| EAI Working Group | B. Leiba |
| Internet-Draft | Huawei Technologies |
| Updates: 5322 (if approved) | July 14, 2012 |
| Intended status: Standards Track | |
| Expires: January 13, 2013 |
Update to Internet Message Format to Allow Group Syntax in the 'From:' Header Field
draft-leiba-5322upd-from-group-02
The Internet Message Format (RFC 5322) allows "group" syntax in some email header fields, such as "To:" and "CC:", but not in "From:". This document updates RFC 5322 to relax that restriction, allowing group syntax in "From:".
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http:/⁠/⁠datatracker.ietf.org/⁠drafts/⁠current/⁠.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2013.
Copyright (c) 2012 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.
The Internet Message Format [RFC5322] allows "group" syntax in some email header fields, such as "To:" and "CC:", but not in "From:". 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, and significant value in allowing it. This document updates RFC 5322 to relax that restriction, allowing group syntax in "From:".
The notational conventions here are the same as those in RFC 5322, and the following two subsections are copied directly from that document.
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].
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).
Section 3.6.2 of RFC 5322 defines the "From:" header field as 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:".
The following normative section replaces Section 3.6.2 of RFC 5322.
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.
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 address (mailbox or group) in the address-list, then the sender field, containing the field name "Sender" and a single mailbox specification, MUST appear in the message. 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 addresses (either mailbox or group syntax).
from = "From:" address-list CRLF
sender = "Sender:" mailbox CRLF
reply-to = "Reply-To:" address-list CRLF
The originator fields indicate the address(es) of the source of the message. The "From:" field specifies the author(s) of the message, that is, the address(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 address of the actual author would appear in the "From:" field. If the originator of the message can be indicated by a single mailbox in the "From:" field and the author and transmitter are identical, the "Sender:" field SHOULD NOT be used. Otherwise, both fields SHOULD appear.
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 address(es) specified in the "From:" field unless otherwise specified by the person composing the reply.
In all cases, the "From:" field SHOULD NOT contain any address 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.
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 group syntax does not exacerbate the problem.
Some protocols attempt to validate that 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, and local policy will determine how such messages are handled. Senders, therefore, need to be aware that using group syntax in the "From" might adversely affect deliverability of the message.
Because group syntax in the "From" header field has previously not been allowed, it is possible that some implementations that conform to RFC 5322 might not be prepared to handle the syntax, and, indeed, might not even recognize that group syntax is being used. Of those implementations, some subset might, when presented with "From" group syntax, 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.
No IANA actions are requested by this document, and the RFC Editor is asked to remove this section before publication.
| [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. |
| [RFC5322] | Resnick, P., "Internet Message Format", RFC 5322, October 2008. |
| [RFC5617] | Allman, E., Fenton, J., Delany, M. and J. Levine, "DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)", RFC 5617, August 2009. |