Trust and security considerations for Structured Email
draft-ietf-sml-trust-01

sml                                                         H.-J. Happel
Internet-Draft                                                   audriga
Intended status: Standards Track                          A. Gulbrandsen
Expires: 14 November 2026                                          ICANN
                                                             13 May 2026

         Trust and security considerations for Structured Email
                        draft-ietf-sml-trust-01

Abstract

   This document discusses trust and security considerations for
   structured email and provides recommendations for message user agents
   on how to deal with structured data in email messages.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Structured Email
   Working Group mailing list (sml@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/sml/.

   Source for this draft and an issue tracker can be found at
   https://github.com/arnt/sml-trust.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 14 November 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Types of security concerns  . . . . . . . . . . . . . . . . .   3
     3.1.  Spam/virus filters  . . . . . . . . . . . . . . . . . . .   3
     3.2.  Formal display of data  . . . . . . . . . . . . . . . . .   3
     3.3.  Additional user interface options . . . . . . . . . . . .   4
     3.4.  Automated processing  . . . . . . . . . . . . . . . . . .   4
     3.5.  External references . . . . . . . . . . . . . . . . . . .   4
     3.6.  Social engineering  . . . . . . . . . . . . . . . . . . .   4
   4.  Trust mechanisms  . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Processing structured data  . . . . . . . . . . . . . . .   5
     4.2.  Inlining data . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Privacy considerations  . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Note to self . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Structured email, as described in [I-D.sml-structured-email], makes
   the content of some email messages machine-readable, such that user
   agents can provide higher-level functions than displaying/replying,
   for example "add this to calendar".

   Naturally, new functions bring new trust and security considerations,
   or bring new urgency to existing issues.  This document recommends
   security and trust mechanisms that should be applied when processing
   machine-readable content in email messages, both by senders and
   receivers.

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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Types of security concerns

   This section gives an overview of the various types of security and
   privacy concerns that arise when email messages contain structured
   data.  The same concerns often arise for other messages, of course.

   This section is informative.

3.1.  Spam/virus filters

   Structured email increases the syntactical and semantic complexity of
   email messages.  If a spam/virus filter parses structured email in
   order to block malevolent messages, the filter's parser will
   necessarily differ from that of the MUA that will finally act on the
   structured data, creating a risk of misclassification.

   These risks are elevated when a structured data format has complex
   syntax, syntax that offers several optional or alternative ways to
   express the same substance, and of course by parsers that deviate
   from the specification for better bug compatibiloty.

3.2.  Formal display of data

   A common example is displaying a received calendar invitation using
   dates/times in the recipient's timezone, in a fixed format.

   Formal display introduces additional possibilities of discrepancy
   between the different representations.  For example, a single message
   might contains a multipart/alternative containing a text/plain
   description of a flight itinerary, a text/html description of the
   same itinerary, and a structured representation.  All three may be
   different, leading to confusion (and in this example, perhaps to
   missing a flight).

   Unintentional discrepancy is a risk for senders; some recipients may
   be misinformed.

   If a message is sent to a group and there is a discrepancy, different
   members of the group may see it differently.

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   If a particular MUA displays the formal representation within the
   message, a malevolent sender could try to mimic the visual
   representation using HTML with CSS, but with misleading content.

3.3.  Additional user interface options

   Structured mail processing may provide the receiving user with
   additional commands.  Returning to the calendar example, many MUAs
   provide the user with additional commands to add something to a
   calendar.

3.4.  Automated processing

   Automated processing covers actions that are taken as soon as the
   message arrives rather than when a human user reads the message.  For
   example, a user might want flight reservations to be automatically
   added to a travel itinerary application and/or a calendar.

   Such automation could be a custom MUA feature or a future extension
   of the Sieve email filtering language ([RFC5228]).  A related example
   for abuse in automated processing is calendar spam ([CalSpam]).

3.5.  External references

   Email messages with a text/html body part ("HTML email messages") may
   contain image resources that link to web servers.  Such links can be
   used for tracking user interaction with the message.

   Similar concerns apply to structured data types which include image
   references, such as the cover image of a music album or the teaser
   image of a news article.

   RDF structured data can be partial by design and include references
   to additional data.  Using a "follow your nose" approach, tools can
   follow URL references to obtain further structured data concerning a
   resource.  For example, a piece of structured data about an article
   could reference the article's authors only by URL.  For a meaningful
   processing of author information, one might try to obtain further
   data using that URL.

3.6.  Social engineering

   While the risks of social engineering are hardly new and the human-
   readable text in a message can in principle be used to persuade the
   human reader to do anything, structured data widens the variety of
   actions the human reader can easily perform.  If there are more
   buttons to click, then there's also a greater variety of attacks.

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   Put differently: A user who might not be able to follow the
   instructions in a long and involved text-based social engineering
   attack may be able to follow simple instructions such as "click this
   then that".

4.  Trust mechanisms

   Several implementations of structured email restrict processing to
   messages that are particularly trusted.  That is to say, an incoming
   message is in one of these three categories:

   1.  Spam.  Structured data is not processed.

   2.  Ordinary.  Structured data is not processed.

   3.  Trusted.  Structured data is processed.

   This section gives an overview of the trust mechanisms used to
   differentiate between 2 and 3.

   It does not attempt to describe whether a trust-based mechanism is
   appopriate in a particular case.

4.1.  Processing structured data

   MUAs SHOULD display structured data in incoming email messages only
   if any of these criteria hold:

   *  Processing the data offers no additional attack surface compared
      to displaying the HTML in which the structured data is embedded.
      This may often be the case for formal display.

   *  Only for MUAs that process calendar invitations/updates: The MUA
      would process a calendar invitation in the same message.

   *  The sender is trusted (e.g., part of the user's address book) and
      the messsage contains a valid personal or domain signature.

   *  The message is part of an ongoing thread with a trusted sender.

   *  The message's content indicate a connection with an older, trusted
      message.  For example, if a calendar invitation was accepted, then
      updates or responses for the same event are connected with the
      original.

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   Structured data that requires or suggests automatic processing may
   benefit from additional precautions before acting on the message.
   Documents that specify such data types should discuss how recipients
   should decide whether to act.

   Open issue: At some point this document needs to mention JSON Web
   Signatures and RFC 7519, ether to use or to ignore.

4.2.  Inlining data

   Structured data included in an email message SHOULD be self-contained
   in order to avoid privacy problems.  This implies that if an MUA is
   able to provide meaningful user interaction (rather than mere
   display), then the data SHOULD be self-contained, such that the
   interaction will not need referenced resources from the web.

5.  Security considerations

   Security considerations are a core subject of this document.

6.  Privacy considerations

   Privacy considerations are a core subject of this document.

7.  IANA Considerations

   This document has no IANA actions at this time.

8.  References

8.1.  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,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

8.2.  Informative References

   [I-D.sml-structured-email]
              "*** BROKEN REFERENCE ***".

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   [RFC5228]  Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
              Filtering Language", RFC 5228, DOI 10.17487/RFC5228,
              January 2008, <https://www.rfc-editor.org/rfc/rfc5228>.

   [RFC6132]  George, R. and B. Leiba, "Sieve Notification Using
              Presence Information", RFC 6132, DOI 10.17487/RFC6132,
              July 2011, <https://www.rfc-editor.org/rfc/rfc6132>.

   [RFC6134]  Melnikov, A. and B. Leiba, "Sieve Extension: Externally
              Stored Lists", RFC 6134, DOI 10.17487/RFC6134, July 2011,
              <https://www.rfc-editor.org/rfc/rfc6134>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/rfc/rfc7942>.

   [CalSpam]  "Calendar operator practices — Guidelines to protect
              against calendar abuse", n.d.,
              <https://standards.calconnect.org/csd/cc-18003.html>.

   [MachineReadable]
              "NIST IR 7511 Rev 4", n.d.,
              <https://csrc.nist.gov/glossary/term/Machine_Readable>.

Appendix A.  Acknowledgements

   The authors wish to thank Ben Bucksch, Alexey Melnikov, Phillip Tao,
   Lisa Dusseault, Orie Steele, Daniel Kahn Gillmor, and others whose
   suggestions were made before this paragraph was started.

Appendix B.  Note to self

   RFC Editor: Please remove this section.

   The charter has this to say about what this document should contain:
   "Recommendations for security and trust mechanisms that should be
   applied when processing machine-readable content in email messages"
   and "security and trust recommendations to prevent abuse of
   structured email".  No more, no less.

Authors' Addresses

   Hans-Joerg Happel
   audriga
   Email: happel@audriga.com
   URI:   https://www.audriga.com

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   Arnt Gulbrandsen
   ICANN
   6 Rond Point Schumann, Bd. 1
   1040 Brussels
   Belgium
   Email: arnt@gulbrandsen.priv.no
   URI:   https://icann.org/ua

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