Re: [dmarc-ietf] ARC vs reject

Jim Fenton <fenton@bluepopcorn.net> Sun, 06 December 2020 02:04 UTC

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From: "Jim Fenton" <fenton@bluepopcorn.net>
To: "John Levine" <johnl@taugh.com>
Cc: dmarc@ietf.org, mike@mtcc.com
Date: Sat, 05 Dec 2020 18:04:44 -0800
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Subject: Re: [dmarc-ietf] ARC vs reject
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On 5 Dec 2020, at 13:03, John Levine wrote:

> In article <4f2d2e0e-c773-95df-0958-12344e963b7a@mtcc.com> you write:
>>
>> As I understand ARC, it is means of transporting the original 
>> auth-res
>> to the destination in case the origin signature is broken by an
>> intermediary. From there the destination can decide one way or the 
>> other
>> to override the DMARC policy of, say, reject.
>
> Right.
>
>> There are, however, use
>> cases where that is exactly wrong and in no case does the originating
>> domain want such an override to happen. Consider my bank sending me
>> transactional email. If somehow somebody managed to get that mail
>> through a mailing list and arc-resigned it, my bank does *not* want 
>> that
>> mail to be delivered regardless of the reputation of the mailing list
>> because something weird and wrong is happening on its face.
>
> If you get a message from a bank via a trustworthy mailing list with a
> valid ARC chain that starts with a DMARC pass, that means someone at
> the bank really did send the message to the list. I don't think it's
> our job to try to guess whether the bank's users are following some
> internal policy we can't see.

I’d like to step back from the specific use case of “a bank”.

If a domain publishes p=reject, they’re requesting particular handling 
of a message they originate. ARC modifies that, which is good for 
mailing lists and similar intermediaries, but depends on a list of 
trusted intermediaries that is not under the control of the originating 
domain. That increases the attack surface for DMARC considerably.

The question I have is: Should DMARC have a policy (or policy modifier) 
that says, “Do not accept modifications to this message?” In other 
words, that the originator values the integrity of their messages over 
deliverability.

-Jim