/rfc822"
(as defined in [<a href="#ref-MIME-TYPES">MIME-TYPES</a>]) or of type "text/rfc822-headers" (as
defined in [<a href="#ref-REPORT" title=""The Multipart/Report Media Type for the Reporting of Mail System Administrative Messages"">REPORT</a>]) and contains a copy of the entire header block
from the original message. This part MUST be included (contrary to
[<a href="#ref-REPORT" title=""The Multipart/Report Media Type for the Reporting of Mail System Administrative Messages"">REPORT</a>], which makes it optional).
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For privacy reasons, report generators might need to redact portions
of a reported message, such as an identifier or address associated
with the end user whose complaint action resulted in the report. A
discussion of relevant issues and a suggested method for doing so can
be found in [<a href="./rfc6590" title=""Redaction of Potentially Sensitive Data from Mail Abuse Reports"">RFC6590</a>].
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. New ARF Header Field Names</span>
The following new ARF field names are defined as extensions to
Section 3.1 of [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>].
<span class="h4"><a class="selflink" id="section-3.2.1" href="#section-3.2.1">3.2.1</a>. Required for All Reports</span>
Auth-Failure: Indicates the failure from an email authentication
method that is being reported. The list of valid values is
enumerated in <a href="#section-3.3">Section 3.3</a>.
<span class="h4"><a class="selflink" id="section-3.2.2" href="#section-3.2.2">3.2.2</a>. Optional for All Reports</span>
Delivery-Result: The final message disposition that was enacted by
the ADministrative Management Domain (ADMD) generating the report.
It MUST NOT appear more than once. Possible values are as
follows:
delivered: The message was delivered (not specific as to where).
spam: The message was delivered to the recipient's spam folder
(or equivalent).
policy: The message was not delivered to the intended inbox due
to a failure from an email authentication method. The specific
action taken is not specified.
reject: The message was rejected.
other: The message had a final disposition not covered by one of
the above values.
<span class="h4"><a class="selflink" id="section-3.2.3" href="#section-3.2.3">3.2.3</a>. Required for DKIM Reports</span>
DKIM-Domain: The domain that signed the message, taken from the "d="
tag of the signature.
DKIM-Identity: The identity of the signature that failed
verification, taken from the "i=" tag of the signature.
DKIM-Selector: The selector of the signature that failed
verification, taken from the "s=" tag of the signature.
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<span class="h4"><a class="selflink" id="section-3.2.4" href="#section-3.2.4">3.2.4</a>. Optional for DKIM Reports</span>
DKIM-Canonicalized-Header: A base64 encoding of the canonicalized
header of the message as generated by the verifier.
DKIM-Canonicalized-Body: A base64 encoding of the canonicalized body
of the message as generated by the verifier. The encoded content
MUST be limited to those octets that contribute to the DKIM body
hash (i.e., the value of the "l=" tag; see Section 3.7 of [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]).
If DKIM-Canonicalized-Header and DKIM-Canonicalized-Body encode
redacted data, they MUST NOT be included. Otherwise, they SHOULD be
included. The data presented there have to be exactly the
canonicalized header and body as defined by [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>] and computed at
the verifier. This is because these fields are intended to aid in
identifying message alterations that invalidate DKIM signatures in
transit. Including redacted data in them renders the data unusable.
(See also Sections <a href="#section-3.1">3.1</a> and <a href="#section-6.6">6.6</a> for further discussion.)
<span class="h4"><a class="selflink" id="section-3.2.5" href="#section-3.2.5">3.2.5</a>. Required for ADSP Reports</span>
DKIM-ADSP-DNS: Includes the Author Domain Signing Practices (ADSP)
policy used to obtain the verifier's ADSP result. This MUST be
formatted per Section 4.2.1 of [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>].
<span class="h4"><a class="selflink" id="section-3.2.6" href="#section-3.2.6">3.2.6</a>. Required for SPF Reports</span>
SPF-DNS: This field MUST appear once for every SPF record [<a href="#ref-SPF" title=""Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1"">SPF</a>] used
to obtain the SPF result. It MUST include the DNS RRTYPE used,
the DNS domain from which the record was retrieved, and the
content of that record. The syntax is defined in <a href="#section-4">Section 4</a>.
<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>. Authentication Failure Types</span>
The list of defined email authentication failure types used in the
"Auth-Failure:" header field (defined above), is as follows:
adsp: The message did not conform to the author domain's published
[<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>] signing practices. The DKIM-ADSP-DNS field MUST be
included in the report.
bodyhash: The body hash in the signature and the body hash computed
by the verifier did not match. The DKIM-Canonicalized-Body field
SHOULD be included in the report (see <a href="#section-3.2.4">Section 3.2.4</a>).
revoked: The DKIM key referenced by the signature on the message has
been revoked. The DKIM-Domain and DKIM-Selector fields MUST be
included in the report.
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signature: The DKIM signature on the message did not successfully
verify against the header hash and public key. The DKIM-Domain
and DKIM-Selector fields MUST be included in the report, and the
DKIM-Canonicalized-Header field SHOULD be included in the report
(see <a href="#section-3.2.4">Section 3.2.4</a>).
spf: The evaluation of the author domain's SPF record produced a
"none", "fail", "softfail", "temperror", or "permerror" result.
("none" is not strictly a failure per [<a href="#ref-SPF" title=""Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1"">SPF</a>], but a service that
demands successful SPF evaluations of clients could treat it like
a failure.)
Supplementary data MAY be included in the form of comments compliant
with [<a href="#ref-MAIL" title=""Internet Message Format"">MAIL</a>]. For example, "Auth-Failure: adsp" could be augmented by
a comment to indicate that the failed message was rejected because it
was not signed when it should have been. See <a href="#appendix-B">Appendix B</a> for an
example.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Syntax for Added ARF Header Fields</span>
The [<a href="#ref-ABNF" title=""Augmented BNF for Syntax Specifications: ABNF"">ABNF</a>] definitions for the new fields are as follows:
auth-failure = "Auth-Failure:" [CFWS]
( "adsp" / "bodyhash" / "revoked" /
"signature" / "spf" ) [CFWS] CRLF
; "CFWS" is defined in [<a href="#ref-MAIL" title=""Internet Message Format"">MAIL</a>]
delivery-result = "Delivery-Result:" [CFWS]
( "delivered" / "spam" / "policy" /
"reject" / "other" ) [CFWS] CRLF
dkim-header = "DKIM-Canonicalized-Header:" [CFWS]
base64string CRLF
; "base64string" is defined in [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]
dkim-sig-domain = "DKIM-Domain:" [CFWS] domain-name [CFWS]
CRLF
; "domain-name" is defined in [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]
dkim-identity = "DKIM-Identity:" [CFWS] [ local-part ] "@"
domain-name [CFWS] CRLF
; "local-part" is defined in [<a href="#ref-MAIL" title=""Internet Message Format"">MAIL</a>]
dkim-selector = "DKIM-Selector:" [CFWS] selector [CFWS] CRLF
; "selector" is defined in [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]
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dkim-adsp-dns = "DKIM-ADSP-DNS:" [CFWS]
quoted-string [CFWS] CRLF
; "quoted-string" is defined in [<a href="#ref-MAIL" title=""Internet Message Format"">MAIL</a>]
dkim-body = "DKIM-Canonicalized-Body:" [CFWS]
base64string CRLF
dkim-selector-dns = "DKIM-Selector-DNS:" [CFWS]
quoted-string [CFWS] CRLF
spf-dns = "SPF-DNS:" [CFWS] ( "txt" / "spf" ) [CFWS] ":" [CFWS]
domain [CFWS] ":" [CFWS] quoted-string [CFWS] CRLF
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. IANA Considerations</span>
As required by [<a href="#ref-IANA" title="">IANA</a>], this section contains registry information for
the extension to [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>].
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Updates to ARF Feedback Types</span>
The following feedback type has been added to the Feedback Report
Type Values registry:
Feedback Type: auth-failure
Description: email authentication failure report
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Updates to ARF Header Field Names</span>
The following headers are added to the Feedback Report Header Fields
registry:
Field Name: Auth-Failure
Description: Type of email authentication method failure
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
Field Name: Delivery-Result
Description: Final disposition of the subject message
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
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Field Name: DKIM-ADSP-DNS
Description: Retrieved DKIM ADSP record
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
Field Name: DKIM-Canonicalized-Body
Description: Canonicalized body, per DKIM
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
Field Name: DKIM-Canonicalized-Header
Description: Canonicalized header, per DKIM
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
Field Name: DKIM-Domain
Description: DKIM signing domain from "d=" tag
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
Field Name: DKIM-Identity
Description: Identity from DKIM signature
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
Field Name: DKIM-Selector
Description: Selector from DKIM signature
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
Field Name: DKIM-Selector-DNS
Description: Retrieved DKIM key record
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
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Field Name: SPF-DNS
Description: Retrieved SPF record
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Published in: [<a href="./rfc6591">RFC6591</a>]
Status: current
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Security Considerations</span>
Security issues with respect to these reports are similar to those
found in [<a href="#ref-DSN" title=""An Extensible Message Format for Delivery Status Notifications"">DSN</a>].
<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. Inherited Considerations</span>
Implementers are advised to consider the Security Considerations
sections of [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>], [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>], [<a href="#ref-SPF" title=""Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1"">SPF</a>], and [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>].
<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. Forgeries</span>
These reports can be forged as easily as ordinary Internet electronic
mail. User agents and automatic mail-handling facilities (such as
mail distribution list exploders) that wish to make automatic use of
Delivery Status Notifications (DSNs) of any kind should take
appropriate precautions to minimize the potential damage from denial-
of-service attacks.
Security threats related to forged DSNs include the sending of
a. A falsified email authentication method failure notification when
the message was in fact delivered to the indicated recipient;
b. Falsified signature information, such as selector, domain, etc.
Perhaps the simplest means of mitigating this threat is to assert
that these reports should themselves be signed with something like
DKIM. On the other hand, if there's a problem with the DKIM
infrastructure at the verifier, signing DKIM failure reports might
produce reports that aren't trusted or even accepted by their
intended recipients.
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<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>. Automatic Generation</span>
Automatic generation of these reports by verifying agents can cause a
denial-of-service attack when a large volume of email is sent that
causes email authentication failures for whatever reason.
Limiting the rate of generation of these messages might be
appropriate but threatens to inhibit the distribution of important
and possibly time-sensitive information.
In general ARF feedback loop terms, it is suggested that report
generators only create these (or any) ARF reports after an out-of-
band arrangement has been made between two parties. This mechanism
then becomes a way to adjust parameters of an authorized abuse report
feedback loop that is configured and activated by private agreement
rather than starting to send them automatically based solely on
discovered data in the DNS.
<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>. Envelope Sender Selection</span>
In the case of transmitted reports in the form of a new message, it
is necessary to consider the construction and transmission of the
message so as to avoid amplification attacks, deliberate or
otherwise. See Section 5 of [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] for further information.
<span class="h3"><a class="selflink" id="section-6.5" href="#section-6.5">6.5</a>. Reporting Multiple Incidents</span>
If it is known that a particular host generates abuse reports upon
certain incidents, an attacker could forge a high volume of messages
that will trigger such a report. The recipient of the report could
then be inundated with reports. This could easily be extended to a
distributed denial-of-service attack by finding a number of report-
generating servers.
The incident count referenced in [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] provides a limited form of
mitigation. The host generating reports may elect to send reports
only periodically, with each report representing a number of
identical or near-identical incidents. One might even do something
inverse-exponentially, sending reports for each of the first ten
incidents, then every tenth incident up to 100, then every 100th
incident up to 1000, etc., until some period of relative quiet after
which the limitation resets.
The use of this technique for "near-identical" incidents in
particular causes a degradation in reporting quality, however. If,
for example, a large number of pieces of spam arrive from one
attacker, a reporting agent might decide only to send a report about
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a fraction of those messages. While this averts a flood of reports
to a system administrator, the precise details of each incident are
similarly not sent.
<span class="h3"><a class="selflink" id="section-6.6" href="#section-6.6">6.6</a>. Redaction of Data in DKIM Reports</span>
This memo requires that the canonicalized header and body be returned
without being subject to redaction when a DKIM failure is being
reported. This is necessary to ensure that the returned
canonicalized forms are useful for debugging, as they must be
compared to the equivalent form at the signer. If a message is
altered in transit, and the returned data are also redacted, the
redacted portion and the altered portion may overlap, rendering the
comparison results meaningless. However, unredacted data can leak
information the reporting entity considers to be private. It is for
this reason the return of the canonicalized forms is not required.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. References</span>
<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>. Normative References</span>
[<a id="ref-ABNF">ABNF</a>] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, <a href="./rfc5234">RFC 5234</a>,
January 2008.
[<a id="ref-ADSP">ADSP</a>] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", <a href="./rfc5617">RFC 5617</a>, August 2009.
[<a id="ref-ARF">ARF</a>] Shafranovich, Y., Levine, J., and M. Kucherawy, "An
Extensible Format for Email Feedback Reports", <a href="./rfc5965">RFC 5965</a>,
August 2010.
[<a id="ref-AUTH-RESULTS">AUTH-RESULTS</a>]
Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", <a href="./rfc5451">RFC 5451</a>, April 2009.
[<a id="ref-BASE64">BASE64</a>] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", <a href="./rfc4648">RFC 4648</a>, October 2006.
[<a id="ref-DKIM">DKIM</a>] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", <a href="./rfc6376">RFC 6376</a>,
September 2011.
[<a id="ref-IANA">IANA</a>] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc5226">RFC 5226</a>,
May 2008.
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[<a id="ref-KEYWORDS">KEYWORDS</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-MAIL">MAIL</a>] Resnick, P., Ed., "Internet Message Format", <a href="./rfc5322">RFC 5322</a>,
October 2008.
[<a id="ref-MIME">MIME</a>] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", <a href="./rfc2045">RFC 2045</a>, November 1996.
[<a id="ref-MIME-TYPES">MIME-TYPES</a>]
Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", <a href="./rfc2046">RFC 2046</a>,
November 1996.
[<a id="ref-REPORT">REPORT</a>] Kucherawy, M., Ed., "The Multipart/Report Media Type for
the Reporting of Mail System Administrative Messages",
STD 73, <a href="./rfc6522">RFC 6522</a>, January 2012.
[<a id="ref-RFC6590">RFC6590</a>] Falk, J., Ed., and M. Kucherawy, Ed., "Redaction of
Potentially Sensitive Data from Mail Abuse Reports",
<a href="./rfc6590">RFC 6590</a>, April 2012.
[<a id="ref-SPF">SPF</a>] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
<a href="./rfc4408">RFC 4408</a>, April 2006.
<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>. Informative References</span>
[<a id="ref-DSN">DSN</a>] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", <a href="./rfc3464">RFC 3464</a>,
January 2003.
[<a id="ref-EMAIL-ARCH">EMAIL-ARCH</a>]
Crocker, D., "Internet Mail Architecture", <a href="./rfc5598">RFC 5598</a>,
July 2009.
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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. Acknowledgements</span>
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: Frank Ellermann, J.D. Falk,
Scott Kitterman, John Levine, Mike Markley, Kelly Wanser, Murray
Kucherawy, and Alessandro Vesely.
<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>. Example</span>
This section contains an example of the use of the extension defined
by this memo.
<span class="h3"><a class="selflink" id="appendix-B.1" href="#appendix-B.1">B.1</a>. Example Use of ARF Extension Headers</span>
An ARF-formatted report using the proposed ARF extension fields:
Message-ID: <433689.81121.example@mta.mail.receiver.example>
From: "SomeISP Antispam Feedback" <feedback@mail.receiver.example>
To: [email protected]
Subject: FW: You have a new bill from your bank
Date: Sat, 8 Oct 2011 15:15:59 -0500 (CDT)
MIME-Version: 1.0
Content-Type: multipart/report;
boundary="------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg";
report-type=feedback-report
Content-Transfer-Encoding: 7bit
--------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg
Content-Type: text/plain; charset="us-ascii"
Content-Disposition: inline
Content-Transfer-Encoding: 7bit
This is an authentication failure report for an email message
received from a.sender.example on 8 Oct 2011 20:15:58 +0000 (GMT).
For more information about this format, please see [<a href="./rfc6591">RFC6591</a>].
--------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg
Content-Type: message/feedback-report
Content-Transfer-Encoding: 7bit
Feedback-Type: auth-failure
User-Agent: Someisp!Mail-Feedback/1.0
Version: 1
Original-Mail-From: [email protected]
Original-Envelope-Id: o3F52gxO029144
Authentication-Results: mta1011.mail.tp2.receiver.example;
dkim=fail (bodyhash) header.d=sender.example
Auth-Failure: bodyhash
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DKIM-Canonicalized-Body: VGhpcyBpcyBhIG1lc3NhZ2UgYm9keSB0
aGF0IGdvdCBtb2RpZmllZCBpbiB0cmFuc2l0LgoKQXQgdGhlIHNhbWU
gdGltZSB0aGF0IHRoZSBib2R5aGFzaCBmYWlscyB0byB2ZXJpZnksIH
RoZQptZXNzYWdlIGNvbnRlbnQgaXMgY2xlYXJseSBhYnVzaXZlIG9yI
HBoaXNoeSwgYXMgdGhlClN1YmplY3QgYWxyZWFkeSBoaW50cy4gIElu
ZGVlZCwgdGhpcyBib2R5IGFsc28gY29udGFpbnMKdGhlIGZvbGxvd2l
uZyB0ZXh0OgoKICAgUGxlYXNlIGVudGVyIHlvdXIgZnVsbCBiYW5rIG
NyZWRlbnRpYWxzIGF0CiAgIGh0dHA6Ly93d3cuc2VuZGVyLmV4YW1wb
GUvCgpXZSBhcmUgaW1wbHlpbmcgdGhhdCwgYWx0aG91Z2ggbXVsdGlw
bGUgZmFpbHVyZXMKcmVxdWlyZSBtdWx0aXBsZSByZXBvcnRzLCBhIHN
pbmdsZSBmYWlsdXJlIGNhbiBiZQpyZXBvcnRlZCBhbG9uZyB3aXRoIH
BoaXNoaW5nIGluIGEgc2luZ2xlIHJlcG9ydC4K
DKIM-Domain: sender.example
DKIM-Identity: @sender.example
DKIM-Selector: testkey
Arrival-Date: 8 Oct 2011 20:15:58 +0000 (GMT)
Source-IP: 192.0.2.1
Reported-Domain: a.sender.example
Reported-URI: <a href="http://www.sender.example/">http://www.sender.example/</a>
--------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg
Content-Type: text/rfc822-headers
Content-Transfer-Encoding: 7bit
Authentication-Results: mta1011.mail.tp2.receiver.example;
dkim=fail (bodyhash) header.d=sender.example;
spf=pass [email protected]
Received: from smtp-out.sender.example
by mta1011.mail.tp2.receiver.example
with SMTP id oB85W8xV000169;
Sat, 08 Oct 2011 13:15:58 -0700 (PDT)
DKIM-Signature: v=1; c=relaxed/simple; a=rsa-sha256;
s=testkey; d=sender.example; h=From:To:Subject:Date;
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
4bmp/YzhwvcubU4=
Received: from mail.sender.example
by smtp-out.sender.example
with SMTP id o3F52gxO029144;
Sat, 08 Oct 2011 13:15:31 -0700 (PDT)
Received: from internal-client-001.sender.example
by mail.sender.example
with SMTP id o3F3BwdY028431;
Sat, 08 Oct 2011 13:15:24 -0700 (PDT)
Date: Sat, 8 Oct 2011 16:15:24 -0400 (EDT)
Reply-To: [email protected]
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From: [email protected]
To: [email protected]
Subject: You have a new bill from your bank
Message-ID: <87913910.1318094604546@out.sender.example>
--------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg--
Example 1: Example ARF Report Using These Extensions
This example ARF message is making the following assertion:
o DKIM verification of the signature added within "sender.example"
failed.
o The cause of the verification failure was a mismatch between the
body contents observed at the verifier and the body hash contained
in the signature.
Author's Address
Hilda L. Fontana
3579 E. Foothill Blvd., Suite 282
Pasadena, CA 91107
US
Phone: +1 626 676 8852
EMail: [email protected]
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