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Re: (DMARC) We've been here before, was Why mailing lists

Brian E Carpenter <brian.e.carpenter@gmail.com> Fri, 18 April 2014 20:47 UTC

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Date: Sat, 19 Apr 2014 08:47:37 +1200
From: Brian E Carpenter <brian.e.carpenter@gmail.com>
Organization: University of Auckland
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To: "Murray S. Kucherawy" <superuser@gmail.com>
Subject: Re: (DMARC) We've been here before, was Why mailing lists
References: <20140417181815.8A5871ACD1@ld9781.wdf.sap.corp> <9451.1397772992@sandelman.ca> <CAL0qLwa0a4nDAdCHkkMJdeemsj+cezcmH3+59CvhF8q7B72ryg@mail.gmail.com>
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Cc: Michael Richardson <mcr+ietf@sandelman.ca>, Pete Resnick <presnick@qti.qualcomm.com>, John R Levine <johnl@taugh.com>, "ietf@ietf.org" <ietf@ietf.org>
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On 19/04/2014 03:20, Murray S. Kucherawy wrote:
...
> One of the key points about DMARC's design is that it's concerned
> specifically with From:.  The reason is that the content of From: is what's
> typically shown to the recipient by MUAs.  If DMARC keyed off Sender:
> instead, then this would work:
> 
> MAIL FROM: haha@badguy.example.com
> 
> From: security@paypal.com
> Sender: haha@badguy.example.com
> DKIM-Signature: v=1; d=badguy.example.com; ...

So, if the From says

From: goodguy@yahoo.com <haha@badguy.example.com>

many UAs would show only goodguy@yahoo.com as the sender,
but badguy could have passed DMARC, no?

This would not exactly enhance goodguy's reputation,
or Yahoo's for that matter. I realise it isn't the exploit
that Yahoo is trying to stop, but it suggests to me that
DMARC is only plugging one small hole in a very leaky dam.

    Brian

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