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
draft-wmills-rrvs-header-field-01
Abstract
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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This Internet-Draft will expire on February 3, 2014.
Copyright Notice
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to this document. Code Components extracted from this document must
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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
ownership.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
document.
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
algorithmically:
1. Extract the set of Require-Recipient-Valid-Since fields from the
message.
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 server.example.com ESMTP ready
C: HELO client.example.net
S: 250 server.example.com
C: MAIL FROM:<sender@example.net>
S: 250 OK
C: RCPT TO:<receiver@example.com>
S: 250 OK
C: DATA
S: 354 Ready for message content
C: From: Mister Sender <sender@example.net>
To: Miss Receiver <receiver@example.com>
Subject: Are you still there?
Date: Fri, 28 Jun 2013 18:01:01 +0200
Require-Recipient-Valid-Since: receiver@example.com;
Sat, 1 Jun 2013 09:23:01 -0700
Are you still there?
.
S: 550 5.7.15 receiver@example.com 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
consideration.
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.
[] 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,
(others)
Authors' Addresses
William J. Mills
Yahoo! Inc.
EMail: wmills_92105@yahoo.com
Murray S. Kucherawy
Facebook, Inc.
1 Hacker Way
Menlo Park, CA 94025
USA
EMail: msk@fb.com
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