| DMARC Working Group | S. Jones |
| Internet-Draft | DMARC.org |
| Intended status: Informational | K. Andersen |
| Expires: October 25, 2018 | |
| J. Rae-Grant | |
| T. Adams, Ed. | |
| Paypal | |
| April 23, 2018 |
Recommended Usage of the Authenticated Received Chain (ARC)
draft-ietf-dmarc-arc-usage-05
The Authentication Received Chain (ARC) provides a means to preserve email authentication results and verify the identity of email message handlers, each of which participates by inserting certain header fields before passing the message on. But the specification does not indicate how intermediaries and receivers should interpret or utilize ARC. This document will provide guidance in these areas.
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[ARC] is intended to be used primarily by intermediaries, or message handlers - those parties who may forward or resend messages, with or without alterations, such that they will no longer pass the SPF, DKIM, and/or [RFC7489] authentication mechanisms. In such cases ARC may provide the final message recipient with useful information about the original sender.
Consider a mailing list as an example, where the message submitter’s domain publishes a DMARC policy other than “p=none”. The message is received, a prefix is added to the RFC5322.Subject header field, some text is appended to the message body, and the message is sent to list members with the original RFC5322.From address intact. In this case SPF may pass because the mailing list operator uses their own domain in the RFC5321.MailFrom header field, but this domain will not match the RFC5322.From address, thus the DMARC SPF result cannot be a “pass.” Any DKIM signature from the message submitter’s domain will be broken as the message body has been altered (and if included in the signature, the RFC5322.Subject header field). Again, the DMARC DKIM result cannot be a “pass.” And if the mailing list operator inserted an Authentication-Results header field it was most likely stripped and/or replaced by the next message receiver.
If the mailing list implemented ARC, it would record the contents of the Authentication-Results header field in the ARC-Authentication-Results header field. It would then create an an ARC-Message-Signature header field, which includes a cryptographic signature of the message itself, and then an ARC-Seal header field, which includes a cryptographic signature of a few key message header fields - including the other ARC header fields.
Any subsequent system participating in ARC that was not performing final delivery of the message within its ADMD boundaries would also generate and insert ARC header fields whose signatures cover all ARC header fields inserted into the message by previous message handlers. Thus the information from any previous ARC participants, including the ARC-Authentication-Results header field from the mailing list operator, would be signed at each ADMD that handled the message.
When the message reaches the final receiving system, the SPF and DKIM results will not satisfy the DMARC policy for the message author’s domain. However if the receiving system implements ARC then it can check for and validate an ARC chain and verify that the contents of the ARC-Authentication-Results header field were conveyed intact from the mailing list operator. At that point the receiving system might choose to use those authentication results in the decision of whether or not to deliver the message, even though it failed to pass the usual authentication checks.
An intact ARC chain conveys authentication results like SPF and DKIM as observed by the first ARC participant. In cases where the message no longer produces passing results for DKIM, SPF, or DMARC but an intact ARC chain is present, the message receiver may choose to use the contents of the ARC-Authentication-Results header field in determining how to handle the message.
Note that not all ADMDs will implement ARC, and receivers will see messages where one or more non-participating ADMDs handled a message before, after, or in between participating ADMDs.
An intact ARC chain is one where the ARC header fields that are present can be validated, and in particular the ARC-Message-Signature header field from the last ARC participant can still be validated. This shows that, whether another ADMD handled the message after the last ARC participant or not, the portions of the message covered by that signature were not altered. If any non-participating ADMDs handled the message between ARC intermediaries but did not alter the message in a way that invalidated the most recent ARC-Message-Signature present at that time, the chain would still be considered intact by the next ARC participant, and recorded as such in the ARC-Seal header field they insert.
Message receivers may make local policy decisions about whether to use the contents of the ARC-Authentication-Results header field in cases where a message no longer passes DKIM, DMARC, and/or SPF checks. Whether an ARC chain is intact can be used to inform that local policy decision.
So for example one message receiver may decide that, for messages with an intact ARC chain where a DMARC evaluation does not pass, but the ARC-Authentication-Results header field indicates a DKIM pass was reported by the first ARC intermediary that matches the domain in the RFC5322.From header field, it will override a DMARC “p=reject” policy. Another message receiver may decide to do so for intact ARC chains where the ARC-Authentication-Results header field indicates an SPF pass. A third message receiver may use very different criteria, according to their requirements, while a fourth may choose not to take ARC information into account at all.
An ARC chain is broken if the signatures in the ARC-Seal header fields cannot be verified or if the most recent AMS can not be verified. For example the remote server delivering the message to the local ADMD is not reflected in any ARC header fields, perhaps because they have not implemented ARC, but they modified the message such that ARC and DKIM signatures already in the message were invalidated.
In case of a broken ARC chain, the message should be treated the same as if there was no ARC chain at all. For example, a message that fails under DMARC and has an invalid ARC chain would be subject to that DMARC policy, which may cause it to be quarantined or rejected.
Email transit can produce broken signatures for a wide variety of benign reasons. This includes possibly breaking one or more ARC signatures. Therefore, receivers need to be wary of ascribing motive to such breakage although patterns of common behaviour may provide some basis for adjusting local policy decisions.
The absence of an ARC chain means nothing. ARC is intended to allow a participating message handler to preserve certain authentication results when a message is being forwarded and/or modified such that the final recipient can evaluate this information. If they are absent, there is nothing extra that ARC requires the final recipient to do.
With sufficient history, ARC can be used to augment DMARC authentication policy (i.e. a message could fail DMARC, but validated ARC information and therefore could be considered as validly authenticated as reported by the first ARC participant).
If the validator does content analysis and reputation tracking, the ARC participants in a message can be credited or discredited for good or bad content. By analyzing different ARC chains involved in “bad” messages, a validator might identify malicious participating intermediaries.
With a valid chain and good reputations for all ARC participants, receivers may choose to apply a “local policy override” to the DMARC policy assertion for the domain authentication evaluation, depending on the ARC-Authentication-Results header field value. Normal content analysis should never be skipped.
This has no impact on the operation of ARC, as ARC is not a reputation system. ARC conveys the results of other authentication mechanisms such that the participating message handlers can be positively identified. Final message recipients may or may not choose to examine these results when messages fail other authentication checks. They are more likely to override, say, a failing DMARC result in the presence of an intact ARC chain where the participating ARC message handlers have been observed to not convey “bad” content in the past, and the initial ARC participant indicates the message they received had passed authentication checks.
Validators may choose to build reputation models for ARC message handlers they have observed. Generally speaking it is more feasible to accrue positive reputation to intermediaries when they consistently send messages that are evaluated positively in terms of content and ARC chains. When messages are received with ARC chains that are not intact, it is very difficult identify which intermediaries may have manipulated the message or injected bad content.
Message handlers should do what they normally do when they detect malicious content in a message - hopefully that means quarantining or discarding the message. ARC information should never make malicious content acceptable.
In such cases it is difficult to determine where the malicious content may have been injected. What ARC can do in such cases is verify that a given intermediary or message handler did in fact handle the message as indicated in the header fields. In such cases a message recipient who maintains a reputation system about email senders may wish to incorporate this information as an additional factor in the score for the intermediaries and sender in question. However reputation systems are very complex, and usually unique to those organizations operating them, and therefore beyond the scope of this document.
ARC itself does not include any mechanism for feedback or reporting. It does however recommend that message receiving systems that use ARC to augment their delivery decisions, who use DMARC and decide to deliver a message because of ARC information, should include a notation to that effect in their normal DMARC reports. These notations would be easily identifiable by report processors, so that senders and domain owners can see where ARC is being used to augment the deliverability of their messages.
ARC does not prevent a malicious actor from doing this. Nor does it prevent a malicious actor from removing all but the first ADMD’s ARC header fields and altering the message, eliminating intervening participants from the ARC chain. Or similar variations.
A valid ARC chain does not provide any automatic benefit. With an intact ARC chain, the final message recipient may choose to use the contents of the ARC-Authentication-Results header field in determining how to handle the message. The decision to use the ARC-Authentication-Results header field is dependent on evaluation of those ARC intermediaries.
In the first case, the bad actor has succeeded in manipulating the message but they have attached a verifiable signature identifying themselves. While not an ideal situation, it is something they are already able to do without ARC involved, but now a signature linked to the domain responsible for the manipulation is present.
Additionally in the second case it is possible some negative reputational impact might accrue to the first ARC participant left in place until more messages reveal the pattern of activity by the bad actor. But again, a bad actor can similarly manipulate a sequence of RFC5322.Received header fields today without ARC, but with ARC that bad actor has verifiably identified themselves.
In the context of ARC, an Intermediary is typically an Administrative Management Domain [RFC5598] that is receiving a message, potentially manipulating or altering it, and then passing it on to another ADMD for delivery. Common examples of Intermediaries are mailing lists, alumni or professional email address providers that forward messages such as universities or professional organizations, et cetera.
A participating ARC intermediary must validate the ARC chain on a message it receives, if one is present. It then attaches its own ARC seal and signature, including an indication if the chain failed to validate upon receipt.
Generally speaking, ARC is designed to operate at the ADMD level. When a message is first received by an ADMD, the traditional authentication results should be captured and preserved - this could be the common case of creating an Authentication-Results header field. But when it is determined that the message is being sent on outside of that ADMD, that is when the ADMD should add itself to the ARC chain - before sending the message outside of the ADMD.
Some organizations may operate multiple ADMDs, with more or less independence between them. While they should make a determination based on their specific circumstances, it may be useful and appropriate to have one or both ADMDs be ARC participants.
In general terms, a participating ARC intermediary will note that an ARC chain was present and invalid, or broken, when it attaches its own ARC seal and signature. However the fact that the ARC chain was invalid should have no impact on whether and how the message is delivered.
A participating ARC intermediary receiving a message with no ARC chain, and which will be delivered outside its ADMD, should start an ARC chain according to the ARC specification. This will include capturing the normal email authentication results for the intermediary (SPF, DKIM, DMARC, etc), which will be conveyed as part of the ARC chain.
Message receivers often operate reputation systems, which build a behavioral profile of various message handlers and intermediaries. The presence or absence of ARC is yet another data point that may be used as an input to such reputation systems. Messages deemed to have good content may provide a positive signal for the intermediaries that handled it, while messages with bad content may provide a negative signal for the those intermediaries. Intact and valid ARC elements may amplify or attenuate such signals, depending on the circumstances.
Reputation systems are complex and usually specific to a given message receiver, and a meaningful discussion of such a broad topic is beyond the scope of this document.
Today it is extremely simple for a malicious actor to construct a message that includes your identity as an intermediary, even though you never handled the message. It is possible that an intermediary implementing ARC on all traffic it handles might receive some reputational benefit by making it easier to detect when their involvement in conveying bad traffic has been “forged.”
As mentioned previously reputation systems are very complex and usually specific to a given message receiver, and a meaningful discussion of such a broad topic is beyond the scope of this document.
Please join the arc-discuss list at arc-discuss@dmarc.org[mailto:arc-discuss@dmarc.org].
To discuss the IETF spec itself, please join the dmarc working group at [https://datatracker.ietf.org/wg/dmarc].
The arc-discuss list is the best place to stay in touch with work in progress.
Prior to ARC, certain DMARC policies on a domain would cause messages using those domains in the RFC5322.From field, and which pass through certain kinds of intermediaries (mailing lists, forwarding services), to fail authentication checks at the message receiver. As a result these messages might not be delivered to the intended recipient.
ARC seeks to provide these so-called “indirect mailflows” with a means to preserve email authentication results as recorded by participating intermediaries. Message receivers may accept validated ARC information to supplement the information that DMARC provides, potentially deciding to deliver the message even though a DMARC check did not pass.
The net result for domain owners and senders is that ARC may allow messages routed through participating ARC intermediaries to be delivered, even though those messages would not have been delivered in the absence of ARC.
Message receivers often operate reputation systems, which build a behavioral profile of various message senders (and perhaps intermediaries). The presence or absence of ARC is yet another data point that may be used as an input to such reputation systems. Messages deemed to have good content may provide a positive signal for the sending domain and the intermediaries that handled it, while messages with bad content may provide a negative signal for the sending domain and the intermediaries that handled it. Intact and valid ARC elements may amplify or attenuate such signals, depending on the circumstances.
Reputation systems are complex and usually specific to a given message receiver, and a meaningful discussion of such a broad topic is beyond the scope of this document.
At present there is no way for a message sender to request that intermediaries not employ ARC.
| [RFC5321] | Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008. |
| [RFC5322] | Resnick, P., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008. |
| [RFC5598] | Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, July 2009. |
| [RFC6377] | Kucherawy, M., "DomainKeys Identified Mail (DKIM) and Mailing Lists", BCP 167, RFC 6377, DOI 10.17487/RFC6377, September 2011. |
| [RFC7601] | Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 7601, DOI 10.17487/RFC7601, August 2015. |
| [ARC] | Andersen, K., Rae-Grant, J., Long, B. and S. Jones, "Authenticated Received Chain (ARC) Protocol", December 2017. |
| [ENHANCED-STATUS] | "IANA SMTP Enhanced Status Codes", n.d.. |
| [OAR] | Chew, M. and M. Kucherawy, "Original-Authentication-Results Header Field", February 2012. |
| [RFC7489] | Kucherawy, M. and E. Zwicky, "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015. |
| [RFC7960] | Martin, F., Lear, E., Draegen. Ed., T., Zwicky, E. and K. Andersen, "Interoperability Issues between Domain-based Message Authentication, Reporting, and Conformance (DMARC) and Indirect Email Flows", RFC 7960, DOI 10.17487/RFC7960, September 2016. |
This draft is the work of OAR-Dev Group.
The authors thanks the entire OAR-Dev group for the ongoing help, innumerable diagrams and discussions from all the participants, especially: Alex Brotman, Brandon Long, Dave Crocker, Elizabeth Zwicky, Franck Martin, Greg Colburn, J. Trent Adams, John Rae-Grant, Mike Hammer, Mike Jones, Steve Jones, Terry Zink, Tim Draegen.
Please address all comments, discussions, and questions to the dmarc working group at [https://datatracker.ietf.org/wg/dmarc].