[Mailman-Users] ARC

Stephen J. Turnbull turnbull.stephen.fw at u.tsukuba.ac.jp
Wed Jul 25 05:53:40 EDT 2018

Grant Taylor via Mailman-Users writes:

 > I would think / hope / expect that such services would be from a 
 > different (sub)domain of the client that they are sending email on 
 > behalf of.

That's not how "on behalf of" worked in practice.  What happened in
April 2014, was that a home business owner (HBO) would send a pile of
completed order notices to intuit.com, and intuit.com would send an
invoice to each customer on behalf of hbo at yahoo.com, from
hbo at yahoo.com.  HBO, of course, doesn't have a vanity domain, just a
Wordpress or SquareSite (or even Facebook) home page.  Tens of
thousands of invoices worth millions of dollars bounced off of
personal accounts and tiny business owner accounts at Yahoo! and other
receivers who take p=reject as a command.

Note that if I were intuit.com's CISO, I would fight tooth and nail
against the system you suggest, because it implies that I have DKIM
private keys for all those subdomains owned by clients.  Every spammer
in the world would be trying to hack the server that has those keys.
I could probably keep them out, but Lordy, the liability involved!

Less financially painful are the services that newspapers and tourist
destinations provided (note past tense, they're mostly dead now) where
you could sit at a terminal and send a "postcard" recommending an
article or with a picture of the attraction to family and friends
"from" yourself, including your email address, simply by typing name
and address in.  Not a huge loss, I guess, but ....

 > > The other *possible* use case for ARC would be non-mailing list 
 > > forwarding.  But these almost never break the DKIM signature of the 
 > > originator.
 > They may not break DKIM.  But depending on how they operate, they may 
 > break SPF directly (re-sending with the original SMTP envelope From: 
 > thus violating SPF) -or- indirectly (re-sending with something like SRS)

I've never seen SRS in the wild, except in the headers of some of the
crazier denizens of IETF mailing lists.  YMMV, of course.

 > thus breaking DMARC alignment.

Simple .forwarding breaks SPF of the author domain, period, unless
entirely within one administrative domain, and that domain is
well-configured so that all the forwarding is internal, and the SPF
checks are all done at the boundary.

I guess in the case where forwarding takes place entirely within an
administrative domain, DMARC From alignment could be broken without
breaking SPF itself because From alignment is validated on a different
host from SPF, but that sounds like a case of "evolution in action".

 > My understanding is that DMARC can be configured to require both SPF and 
 > DKIM alignment.

That's non-conforming to RFC 7489.  Section 4.2:

    A message satisfies the DMARC checks if AT LEAST ONE of the supported
    authentication mechanisms:

    1.  produces a "pass" result, and

    2.  produces that result based on an identifier that is in alignment,
        as defined in Section 3.

[Emphasis mine.]

 > The point being that simple .forward(ing) may still break things.
 > I maintain that detecting such is one of the functional purposes of 

Of course.  That's why it's not "DMAC" (although either would be a
great street name for a hip-hop DJ!)

 > I question the wisdom of making processing of ARC conditional on RFC 
 > 2369 List-* headers.  I mainly say this because there is nothing that 
 > prevents malicious actors from inserting (possibly bogus) List-* 
 > headers.  (Or lots of tiny lists of single recipients.)

I'm afraid I failed to make the concept explicit again.  Remember,
authenticating mail origin doesn't prevent anyone from sending
malicious mail, or even from having it delivered.  So while I suggest
that ARC processing results be taken seriously in the presence of
(List-* headers) for DMARC alignment purposes, delivery is also
conditional on (not malicious).  If a site wants to be conservative
and assume some degree of malice until it's seen "enough" benign
traffic, that's consistent with what I intended.

Also, sites like GMail and Yahoo! might very well be willing to
specially handle From addresses at sites known to have leaked a
billion address books.  Similarly, they could "throttle" on floods of
ARC email via purported mailing lists that have never been seen
before.  I acknowledge that sites managed by single admins would
likely be unwilling, and perhaps even unable, to come up with such

N.B. By "intended," I mean to admit that my wording in the previous
post strongly suggested that they would start with the assumption that
mail is benign until proven otherwise.  I can't fault you for taking
my words literally. :-)


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