Signaling One-Click Functionality for List Email Headers
A signed email header that tells mail systems they can unsubscribe a user with one automatic HTTPS POST.
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>.
@misc{rfc8058,
series = {Request for Comments},
number = 8058,
howpublished = {RFC 8058},
publisher = {RFC Editor},
doi = {10.17487/RFC8058},
url = {https://www.rfc-editor.org/info/rfc8058},
author = {J. Levine and T. Herkula},
title = {{Signaling One-Click Functionality for List Email Headers}},
pagetotal = 9,
year = 2017,
month = jan,
} RFC text
Contents
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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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
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.
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.
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
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
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:listrequest@example.com?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:listrequest@example.com?subject=unsubscribe>,
<https://example.com/unsubscribe.html/opaque123456789>
List-Unsubscribe-Post: List-Unsubscribe=One-Click
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>.
Authors' Addresses
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
United States of America
Phone: +1 831 480 2300
Email: standards@taugh.com
URI: http://jl.ly
Tobias Herkula
optivo GmbH
Wallstrasse 16
Berlin 10179
Germany
Phone: +49 30 768078 129
Email: t.herkula@optivo.com
URI: https://www.optivo.com
AI ELI5
On this page
§1The Accidental Unsubscribe Problem
A List-Unsubscribe header field (RFC 2369) can hold HTTPS URIs meant to unsubscribe the recipient. But anti-spam software often fetches resources in mail headers without any action by the user, and a sender cannot mechanically tell an automatic fetch from a manual request.
To avoid accidents, senders return landing pages with a confirmation step. A person recognizes it, an automated system does not, so unsubscribing becomes more than one click.
Mailers care about deliverability and want unsubscribing to be easy, because a recipient who finds it hard may report the mail as junk instead. Recipient systems know users do not clearly separate unsubscribing from reporting junk. When an unsubscribe is combined with a junk report, or a system removes closed or abandoned mailboxes, nobody is there to click a confirmation page.
If a robot tries every link in your mail, it might press "stop sending me this" for you. A confirmation page stops that, but then a robot can never press it on purpose.
§2Goals and the Basic Idea
The RFC adds an HTTPS POST action for mail receivers. Senders can tell it apart from other unsubscribe requests and handle it as a one-click unsubscription with no manual step.
It has two goals: let senders signal that a List-Unsubscribe header has one-click functionality, and let users of a Mail User Agent (MUA) unsubscribe in a familiar place without leaving it. The receiving system can process the request in the background, knowing the sender's system can fully handle it.
The document does not update RFC 2369, so List-Unsubscribe URIs used any other way work as before.
The mail program sends one small, special message to the sender's website, and the website knows exactly what to do.
§3What Mail Senders Must Do
A sender places one List-Unsubscribe header and one List-Unsubscribe-Post header in the message. The first MUST contain one HTTPS URI and MAY contain other non-HTTP/S URIs such as MAILTO:. The second MUST contain the single pair List-Unsubscribe=One-Click.
Because there is no provision for extra POST arguments, the URI MUST carry enough to identify the recipient and the list. It SHOULD include an opaque or hard-to-forge component, and the server SHOULD verify it. This deters attackers who send spam with unsubscribe links for a victim list, or who POST directly to the unsubscription server.
The POST request MUST NOT include cookies, HTTP authorization or other context, since unsubscribing is unrelated to earlier web activity. The sender MUST NOT return an HTTPS redirect, because redirected POSTs have historically not worked reliably and many browsers turned them into GETs.
The sender puts a secret-code link in each email. The link has to say who you are and which list, because the special message carries nothing else.
§4What Mail Receivers Do
A receiver does a one-click unsubscription by sending an HTTPS POST to the HTTPS URI in List-Unsubscribe, with the key/value pair from List-Unsubscribe-Post as the request body.
The body SHOULD be sent as multipart/form-data (RFC 7578) and MAY be sent as application/x-www-form-urlencoded. These are the encodings browsers use for forms, and the target is the same as for a manual GET unsubscription, so the same server code can handle both.
The receiver MUST NOT POST without user consent. How and when consent is obtained is outside this specification.
The mail program only presses the button after you say it is okay, and it presses it the same way a web form would.
§5The DKIM Requirement
The message needs at least one valid authentication identifier. In this version only DKIM (RFC 6376) is supported, so senders MUST apply at least one valid DKIM signature.
The List-Unsubscribe and List-Unsubscribe-Post headers MUST be covered by the signature and listed in the h= tag of a valid DKIM-Signature header. If the signature is missing, the receiver SHOULD NOT offer one-click unsubscribe for that message.
The sender signs the letter so the mail program can tell the unsubscribe link really came from the sender and was not swapped.
§6Header Syntax and IANA Entry
The new header is defined in ABNF, importing fields, WSP and CRLF from RFC 5322. Its only legal argument is List-Unsubscribe=One-Click.
IANA added List-Unsubscribe-Post to the "Permanent Message Header Field Names" registry, with applicable protocol mail, status standard and the IETF as change controller.
The rules for writing the new header are very strict: there is only one allowed sentence.
fields =/ list-unsubscribe-post
list-unsubscribe-post = "List-Unsubscribe-Post:" 0*1WSP postarg CRLF
postarg = "List-Unsubscribe=One-Click"§7Security Considerations
The unsubscribe header can include the recipient address, but it is usually also in the To: header. The specification lets anyone with access to a message unsubscribe its recipient, which the RFC notes is typically already true of List-Unsubscribe, just with more steps.
A malicious mailer could try to provoke many unsubscriptions with crafted headers. The RFC judges this unlikely to add much annoyance, since provoking GET requests has long been possible and is easier. The header content is limited to one known key/value pair so an attacker cannot make a POST that simulates a user filling in an arbitrary form on a victim site.
An unsubscribe tells the mailer the address was valid, but the RFC notes simpler ways exist, such as image links. The URI should hold an opaque identifier, and the request carries no cookies, so the server cannot link it to earlier requests.
The only message allowed is one fixed sentence, so a trickster cannot use it to make your mail program fill in some other website's forms.
§8Worked Examples
The RFC gives three examples. The simplest has a single HTTPS URI in List-Unsubscribe, and the resulting POST sends List-Unsubscribe=One-Click as application/x-www-form-urlencoded with a Content-Length of 26.
A second example has both a mailto: URI and an HTTPS URI with an opaque query value. The third shows the same pair sent as multipart/form-data with a form boundary, where the form field is named List-Unsubscribe and its content is One-Click.
The examples show a real header and the exact little message that gets sent back when the button is pressed.
List-Unsubscribe: <https://example.com/unsubscribe/opaquepart>
List-Unsubscribe-Post: List-Unsubscribe=One-Click
POST /unsubscribe/opaquepart HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 26
List-Unsubscribe=One-Click