RFC 9477: Complaint Feedback Loop Address Header

RFC 9477	CFBL Address Header	September 2023
Benecke	Experimental	[Page]

Abstract

This document describes a method that allows a Message Originator to specify a Complaint Feedback Loop (CFBL) address as a message header field. It also defines the rules for processing and forwarding such a complaint. The motivation for this arises out of the absence of a standardized and automated way to provide Mailbox Providers with an address for a CFBL. Currently, providing and maintaining such an address is a manual and time-consuming process for Message Originators and Mailbox Providers.¶

The mechanism specified in this document is being published as an experiment to gather feedback and gauge the interest of implementers and deployers. This document is produced through the Independent RFC Stream and was not subject to the IETF's approval process.¶

1. Introduction and Motivation

This memo extends the CFBL recommendations described in [RFC6449] with an automated way to provide the necessary information by the Message Originator to Mailbox Providers. The reader should be familiar with the terminology and concepts in that document. Terms beginning with capital letters used in this memo are described in that document.¶

As described in [RFC6449], the registration for such a CFBL needs to be done manually by a human at any Mailbox Provider that provides a CFBL. The key underpinning of [RFC6449] is that access to the CFBL is a privilege and Mailbox Providers are not prepared to send feedback to anyone they cannot reasonably believe are legitimate. However, manual registration and management can be quite time-consuming if there are new feedback loops rising up or if the Message Originator wants to add new IP addresses, DomainKeys Identified Mail (DKIM) domains, or change their complaint address. In addition, a manual process is not well suited or feasible for smaller Mailbox Providers.¶

Here, we propose that Message Originators add a header field without the need to manually register with each Feedback Provider and willing Mailbox Providers can use it to send the Feedback Messages to the specified complaint address. This simplification or extension of a manual registration and verification process would be another advantage for the Mailbox Providers.¶

A new message header field, rather than a new DNS record, was chosen to easily distinguish between multiple Message Originators without requiring user or administrator intervention. For example, if a company uses multiple systems, each system can set this header field on its own without requiring users or administrators to make any changes to their DNS. No additional DNS lookup is required of the Mailbox Provider side to obtain the complaint address.¶

The proposed mechanism is capable of being operated in compliance with data privacy laws, e.g., the EU's General Data Protection Regulation (GDPR) or the California Consumer Privacy Act (CCPA). As described in Section 6.4, a Feedback Message may contain personal data. This document describes a way to omit this personal data when sending the Feedback Message and only send back a header field.¶

Nevertheless, the described mechanism below potentially permits a kind of person-in-the-middle attack between the domain owner and the recipient. A bad actor can generate forged reports to be "from" a domain name the bad actor is attacking and send these reports to the CFBL address. These fake messages can result in a number of actions, such as blocking accounts or deactivating recipient addresses. This potential harm and others are described with potential countermeasures in Section 6.¶

In summary, this document has the following objectives:¶

Allow Message Originators to signal that a complaint address exists without requiring manual registration with all providers.¶
Allow Mailbox Providers to obtain a complaint address without developing their own manual registration process.¶
Have the ability to provide a complaint address to smaller Mailbox Providers who do not have a feedback loop in place¶
Provide a data privacy safe option for a CFBL.¶

1.1. Scope of this Experiment

The CFBL-Address header field and the CFBL-Feedback-ID header field comprise an experiment. Participation in this experiment consists of adding the CFBL-Address header field on the Message Originator side or by using the CFBL-Address header field to send Feedback Messages to the provided address on the Mailbox Provider side. Feedback on the results of this experiment can be emailed to the author, raised as an issue at <https://github.com/jpbede/rfc-cfbl-address-header/>, or can be emailed to the IETF cfbl mailing list (cfbl@ietf.org).¶

The goal of this experiment is to answer the following questions based on real-world deployments:¶

Is there interest among Message Originators and Mailbox Providers?¶
If the Mailbox Provider adds this capability, will it be used by the Message Originators?¶
If the Message Originator adds this capability, will it be used by the Mailbox Providers?¶
Does the presence of the CFBL-Address and CFBL-Feedback-ID header fields introduce additional security issues?¶
What additional security measures/checks need to be performed at the Mailbox Provider before a Feedback Message is sent?¶
What additional security measures/checks need to be performed at the Message Originator after a Feedback Message is received?¶

This experiment will be considered successful if the CFBL-Address header field is used by a leading Mailbox Provider and by at least two Message Originators within the next two years. It will also be considered a success if these parties successfully use the address specified in the header field to exchange Feedback Messages.¶

If this experiment is successful and these header fields prove to be valuable and popular, the header fields may be taken to the IETF for further discussion and revision.¶

1.2. How CFBL Differs from One-Click-Unsubscribe

For good reasons, the One-Click-Unsubscribe [RFC8058] signaling already exists and may have several interests in common with this document. However, this header field requires the List-Unsubscribe header field. The purpose of this header field is to provide the link to unsubscribe from a list. For this reason, this header field is only used by operators of broadcast marketing lists or mailing lists and not in normal email traffic.¶

3. Requirements

3.1. Received Message

This section describes the requirements that must be met for the following: a received message, the message that is sent from the Message Originator to the Mailbox Provider, and a report that is to be sent later.¶

3.1.1. Strict

If the domain in the RFC5322.From and the domain in the CFBL-Address header fields are identical, this domain MUST be matched by a valid [DKIM] signature. In this case, the DKIM "d=" parameter and the RFC5322.From field have identical domains. This signature MUST meet the requirements described in Section 3.1.4.¶

The following example meets this case:¶

Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: receiver@example.org
Subject: Super awesome deals for you
CFBL-Address: fbl@example.com; report=arf
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
       h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

This is a super awesome newsletter.

3.1.2. Relaxed

If the domain in CFBL-Address header field is a child domain of RFC5322.From, the RFC5322.From domain MUST be matched by a valid [DKIM] signature. In this case, the DKIM "d=" parameter and the RFC5322.From domain have an identical (Example 1) or parent (Example 2) domain. This signature MUST meet the requirements described in Section 3.1.4.¶

Example 1:¶

Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@mailer.example.com>
To: receiver@example.org
Subject: Super awesome deals for you
CFBL-Address: fbl@mailer.example.com; report=arf
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com;
      h=Content-Type:Subject:From:To:Message-ID:
      CFBL-Feedback-ID:CFBL-Address;

This is a super awesome newsletter.

Example 2:¶

Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: receiver@example.org
Subject: Super awesome deals for you
CFBL-Address: fbl@mailer.example.com; report=arf
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com;
      h=Content-Type:Subject:From:To:Message-ID:
      CFBL-Feedback-ID:CFBL-Address;

This is a super awesome newsletter.

3.1.3. Third Party Address

If the domain in RFC5322.From differs from the domain in the CFBL-Address header field, an additional valid [DKIM] signature MUST be added that matches the domain in the CFBL-Address header field. The other existing valid [DKIM] signature MUST match the domain in the RFC5322.From header field. This double DKIM signature ensures that both the domain owner of the RFC5322.From domain and the domain owner of the CFBL-Address domain agree on who should receive the Feedback Messages. Both signatures MUST meet the requirements described in Section 3.1.4.¶

The following example meets this case:¶

Return-Path: <sender@saas-mailer.example>
From: Awesome Newsletter <newsletter@example.com>
To: receiver@example.org
Subject: Super awesome deals for you
CFBL-Address: fbl@saas-mailer.example; report=arf
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=saas-mailer.example; s=system;
       h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
       h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

This is a super awesome newsletter.

An Email Service Provider may accept pre-signed messages from its Message Authors, making it impossible for it to apply the double signature described above; in this case, the double signature MUST be omitted and the Email Service Provider MUST sign with its domain. Therefore, the pre-signed message MUST NOT include "CFBL-Address" and "CFBL-Feedback-ID" in its "h=" tag.¶

This way, the Email Service Provider has the possibility to accept the pre-signed messages and can inject their own CFBL-Address.¶

The following example meets this case:¶

Return-Path: <newsletter@example.com>
From: Awesome Newsletter <newsletter@example.com>
To: receiver@example.org
Subject: Super awesome deals for you
CFBL-Address: fbl@saas-mailer.example; report=arf
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
       h=Subject:From:To:Message-ID;
DKIM-Signature: v=1; a=rsa-sha256; d=saas-mailer.example; s=system;
       h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

This is a super awesome newsletter.

3.1.4. DKIM Signature

When present, CFBL-Address and CFBL-Feedback-ID header fields MUST be included in the "h=" tag of the aforementioned valid DKIM signature.¶

If the domain is not matched by a valid DKIM signature or the header field is not covered by the "h=" tag, the Mailbox Provider SHALL NOT send a report message.¶

3.2. Multiple CFBL-Address Header Fields

A Message can contain multiple CFBL-Address header fields. These multiple header fields MUST be treated as a list of addresses, each of which should receive a report.¶

3.3. CFBL-Feedback-ID Header Field

The Message Originator MAY include a CFBL-Feedback-ID header field in its messages for various reasons, e.g., their feedback loop processing system can't do anything with the Message-ID header field.¶

It is RECOMMENDED that the header field include a hard-to-forge protection component, such as an [HMAC] using a secret key, instead of a plaintext string.¶

3.4. Receiving Report Address

The receiving report address provided in the CFBL-Address header field MUST accept [ARF] reports.¶

It is OPTIONAL for the Message Originator to request a [XARF] report, as described in Section 3.5.1.¶

3.5. Feedback Message

The Feedback Message (sent by Mailbox Provider to the address provided in the CFBL-Address header field) MUST have a valid [DKIM] signature. This signature MUST match the RFC5322.From domain of the Feedback Message.¶

If the message does not have the required valid [DKIM] signature, the Message Originator SHALL NOT process this Feedback Message.¶

The Feedback Message MUST be an [ARF] or [XARF] report. If the Message Originator requests it (described in Section 3.5.1) and it is technically possible for the Mailbox Provider to do so, the Feedback Message MUST be a [XARF] report. Otherwise, the Feedback Message MUST be an [ARF] report.¶

The third MIME part of the [ARF] or the "Samples" section of the [XARF] report MUST contain the Message-ID [RFC5322] of the received message. If present, the CFBL-Feedback-ID header field of the received message MUST be added to the third MIME part of the [ARF] or to the "Samples" section of the [XARF] report.¶

The Mailbox Provider MAY omit or redact all further header fields and/or body to comply with any data regulation laws as described in [RFC6590].¶

3.5.1. XARF Report

A Message Originator wishing to receive a [XARF] report MUST append "report=xarf" to the CFBL-Address header field (Section 5.1). The report parameter is separated from the report address by a ";".¶

The resulting header field would appear as shown below.¶

CFBL-Address: fbl@example.com; report=xarf

6. Security Considerations

This section discusses possible security issues of a CFBL-Address header field and their solutions.¶

6.1. Attacks on the Feedback Loop Address

Like any other email address, a CFBL address can be an attack vector for malicious messages. For example, CFBL addresses can be flooded with spam. This is an existing problem with any existing email address and is not created by this document.¶

6.2. Automatic Suspension of an Account

Receiving a Feedback Message regarding a Message Author can cause the Message Author to be unreachable if an automatic account suspension occurs too quickly. For example, someone sends an invitation to their friends, and someone else marks this message as spam for some reason.¶

If automatic account suspension is too fast, the Message Author's account will be blocked and the Message Author will not be able to access their emails or send further messages, depending on the account suspension the Message Originator has chosen.¶

Message Originators must take appropriate measures to prevent account suspensions that happen too fast. Therefore, Message Originators have -- mostly proprietary -- ways to assess the trustworthiness of an account. For example, Message Originators may take into account the age of the account and/or any previous account suspension before suspending an account.¶

6.3. Enumeration Attacks / Provoking Unsubscription

A malicious person may send a series of spoofed Abuse Reporting Format (ARF) messages to known CFBL addresses and attempt to guess a Message-ID / CFBL-Feedback-ID or any other identifiers. The malicious person may attempt to mass unsubscribe/suspend if such an automated system is in place. This is also an existing problem with the current feedback loop implementation and/or One-Click Unsubscription [RFC8058].¶

The Message Originator must take appropriate measures. For example, the CFBL-Feedback-ID header field (if used) can use a hard-to-forge component, such as an [HMAC] with a secret key, instead of a plaintext string, to make an enumeration attack impossible.¶

6.4. Data Privacy

The provision of such a header field itself does not pose a data privacy issue. The resulting ARF/XARF report sent by the Mailbox Provider to the Message Originator may violate a data privacy law because it may contain personal data.¶

This document already addresses some parts of this problem and describes a way to send a Feedback Message that keeps data privacy safe. As described in Section 3.5, the Mailbox Provider can omit the entire body and/or header field and send only the required fields. As recommended in [RFC6590], the Mailbox Provider can also redact the data in question. Nevertheless, each Mailbox Provider must consider for itself whether this implementation is acceptable and complies with existing data privacy laws in their country.¶

As described in Sections 3.5 and 3.3, it is also strongly RECOMMENDED that the Message-ID and CFBL-Feedback-ID (if used) contain a component that is difficult to forge, such as an [HMAC] that uses a secret key, rather than a plaintext string. See Section 8.3 for an example.¶

6.5. Abusing for Validity and Existence Queries

This mechanism could be abused to determine the validity and existence of an email address, exhibiting another potential data privacy issue. If the Mailbox Provider has an automatic process to generate a Feedback Message for a received message, it may not be doing the mailbox owner any favors. As the Mailbox Provider generates an automatic Feedback Message for the received message, the Mailbox Provider proves to the Message Originator that this mailbox exists for sure because it is based on a manual action of the mailbox owner.¶

The receiving Mailbox Provider must take appropriate measures. One possible countermeasure could be pre-existing reputation data (usually proprietary data), for example. Using this data, the Mailbox Provider can assess the trustworthiness of a Message Originator and decide whether to send a Feedback Message based on this information.¶

8. Examples

For simplicity, the DKIM header field has been shortened, and some tags have been omitted.¶

8.1. Simple

Email about the report will be generated:¶

Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: me@example.net
Subject: Super awesome deals for you
CFBL-Address: fbl@example.com; report=arf
CFBL-Feedback-ID: 111:222:333:4444
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
       h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

This is a super awesome newsletter.

Resulting ARF report:¶

------=_Part_240060962_1083385345.1592993161900
Content-Type: message/feedback-report
Content-Transfer-Encoding: 7bit

Feedback-Type: abuse
User-Agent: FBL/0.1
Version: 0.1
Original-Mail-From: sender@mailer.example.com
Arrival-Date: Tue, 23 Jun 2020 06:31:38 GMT
Reported-Domain: example.com
Source-IP: 192.0.2.1

------=_Part_240060962_1083385345.1592993161900
Content-Type: text/rfc822; charset=UTF-8
Content-Transfer-Encoding: 7bit

Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: me@example.net
Subject: Super awesome deals for you
CFBL-Address: fbl@example.com; report=arf
CFBL-Feedback-ID: 111:222:333:4444
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
       h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

This is a super awesome newsletter.
------=_Part_240060962_1083385345.1592993161900--

8.2. Data Privacy Safe Report

Email about the report will be generated:¶

Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: me@example.net
Subject: Super awesome deals for you
CFBL-Address: fbl@example.com; report=arf
CFBL-Feedback-ID: 111:222:333:4444
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
       h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

This is a super awesome newsletter.

Resulting ARF report that only contains the CFBL-Feedback-ID:¶

------=_Part_240060962_1083385345.1592993161900
Content-Type: message/feedback-report
Content-Transfer-Encoding: 7bit

Feedback-Type: abuse
User-Agent: FBL/0.1
Version: 0.1
Original-Mail-From: sender@mailer.example.com
Arrival-Date: Tue, 23 Jun 2020 06:31:38 GMT
Reported-Domain: example.com
Source-IP: 2001:DB8::25

------=_Part_240060962_1083385345.1592993161900
Content-Type: text/rfc822-headers; charset=UTF-8
Content-Transfer-Encoding: 7bit

CFBL-Feedback-ID: 111:222:333:4444
------=_Part_240060962_1083385345.1592993161900--

8.3. Data Privacy Safe Report with HMAC

Email about the report will be generated:¶

Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: me@example.net
Subject: Super awesome deals for you
CFBL-Address: fbl@example.com; report=arf
CFBL-Feedback-ID: 3789e1ae1938aa2f0dfdfa48b20d8f8bc6c21ac34fc5023d
       63f9e64a43dfedc0
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
       h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

This is a super awesome newsletter.

Resulting ARF report that only contains the CFBL-Feedback-ID:¶

------=_Part_240060962_1083385345.1592993161900
Content-Type: message/feedback-report
Content-Transfer-Encoding: 7bit

Feedback-Type: abuse
User-Agent: FBL/0.1
Version: 0.1
Original-Mail-From: sender@mailer.example.com
Arrival-Date: Tue, 23 Jun 2020 06:31:38 GMT
Reported-Domain: example.com
Source-IP: 2001:DB8::25

------=_Part_240060962_1083385345.1592993161900
Content-Type: text/rfc822-headers; charset=UTF-8
Content-Transfer-Encoding: 7bit

CFBL-Feedback-ID: 3789e1ae1938aa2f0dfdfa48b20d8f8bc6c21ac34fc5023d
       63f9e64a43dfedc0
------=_Part_240060962_1083385345.1592993161900--

9. References

9.1. Normative References

[ARF]: Shafranovich, Y., Levine, J., and M. Kucherawy, "An Extensible Format for Email Feedback Reports", RFC 5965, DOI 10.17487/RFC5965, August 2010, <https://www.rfc-editor.org/info/rfc5965>.
[DKIM]: Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011, <https://www.rfc-editor.org/info/rfc6376>.
[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC5234]: Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>.
[RFC5322]: Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, <https://www.rfc-editor.org/info/rfc5322>.
[RFC6449]: Falk, J., Ed., "Complaint Feedback Loop Operational Recommendations", RFC 6449, DOI 10.17487/RFC6449, November 2011, <https://www.rfc-editor.org/info/rfc6449>.
[RFC6532]: Yang, A., Steele, S., and N. Freed, "Internationalized Email Headers", RFC 6532, DOI 10.17487/RFC6532, February 2012, <https://www.rfc-editor.org/info/rfc6532>.
[RFC7405]: Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014, <https://www.rfc-editor.org/info/rfc7405>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[XARF]: "XARF - eXtended Abuse Reporting Format", commit cc1a6e6, March 2023, <https://github.com/abusix/xarf>.

9.2. Informative References

[HMAC]: Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, <https://www.rfc-editor.org/info/rfc2104>.
[RFC3864]: Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004, <https://www.rfc-editor.org/info/rfc3864>.
[RFC6590]: Falk, J., Ed. and M. Kucherawy, Ed., "Redaction of Potentially Sensitive Data from Mail Abuse Reports", RFC 6590, DOI 10.17487/RFC6590, April 2012, <https://www.rfc-editor.org/info/rfc6590>.
[RFC8058]: Levine, J. and T. Herkula, "Signaling One-Click Functionality for List Email Headers", RFC 8058, DOI 10.17487/RFC8058, January 2017, <https://www.rfc-editor.org/info/rfc8058>.