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Improving Privacy for the email "Received" Header
draft-josefsson-email-received-privacy-00

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Document Type This is an older version of an Internet-Draft whose latest revision is Expired
Authors Simon Josefsson , Linus Nordberg
Last updated 2015-10-19
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draft-josefsson-email-received-privacy-00
Network Working Group                                       S. Josefsson
Internet-Draft
Updates: 5321, 4409 (if approved)                            L. Nordberg
Intended status: Informational                                      DFRI
Expires: April 22, 2016                                 October 20, 2015

           Improving Privacy for the email "Received" Header
               draft-josefsson-email-received-privacy-00

Abstract

   The email "Received" header has a problematic privacy concern
   affecting email routing before or after handling by public SMTP
   relays.  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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   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 April 22, 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
   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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

1.  Introduction

   As discussed 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.

   For example, when SMTP is used for message submission [RFC4409], this
   allows an attacker to learn the IP address of the host used by the
   individual who sent the email.  This consitutes a privacy leak.  The
   knowlege 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 leaks 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.

   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.

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   Therefore we generalize the privacy problem we are interested in
   resolving to that which affect SMTP transfer or submission agents
   that the organization operating it considers appropriate to not leak
   potentially privacy sensitive information about.

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

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Privacy-sensitive Received header Convention

   If the operator of an SMTP protocol entity, including transfer agents
   and submission agents, desires for improved privacy of the submitting
   entity, it MUST NOT add a Received header as discussed in section 4.4
   of RFC 5321.

3.  Acknowledgements

   Thanks to Philipp Winter for valuable feedback.

4.  Security Considerations

   This document resolves a privacy concern with 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 with the problem to "opt
   out".

   The header is primarily intended to aid debugging, and according to
   RFC 5321 systems SHOULD be robust against unexpected information in
   the header.  Therefore, we believe no security considerations are
   introduced by the proposal in this document.

5.  IANA Considerations

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

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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,
              <http://www.rfc-editor.org/info/rfc4409>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <http://www.rfc-editor.org/info/rfc5321>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI
              10.17487/RFC5322, October 2008,
              <http://www.rfc-editor.org/info/rfc5322>.

6.2.  Informative References

   [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,
              <http://www.rfc-editor.org/info/rfc7624>.

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

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   Linus Nordberg
   DFRI

   Email: linus@dfri.se

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