Internet Engineering Task Force (IETF) H. Fontana Request for Comments: 6591 April 2012 Category: Standards Track ISSN: 2070-1721
Authentication Failure Reporting Using the Abuse Reporting Format
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 Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6591.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................2 2. Definitions .....................................................3 2.1. Key Words ..................................................3 2.2. Email Architecture .........................................3 2.3. Base64 .....................................................3 2.4. Technologies ...............................................3 3. ARF Extension for Authentication Failure Reporting ..............3 3.1. New ARF Feedback Type ......................................4 3.2. New ARF Header Field Names .................................5 3.2.1. Required for All Reports ............................5 3.2.2. Optional for All Reports ............................5 3.2.3. Required for DKIM Reports ...........................5 3.2.4. Optional for DKIM Reports ...........................6 3.2.5. Required for ADSP Reports ...........................6 3.2.6. Required for SPF Reports ............................6 3.3. Authentication Failure Types ...............................6 4. Syntax for Added ARF Header Fields ..............................7 5. IANA Considerations .............................................8 5.1. Updates to ARF Feedback Types ..............................8 5.2. Updates to ARF Header Field Names ..........................8 6. Security Considerations ........................................10 6.1. Inherited Considerations ..................................10 6.2. Forgeries .................................................10 6.3. Automatic Generation ......................................11 6.4. Envelope Sender Selection .................................11 6.5. Reporting Multiple Incidents ..............................11 6.6. Redaction of Data in DKIM Reports .........................12 7. References .....................................................12 7.1. Normative References ......................................12 7.2. Informative References ....................................13 Appendix A. Acknowledgements ......................................14 Appendix B. Example ...............................................14 B.1. Example Use of ARF Extension Headers .......................14
The Abuse Reporting Format [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 to include the reporting of messages that fail to authenticate using known message authentication methods, such as DomainKeys Identified Mail [DKIM] and Sender Policy Framework [SPF], 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.
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].
The values that are base64 encodings MAY contain folding whitespace (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]. No other extensions to the valid base64 character set are permitted.
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 [AUTH-RESULTS].
3. ARF Extension for Authentication Failure Reporting
The current report format defined in [ARF] 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.
A new feedback type, "auth-failure", is defined in this document as an extension, per Section 7.3 of [ARF].
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: Syntax as specified in [AUTH-RESULTS]. Furthermore, [ARF] 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 [ARF]. Furthermore, [ARF] 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 [ARF]. Furthermore, [ARF] 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 [ARF]. Furthermore, [ARF] 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 [ARF]. Furthermore, [ARF] 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 Section 3.2.2. 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 [MIME-TYPES]) or of type "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], 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 [RFC6590].
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.
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 [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 Sections 3.1 and 6.6 for further discussion.)
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 [ADSP].
SPF-DNS: This field MUST appear once for every SPF record [SPF] 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 Section 4.
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 [ADSP] 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 Section 3.2.4).
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 Section 3.2.4).
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 [SPF], 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 [MAIL]. 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.
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: [RFC6591] Status: current
Field Name: Delivery-Result Description: Final disposition of the subject message Multiple Appearances: No Related "Feedback-Type": auth-failure Published in: [RFC6591] 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: [RFC6591] Status: current
Field Name: DKIM-Canonicalized-Body Description: Canonicalized body, per DKIM Multiple Appearances: No Related "Feedback-Type": auth-failure Published in: [RFC6591] Status: current
Field Name: DKIM-Canonicalized-Header Description: Canonicalized header, per DKIM Multiple Appearances: No Related "Feedback-Type": auth-failure Published in: [RFC6591] Status: current
Field Name: DKIM-Domain Description: DKIM signing domain from "d=" tag Multiple Appearances: No Related "Feedback-Type": auth-failure Published in: [RFC6591] Status: current
Field Name: DKIM-Identity Description: Identity from DKIM signature Multiple Appearances: No Related "Feedback-Type": auth-failure Published in: [RFC6591] Status: current
Field Name: DKIM-Selector Description: Selector from DKIM signature Multiple Appearances: No Related "Feedback-Type": auth-failure Published in: [RFC6591] Status: current
Field Name: DKIM-Selector-DNS Description: Retrieved DKIM key record Multiple Appearances: No Related "Feedback-Type": auth-failure Published in: [RFC6591] Status: current
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Field Name: SPF-DNS Description: Retrieved SPF record Multiple Appearances: No Related "Feedback-Type": auth-failure Published in: [RFC6591] Status: current
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.
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.
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.
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 [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 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.
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.
[ABNF] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, "DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)", 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.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006.
[DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, September 2011.
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
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[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[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] Kucherawy, M., Ed., "The Multipart/Report Media Type for the Reporting of Mail System Administrative Messages", STD 73, RFC 6522, January 2012.
[RFC6590] Falk, J., Ed., and M. Kucherawy, Ed., "Redaction of Potentially Sensitive Data from Mail Abuse Reports", RFC 6590, April 2012.
[SPF] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 4408, April 2006.
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.
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: arf-failure@sender.example 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
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 [RFC6591].
Authentication-Results: mta1011.mail.tp2.receiver.example; dkim=fail (bodyhash) header.d=sender.example; spf=pass smtp.mailfrom=anexample.reply@a.sender.example 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: anexample.reply@a.sender.example
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From: anexample@a.sender.example To: someuser@receiver.example 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