Internet DRAFT - draft-josefsson-email-received-privacy
draft-josefsson-email-received-privacy
Network Working Group S. Josefsson
Internet-Draft
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
draft-josefsson-email-received-privacy-01
Abstract
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.
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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",
"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
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 ([155.4.17.2])
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 ([155.4.17.2])".
3. Acknowledgements
The following individuals provided valuable feedback: Philipp Winter,
Georg Koppen, Jacob Appelbaum, Christian Huitema, Ned Freed, John
Levine
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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,
<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
[RFC2821] Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC
2821, DOI 10.17487/RFC2821, April 2001,
<http://www.rfc-editor.org/info/rfc2821>.
[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>.
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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
DFRI
Email: linus@dfri.se
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