MARF Working Group H. Fontana
Internet-Draft eCert Inc.
Intended status: Standards Track June 29, 2011
Expires: December 31, 2011
Authentication Failure Reporting using the Abuse Report Format
draft-ietf-marf-authfailure-report-00
Abstract
This memo registers an extension report type to ARF to be used for
reporting forensic information about messages that fail one or more
message authentication schemes in use by the purported sender of the
message.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 31, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Imported Definitions . . . . . . . . . . . . . . . . . . . 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. Required For DKIM Reports . . . . . . . . . . . . . . 6
3.3. Authentication Failure Types . . . . . . . . . . . . . . . 7
4. Syntax For Added ARF Header Fields . . . . . . . . . . . . . . 8
5. Redacting Data . . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. Updates to ARF Feedback Types . . . . . . . . . . . . . . 10
6.2. Updates to ARF Header Field Names . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7.1. Inherited Considerations . . . . . . . . . . . . . . . . . 12
7.2. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 12
7.3. Automatic Generation . . . . . . . . . . . . . . . . . . . 12
7.4. Envelope Sender Selection . . . . . . . . . . . . . . . . 13
7.5. Reporting Multiple Incidents . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 16
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 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 toward automating both the
generating and consumption of those reports. This memo presents
extensions to the Abuse Reporting Format (ARF) 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. Imported Definitions
The ABNF token "qp-section" is imported from [MIME].
base64 is defined in [MIME].
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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: This field MUST be formatted as defined in
[AUTH-RESULTS], except that it MUST include explicit results for
both DKIM and SPF even if those tests were not actually performed.
This field MUST appear at least once, and it is RECOMMENDED that
the corresponding header fields be copied directly from the
message about which a report is being generated.
Original-Envelope-Id: As specified in [ARF]. This field MUST appear
exactly once.
Original-Mail-From: As specified in [ARF]. This field MUST appear
exactly once.
Arrival-Date: As specified in [ARF]. This field MUST appear exactly
once.
Source-IP: As specified in [ARF]. This field MUST appear exactly
once. If this information is either not available at the time the
report is generated, or the generating ADMD's policy requires it
be redacted, a value of 0.0.0.0 MUST be used.
Message-ID: As specified in [ARF]. This field MUST appear exactly
once.
Reported-Domain: As specified in [ARF]. This field MUST appear
exactly once.
Delivery-Result: As specified in Section 3.2.1. This field MUST
appear exactly once.
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
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[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
of a reported message such as the end user whose complaint action
resulted in the report. See Section 5 for a discussion of this.
3.2. New ARF Header Field Names
The following new ARF field names are defined as extensions to
Section 6.2 of [ARF].
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].
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 below.
Delivery-Result: The final message disposition that was enacted by
the ADMD generating the report. Possible values are:
inbox: The message was delivered to the intended inbox.
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.
3.2.2. Required For DKIM Reports
DKIM-Canonicalized-Header: A base64 encoding of the canonicalized
header of the message as generated by the verifier.
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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.
3.3. Authentication Failure Types
The list of defined authentication failure types, used in the "Auth-
Failure:" header (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.
granularity: The DKIM key referenced by the signature on the message
was not authorized for use by the sender. The DKIM-Domain and
DKIM-Selector fields MUST be included in the report, and the DKIM-
Identity field SHOULD be included.
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,
DKIM-Selector and DKIM-Canonicalized-Header fields MUST be
included in the report.
spf: The evaluation of the sending domain's SPF record produced a
"fail" or "softfail" 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
examples.
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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
delivery-result = "Delivery-Result:" [CFWS]
( "inbox" / "spam" / "policy" /
"reject" / "other" ) [CFWS] CRLF
dkim-header = "DKIM-Canonicalized-Header:" [CFWS]
base64string CRLF
; "base64string" is imported from [DKIM]
dkim-domain = "DKIM-Domain:" [CFWS] domain [CFWS] CRLF
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:" [CFWS] quoted-string [CFWS] CRLF
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5. Redacting Data
For privacy considerations it might be the policy of a report
generator to redact, or obscure, portions of the report that might
identify an end user that caused the report to be generated.
Precisely how this is done is unspecified in [ARF] as it will
generally be a matter of local policy. That specification does
admonish generators against being overly zealous with this practice,
as obscuring too much data makes the report inactionable.
Previous redaction practices, such as replacing local-parts of
addresses with a uniform string like "xxxxxxxx", often frustrated any
kind of prioritizing or grouping of reports.
Generally, it is assumed that the recipient fields of a message (i.e.
those containing recipient addresses), when copied into a report, are
to be obscured to protect the identify of an end user that submitted
a complaint about a message. However, it is also presumed that other
data will be left intact, data that could be correlated against logs
to determine the source of the message that drew a complaint.
See [I-D.IETF-MARF-REDACTION] for further details.
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6. IANA Considerations
As required by [IANA-CONSIDERATIONS], this section contains registry
information for the new tag, and the extension to [ARF].
6.1. 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)
6.2. 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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7. Security Considerations
Security issues with respect to these reports are similar to those
found in [DSN].
7.1. Inherited Considerations
Implementors are advised to consider the Security Considerations
sections of [DKIM], [ADSP] [SPF] and [ARF].
7.2. 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.
7.3. 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.
7.4. Envelope Sender Selection
In the case of transmitted reports in the form of a new message, it
is necessary to construct the message so as to avoid amplification
attacks, deliberate or otherwise. Thus, per Section 2 of [DSN], the
envelope sender address of the report SHOULD be chosen to ensure that
no delivery status reports will be issued in response to the report
itself, and MUST be chosen so that these reports will not generate
mail loops. Whenever an [SMTP] transaction is used to send a report,
the MAIL FROM command MUST use a NULL return address, i.e. "MAIL
FROM:<>".
7.5. 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.
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8. References
8.1. 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] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[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-CONSIDERATIONS]
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 2822,
April 2001.
[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.
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[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
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.
8.2. 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: Mike Markley, Kelly Wanser
and Murray Kucherawy.
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Appendix B. Examples
This section contains examples of the use of each the extension
defined by this memo.
B.1. Example Use of ARF Extension Headers
An ARF-formatted report using some of the proposed ARF extension
fields:
From: arf-daemon@example.com
To: recipient@example.net
Subject: This is a test
Date: Wed, 14 Apr 2010 12:17:45 -0700 (PDT)
MIME-Version: 1.0
Content-Type: multipart/report; report-type=feedback-report;
boundary="part1_13d.2e68ed54_boundary"
--part1_13d.2e68ed54_boundary
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
This is an email abuse report for an email message received
from IP 192.0.2.1 on Wed, 14 Apr 2010 12:15:31 PDT. For more
information about this format please see
http://www.mipassoc.org/arf/.
--part1_13d.2e68ed54_boundary
Content-Type: message/feedback-report
Feedback-Type: auth-failure
User-Agent: SomeDKIMFilter/1.0
Version: 1.0
Original-Mail-From: <randomuser@example.net>
Original-Rcpt-To: <user@example.com>
Received-Date: Wed, 14 Apr 2010 12:15:31 -0700 (PDT)
Source-IP: 192.0.2.1
Authentication-Results: mail.example.com; dkim=fail
header.d=example.net
Reported-Domain: example.net
DKIM-Domain: example.net
DKIM-Failure: bodyhash
--part1_13d.2e68ed54_boundary
Content-Type: message/rfc822
DKIM-Signature: v=1; c=relaxed/simple; a=rsa-sha256;
s=testkey; d=example.net; h=From:To:Subject:Date;
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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;
Wed, 14 Apr 2010 12:15:31 -0700 (PDT)
Received: from internal-client-001.example.com
by mail.example.com
with SMTP id o3F3BwdY028431;
Wed, 14 Apr 2010 12:12:09 -0700 (PDT)
From: randomuser@example.net
To: user@example.com
Date: Wed, 14 Apr 2010 12:12:09 -0700 (PDT)
Subject: This is a test
Hi, just making sure DKIM is working!
--part1_13d.2e68ed54_boundary--
Example 3: Example ARF report using these extensions
This example ARF message is making the following assertion:
o DKIM verification of the signature added within "example.net"
failed when it was processed on arrival at "mail.example.com".
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
eCert Inc.
One Market Street Suite 3600
San Francisco, CA 94107
US
Phone: +1 626 676 8852
Email: hfontana@ecertsystems.com
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