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.
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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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