Complaint Feedback Loop Address Header

Introduction and Motivation The topic and goal of this document is to extend the complaint feedback loop recommendations described in (the terminology used in this document hereafter is taken from there) with an automated way to provide the necessary information to Mailbox Providers, to report message handling actions taken by message recipients, such as "mark as spam", back to the Message Originator. As described in , the registration for such a complaint feedback loop needs to be done manually by a human at any Mailbox Provider who provides a complaint feedback loop. The key underpinning of is that access to the complaint feedback loop is a privilege, and that 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, DKIM domains or change their complaint address. In addition, a manual process is not well suited and/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 that 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 the data privacy laws e.g. GDPR or CCPA. Nevertheless, the described mechanism below potentially permits a kind of man-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 this reports to the complaint feedback loop address. These fake messages can result in a number of actions, such as blocking of accounts or deactivating recipient addresses. This potential harm and others are described with potential countermeasures in . 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. Be able 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 complaint feedback loop.

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 Message Originators side or by using the CFBL-Address header field to send Feedback Messages to the provided address on 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 Originator 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 field 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 and 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.

How CFBL differs from One-Click-Unsubscribe For good reasons, the One-Click-Unsubscribe signaling already exists, which may have several interests in common with this document. However, this header field requires the List-Unsubscribe header field, whose purpose 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, not in normal email traffic.

Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. The key word "CFBL" in this document is the abbreviation for "complaint feedback loop" and will hereafter be used. Syntax descriptions use ABNF .

Requirements

Received Message This section describes the requirements that a received message, the message that is sent from the Message Originator to the Mailbox Provider and about which a report is to be sent later, must meet.

Strict If the domain in the .From and the domain in the CFBL-Address header field are identical, this domain MUST be matched by a valid signature. In this case, the DKIM "d=" parameter and the .From field have identical domains. This signature MUST meet the requirements described in . The following example meets this case:

From: Awesome Newsletter To: receiver@example.org Subject: Super awesome deals for you CFBL-Address: fbl@example.com; report=arf Message-ID: 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. ]]>

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

From: Awesome Newsletter To: receiver@example.org Subject: Super awesome deals for you CFBL-Address: fbl@mailer.example.com; report=arf Message-ID: 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:

Third Party Address If the domain in .From differs from the domain in the CFBL-Address header field, an additional valid signature MUST be added that matches the domain in the CFBL-Address header field. The other existing valid signature MUST match the domain in the .From header field. This double DKIM signature ensures that both, the domain owner of the .From domain and the domain owner of the CFBL-Address domain, agree who should receive the Feedback Messages. Both signature MUST meet the requirements described in . The following example meets this case:

From: Awesome Newsletter To: receiver@example.org Subject: Super awesome deals for you CFBL-Address: fbl@saas-mailer.example; report=arf Message-ID: 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 which 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:

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 neither matched by a valid DKIM signature nor the header field is covered by the "h=" tag, the Mailbox Provider SHALL NOT send a report message.

Multiple CFBL-Address Header Fields A Message can only have exactly one CFBL-Address header field. All further occurrences of the CFBL-Address header field MUST be ignored after the first valid occurrence.

CFBL-Feedback-ID Header Field The Message Originator MAY include a CFBL-Feedback-ID header field in its messages out of 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 using a secret key, instead of a plain-text string.

Receiving Report Address The receiving report address provided in the CFBL-Address header field MUST accept reports. The Message Originator can OPTIONALLY request a report, as described in .

Feedback Message The Feedback Message (sent by Mailbox Provider to the address provided in the CFBL-Address header field) MUST have a valid signature. This signature MUST match the .From domain of the Feedback Message. If the message does not have the required valid signature, the Message Originator SHALL NOT process this Feedback Message. The Feedback Message MUST be a or report. If the Message Originator requests it (described in ), and it is technically possible for the Mailbox Provider to do so, the Feedback Message MUST be a report, otherwise the Feedback Message MUST be a report. The third MIME part of the or the "Samples" section of the report MUST contain the Message-ID of the received message. If present, the header field "CFBL-Feedback-ID" of the received message MUST be added additionally to the third MIME part of the or to "Samples" section of the report. The Mailbox Provider MAY omit or redact, as described in , all further header fields and/or body to comply with any data-regulation laws.

XARF Report A Message Originator wishing to receive a report MUST append "report=xarf" to the CFBL-Address header field. The report parameter is separated from the report address by a ";". The resulting header field would look like the following:

Implementation

Message Originator A Message Originator who wishes to use this new mechanism to receive Feedback Messages MUST include a CFBL-Address header field in their messages. It is RECOMMENDED that these Feedback Messages be processed automatically. Each Message Originator must decide for themselves what action to take after receiving a Feedback Message. The Message Originator MUST take action to address the described requirements in .

Mailbox Provider A Mailbox Provider who wants to collect user actions that indicate the message was not wanted and send a Feedback Message to the Message Originator, they MAY query the CFBL-Address header field and forward the report to the provided complaint feedback loop address. The Mailbox Provider MUST validate the requirements described in and MUST take action to address the requirements described in when sending Feedback Messages.

Header Field Syntax

CFBL-Address The following ABNF imports fields, CFWS, CRLF and addr-spec from . The CFBL-Address header field is compatible with .

CFBL-Feedback-ID The following ABNF imports fields, WSP, CRLF and atext from .

Whitespace is ignored in the fid value and MUST be ignored when reassembling the original feedback id.
In particular, when adding the header field the Message Originator can safely insert CFWS in the fid value in arbitrary places to conform to line-length limits.

Security Considerations This section discusses possible security issues, and their possible solutions, of a complaint feedback loop address header field.

Attacks on the Feedback Loop Address Like any other email address, a complaint feedback loop address can be an attack vector for malicious messages. For example, complaint feedback loop addresses can be flooded with spam. This is an existing problem with any existing email address and is not created by this document.

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. An example: someone sends an invitation to their friends. For some reason, someone marks this message as spam. Now, if there is too fast automatic account suspension, the Message Author's account will be blocked and the Message Author will not be able to access their emails or is able to send further messages, depending on the account suspension the Message Originator has chosen. Message Originators must take appropriate measures to prevent too fast account suspensions. Message Originators therefore 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.

Enumeration Attacks / Provoking Unsubscription A malicious person may send a series of spoofed ARF messages to known complaint feedback loop 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 . The Message Originator must take appropriate measures, a countermeasure would be, that the CFBL-Feedback-ID header field, if used, use a hard-to-forge component such as a with a secret key instead of a plaintext string to make an enumeration attack impossible.

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 data privacy safe way to send a Feedback Message. As described in , the Mailbox Provider can omit the entire body and/or header field and send only the required fields. As recommended in , 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 and in , it is also strongly RECOMMENDED that the Message-ID and, if used, the CFBL-Feedback-ID. contain a component that is difficult to forge, such as a that uses a secret key, rather than a plaintext string. See for an example.

Abusing for Validity and Existence Queries This mechanism could be abused to determine the validity and existence of an email address, which exhibits another potential data privacy issue. Now, 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 now generates an automatic Feedback Message for the received message, the Mailbox Provider now 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, for example, pre-existing reputation data, usually proprietary data. 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.

IANA Considerations

CFBL-Address The IANA is requested to register a new header field, per , into the "Provisional Message Header Field Names" registry:

Specification document: this document ]]>

CFBL-Feedback-ID The IANA is requested to register a new header field, per , into the "Provisional Message Header Field Names" registry:

Specification document: this document ]]>

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

Simple Email about the report will be generated:

From: Awesome Newsletter 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: 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:

Data Privacy Safe Report Email about the report will be generated:

Data Privacy Safe Report with HMAC Email about the report will be generated:

From: Awesome Newsletter To: me@example.net Subject: Super awesome deals for you CFBL-Address: fbl@example.com; report=arf CFBL-Feedback-ID: 3789e1ae1938aa2f0dfdfa48b20d8f8bc6c21ac34fc5023d 63f9e64a43dfedc0 Message-ID: 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 contains only the CFBL-Feedback-ID:

Acknowledgments Technical and editorial reviews were provided by the colleagues at CleverReach, the colleagues at Certified Senders Alliance and eco.de, Arne Allisat, Tobias Herkula and Levent Ulucan (1&1 Mail & Media) and Sven Krohlas (BFK Edv-consulting).