Internet Engineering Task Force (IETF) J. Levine

Internet Engineering Task Force (IETF) J. Levine
Request for Comments: 8058 Taughannock Networks
Category: Standards Track T. Herkula
ISSN: 2070-1721 optivo GmbH
January 2017

Signaling One-Click Functionality for List Email Headers

Abstract

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

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 7841.

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/rfc8058.

Copyright Notice

Copyright (c) 2017 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
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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 and Motivation . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Implementation . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Mail Senders . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Mail Receivers . . . . . . . . . . . . . . . . . . . . . 5
4. Additional Requirements . . . . . . . . . . . . . . . . . . . 5
5. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Simple . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.2. Complex . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.3. Complex with 'multipart/form-data' . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8

1. Introduction and Motivation

A List-Unsubscribe email header field [RFC2369] can contain HTTPS
[RFC7230] URIs. In that header field, the HTTPS URI is intended to
unsubscribe the recipient of the message from the list. But anti-
spam software often fetches all resources in mail header fields
automatically, without any action by the user, and there is no
mechanical way for a sender to tell whether a request was made
automatically by anti-spam software or manually requested by a user.
To prevent accidental unsubscriptions, senders return landing pages
with a confirmation step to finish the unsubscribe request. A live
user would recognize and act on this confirmation step, but an
automated system would not. That makes the unsubscription process
more complex than a single click.

Operators of broadcast marketing lists tend to be primarily concerned
about deliverability of their mail: whether the mail is delivered to
the recipients and how the messages are presented, e.g., whether in
the primary inbox or in a junk folder. Many mail systems allow
recipients to report mail as spam or junk, and mail streams from
senders whose mail is often reported as junk tend to have poor
deliverability. Hence, the mailers want to make it as easy as
possible for recipients to unsubscribe; if an unsubscription process
is too difficult, the recipient's alternative is to report mail from
the sender as junk until the mail no longer appears in the
recipient's inbox.

Operators of recipient mail systems are aware that their users do not
make a clear distinction between unsubscription and junk. In some
cases, they allow trustworthy mailers to request notification when

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their mail is reported as junk so they can unsubscribe the recipient,
but the process of identifying trustworthy mailers and notifying them
does not scale well to large numbers of small mailers. This
specification provides a way for recipient systems to notify the
mailer automatically, using only information within the mail message,
and without prearrangement. Some recipient systems might wish to
send an unsubscription notice to mailers whenever a user reports a
message as junk, or they might offer the user the option of reporting
and unsubscribing.

If a mail recipient is unsubscribing manually and the unsubscription
process requires confirmation, the resulting web page is presented to
the recipient who can then click the appropriate button. But when
the unsubscribe action is combined with a user junk report, there is
no direct user interaction with the mailer's website. Similarly, if
a mail system automatically unsubscribes recipient mailboxes that
have been closed or abandoned, there can be no interaction with a
user who is not present. 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 an HTTPS POST
action for mail receivers. Mail senders can distinguish this action
from other unsubscribe requests and handle it as a one-click
unsubscription without manual intervention by the mail recipient.

This document has two goals:

o Allow email senders to signal that a List-Unsubscribe header field
[RFC2369] has one-click functionality.

o Allow MUA (Mail User Agent) users to unsubscribe from mailing
lists in a familiar environment and without leaving the MUA
context. A receiving system can process an unsubscription request
in the background without further interaction and know that it can
be fully processed by the mail sender's system.

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.

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3. Implementation

3.1. Mail Senders

A mail sender that wishes to enable one-click unsubscriptions places
one List-Unsubscribe header field and one List-Unsubscribe-Post
header field in the message. The List-Unsubscribe header field MUST
contain one HTTPS URI. It MAY contain other non-HTTP/S URIs such as
MAILTO:. The List-Unsubscribe-Post header MUST contain the single
key/value pair "List-Unsubscribe=One-Click". As described below, the
message MUST have a valid DomainKeys Identified Mail (DKIM) signature
that covers at least the List-Unsubscribe and List-Unsubscribe-Post
headers.

The URI in the List-Unsubscribe header MUST contain enough
information to identify the mail recipient and the list from which
the recipient is to be removed, so that the unsubscription process
can complete automatically. Since there is no provision for extra
POST arguments, any information about the message or recipient is
encoded in the URI. In particular, one-click has no way to ask the
user what address or from what list the user wishes to unsubscribe.

The POST request MUST NOT include cookies, HTTP authorization, or any
other context information. The unsubscribe operation is logically
unrelated to any previous web activity, and context information could
inappropriately link the unsubscribe to previous activity.

The URI SHOULD include an opaque identifier or another hard-to-forge
component in addition to, or instead of, the plaintext names of the
list and the subscriber. The server handling the unsubscription
SHOULD verify that the opaque or hard-to-forge component is valid.
This will deter attacks in which a malicious party sends spam with
List-Unsubscribe links for a victim list, with the intention of
causing list unsubscriptions from the victim list as a side effect of
users reporting the spam, or where the attacker does POSTs directly
to the mail sender's unsubscription server.

The mail sender needs to provide the infrastructure to handle POST
requests to the specified URI in the List-Unsubscribe header, and to
handle the unsubscribe requests that its mail will provoke.

The mail sender MUST NOT return an HTTPS redirect, since redirected
POST actions have historically not worked reliably, and many browsers
have turned redirected HTTP POSTs into GETs.

This document does not update [RFC2369], so the usage of List-
Unsubscribe URIs other than for one-click remains unchanged.

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3.2. Mail Receivers

A mail receiver can do a one-click unsubscription by performing an
HTTPS POST to the HTTPS URI in the List-Unsubscribe header. It sends
the key/value pair in the List-Unsubscribe-Post header as the request
body.

The POST content SHOULD be sent as 'multipart/form-data' [RFC7578] or
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 is the same as the one in the GET action for a manual
unsubscription, so this is intended to allow the same server code to
handle both.

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

4. Additional Requirements

The message needs at least one valid authentication identifier. In
this version of the specification, the only supported identifier type
is DKIM [RFC6376]. Hence, senders MUST apply at least one valid DKIM
signature to the message.

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

If the message does not have the required DKIM signature, the mail
receiver SHOULD NOT offer a one-click unsubscribe for that message.

5. Header Syntax

The following ABNF imports fields, WSP, and CRLF from [RFC5322].

fields =/ list-unsubscribe-post

list-unsubscribe-post = "List-Unsubscribe-Post:" 0*1WSP postarg CRLF

postarg = "List-Unsubscribe=One-Click"

6. Security Considerations

The List-Unsubscribe header can contain a plaintext or encoded
version of the recipient address, but that address is usually also in
the To: header. This specification allows anyone with access to a

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message to unsubscribe the recipient of the message, but that's
typically the case with existing List-Unsubscribe, just with more
steps.

A malicious mailer could send spam with content intended to provoke
large numbers of unsubscriptions and with suitably crafted headers to
send POST requests 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. The content
of the List-Unsubscribe-Post header is limited to a single known key/
value pair to prevent an attacker from creating malicious messages
where the POST operation could simulate a user filling in an
arbitrary form on a victim website.

The unsubscribe operation provides a strong hint to the mailer that
the address to which the message was sent was valid, and could in
principle be used as a way to test whether an email address is valid.
In practice, though, there are simpler ways such as embedding image
links into the HTML of a message and seeing whether the recipient
fetches the images.

Since the mailer's server that receives the POST request cannot in
general tell where the request is coming from, the URI SHOULD contain
an opaque identifier or another hard-to-forge component to identify
the list and recipient address. That can ensure that the request
originated from the List-Unsubscribe and List-Unsubscribe-Post
headers in a message the mailer sent. Also, the request MUST NOT
include cookies or other context information to prevent the server
from associating the request with previous web requests.

7. IANA Considerations

IANA has added 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: RFC 8058

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8. Examples

8.1. Simple

Header in Email

List-Unsubscribe: <https://example.com/unsubscribe/opaquepart>
List-Unsubscribe-Post: List-Unsubscribe=One-Click

Resulting POST request

POST /unsubscribe/opaquepart HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 26

List-Unsubscribe=One-Click

8.2. Complex

Header in Email

List-Unsubscribe:
<mailto:[email protected]?subject=unsubscribe>,
<https://example.com/unsubscribe.html?opaque=123456789>
List-Unsubscribe-Post: List-Unsubscribe=One-Click

Resulting POST request

POST /unsubscribe.html?opaque=123456789 HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 26

List-Unsubscribe=One-Click

8.3. Complex with 'multipart/form-data'

Header in Email

List-Unsubscribe:
<mailto:[email protected]?subject=unsubscribe>,
<https://example.com/unsubscribe.html/opaque123456789>
List-Unsubscribe-Post: List-Unsubscribe=One-Click

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Resulting POST request

POST /unsubscribe.html/opaque=123456789 HTTP/1.1
Host: example.com
Content-Type: multipart/form-data; boundary=---FormBoundaryjWmhtjORrn
Content-Length: 124

---FormBoundaryjWmhtjORrn
Content-Disposition: form-data; name="List-Unsubscribe"

One-Click
---FormBoundaryjWmhtjORrn--

9. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, DOI 10.17487/RFC2369,
July 1998, <http://www.rfc-editor.org/info/rfc2369>.

[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>.

[RFC6376] 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,
<http://www.rfc-editor.org/info/rfc6376>.

[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.

[RFC7578] Masinter, L., "Returning Values from Forms: multipart/
form-data", RFC 7578, DOI 10.17487/RFC7578, July 2015,
<http://www.rfc-editor.org/info/rfc7578>.

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Authors' Addresses

John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
United States of America

Phone: +1 831 480 2300
Email: [email protected]
URI: http://jl.ly

Tobias Herkula
optivo GmbH
Wallstrasse 16
Berlin 10179
Germany

Phone: +49 30 768078 129
Email: [email protected]
URI: https://www.optivo.com

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