Network Working Group S. Josefsson
Updates: 5321, 4409 (if approved) L. Nordberg
Intended status: Informational DFRI
Expires: April 21, 2016 October 19, 2015

Improving Privacy for the email "Received" Header


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

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This Internet-Draft will expire on April 21, 2016.

Copyright Notice

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Table of Contents

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.

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.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 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., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008.

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.

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:
Linus Nordberg DFRI EMail: