Email Author Header Field
Preserving the original author when mailing lists modify the From field
Crocker, D., "Email Author Header Field", RFC 9057, DOI 10.17487/RFC9057, June 2021, <https://www.rfc-editor.org/info/rfc9057>.
@misc{rfc9057,
series = {Request for Comments},
number = 9057,
howpublished = {RFC 9057},
publisher = {RFC Editor},
doi = {10.17487/RFC9057},
url = {https://www.rfc-editor.org/info/rfc9057},
author = {D. Crocker},
title = {{Email Author Header Field}},
pagetotal = 7,
year = 2021,
month = jun,
} RFC text
Contents
Independent Submission D. Crocker
Request for Comments: 9057 Brandenburg InternetWorking
Category: Experimental June 2021
ISSN: 2070-1721
Email Author Header Field
Abstract
Internet mail defines the From: header field to indicate the author
of the message's content and the Sender: field to indicate who
initially handled the message on the author's behalf. The Sender:
field is optional if it has the same information as the From: field.
This was not a problem until development of stringent protections on
use of the From: field. It has prompted Mediators, such as mailing
lists, to modify the From: field to circumvent mail rejection caused
by those protections. In effect, the From: field has become
dominated by its role as a handling identifier.
The current specification augments the altered use of the From: field
by specifying the Author: field, which ensures identification of the
original author of the message and is not subject to modification by
Mediators. This document is published as an Experimental RFC to
assess community interest, functional efficacy, and technical
adequacy.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see 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
https://www.rfc-editor.org/info/rfc9057.
Copyright Notice
Copyright (c) 2021 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
(https://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.
Table of Contents
1. Introduction
2. Terminology
3. Author Header Field
4. Discussion
5. Security Considerations
6. IANA Considerations
7. Experimental Goals
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Author's Address
1. Introduction
Internet mail conducts asynchronous communication from an author to
one or more recipients and is used for ongoing dialog amongst them.
Email has a long history of serving a wide range of human uses and
styles, within that simple framework, and the mechanisms for making
email robust and safe serve that sole purpose.
Internet mail defines the content header's From: field to indicate
the author of the message and the Sender: field to indicate who
initially handled the message on the author's behalf [Mail-Fmt]. The
Sender: field is optional if it has the same information as the From:
field. That is, when the Sender: field is absent, the From: field
has conflated semantics as both a handling identifier and a content
creator identifier. These fields were initially defined in [RFC733],
and making the redundant Sender: field optional was a small, obvious
optimization in the days of slower communications, expensive storage,
and less powerful computers.
The dual semantics were not a problem until development of stringent
protections on use of the From: field. It has prompted Mediators,
such as mailing lists, to modify the From: field to circumvent
receiver mail rejection caused by those protections. This affects
end-to-end usability of email between the author and the final
recipients, because mail received from the same author is treated
differently by the recipient's software, depending on what path the
message followed.
By way of example, mail originating with:
From: Example User <user@example.com>
which is sent directly to a recipient, will show the author's display
name correctly and can correctly analyze, filter, and aggregate mail
from the author based on their email address. However, if the author
sends through a mailing list and the mailing list conducts a common
form of From: modification needed to bypass enforcement of stringent
authentication policies, then the received message might instead have
a From: field showing:
From: Example User via Example List <listname@list.example.org>
The change inserts an operational address, for the Mediator, into the
From: field and distorts the field's display name as a means of
recording the modification.
In terms of email identification semantics, this is a profound
change:
* The result is that the recipient's software will see the message
as being from an entirely different author and will handle it
separately, such as for sorting or filtering. In effect, the
recipient's software will see the same person's email as being
from a different address; this includes the person's actual
address and each of the mailing lists that person's mail transits.
* Mediators might create a Reply-To: field with the original From:
field email address. This facilitates getting replies back to the
original author, but it does nothing to aid other processing or
presentation done by the recipient's Mail User Agent (MUA) based
on what it believes is the author's address or original display
name. This Reply-To action represents another knock-on effect
(e.g., collateral damage) by distorting the meaning of that header
field, as well as creating an issue if the field already exists.
In effect, the From: field has become dominated by its role as a
handling identifier. The current specification augments this altered
use of the From: field by specifying the Author: field, which
identifies the original author of the message and is not subject to
modification by Mediators.
While it might be cleanest to move towards more reliable use of the
Sender: field and then to target it as the focus of authentication
concerns, enhancement of existing standards works best with
incremental additions, rather than with efforts at replacement. To
that end, this specification provides a means of supplying author
information that is not subject to modification by processes seeking
to enforce stringent authentication.
This version is published as an Experimental RFC to assess community
interest, functional efficacy, and technical adequacy. See
Section 7.
2. Terminology
Terminology and architectural details in this document are
incorporated from [Mail-Arch].
Normative language, per [RFC8174]:
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Author Header Field
Author: is a new message header field being defined. It has the same
syntax as the From: header field [Mail-Fmt]. As with the original
and primary intent for the From: field, the Author: field is intended
to contain the email address of the author of the message content.
It also can contain the displayable human name of the author.
The [ABNF] for the field's syntax is:
author = "Author:" mailbox-list CRLF
which echos the syntax for the From: header field.
This header field can be added as part of the original message
creation process, or it can be added later, by a Mediator, to
preserve the original author information from the From: field.
The goal of the Author: field is to reflect information about the
original author. However, it is possible that the author's MUA or
Mail Submission Agent (MSA) will not create it but that a Mediator
might know it will be modifying the From: field and wish to preserve
the author information. Hence, it needs to be allowed to create the
Author: field for this if the field does not already exist.
Processing of the Author: field follows these rules:
* If an Author: field already exists, a new one MUST NOT be created,
and the existing one MUST NOT be modified.
* An author's MUA or MSA MAY create an Author: field, and its value
MUST be identical to the value in the From: field.
* A Mediator MAY create an Author: field if one does not already
exist, and this new field's value MUST be identical to the value
of the From: field at the time the Mediator received the message
(and before the Mediator causes any changes to the From: field).
4. Discussion
The Author: header field, here, is intended for creation during
message generation or during mediation. It is intended for use by
recipient MUAs, as they typically use the From: field. In that
regard, it would be reasonable for an MUA that would normally
organize, filter, or display information based on the From: field to
give the Author: header field preference.
Original-From: is a similar header field referenced in [RFC5703]. It
is registered with IANA, which cites [RFC5703] as the controlling
source for the entry. However, that document only has a minimal
definition for the field. Also, the field is solely intended for use
by Mediators to preserve information from a modified From: field.
The current specification can be used during either origination or
mediation.
While the basic model of email header fields is highly extensible,
there well might be implementation and usability considerations for
carrying this field through to end users, such as via [IMAP].
Obviously, any security-related processing of a message needs to
distinguish the From: field from the Author: field and treat their
information accordingly.
5. Security Considerations
Any header field containing identification information is a source of
security and privacy concerns, especially when the information
pertains to content authorship. Generally, the handling of the
Author: header field needs to receive scrutiny and care, comparable
to that given to the From: header field, but preferably not in a way
that defeats its utility.
Given the semantics of the Author: header field, it is easy to
believe that use of this field will create a new attack vector for
tricking end users. However (and perhaps surprisingly), for all of
the real and serious demonstrations of users being tricked by
deceptive or false content in a message, there is no evidence that
problematic content in a header field, which is providing information
about message's author, directly contributes to differential and
problematic behavior by the end user. (The presents an obvious
exercise for the reader to find credible, documented evidence.)
6. IANA Considerations
IANA has registered the Author: header field, per [RFC3864], in the
"Provisional Message Header Field Names" registry:
Header field name: Author
Applicable protocol: mail
Status: Provisional
Author/Change controller: Dave Crocker <dcrocker@bbiw.net>
Specification document(s): RFC 9057
7. Experimental Goals
Given that the semantics of this field echo the long-standing From:
header field, the basic mechanics of the field's creation and use are
well understood. Points of concern, therefore, are with possible
interactions with the existing From: field, anti-abuse systems, and
MUA behavior, along with basic market acceptance. So the questions
to answer while the header field has experimental status are:
* Is there demonstrated interest by MUA developers?
* If MUA developers add this capability, is it used by authors?
* Does the presence of the Author: field, in combination with the
From: field, create any operational problems, especially for
recipients?
* Does the presence of the Author: field demonstrate additional
security issues?
* Does the presence of the Author: field engender problematic
behavior by anti-abuse software, such as defeating its utility?
8. References
8.1. Normative References
[ABNF] 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>.
[Mail-Arch]
Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
[Mail-Fmt] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[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>.
[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>.
[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>.
8.2. Informative References
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC5703] Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part
Tests, Iteration, Extraction, Replacement, and Enclosure",
RFC 5703, DOI 10.17487/RFC5703, October 2009,
<https://www.rfc-editor.org/info/rfc5703>.
[RFC733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
"Standard for the format of ARPA network text messages",
RFC 733, DOI 10.17487/RFC0733, November 1977,
<https://www.rfc-editor.org/info/rfc733>.
Acknowledgements
The idea for this field was prompted by discussions in the IETF's
DMARC Working Group, with participation from: Benny Lyne Amorsen,
Kurt Anderson, Laura Atkins, Adrian Farrel, Murray S. Kucherawy, Mike
Hammer, John Levine, Alexey Melnikov, Jesse Thompson, and Alessandro
Vesely.
Author's Address
Dave Crocker
Brandenburg InternetWorking
Email: dcrocker@bbiw.net
AI ELI5
On this page
§1The Problem With From
Internet mail has long used the From: header field to identify the message author. In the original design, the Sender: field identified who handled the message on behalf of the author. When the two were the same, Sender was omitted as redundant.
Modern email authentication systems like DMARC and SPF protect the From: field strictly to prevent spoofing. This has created a problem: when a message passes through a mailing list, the list must modify the From: field to avoid authentication failures, changing it from the original author to the list's address. The recipient's mail client then sees many messages from the same person as coming from different addresses—each mailing list address—making it impossible to reliably group or filter by author.
The Author: header field solves this by giving message authors and mailing list mediators a way to preserve the original authorship information separately from the operational From: field.
When your letter goes through a mailing list, the list changes who the letter says it is from, so email security checks pass. The new Author field lets you write down who really sent it first, so it does not get forgotten.
§2The Author Header Field
The Author: field has identical syntax and semantics to the From: field. It contains the email address and optional display name of the original message author.
Author: can be created at two points in a message's life. The author's mail client can add it when composing (with the same value as From:). More commonly, a mediator such as a mailing list adds it before modifying the From: field, preserving the author's original address. Once an Author: field exists in a message, no other entity may modify or replace it—this ensures it remains immutable end-to-end.
The Author field looks the same as the From field and holds the writer's name and email. Once someone writes it down, nobody is allowed to change it.
§3Processing Rules
The RFC specifies strict rules to ensure Author: stays reliable. If an Author: field already exists, it MUST never be touched. An author's mail client MAY create one with the value from From:. A Mediator MAY create one if it does not exist, capturing the From: value before the Mediator modifies it.
This means Author: represents what the original author put in From:, frozen in place. Mail clients can now organize, filter, and display messages grouped by Author: instead of From:, giving users a consistent view of all mail from a person regardless of which list it came through.
The rules say: if Author exists, leave it alone; mail programs can add it when they send; list operators can add it if it is not there. Once it is written, it stays the same forever.
§4Security Considerations
Like any identification header, Author: carries authentication and privacy risks. Mail systems should apply scrutiny comparable to From:, but the RFC notes something important: despite many real demonstrations of users being tricked by email content, there is no documented evidence that false author identification in a header field directly causes problematic behavior. The concern exists in principle, but lacks a track record of actual harm.
Security systems must distinguish Author: from From: and treat their information separately. An Author: field claiming to be from an attacker is no more dangerous than the From: field already is, because the RFC's immutability rule means an attacker cannot add a false Author: to an existing message—only the original sender can.
The Author field is like other name fields in email—it can be faked, so antispam tools need to care about it. But since no one can change it after it is sent, only the real sender can put a false name in it, just like with the From field.
§5Experimental Status
RFC 9057 is Experimental, published to gauge whether mail client vendors, mailing list operators, and end users find Author: valuable. The experiment aims to answer several questions: Will mail clients implement it? If they do, will users adopt it? Does the field create operational problems? Does it introduce new security issues?
The Experimental status is temporary; if adoption is strong and no problems emerge, a later Standards Track document could make Author: a standard part of Internet mail.
This is a test: the RFC asks mail program makers to try it and see if people use it and if it causes trouble. If it works well, it can become a permanent part of email.