MARF Working Group H. Fontana
Internet-Draft December 2, 2011
Intended status: Standards Track
Expires: June 4, 2012
Authentication Failure Reporting using the Abuse Report Format
draft-ietf-marf-authfailure-report-05
Abstract
This memo registers an extension report type to ARF for use in
reporting messages that fail one or more authentication checks
performed on receipt of a message, with the option to include
forensic information describing the specifics of the failure.
Status of this Memo
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This Internet-Draft will expire on June 4, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Base 64 . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Extension ARF Fields for Authentication Failure Reporting . . 5
3.1. New ARF Feedback Type . . . . . . . . . . . . . . . . . . 5
3.2. New ARF Header Field Names . . . . . . . . . . . . . . . . 6
3.2.1. Required For All Reports . . . . . . . . . . . . . . . 6
3.2.2. Optional For All Reports . . . . . . . . . . . . . . . 6
3.2.3. Required For DKIM Reports . . . . . . . . . . . . . . 6
3.2.4. Optional For DKIM Reports . . . . . . . . . . . . . . 7
3.2.5. Required For ADSP Reports . . . . . . . . . . . . . . 7
3.2.6. Required For SPF Reports . . . . . . . . . . . . . . . 7
3.3. Authentication Failure Types . . . . . . . . . . . . . . . 7
4. Syntax For Added ARF Header Fields . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. Updates to ARF Feedback Types . . . . . . . . . . . . . . 10
5.2. Updates to ARF Header Field Names . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6.1. Inherited Considerations . . . . . . . . . . . . . . . . . 12
6.2. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Automatic Generation . . . . . . . . . . . . . . . . . . . 12
6.4. Envelope Sender Selection . . . . . . . . . . . . . . . . 13
6.5. Reporting Multiple Incidents . . . . . . . . . . . . . . . 13
6.6. Redaction of Data in DKIM Reports . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 16
Appendix B. Example . . . . . . . . . . . . . . . . . . . . . . . 17
B.1. Example Use of ARF Extension Headers . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
[ARF] 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 format to include reporting of messages that
fail to authenticate using known authentication methods, 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 specific reason the authentication method
failed. Thus, this memo presents such extensions to the Abuse
Reporting Format to allow for detailed reporting of message
authentication failures.
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2. Definitions
2.1. Keywords
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 [KEYWORDS].
2.2. Base 64
base64 is defined in [MIME].
The values that are base64 encodings may contain FWS for formatting
purposes as per the usual header field wrapping defined in [MAIL].
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 [DKIM].
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3. Extension ARF Fields for Authentication Failure Reporting
The current report format defined in [ARF] lacks some specific
features required to do effective sender authentication reporting.
This section defines extensions to ARF to accommodate this
requirement.
3.1. New ARF Feedback Type
A new feedback type of "auth-failure" is defined as an extension to
Section 8.2 of [ARF]. See Section 3.3 for details.
A message that uses this feedback type has the following modified
header field requirements for the second (machine-parseable) [MIME]
part of the report:
Authentication-Results: MUST appear exactly once. It MUST be
formatted according to [AUTH-RESULTS], and MUST reflect only a
single authentication failure. To report multiple failures for a
single message, multiple reports MUST be generated. This
indicates to the receiver the failure that generated the report.
Original-Envelope-Id: As specified in [ARF]. This field SHOULD be
included exactly once if available to the entity generating the
report.
Original-Mail-From: As specified in [ARF]. This field SHOULD be
included exactly once for SPF, or for other methods that evaluate
authentication during the SMTP phase.
Source-IP: As specified in [ARF]. This field SHOULD be included
exactly once for SPF, or for other methods that evaluate
authentication during the SMTP phase.
Reported-Domain: As specified in [ARF]. This field MUST appear at
least once.
Delivery-Result: As specified in Section 3.2.1. This field is
OPTIONAL, but MUST NOT appear more than once. If present, it
SHOULD indicate the outcome of the message in some meaningful way,
but might be redacted to 'other' for local policy reasons.
The third MIME part of the message is either of type "message/rfc822"
(as defined in [MIME-TYPES]) or "text/rfc822-headers" (as defined in
[REPORT]) and contains a copy of the entire header block from the
original message. This part MUST be included (contrary to [REPORT]).
For privacy reasons, report generators might need to redact portions
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of a reported message such as the end user whose complaint action
resulted in the report. See [I-D.IETF-MARF-REDACTION] for a
discussion of this.
3.2. New ARF Header Field Names
The following new ARF field names are defined as extensions to
Section 3.1 of [ARF].
3.2.1. Required For All Reports
Auth-Failure: Indicates the type of authentication failure that is
being reported. The list of valid values is enumerated in
Section 3.3.
3.2.2. Optional For All Reports
Delivery-Result: The final message disposition that was enacted by
the ADMD generating the report and MUST NOT appear more than once.
Possible values are:
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 authentication failure. 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.
3.2.3. Required For DKIM Reports
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.
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DKIM-Selector: The selector of the signature that failed
verification, taken from the "s=" tag of the signature.
3.2.4. Optional For DKIM Reports
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 bytes that contribute to the DKIM body
hash (i.e., the value of the "l=" tag; see Section 3.7 of [DKIM].
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 [DKIM] 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 Section 5 and Section 7.6 for further discussion.)
3.2.5. Required For ADSP Reports
DKIM-ADSP-DNS: Includes the ADSP record discovered and applied by the
entity generating this report.
3.2.6. Required For SPF Reports
SPF-DNS MUST appear once for every SPF record used to obtain the SPF
result.
3.3. Authentication Failure Types
The list of defined authentication failure types, used in the "Auth-
Failure:" header field (defined above), is as follows:
adsp: The message did not conform to the sender's published [ADSP]
signing practises. 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 Section 3.2.4).
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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.
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 Section 3.2.4).
spf: The evaluation of the sending domain's SPF record produced a
"fail", "softfail", "temperror" or "permerror" result.
Supplementary data MAY be included in the form of [MAIL]-compliant
comments. 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 Appendix B for an
example.
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4. Syntax For Added ARF Header Fields
The ABNF definitions for the new fields are as follows:
auth-failure = "Auth-Failure:" [CFWS] token [CFWS] CRLF
; "token" must be a registered authentication failure type
; as specified elsewhere in this memo
; "token" is imported from [MIME]
; "CFWS" is imported from [MAIL]
delivery-result = "Delivery-Result:" [CFWS]
( "delivered" / "spam" /"policy" /
"reject" / "other" ) [CFWS] CRLF
dkim-header = "DKIM-Canonicalized-Header:" [CFWS]
base64string CRLF
; "base64string" is imported from [DKIM]
dkim-sig-domain = "DKIM-Domain:" [CFWS] dkim-domain [CFWS] CRLF
; "dkim-domain" is imported from [DKIM]
dkim-identity = "DKIM-Identity:" [CFWS] [ local-part ] "@"
domain-name [CFWS] CRLF
; "local-part" is imported from [MAIL]
dkim-selector = "DKIM-Selector:" [CFWS] token [CFWS] CRLF
dkim-adsp-dns = "DKIM-ADSP-DNS:" [CFWS]
quoted-string [CFWS] CRLF
; "quoted-string" is imported from [MAIL]
dkim-body = "DKIM-Canonicalized-Body:" [CFWS]
base64string CRLF
dkim-selector-dns = "DKIM-Selector-DNS:" [CFWS]
quoted-string [CFWS] CRLF
spf-dns = "SPF-DNS:" : { "txt" / "spf" } [FWS] ":" [FWS]
domain [FWS] ":" [FWS] quoted-string
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5. IANA Considerations
As required by [IANA], this section contains registry information for
the new tag, and the extension to [ARF].
5.1. Updates to ARF Feedback Types
The following feedback type is added to the Feedback Report Feedback
Type Registry:
Feedback Type: auth-failure
Description: sender authentication failure report
Registration: (this document)
5.2. Updates to ARF Header Field Names
The following headers are added to the Feedback Report Header Names
Registry:
Field Name: Auth-Failure
Description: Type of authentication failure
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Field Name: Delivery-Result
Description: Final disposition of the subject message
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Field Name: DKIM-ADSP-DNS
Description: Retrieved DKIM ADSP record
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Field Name: DKIM-Canonicalized-Body
Description: Canonicalized body, per DKIM
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Field Name: DKIM-Canonicalized-Header
Description: Canonicalized header, per DKIM
Multiple Appearances: No
Related "Feedback-Type": auth-failure
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Field Name: DKIM-Domain
Description: DKIM signing domain from "d=" tag
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Field Name: DKIM-Identity
Description: Identity from DKIM signature
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Field Name: DKIM-Selector
Description: Selector from DKIM signature
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Field Name: DKIM-Selector-DNS
Description: Retrieved DKIM key record
Multiple Appearances: No
Related "Feedback-Type": auth-failure
Field Name: SPF-DNS
Description: Retrieved SPF record
Multiple Appearances: No
Related "Feedback-Type": auth-failure
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6. Security Considerations
Security issues with respect to these reports are similar to those
found in [DSN].
6.1. Inherited Considerations
Implementers are advised to consider the Security Considerations
sections of [DKIM], [ADSP] [SPF] and [ARF].
6.2. Forgeries
These reports may 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
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 authentication 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 may
produce reports that aren't trusted or even accepted by their
intended recipients.
6.3. Automatic Generation
Automatic generation of these reports by verifying agents can cause a
denial-of-service attack when a large volume of e-mail is sent that
causes sender authentication failures for whatever reason.
Limiting the rate of generation of these messages may 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
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discovered data in the DNS.
6.4. Envelope Sender Selection
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 [ARF] for further information.
6.5. Reporting Multiple Incidents
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 innundated 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 [ARF] 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 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
may decide only to send a report about 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.
6.6. Redaction of Data in DKIM Reports
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.
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7. References
7.1. Normative References
[ADSP] Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM
Sender Signing Practises", RFC 5617, August 2009.
[ARF] Shafranovich, Y., Levine, J., and M. Kucherawy, "An
Extensible Format for Email Feedback Reports", RFC 5965,
August 2010.
[AUTH-RESULTS]
Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[DKIM] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", RFC 6376,
September 2011.
[I-D.IETF-MARF-REDACTION]
Falk, JD., "Redaction of Potentially Sensitive Data from
Mail Abuse Reports", I-D draft-ietf-marf-redaction,
March 2011.
[IANA] Alvestrand, H. and T. Narten, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226, May 2008.
[KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[MAIL] Resnick, P., "Internet Message Format", RFC 5322,
October 2008.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[MIME-TYPES]
Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[REPORT] Vaudreuil, G., "The Multipart/Report Content Type for the
Reporting of Mail System Administrative Messages",
RFC 3462, January 2003.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
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October 2008.
[SPF] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.
7.2. Informative References
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
January 2003.
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Appendix A. Acknowledgements
The authors wish to acknowledge the following for their review and
constructive criticism of this proposal: Frank Ellerman, J.D. Falk,
Scott Kitterman, John Levine, Mike Markley, Kelly Wanser, Murray
Kucherawy and Alessandro Vesely.
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Appendix B. Example
This section contains an example of the use of the extension defined
by this memo.
B.1. Example Use of ARF Extension Headers
An ARF-formatted report using the proposed ARF extension fields:
Received: from mail.example.com (mail.example.com [192.0.2.1])
by mx.example.net (8.14.4/8.14.4) with ESMTP id c6cs67945pbm;
Sat, 8 Oct 2011 13:16:24 +0000 (GMT)
Return-Path: feedback@arf.mail.example.net
Authentication-Results: mx.example.net; spf=pass (example.net: domain
of feedback@arf.mail.example.net designates 192.0.2.1 as permitted
sender) smtp.mail=feedback@arf.mail.example.net
Message-ID: <433689.81121.example@mta.mail.example.net>
From: "Someisp Mail Antispam Feedback" <feedback@mail.example.net>
To: arf-failure@example.com
Subject: FW: You have a new bill from your bank
Date: 8 Oct 2011 13:16:24 +0000 (GMT)
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 anexample.example.com on 8 Oct 2011 13:16:24
+0000(GMT). For more information about this format please see
http://datatracker.ietf.org/doc/draft-ietf-marf-authfailure-report
--------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg
Content-Type: message/feedback-report
Content-Disposition: inline
Content-Transfer-Encoding: 7bit
Feedback-Type: auth-failure
User-Agent: Someisp!-Mail-Feedback/1.0
Version: 0.1
Original-Mail-From: anexample@anexample.example.com
Authentication-Results: mta1011.mail.tp2.example.net
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from=anexample.example.com; dkim=fail (bodyhash)
Auth-Failure: bodyhash
DKIM-Canonicalized-Body:VGhpcyBpcyBhIG1lc3NhZ2UgYm9ke
SB0aGF0IGdvdCBtb2RpZmllZCBpbiB0cmFuc2l0Lgo=
Arrival-Date: 8 Oct 2011 13:16:24 +0000(GMT)
Source-IP: 192.0.2.1
Reported-Domain: anexample.example.com
Policy-Action: none
Reported-URI:http://www.example.com/
--------------Boundary-00=_3BCR4Y7kX93yP9uUPRhg
Content-Type: text/rfc822-headers
Content-Transfer-Encoding: 7bit
Authentication-Results: mta1011.mail.tp2.example.net
from=anexample.example.com; dkim=fail (bodyhash); spf=pass
DKIM-Signature: v=1; c=relaxed/simple; a=rsa-sha256;
s=testkey; d=example.net; h=From:To:Subject:Date;
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
4bmp/YzhwvcubU4=
Received: from smtp-out.example.net by mail.example.com
with SMTP id o3F52gxO029144;
Sat, 08 Oct 2011 13:15:31 -0700 (PDT)
Received: from internal-client-001.example.com
by mail.example.com
with SMTP id o3F3BwdY028431;
Sat, 08 Oct 2011 13:12:09 -0700 (PDT)
Date: Sat, 8 Oct 2011 13:16:24 -0400 (EDT)
Reply-To: anexample.reply@anexample.example.com
From: anexample@anexample.example.com
Subject: You have a new bill
Message-ID: <87913910.1318094604546>
--------------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 "example.com"
failed
o The cause for the verification failure was a mismatch between the
body contents observed at the verifier and the body hash contained
in the signature.
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Author's Address
Hilda L. Fontana
3579 E. Foothill Blvd., suite 282
Pasadena, CA 91107
US
Phone: +1 626 676 8852
Email: hilda@hfontana.com
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