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Versions: 00 01                                                         
Network Working Group                                       S. Josefsson
Updates: 5321, 4409 (if approved)                            L. Nordberg
Intended status: Informational                                      DFRI
Expires: May 5, 2016                                    November 2, 2015

           Improving Privacy for the email "Received" Header


   The email "Received" header raises a privacy concern with email
   routing.  This document discusses the problem and describes a
   solution that relevant Message Transfer Agents (MTAs) and Mail
   Submission Agents (MSAs) may adopt.

Status of This Memo

   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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

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   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 May 5, 2016.

Copyright Notice

   Copyright (c) 2015 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
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   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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Privacy-sensitive Received header Convention  . . . . . . . .   3
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Appendix A.  Copying conditions . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   As mentioned in RFC 7624 section 3.3.4 [RFC7624], the Simple Mail
   Transfer Protocol (SMTP) [RFC5321] requires that each successive SMTP
   relay adds a "Received" header to the mail headers.  The purpose of
   these headers is to enable audit of mail transmission, and perhaps to
   distinguish between regular mail and spam.  An attacker that can
   observe sufficient email traffic can regularly update the mapping
   between public IP addresses and individual email identities.  Even if
   the SMTP traffic was encrypted on submission and relaying, the
   attacker can still receive a copy of public mailing lists.

   While not mentioned in RFC 7624, the Received header is used for loop
   detection, as discussed in section 6.3 of SMTP [RFC5321].

   To give an example of a privacy violation, consider the following
   scenario.  When SMTP is used for message submission [RFC4409], the
   SMTP server accepting the email from the user MUA will add a Received
   header that will record the IP address of the user's host.  When the
   email is circulated further in the Internet environment, possibly
   ending up publicly archives, it will be possible to read this
   Received header.  This allows an attacker to learn the IP address of
   the host used by the individual who sent the email.  This consitutes
   a privacy violation.  The knowledge of the IP address of the user may
   be used to gather additional information about the user, or to
   simplify direct attacks against the host of the user.

   Privacy violations may also happen when adding additional Received
   headers after an email has been delivered to the MX for the
   destination domain, where anyone who can observe the Received header
   can learn additional information about the internal network topology
   of a single organization.  The privacy relevance of this information
   depends on each organization.

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   There may be other situations where adding Received headers would
   leak unintended information to an observing party.  For example, an
   organization may use different SMTP relays depending on the category
   of a customer.  By knowing the mapping between SMTP relay and
   customer category, an observing party would learn the customer
   category for the organization.

   The privacy problem we are interested in resolving is the part of an
   SMTP agent (be it an MTA or an MSA) that persistently records the IP
   address of the client in the Received header.

   The purpose of this document is to propose a mechanism that
   implementers and operators of SMTP agents may adopt to mitigate the
   privacy violation.

   For ease of reference, the syntax of the Received header is defined
   in RFC 5322 section 3.6.7 [RFC5322] and the SMTP protocol requirement
   to add them is described in RFC 5321 section 4.4 [RFC5321].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Privacy-sensitive Received header Convention

   The "from clause" of the Received header MUST NOT be added by SMTP
   entities concerned with the privacy of their clients.

   With "from clause", we intend what in [RFC2821] is denoted as "From-
   domain" in the ABNF.  To illustrate what is intended, consider the
   following Received header that were added by a MSA and thus leaked
   the then-current IP address of the submitter's host.

   Received: from latte.josefsson.org ([])
    by duva.sjd.se (8.14.4/8.14.4/Debian-4) with ESMTP id t9QFWqNO022103
    (version=TLSv1/SSLv3 cipher=AES128-GCM-SHA256 bits=128 verify=NOT);
    Mon, 26 Oct 2015 16:32:53 +0100

   The from clause is the part of the header that reads "from
   latte.josefsson.org ([])".

3.  Acknowledgements

   The following individuals provided valuable feedback: Philipp Winter,
   Georg Koppen, Jacob Appelbaum, Christian Huitema, Ned Freed, John

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4.  Security Considerations

   This document resolves a privacy concern with the Received header.
   The privacy concern is discussed as a security consideration in
   section 7.6 of SMTP [RFC5321] however that document does not provide
   any mechanism for implementers who are concerned about the problem to
   "opt out".

5.  IANA Considerations

   IANA is adviced to add this document to the Reference column of the
   "Permanent Message Header Field Names" registry for "Received".

6.  References

6.1.  Normative References

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

   [RFC4409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              RFC 4409, DOI 10.17487/RFC4409, April 2006,

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI
              10.17487/RFC5322, October 2008,

6.2.  Informative References

   [RFC2821]  Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC
              2821, DOI 10.17487/RFC2821, April 2001,

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624, DOI
              10.17487/RFC7624, August 2015,

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Appendix A.  Copying conditions

   Regarding this entire document or any portion of it, the authors
   makes no guarantees and is not responsible for any damage resulting
   from its use.  The authors grants irrevocable permission to anyone to
   use, modify, and distribute it in any way that does not diminish the
   rights of anyone else to use, modify, and distribute it, provided
   that redistributed derivative works do not contain misleading author
   or version information.  Derivative works need not be licensed under
   similar terms.

Authors' Addresses

   Simon Josefsson

   Email: simon@josefsson.org

   Linus Nordberg

   Email: linus@dfri.se

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