Re: [dmarc-ietf] attack on reports

[Todd Herr <[e-mail]>]

On Tue, Jan 26, 2021 at 2:24 PM Michael Thomas <[e-mail]> wrote:

>
> On 1/26/21 10:56 AM, Todd Herr wrote:
>
>> > In addition, if I recover that message from the log, I might find no
>> > relationship with the reporting domain or the reported source IP.
>> > That is to say, I won't be able to deduce if the report is fake or real.
>>
>>
>> My main point here is to point out the attack.
>>
>>
> The attack scenario you have described relies on several possible but
> perhaps implausible conditions all being true:
>
> 1. There exists a domain run by people who are savvy enough to want to
> implement DMARC and can consume reports, but don't have a good grasp on
> which IPs are likely to be theirs and which aren't, and don't have an
> understanding of how to use common tools to figure out whether an IP
> address might belong to their provider's ASN or one halfway around the
> world, and
>
> Here's a very basic question: if I do not know all of the IP addresses
> that send on my behalf, are DMARC reports of any value? Enterprises farm
> out email all of the time and it could be difficult to know when they
> change their server addresses, etc. If the reporting is predicated on your
> having in effect and up to date SPF record (ie, do all of the work to be
> able to produce one), then that negates anybody who just uses DKIM alone
> which should be a completely acceptable use case. And no, the
> domain/selector tells you nothing when it doesn't verify.
>
> If it is the case that you MUST know all of your sending IP addresses,
> that should be in blinking bold right up front in section 7.
>
>
> Yes, DMARC reports are of value if you don't know all of the IP addresses
that send on your behalf. Some have even written blog posts on the topic of
using DMARC aggregate reports as a tool to audit one's authentication
practices, by publishing a policy of p=none, collecting the reports,
analyzing the data, fixing problems, iterate, iterate, iterate until one is
ready to move on to the ultimate goal of p=reject.

In my experience, there is enough information in the DMARC aggregate
reports to allow for someone to make an educated guess as to whether an IP
that's generating mail that's not authenticating is perhaps a server
sending reports from the side of the desk of Emily in Engineering, or a
third party that Mike in Marketing contracted to send mail on the company's
behalf, or whether it's almost certainly an IP that's not one used by the
organization at all. This presumes, of course, that one at least has a clue
as to what public IP range(s) are in use by the organization, not just for
mail but for general connectivity, but with the security organization
and/or IT typically running (or at least ordering and participating in the
analysis of) the DMARC deployment, that seems a safe assumption. Having the
source IP in the aggregate report isn't necessarily just to make sure it
gets added to the SPF record, either; many times it (along with the other
information in that tuple) gives the report consumer a clue as to where to
start hunting in his infrastructure for the MTA that isn't correctly DKIM
signing mail at this time.

As to third parties changing email addresses all the time, you're correct,
they do, but they mitigate the impact on their customers by instructing
them to use the include: mechanism in their SPF records, rather than trying
to enumerate all of the vendor's IP ranges, e.g., include:_
spf.salesforce.com, include:_spf.google.com, etc.



-- 

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


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