Signalling one-click functionality for list email headers
draft-levine-herkula-oneclick-04

Abstract

This document describes a method for signaling a one-click function for the list-unsubscribe email header. The need for this arises out of the actuality that mail software sometimes fetches URLs in mail headers, and thereby accidentally triggers unsubscriptions in the case of the list-unsubscribe header.

Status of This Memo

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This Internet-Draft will expire on February 24, 2017.

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Table of Contents

• 1. Introduction
• 2. Definitions
• 3. High-Level Goals
• 4. Out of Scope
• 5. Implementation
• 5.1. Mail senders
• 5.2. Mail receivers
• 6. Additional Requirements
• 7. IANA Considerations
• 8. Examples
• 8.1. Simple
• 8.2. Complex
• 9. Security Considerations
• 10. Normative References
• Appendix A. Change Log
• A.1. Changes from -03 to -04
• A.2. Changes from -02 to -03
• Authors' Addresses

1. Introduction

An [RFC2369] email header can contain HTTPS URIs. In a List-Unsubscribe Header the HTTPS URI is intended to unsubscribe the recipient of the email from the list. But anti-spam software often fetches all resources in mail headers automatically, without any action by the user. As a result of this unintended malicious behavior, senders implement landing pages with a confirmation step to finish the unsubscribe request.

If a mail recipient is unsubscribing manually, the confirmation page is presented to the recipient who can then click the appropriate button. But in some cases, there is no direct user interaction with the target web site, as when the unsubscription is a side effect of a spam report, or is performed automatically on mail sent to an abandoned mailbox. In those cases, the unsubscription process has to work without manual intervention, and in particular without requiring that software attempt to interpret the contents of a confirmation page.

This document addresses this part of the problem, with a POST action for receivers that senders can distinguish from other requests and handle as a one-click unsubscription without manual intervention by the mail recipient.

A List-Unsubscribe header can also contain a mailto: URI with an address to which an unsubscription request is sent. While these URIs could be used in a one-click fashion, experience has shown that they do not work well in high volume environments, because the recipient mail systems (typically e-mail service providers that are optimized to send large volumes of mail) cannot keep up with the required number of mailed removal requests. Hence this document considers only HTTPS URIs.

2. Definitions

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 [RFC2119] when written in all capital letters.

"One-click" describes an action that directly triggers a change in a system's state, intended to be applied only with a user's intent.

3. High-Level Goals

This document has several goals.

• Allow email senders to signal One-Click functionality for specific HTTPS URIs used in [RFC2369] email headers.
• Allow MUA designers to implement independent user interface features for a better user experience.
• Allow MUA users to trigger intended actions in a familiar environment and without leaving the MUA context.

4. Out of Scope

This document does not address problems associated with deliberate malicious behavior.

5. Implementation

5.1. Mail senders

An entity which is responsible for sending an email that wishes to add an HTTPS URI for one-click unsubscriptions places both a List-Unsubscribe and a List-Unsubscribe-Post header in the message. The List-Unsubscribe-Post header may contain multiple key value pairs needed by the sending entity. It also MUST contain the key value pair "List-Unsubscribe=One-Click".

The combination of the URI in the List-Unsubscribe header and the POST arguments in the List-Unsubscribe-Post header MUST identify the mail recipient, so that the unsubscription process knows what address to handle. In particular, One-click has no way to manually ask the user what address he or she wishes to unsubscribe.

The sending entity needs to provide the infrastructure to handle POST requests to the specified URI in the List-Unsubscribe header, and to handle the reasonably foreseeable volume of unsubscribe requests that its mail will provoke.

The One-Click action triggered by this URI SHOULD complete promptly and not burden the requester in an inappropriate way. The sending entity cannot expect that HTTP redirects are followed by the requester.

5.2. Mail receivers

A receiving entity which wants to use a List-Unsubscribe HTTP URI from an email that also contains a List-Unsubscribe-Post header performs an HTTPS POST to the first HTTPS URI in the List-Unsubscribe header and send the content of the List-Unsubscribe-Post header as the request body.

The POST content SHOULD be sent as "multipart/form-data" [RFC7578] and MAY be sent as "application/x-www-form-urlencoded". These encodings are the ones used by web browsers when sending forms. The target of the POST action will typically be the same as or similar to the one in the manual confirmation page when doing a two-click unsubscribe, so this is intended to allow the same server code to handle both.

The receiving entity MUST NOT perform a POST on the the HTTPS URI without user consent. When and how the user consent is obtained is not part of this specification.

The Request uses the HTTPS verb POST. The HEAD and GET requests are not intended to be used to trigger a state change. PUT and DELETE would offer similar functionality but are often unavailable.

6. Additional Requirements

The email needs at least one valid authentication identifier. In this version of the specification the only supported identifier type is DKIM [RFC6376], that provides a domain-level identifier in the content of the "d=" tag of a validated DKIM-Signature header field.

The List-Unsubscribe and List-Unsubscribe-Post headers need to be covered by the signature, and hence must be included in the "h=" tag of a valid DKIM-Signature header field.

7. IANA Considerations

IANA is requested to add a new entry to the Permanent Message Header Field Names registry.

Header field name: List-Unsubscribe-Post

Applicable protocol: mail

Status: standard

Author/Change controller: IETF

Specification document: this document

8. Examples

8.1. Simple

Header in Email

List-Unsubscribe: <https://example.com/unsubscribe.html>
List-Unsubscribe-Post: List-Unsubscribe=One-Click&recip=user@example.com

Resulting POST request

POST /unsubscribe.html HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 49

List-Unsubscribe=One-Click&recip=user@example.com

8.2. Complex

Header in Email

List-Unsubscribe: <mailto:listrequest@example.com?subject=unsubscribe>,
    <https://example.com/unsubscribe.html?campaign=123456789>
List-Unsubscribe-Post: List-Unsubscribe=One-Click&recip=user@example.com

Resulting POST request

POST /unsubscribe.html?campaign=123456789 HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 49

List-Unsubscribe=One-Click&recip=user@example.com

9. Security Considerations

The List-Unsubscribe-Post header will typically contain the recipient address, but that address is usually also in the To: header. This specification allows anyone with access to a message to unsubscribe the recipient of the message, but that's typically the case with existing List-Unsubscribe, just with more steps.

A creative mailer could send spam with content intended to provoke large numbers of unsubscriptions, with suitably crafted headers to send POST requests with arbitrary contents to servers that perhaps don't want them. But it's been possible to provoke GET requests in a similar way for a long time (and much easier, due to spam filter auto-fetches) so the chances of significantly increased annoyance seem low.

10. Normative References

Appendix A. Change Log

Remove this section before publication, please.

A.1. Changes from -03 to -04

Require HTTPS. More motivation.

A.2. Changes from -02 to -03

Describe motivation in intro. Clarify required DKIM. More paranoid scenarios.

Authors' Addresses