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Re: [ietf-dkim] Mailsploit

Steve Atkins <steve@wordtothewise.com> Tue, 05 December 2017 21:50 UTC

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From: Steve Atkins <steve@wordtothewise.com>
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Date: Tue, 05 Dec 2017 13:44:36 -0800
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Subject: Re: [ietf-dkim] Mailsploit
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> On Dec 5, 2017, at 1:36 PM, Dave Crocker <dcrocker@bbiw.net> wrote:
> 
> On 12/5/2017 1:33 PM, Steve Atkins wrote:
>> It's a DMARC issue rather than a DKIM one.
> 
> 
> How is it a DMARC issue?

From: {spoo-that-expands-to billing@paypal.com\0}@badpeople.ru will be delivered and (on some clients) have a recipient-visible 822.From that looks like "From: billing@paypal.com" despite not having a valid DKIM signature with a d=paypal.com nor matching paypal.com's published SPF record.

That's DMARC working exactly as designed but not as commonly understood, which makes it a DMARC issue (though a usability one of unmet expectations rather than anything technical).

Much the same as "From: billing@paypal.com <whoever@badpeople.ru>", or the various approaches that pad headers with various sorts of whitespace or v______e_____r_____y long local parts to hide the real domain part on mobile devices, etc.

Cheers,
  Steve
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