Update to Internet Message Format to Allow Group Syntax in the "From:" and "Sender:" Header Fields

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Document Type Active Internet-Draft (individual in app area)
Author Barry Leiba 
Last updated 2012-11-15 (latest revision 2012-11-06)
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EAI Working Group                                               B. Leiba
Internet-Draft                                       Huawei Technologies
Updates: 5322 (if approved)                             November 6, 2012
Intended status: Standards Track
Expires: May 10, 2013

 Update to Internet Message Format to Allow Group Syntax in the "From:"
                      and "Sender:" Header Fields


   The Internet Message Format (RFC 5322) allows "group" syntax in some
   email header fields, such as "To:" and "CC:", but not in "From:" nor
   "Sender:".  This document updates RFC 5322 to relax that restriction,
   allowing group syntax in those latter fields, as well as in "Resent-
   From:" and "Resent-Sender:", in certain situations.

Status of this Memo

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   This Internet-Draft will expire on May 10, 2013.

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   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.      Introduction  . . . . . . . . . . . . . . . . . . . . . . . 3
   1.1.    Notational Conventions  . . . . . . . . . . . . . . . . . . 3
   1.1.1.  Requirements Notation . . . . . . . . . . . . . . . . . . . 3
   1.1.2.  Syntactic Notation  . . . . . . . . . . . . . . . . . . . . 3

   2.      Allowing Group Syntax in "From:" and "Sender:"  . . . . . . 3
   2.1.    Replacement of RFC 5322, Section 3.6.2. Originator
           Fields  . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   2.2.    Update to RFC 5322, Section 3.6.6. Resent Fields  . . . . . 5

   3.      Applicability Statement . . . . . . . . . . . . . . . . . . 6

   4.      Security Considerations . . . . . . . . . . . . . . . . . . 6

   5.      IANA Considerations . . . . . . . . . . . . . . . . . . . . 7

   6.      References  . . . . . . . . . . . . . . . . . . . . . . . . 8
   6.1.    Normative References  . . . . . . . . . . . . . . . . . . . 8
   6.2.    Informative References  . . . . . . . . . . . . . . . . . . 9

           Author's Address  . . . . . . . . . . . . . . . . . . . . . 9

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1.  Introduction

   The Internet Message Format [RFC5322] allows "group" syntax in some
   email header fields, such as "To:" and "CC:", but not in "From:" nor
   "Sender:".  As use cases for group syntax evolve, particularly with
   respect to email address internationalization issues, it is becoming
   clear that there is little value in forbidding that usage completely,
   and significant value in allowing it in certain situations:
   o  There are times when it is desirable to represent the creator of a
      message as a group or role designation, rather than as one or more
   o  There are times when it is desirable to represent the creator
      and/or sender of a message in a manner that is specifically not
      valid in a reply.

   The requirement to represent these identities as replyable mailboxes
   has thus become unnecessarily restrictive, and this document updates
   RFC 5322 to relax that restriction, allowing group syntax in "From:",
   "Sender:", "Resent-From:", and "Resent-Sender:" for limited use (see
   Section 3).

1.1.  Notational Conventions

   The notational conventions here are the same as those in RFC 5322,
   and the following two subsections are copied directly from that

1.1.1.  Requirements Notation

   This document occasionally uses terms that appear in capital letters.
   When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD
   NOT", and "MAY" appear capitalized, they are being used to indicate
   particular requirements of this specification.  A discussion of the
   meanings of these terms appears in [RFC2119].

1.1.2.  Syntactic Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   [RFC5234] notation for the formal definitions of the syntax of
   messages.  Characters will be specified either by a decimal value
   (e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by
   a case-insensitive literal value enclosed in quotation marks (e.g.,
   "A" for either uppercase or lowercase A).

2.  Allowing Group Syntax in "From:" and "Sender:"

   Section 3.6.2 of RFC 5322 defines the "From:" header field as

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   containing a <mailbox-list> syntax element.  This changes that
   definition to use the <address-list> syntax element, as is used in
   other fields, such as "To:", "CC:", and "Reply-To:".  This also
   changes the definition of the "Sender:" header field from the
   <mailbox> syntax element to the <address> syntax element.  While the
   <address> element includes the <mailbox> element already, we have
   chosen to specify both in the updated syntax as a way of highlighting
   the limited use intended for the change (see Section 3).

   Section 2.1 below is a full replacement for Section 3.6.2 of RFC
   5322, containing the new syntax as well as a new description of the
   semantics for the "From:" and "Sender:" fields.  Section 2.2 below is
   a replacement of only the ABNF syntax for the "Resent-From:" and
   "Resent-Sender:" fields in section 3.6.6 of RFC 5322; the rest of the
   syntax as well as the descriptive text of section 3.6.6 of RFC 5322
   remains unchanged.

   [[anchor5: RFC Editor (please remove this paragraph before
   publication): I know that the text in the following section is not
   consistent within itself nor with the rest of this document in how it
   refers to message header fields, sometimes putting the field name in
   quotation marks and sometimes not, sometimes capitalizing the field
   name and sometimes not, and sometimes including the final colon and
   sometimes not.  It's the document editor's judgment that minimizing
   changes to the original text is more important, in this case, than
   attaining consistency.  Please, therefore, hold back edits to the
   following section, 2.1, as well as to Sections 1.1.1 and 1.1.2 above.
   If you think there are editorial changes that you must make, let's
   please discuss them explicitly during AUTH48.]]

2.1.  Replacement of RFC 5322, Section 3.6.2. Originator Fields

   In version -00, this section is unchanged from RFC 5322, to make it
   easier to use DIFF to see the actual changes that this version
   contains.  Compare this version with version -00. [[anchor6: RFC
   Editor: Please remove this paragraph before publication.]]

   The originator fields of a message consist of the from field, the
   sender field (when applicable), and optionally the reply-to field.
   The from field consists of the field name "From" and a comma-
   separated list of one or more addresses (either mailbox or group
   syntax).  If the from field contains more than one mailbox
   specification (including all mailboxes included in any groups), then
   the sender field, containing the field name "Sender" and a single
   address, MUST appear in the message.  The from field and the sender
   field SHOULD NOT use group syntax; rather, the from field SHOULD use
   only the mailbox-list syntax and the sender field SHOULD use only
   mailbox syntax (see Section 3).  If the sender field uses group

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   syntax, the group MUST NOT contain more than one mailbox.  In either
   case, an optional reply-to field MAY also be included, which contains
   the field name "Reply-To" and a comma-separated list of one or more

   from = "From:" (mailbox-list / address-list) CRLF

   sender = "Sender:" (mailbox / address) CRLF

   reply-to = "Reply-To:" address-list CRLF

   The originator fields indicate the mailbox(es) of the source of the
   message.  The "From:" field specifies the author(s) of the message,
   that is, the mailbox(es) of the person(s) or system(s) responsible
   for the writing of the message.  The "Sender:" field specifies the
   mailbox of the agent responsible for the actual transmission of the
   message.  For example, if a secretary were to send a message for
   another person, the mailbox of the secretary would appear in the
   "Sender:" field and the mailbox of the actual author would appear in
   the "From:" field.  If the originator of the message can be indicated
   by a single mailbox and the author and transmitter are identical, the
   "Sender:" field SHOULD NOT be used.  Otherwise, both fields SHOULD

      Note: The transmitter information is always present.  The absence
      of the "Sender:" field is sometimes mistakenly taken to mean that
      the agent responsible for transmission of the message has not been
      specified.  This absence merely means that the transmitter is
      identical to the author and is therefore not redundantly placed
      into the "Sender:" field.

   The originator fields also provide the information required when
   replying to a message.  When the "Reply-To:" field is present, it
   indicates the address(es) to which the author of the message suggests
   that replies be sent.  In the absence of the "Reply-To:" field,
   replies SHOULD by default be sent to the mailbox(es) specified in the
   "From:" field unless otherwise specified by the person composing the

   In all cases, the "From:" field SHOULD NOT contain any mailbox that
   does not belong to the author(s) of the message.  See also [RFC5322]
   Section 3.6.3 for more information on forming the destination
   addresses for a reply.

2.2.  Update to RFC 5322, Section 3.6.6. Resent Fields

   This updates RFC 5322, Section 3.6.6, to allow groups (via the
   address-list ABNF production) in the "Resent-From:" and "Resent-

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   Sender:" fields, to parallel the change to "From:" and "Sender:"
   above.  The ABNF for those fields is changed as follows:

   resent-from = "Resent-From:" (mailbox-list / address-list) CRLF

   resent-sender = "Resent-Sender:" (mailbox / address) CRLF

3.  Applicability Statement

   Mailbox syntax is the normal use in the "From:" and "Sender:" header
   fields; the address syntax defined in Section 2.1, which allows the
   specification of a group, is only for Limited Use (see [RFC2026],
   Section 3.3, item (d)) for the reasons described below.

   Very many Internet email procedures and software assume that the
   addresses in "From:" and "Sender:" fields can be replied to and are
   suitable for use in mail organizing and filtering.  The use of groups
   instead of mailboxes can disrupt those uses.  Consequently, while
   this specification legitimizes the use of groups, it does so only to
   enable circumstances when that use is necessary, and it is important
   that its use be limited to those circumstances and that it be used
   with caution.  In particular, user agents SHOULD NOT permit the use
   of groups in those fields in outgoing messages.

4.  Security Considerations

   See the Internet Message Format specification [RFC5322] for general
   discussion of security considerations related to the formatting of
   email messages.

   The "From:" address is special, in that most user agents display that
   address, or the "friendly" text associated with it, to the end user,
   and label that so as to identify it as the origin of the message (as
   implied in Section 3.6.2 of RFC 5322).  Group syntax in the "From:"
   header field can be used to hide the identity of the message
   originator.  It is as easy to use a fabricated "From:" address to
   accomplish the same thing, so allowing groups there does not
   exacerbate the security problem.

   Some protocols attempt to validate the originator address by matching
   the "From:" address to a particular verified domain (see Author
   Domain Signing Practices (ADSP) [RFC5617] for one such protocol).
   Such protocols will not be applicable to messages that lack an actual
   email address (whether real or fake) in the "From:" field.  Local
   policy will determine how such messages are handled, and senders,
   therefore, need to be aware that using groups in the "From:" might

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   adversely affect deliverability of the message.

   Because groups have previously not been allowed in the "From:" and
   "Sender:" header fields, it is possible that some implementations
   that conform to RFC 5322 might not be prepared to handle that syntax,
   and, indeed, might not even recognize that group syntax is being
   used.  Of those implementations, some subset might, when presented
   with group syntax in those header fields, behave in a way that is
   exploitable by an attacker.  It is deemed unlikely that this will be
   a serious problem in practice: address field parsing is generally an
   integral component of implementations, and address field parsers are
   required to understand group syntax.  In addition, if any
   implementations should be exploitable through this mechanism, it is
   already possible for attackers to do it by violating RFC 5322, and
   other RFC 5322 violations are commonly used by malefactors.

5.  IANA Considerations

   IANA is asked to update the Permanent Message Header Field Names
   registry (
   http://www.iana.org/assignments/message-headers/perm-headers.html )
   as follows:

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   |  From          |  mail  |  standard  |  [RFC5322]               |

   |  Sender        |  mail  |  standard  |  [RFC5322]               |

   |  Resent-From   |  mail  |  standard  |  [RFC5322]               |

   |  Resent-Sender |  mail  |  standard  |  [RFC5322]               |

   |  From          |  mail  |  standard  |  [RFC5322] [[this RFC]]  |

   |  Sender        |  mail  |  standard  |  [RFC5322] [[this RFC]]  |

   |  Resent-From   |  mail  |  standard  |  [RFC5322] [[this RFC]]  |

   |  Resent-Sender |  mail  |  standard  |  [RFC5322] [[this RFC]]  |

6.  References

6.1.  Normative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

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   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

6.2.  Informative References

   [RFC5617]  Allman, E., Fenton, J., Delany, M., and J. Levine,
              "DomainKeys Identified Mail (DKIM) Author Domain Signing
              Practices (ADSP)", RFC 5617, August 2009.

Author's Address

   Barry Leiba
   Huawei Technologies

   Phone: +1 646 827 0648
   Email: barryleiba@computer.org
   URI:   http://internetmessagingtechnology.org/

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