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
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provisions of BCP 78 and BCP 79.
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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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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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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. 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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