Internet DRAFT - draft-gondwana-email-mailpath

draft-gondwana-email-mailpath







DMARC                                                   B. Gondwana, Ed.
Internet-Draft                                                  Fastmail
Intended status: Standards Track                         11 October 2022
Expires: 14 April 2023


     Email extension for specifying the next hop path for delivery
                    draft-gondwana-email-mailpath-01

Abstract

   Much work has been put into adding authentication methods (DKIM,
   ARC), source verification (SPF) and policy support (DMARC) to email
   flows, however all these specifications have focused on looking
   backwards through email flow only, and only add new headers to
   messages, causing them all to be susceptible to replay or re-use.

   In particular, in early 2022, a type of attack called "DKIM Replay"
   was widely seen, where correctly DKIM-signed messages were sent to a
   different envelope sender.  The "To" address would not be aligned,
   but such messages can also be the result of legitimate mailflow, so
   these messages were delivered to end-recipient mailboxes, and caused
   reputation issues for the signers of the original message.

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
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   This Internet-Draft will expire on 14 April 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.






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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used In This Document . . . . . . . . . . . . . .   2
   3.  Discussion on Design  . . . . . . . . . . . . . . . . . . . .   3
   4.  Mailpath - a chain of custody . . . . . . . . . . . . . . . .   3
     4.1.  Email Ingress . . . . . . . . . . . . . . . . . . . . . .   3
     4.2.  Email Modification  . . . . . . . . . . . . . . . . . . .   3
     4.3.  Email Egress  . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Advertising support for Mailpath  . . . . . . . . . . . . . .   4
   6.  Delayed SRS . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Operational considerations - cost . . . . . . . . . . . . . .   5
   8.  Security considerations . . . . . . . . . . . . . . . . . . .   5
   9.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   NOTE: this is an early draft of an idea.  Expect significant
   revision.

   Goals:

   1.  to ensure that there's never a message that can be replayed to an
       arbitrary server with authentication claims from this protocol.
   2.  to be able to build up an ecosystem over time such that if every
       hop in a mail flow supports this specification, then additional
       trust properties can apply to the mail flow.

2.  Conventions Used In This Document

   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.




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3.  Discussion on Design

   The presence of Mailpath-Disposition and the checking of Mailpath-
   Disposition on delivery is sufficient to ensure that mail can't be
   replayed down additional paths.  I did consider stripping signatures
   at each stage (and removal entirely when delivering to a host outside
   the ecosystem of mailpath supporting servers) but it doesn't actually
   win anything since you can't replay a mailpath message to another
   domain, since the disposition specifies an explicit path.

   At the moment there's no encryption of headers TO the destination.
   We could also add encryption of headers, which might useful - since
   we're doing a DNS lookup and the record could return a public key to
   encrypt things for.

4.  Mailpath - a chain of custody

   The Mailpath process adds information at three different stages of an
   email's lifecycle within each site along the email's flow ("site" is
   maybe a single server, or maybe a complex system all under the
   control of a single organisation and with its own internal trust
   patterns).

   I'll use "site" in the rest of this document, but maybe there's a
   better term?

4.1.  Email Ingress

   Upon receiving the email, the site adds two headers:

   "Mailpath-Authentication-Results" - as with ARC, a list of all the
   checks which were done and the results of those checks at the time of
   Ingress.

   "Mailpath-Receipt-Signature" - a signature of the state of the
   message as it arrived, covering the "Mailpath-Authentication-Results"
   and any other headers required to confirm alignment.

   These headers have an i=(number) field, similar to ARC.

4.2.  Email Modification

   If the site performs any modification to the email content or the
   DKIM signed headers; e.g. to, from, subject, ... then it adds a
   "Mailpath-Modification" header specifying which fields were changed,
   and why.





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   TBD: do we want a registry of change reason codes, e.g.  "SRS
   rewrite", "mailing list", "virus/malware scan", "add disclaimer" - or
   just plain text?

4.3.  Email Egress

   The site looks up whether the next hop advertises support for
   Mailpath (see below for how) and based on this it creates a
   "Mailpath-Disposition" header specifying where the email is going
   next.

   The site also adds a "Mailpath-Transit-Signature" covering all the
   Ingress and Modification headers as well as the regular contents of a
   DKIM or ARC signature, and also the Mailpath-Disposition header.

   E.g.

Mailpath-Disposition: protocol=mailbox; to=example@fastmail.com

Mailpath-Authentication-Results: i=2; spf=fail; arc=pass; mailpath=pass
Mailpath-Disposition: i=2; protocol=smtp; mx=example.com; to=fred@example.com;
 mailpath=selector._mailpath.example.com
Mailpath-Transit-Signature: (...)
Mailpath-Action: i=2; changeto=yes; changebody=no; changefrom=srs; ...

Mailpath-Authentication-Results: i=3; spf=fail; arc=pass; mailpath=pass
Mailpath-Disposition: i=3; protocol=smtp; mx=example.com; to=fred@example.com;
 mailpath=none
Mailpath-Transit-Signature: (...)
Mailpath-Action: i=2; changeto=yes; changebody=no; changefrom=srs; ...

5.  Advertising support for Mailpath

   A site advertises support for mailpath by creating a record for the
   hostname in the each MX record.  This might be a TXT on that
   hostname, or a sub-name, or indeed a _srv record on the site's
   domain(s).  I'm happy to be guided by advice on this from those with
   more DNS structure experience.

   The important thing is, having calculated the next hop for a message,
   the site does a DNS lookup to see if Mailpath is supported.  The
   record will contain a version, a "yes or no" and - potentially (TBD)
   a key which can be used to sign information to it.








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6.  Delayed SRS

   An option when deploying this, since we're checking for feature
   support on the NEXT hop of email flow, is to delay SRS rewriting (TBD
   - a reference for SRS (https://en.wikipedia.org/wiki/
   Sender_Rewriting_Scheme)).  Instead of performing SRS immediately, a
   site could request that the next hop perform SRS only if exiting the
   mailpath ecosystem, or otherwise pass the same request on.

   In this case, a flow which consisted entirely of mailpath-aware sites
   could avoid doing SRS entirely by using the Mailpath-Modification
   statements and list of involved sites to know which site to direct
   the bounces to, with a additional Mailpath-Bounce headers specifying
   that they were responsible for forwarding the bounce back the way it
   came.

7.  Operational considerations - cost

   The major additional cost will be borne by mailing list servers which
   expand one message to lots of copies.  I consider that acceptable and
   even somewhat desirable, given that most of them already do SRS
   rewriting.  They can still batch message to the same domain, since
   this spec (unlike Marc's EVE spec) only specifies the destination MX
   domain, not the explicit target address(es).  THOUGH - I still see
   benefits in Marc's one to stop replay to arbitrary mailboxes on the
   same domain, i.e. spamming all gmail users with a replay of a single
   message to gmail.

   Another way to work around the replay issue is to strip some
   signature material on delivery to the next hop or to a mailbox, so
   gmail could deliver a raw message to their users' mailboxes which
   COULD NOT be replayed over the wire to a gmail MX as if it was signed
   by the source.  The retention of all the key material on delivery
   does allow more replay attacks if you only control the destination
   mailbox and not any of the servers along the way.

8.  Security considerations

   DNS lookups will need the same security checks as MX lookups, in
   particular an attacker would could either suppress these DNS lookups
   (hence: break the chain of custody and reduce trust in the messages)
   or fake them (leading a naive forwarding server to look like a replay
   rather than a legitimate actor).

   Obviously the crypto will need to be checked by people who know more
   about this stuff than me!





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   I'm sure the "delayed SRS" stuff has some gnarly edges I haven't
   considered.

   Maybe the list will come up with other things.

9.  IANA considerations

   TBD.  We'll need to register the headers at least, and maybe the DNS
   records

10.  Acknowledgements

   TBD

11.  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/info/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/info/rfc8174>.

Author's Address

   Bron Gondwana (editor)
   Fastmail
   Level 2, 114 William St
   Melbourne  VIC 3000
   Australia
   Email: brong@fastmailteam.com
   URI:   https://www.fastmail.com

















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