Keywords: domain, email, security, messaging, dkim, spf, authentication, reporting, conformance, author, origination, original, from, sender
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
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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