Re: [dmarc-ietf] Reports helping spammers? (#81)

[Todd Herr <[e-mail]>]

On Fri, Jan 22, 2021 at 7:00 AM Douglas Foster <
[e-mail]> wrote:

> My specific concern is with disposition information.    Whether the
> message presents with acceptable credentials is the sender's
> responsibility, and I have no problem with documenting whether it passed
> SPF or DKIM or both, including which DKIM scope was used for the
> evaluation.     I have no problem with reporting the sender policy that I
> retrieved from DNS.
>
> But should I report the disposition applied, and the reason that my
> disposition was different from the sender policy?    That seems like
> information leakage about my defenses which should only be revealed to
> highly trusted correspondent domains.
>
> Consider what is likely to happen If I call an organization's help desk
> and say,
>      "My emails to Sally are not getting delivered, and I want to know
> why?"
> The answer should be, and usually is,
>      "Have Sally open a ticket and we will discuss the problem with her!"
> Should not the same policy apply here?
>



>
> Possible misuse of disposition information:
> - DMARC=(Fail), Disposition = (120 delivered) -- probably means that my
> system does not enforce DMARC at all
> - DMARC=(Pass), Disposition = (20 delivered, 100 rejected)  -- possibly
> means that my system needs 20 messages to learn how to identify bad content
>
> I suggest that disposition information should be redacted by default, and
> only included on an exception basis for highly trusted source domains.
>
>
You're using the phrase "disposition was different from the sender policy"
in a way that I don't understand.

Sender policy is a request for handling when a message fails DMARC
validation checks. In your examples of possible misuse of disposition
information, one you're citing is 100 rejected messages when DMARC passes;
that's not a disposition that's different from sender policy, because DMARC
pass is no guarantee of a message being accepted, and again, sender policy
only concerns the state of DMARC failing validation checks. The DMARC
policy statement isn't a vehicle for requesting handling when the message
passes DMARC checks. Beyond that, I'm not even sure that a condition exists
where a message would have a disposition of "rejected due to DMARC" when
the DMARC validation result is pass, but I've been accused in the past of
lacking imagination, so perhaps it could happen.

For your example of DMARC failing and all 120 messages being delivered,
I've never personally met a spammer (every conversation I ever had started
with "I'm not a spammer")  but I can't conceive of a spammer configuring
his domain with a DMARC policy of p=reject and sending mail that doesn't
authenticate as a way of probing things, but I suppose it could happen.
Because aggregate reports only come in once every 24 hours from most
places, he's not going to get immediate gratification like he would simply
by having a few test or seed accounts at the target domain, but maybe he's
patient. Of course, DMARC isn't the sole arbiter of whether or not the
message made it to the Inbox and not the Junk or Spam folder, so results
would be inconclusive at best. His test accounts will tell him much more
than DMARC reports will tell him.

I can't speak for any mailbox providers, but I suspect that the work of
updating their reporting tools to handle an exception list and curating
such a list is more expensive to them than whatever risk might exist in
generating a DMARC aggregate report for a few spam sending domains. Maybe a
note in Security Considerations or something? *shrug*


-- 

*Todd Herr* | Sr. Technical Program Manager
*e:* [e-mail]
*p:* 703.220.4153


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