Network Working Group                                           W. Mills
Internet-Draft                                               Yahoo! Inc.
Intended status: Informational                              M. Kucherawy
Expires: February 3, 2014                                 Facebook, Inc.
                                                          August 2, 2013

             The Require-Recipient-Valid-Since Header Field


   This document defines an email header field, Require-Recipient-Valid-
   Since, to provide a method for senders to indicate to receivers the
   time when the sender last confirmed the ownership of the target
   mailbox.  This can be used to detect changes of mailbox ownership,
   and thus prevent mail from being delivered to the wrong party.

   The intended use of this header field is on automatically generated
   messages that might contain sensitive information.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 3, 2014.

Copyright Notice

   Copyright (c) 2013 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Description  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Use with Mailing Lists . . . . . . . . . . . . . . . . . . . .  5
   5.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Continuous Ownership . . . . . . . . . . . . . . . . . . . . .  6
   7.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   9.  Privacy Considerations . . . . . . . . . . . . . . . . . . . .  8
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     10.1.  Header Field Registration . . . . . . . . . . . . . . . .  8
     10.2.  Enhanced Status Code Registration . . . . . . . . . . . .  9
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     11.1.  Normative References  . . . . . . . . . . . . . . . . . .  9
     11.2.  Informative References  . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   Mailbox Service Providers (MSPs) are public, often free, services
   that provide email sending and receiving capabilities to users.  Some
   of them have policies that allow for expiration of account names when
   they have been unused for a protracted period.  If an expired account
   name can be reclaimed, there is a risk of delivery of mail to the
   wrong party if some message author is unaware of this change of

   This document defines a header field called Require-Recipient-Valid-
   Since.  The content of this header field includes an intended
   recipient mailbox and a timestamp indicating at what point in time
   the message author believed that mailbox to be under confirmed
   ownership of a specific party.  If the receiving system observes this
   field and can determine that the intended recipient mailbox has
   changed ownership since the provided timestamp, it can decline
   delivery, preventing possible misdelivery of mail.

   The primary application is automatically generated messages rather
   than user-authored content.

2.  Definitions

   For a description of the email architecture, consult [EMAIL-ARCH].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [KEYWORDS].

3.  Description

   The Require-Recipient-Valid-Since header field includes an intended
   recipient coupled with a timestamp indicating the most recent date
   and time when the message author believed the destination mailbox to
   be under the continuous ownership (see Section 6) of a specific
   party.  Presumably there has been some confirmation process applied
   to establish this ownership; however, the method of making such
   determinations is a local matter and outside the scope of this

   The general constraints on syntax and placement of header fields in a
   message are defined in Internet Message Format [MAIL].

   Using Augmented Backus-Naur Form [ABNF], the syntax for the field is:

       rrvs = "Require-Recipient-Valid-Since:" mailbox; date-time CRLF

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   "CFWS" is defined in Section 3.2.2, "date-time" is defined in Section
   3.3, and "mailbox" is defined in Section 3.4, of [MAIL].

   A receiving system that implements this specification determines
   whether the named mailbox is based at that receiving system, is a
   current intended recipient of the message, and has been under
   continuous ownership since the specified date.  If that address does
   not match an intended local recipient (in terms of the email
   transaction details), the header field is ignored.  Otherwise, if
   continuous ownership since the indicated time can be established, the
   message is delivered normally; if not, the message is rejected.
   Finally, when delivery is being performed (and the message is not
   being forwarded), the header field is removed.  Expressed

   1.  Extract the set of Require-Recipient-Valid-Since fields from the

   2.  Discard any such fields that are syntactically invalid.

   3.  Discard any such fields that name a role account as listed in
       Mailbox Names For Common Services, Roles And Functions [ROLES].

   4.  Discard any such fields for which the "mailbox" portion does not
       match a current recipient, as listed in the RCPT TO commands in
       the corresponding Simple Mail Transfer Protocol [SMTP] session.

   5.  For each field remaining, determine if the named mailbox has been
       under continuous ownership since the corresponding timestamp.  If
       it has not, reject the message.

   6.  RECOMMENDED: If local delivery is being performed, remove all
       instances of this field prior to delivery to a mailbox; if the
       message is being forwarded, remove those instances of this header
       field that were not discarded by steps 1-4 above.

   The final step is not mandatory as not all mail handling agents are
   capable of stripping away header fields.

   It is preferred that the rejection be enacted as an error response to
   the SMTP command verb, but this is not strictly necessary.  When
   performing the "DATA" rejection, servers use an SMTP error code (and
   Enhanced Mail System Status Code [ESC], if supported) as described in
   Section 10.2.

   Implementation is expected to be transparent to non participants,
   since they would typically ignore this header field.

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   This header field SHOULD NOT be added to a message that is addressed
   to multiple recipients.  Because of the nature of SMTP, a message
   bearing this header field for multiple mailboxes could result in a
   single delivery attempt for multiple recipients (in particular, if
   two of the recipients are handled by the same server), and if any one
   of them fails the test, the delivery fails to all of them.  It is
   presumed that an author making use of this field is seeking to
   protect transactional or otherwise sensitive data intended for a
   single recipient, and thus generating independent messages for each
   individual recipient is RECOMMENDED.

   If the agent generating the message uses any kind of message
   authentication technology, the authentication SHOULD cover this
   header field.  An agent receiving a message bearing this header field
   that is covered by some kind of authentication SHOULD ignore it if
   the authentication does not succeed.

   To further obscure account details on the receiving system, the
   receiver SHOULD ignore the header field if the address within it has
   had one continuous owner since it was created, regardless of the
   purported confirmation date of the address.  This is further
   discussed in Section 8.

4.  Use with Mailing Lists

   Mailing list services can store the timestamp at which a subscriber
   was added to a mailing list.  Thus, when generating a message for
   distribution, the list service can use this field as a means of
   preventing mailing list traffic from going to the wrong recipient,
   and instead remove that address from further distribution.

   A mailing list service that receives a message containing this field
   removes it from the message prior to redistributing it, limiting
   exposure of information regarding the relationship between the
   message's author and mailing list.

5.  Discussion

   It can be argued that the architecturally better decision would be to
   introduce this capability as an extension to SMTP.  The exchange of
   meta data about the target mailbox is not part of the actual message
   content, nor is it meta data about the content.  However, if the
   author and the target mailbox are separated by an SMTP server that
   does not implement the SMTP extension, the check will not be able to
   propagate to the intended receiving system.  Implementing this
   service as a header field allows the check to occur even across non-
   participating systems, effectively tunneling the request.

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   The presence of the intended mailbox in the field content supports
   the case where a message bearing this header field is forwarded.  The
   specific use case is as follows:

   1.  A user subscribes to a service "S" on date "D" and confirms an
       email address at the user's current location, "A";

   2.  At some later date, the user intends to leave the current
       location, and thus creates a new mailbox elsewhere, at "B";

   3.  The user replaces mailbox "A" with forwarding to "B";

   4.  "S" constructs a message to "A" claiming that address was valid
       at date "D" and sends it to "A";

   5.  The receiving MTA at "A" determines that the forwarding in effect
       was created by the same party that owned the mailbox there, and
       thus concludes the continuous ownership test has been satisfied;

   6.  If possible, "A" removes this header field from the message, and
       in either case, forwards it to "B";

   7.  On receipt at "B", either the header field has been removed, or
       the header field does not refer to a current envelope recipient,
       and in either case delivers the message.

   Some services generate messages with an RFC5322.To field that does
   not contain a valid address, in order to obscure the intended
   recipient.  For this reason, the original intended recipient is
   included in this header field.

6.  Continuous Ownership

   Determining continuous ownership of a mailbox is entirely a local
   matter.  In particular, the only possible answers to that question
   are "yes", "no", and "unknown"; the action to be taken in the
   "unknown" case is a matter of local policy.

   For example, when control of a domain name is transferred, the new
   domain owner may be unable to determine whether the owner of the
   subject mailbox has been under continuous ownership since the stated
   date if the mailbox history is not also transferred (or was not
   previously maintained).

   It will also be "unknown" if whatever database contains mailbox
   ownership data is temporarily unavailable at the time a message
   arrives for delivery.  In this case, typical SMTP temporary failure
   handling is appropriate.

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7.  Example

   In the following example, "C:" indicates data sent by an SMTP client,
   and "S:" indicates responses by the SMTP server.  Message content is
   CRLF terminated, though these are omitted here for ease of reading.

     C: [connection established]
     S: 220 ESMTP ready
     C: HELO
     S: 250
     C: MAIL FROM:<>
     S: 250 OK
     C: RCPT TO:<>
     S: 250 OK
     C: DATA
     S: 354 Ready for message content
     C: From: Mister Sender <>
        To: Miss Receiver <>
        Subject: Are you still there?
        Date: Fri, 28 Jun 2013 18:01:01 +0200
          Sat, 1 Jun 2013 09:23:01 -0700

        Are you still there?
     S: 550 5.7.15 is no longer valid
     C: QUIT
     S: 221 So long!

8.  Security Considerations

   The response of a server implementing this protocol can disclose
   information about the age of existing email mailbox.  Implementation
   of countermeasures against probing attacks is advised.  For example,
   an operator could track appearance of this field with respect to a
   particular mailbox and observe the timestamps being submitted for
   testing; if it appears a variety of timestamps is being tried against
   a single mailbox in short order, the field could be ignored and the
   message silently discarded.  This concern is discussed further in
   Section 9.

   If the mailbox named in the field is known to have had only a single
   continuous owner since creation, or not to have existed at all (under
   any owner) prior to the date specified in the field, then the field
   can be silently ignored and normal message handling applied so that
   this information is not disclosed.  Such fields are likely the
   product of either an attack or gross error.

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9.  Privacy Considerations

   As described above, use of this header field in probing attacks can
   disclose information about the history of the mailbox.  In the
   terminology defined in Privacy Considerations for Internet Protocols
   [PRIVACY], this is an Item of Interest.  The harm that can be done by
   leaking any kind of private information varies widely and cannot be
   predicted, so it is prudent to be sensitive to this sort of
   disclosure, either inadvertently or in response to probing by an
   attacker.  It bears restating, then, that implementing
   countermeasures to abuse of this capability needs strong

   That some MSPs allow for expiration of account names when they have
   been unused for a protracted period forces a choice between two
   potential types of privacy vulnerabilities, one of which presents
   significantly greater threats to users than the other.  Automatically
   generated mail is often used to convey authentication credentials
   that can potentially provide access to extremely sensitive
   information.  Supplying such credentials to the wrong party after a
   mailbox ownership change could allow the previous owner's data to be
   exposed without his or her authorization or knowledge.  In contrast,
   the information that may be exposed to a third party via the proposal
   in this document is limited to information about the mailbox history.
   Given that MSPs have chosen to allow transfers of mailbox ownership
   without the prior owner's involvement, the information leakage from
   the header field specified here creates far fewer risks than the
   potential for delivering mail to the wrong party.

10.  IANA Considerations

10.1.  Header Field Registration

   IANA is requested to add the following entry to the Permanent Message
   Header Field registry, as per the procedure found in [IANA-HEADERS]:

     Header field name: Require-Recipient-Valid-Since
     Applicable protocol: mail ([MAIL])
     Status: Standard
     Author/Change controller: IETF
     Specification document(s): [this document]
     Related information:
       Requesting review of any proposed changes and additions to
       this field is recommended.

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10.2.  Enhanced Status Code Registration

   IANA is requested to register the following in the SMTP Enhanced
   Status Codes registry:

      Code:               X.7.15
      Sample Text:        Mailbox owner has changed
      Associated basic status code:  5
      Description:        This status code is returned when a message is
                          received with a Require-Recipient-Valid-Since
                          field and the receiving system is able to
                          determine that the intended recipient mailbox
                          has not been under continuous ownership since
                          the specified date.
      Reference:          [this document]
      Submitter:          M. Kucherawy
      Change controller:  IESG

11.  References

11.1.  Normative References

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

   [IANA-HEADERS]  Klyne, G., Nottingham, M., and J. Mogul,
                   "Registration Procedures for Message Header Fields",
                   BCP 90, RFC 3864, September 2004.

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

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

   [ROLES]         Crocker, D., "Mailbox Names For Common Services,
                   Roles And Functions", RFC 2142, May 1997.

   [SMTP]          Klensin, J., "Simple Mail Transfer Protocol",
                   RFC 5321, October 2008.

11.2.  Informative References

   [EMAIL-ARCH]    Crocker, D., "Internet Mail Architecture", RFC 5598,
                   July 2009.

   [ESC]           Vaudreuil, G., "Enhanced Mail System Status Codes",
                   RFC 3463, January 2003.

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   [PRIVACY]       Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
                   Morris, J., Hansen, M., and R. Smith, "Privacy
                   Considerations for Internet Protocols", RFC 6973,
                   July 2013.

Appendix A.  Acknowledgments

   Erling Ellingsen proposed the idea.

   Reviews and comments were provided by Michael Adkins, Kurt Andersen,
   Alissa Cooper, Ned Freed, John Levine, Gregg Stefancik, Ed Zayas,

Authors' Addresses

   William J. Mills
   Yahoo! Inc.


   Murray S. Kucherawy
   Facebook, Inc.
   1 Hacker Way
   Menlo Park, CA  94025


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