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Versions: (draft-wmills-rrvs-header-field) 00
01 02 03 04 05 06 07 08 09 10 11 RFC 7293
Network Working Group W. Mills
Internet-Draft Yahoo! Inc.
Intended status: Standards Track M. Kucherawy
Expires: May 10, 2014 Facebook, Inc.
November 6, 2013
The "Require Recipient Valid Since" SMTP Service Extension
draft-ietf-appsawg-rrvs-header-field-02
Abstract
This document defines an extension for the Simple Mail Transfer
Protocol, called RRVS (for "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 facility is on automatically generated
messages that might contain sensitive information, though it may also
be useful in other applications.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 10, 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
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Description . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. The 'RRVS' SMTP Extension . . . . . . . . . . . . . . . . . 4
4. Handling By Receivers . . . . . . . . . . . . . . . . . . . . . 4
4.1. SMTP Extension Offered . . . . . . . . . . . . . . . . . . 4
4.2. SMTP Extension Not Offered . . . . . . . . . . . . . . . . 5
5. Role Accounts . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Use with Mailing Lists . . . . . . . . . . . . . . . . . . . . 5
7. Continuous Ownership . . . . . . . . . . . . . . . . . . . . . 6
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. SMTP Extension Example . . . . . . . . . . . . . . . . . . 6
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
9.1. Abuse Countermeasures . . . . . . . . . . . . . . . . . . . 6
9.2. Suggested Use Restrictions . . . . . . . . . . . . . . . . 7
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 7
10.1. Probing Attacks . . . . . . . . . . . . . . . . . . . . . . 7
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
11.1. SMTP Extension Registration . . . . . . . . . . . . . . . . 8
11.2. Enhanced Status Code Registration . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
12.1. Normative References . . . . . . . . . . . . . . . . . . . 8
12.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Email addresses sometimes get reassigned to a different person. For
example, employment changes at a company can cause an address used
for an ex-employee to be assigned to a new employee, or a mail
service provider (MSP) might expire an account and then let someone
else register for the local-part that was previously used. Those who
sent mail to the previous owner of an address might not know that it
has been reassigned. This can lead to the sending of email to the
correct address, but the wrong recipient.
What is needed is a way to indicate an attribute of the recipient
that will distinguish between the previous owner of an address and
its current owner, if they are different. Further, this needs to be
done in a way that respects privacy.
The mechanism specified here allows the sender of the mail to
indicate how "old" the address assignment is expected to be. In
effect, the sender is saying, "The person to whom I am sending to had
this address assigned to as far back as this date-time." A receiving
system can then compare this information against the date and time
the address was assigned to its current user. If the assignment was
made later than the date-time indicated in the message, there is a
good chance the current user of the address is not the intended
recipient. The receiving system can then choose to prevent delivery
and, possibly, to notify the original sender of the problem.
The primary application is automatically generated messages rather
than user-authored content, though it may be useful in other
contexts.
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
To address the problem described above, a mail sending client needs
to indicate to the server to which it is connecting that there is an
expectation that the destination of the message has been under
continuous ownership since some date-time, presumably the most recent
time the message author had confirmed its understanding of who owned
that mailbox. The mechanism defined here is an extension to the
Simple Mail Transfer Protocol [SMTP] for use between a client and
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server that both implement the extension.
The SMTP extenion is called "RRVS" (Require Recipient Valid Since),
and adds a parameter to the SMTP "RCPT" command that indicates the
most recent date and time when the message author believed the
destination mailbox to be under the continuous ownership (see
Section 7) 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.
3.1. The 'RRVS' SMTP Extension
Extensions to SMTP are described in Section 2.2 of [SMTP].
The name of the extension is "RRVS", an abbreviation of "Require
Recipient Valid Since". Servers implementing the SMTP extension
advertise an additional EHLO keyword of "RRVS", which has no
associated parameters, introduces no new SMTP verbs, and does not
alter the MAIL verb.
An MTA implementing RRVS can transmit or accept a new parameter to
the RCPT command. The new parameter is "RRVS", which takes a value
that is a timestamp expressed as a "date-time" as defiend in
[DATETIME], with the added restriction that a "time-secfrac" MUST NOT
be used. Accordingly, this extension increases the maximum command
length for the RCPT verb by 31 characters.
The meaning of this extension, when used, is described in
Section 4.1.
4. Handling By Receivers
If a receiver implements the RRVS SMTP extension, then there are two
possible evaluation paths:
1. The sending client implements the extension, and so there was an
RRVS parameter on a RCPT TO command in the SMTP session; or
2. The sending client does not (or elected not to) implement the
extension, so the RRVS parameter was not present on the RCPT TO
commands in the SMTP session.
4.1. SMTP Extension Offered
A receiving system that implements the SMTP extension declared above
and observes an RRVS parameter on a RCPT TO command checks whether
the current owner of the destination mailbox has held it
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continuously, far enough back to inclue the given date-time, and
delivers it unless that check returns in the negative. Expressed as
a sequence of steps:
1. Ignore the parameter if the named mailbox is a role account as
listed in Mailbox Names For Common Services, Roles And Functions
[ROLES]. (See Section 5.)
2. Determine if the named address is serviced for local delivery.
If so, and if that address has not been under continuous
ownership since the specified timestamp, return a 550 error to
the RCPT command. If the server implements Enhanced Mail System
Status Codes [ESC], it SHOULD use the code defined in
Section 11.2.
3. Otherwise, continue with normal delivery.
To further obscure account details on the receiving system, the
receiver SHOULD ignore the SMTP extension if the address specified
has had one continuous owner since it was created, regardless of the
purported confirmation date of the address. This is further
discussed in Section 9.
4.2. SMTP Extension Not Offered
For a message being sent that includes content meant to be protected
by this extension, the client MUST NOT continue to deliver a message
to a server where the server does not advertise the RRVS SMTP
extension.
5. Role Accounts
It is necessary not to interfere with delivery of messages to role
mailboxes (see [ROLES]), but it could be useful to indicate to users
handling those mailboxes that a change of ownership might have taken
place where such notification is possible.
6. Use with Mailing Lists
Mailing list services can store the timestamp at which a subscriber
was added to a mailing list. This specification can be used in
conjunction with that information in order to restrict traffic to the
original subscriber, rather than a different person now in possession
of an address under which the original subscriber registered. Upon
receiving a rejection caused by this specification, the list service
can remove that address from further distribution.
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7. Continuous Ownership
Determining continuous ownership of a mailbox is a local matter at
the receiving site. In particular, the only possible answers to the
continuous-ownership-since 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 might be unable to determine whether the owner of the
subject address 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.
8. Examples
In the following examples, "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.
8.1. SMTP Extension Example
C: [connection established]
S: 220 server.example.com ESMTP ready
C: EHLO client.example.net
S: 250-server.example.com
S: 250 RRVS
C: MAIL FROM:<sender@example.net>
S: 250 OK
C: RCPT TO:<receiver@example.com> RRVS=2013-12-31T23:59:59
S: 550 5.7.15 receiver@example.com is no longer valid
C: QUIT
S: 221 So long!
9. Security Considerations
9.1. Abuse Countermeasures
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,
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an operator could track appearance of this extension 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 extension could be ignored and
the message silently discarded. This concern is discussed further in
Section 10.
9.2. Suggested Use Restrictions
If the mailbox named in the RCPT TO command 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 time specified in the parameter,
then the field can be silently ignored and normal message handling
applied so that this information is not disclosed. Such parameters
are likely the product of either gross error or an attack.
A message author using this specification might restrict use of the
extension such that it is only done for recipients known also to
implement this specification, in order to reduce the possibility of
revealing information about the relationship between the author and
the mailbox.
If ownership of an entire domain is transferred, the new owner may
not know what addresses were assigned in the past by the prior owner.
Hence, no address can be known not to have had a single owner, or to
have existed (or not) at all.
10. Privacy Considerations
10.1. Probing Attacks
As described above, use of this extension in probing attacks can
disclose information about the history of the mailbox. The harm that
can be done by leaking any kind of private information is difficult
to predict, so it is prudent to be sensitive to this sort of
disclosure, be it 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
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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.
11. IANA Considerations
11.1. SMTP Extension Registration
IANA is requested to register the SMTP extension described in
Section 3.1.
11.2. 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
12. References
12.1. Normative References
[DATETIME] Klyne, G. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
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12.2. 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.
Appendix A. Acknowledgments
Erling Ellingsen proposed the idea.
Reviews and comments were provided by Michael Adkins, Kurt Andersen,
Alissa Cooper, Dave Cridland, Dave Crocker, Ned Freed, John Levine,
Hector Santos, 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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