On Mon 12/Dec/2022 15:50:44 +0100 Laura Atkins wrote:
On 12 Dec 2022, at 14:34, Murray S. Kucherawy <superu...@gmail.com> wrote:
On Mon, Dec 12, 2022 at 1:13 AM Alessandro Vesely <ves...@tana.it
<mailto:ves...@tana.it>> wrote:
The alternative is to say: Well, if you can't make at least one of those
two quantities bulletproof, then don't sign your mail. That, though,
sounds a lot to me like tossing DKIM in the bin.
On the opposite, if Gmail restricted signing to accountable users only, its
signatures would gain value. If they started doing so it would soon be
noticed, and signatures would acquire a meaning in delivery decisions.
Is the cost of imposing a program that vets every user comparable to that of
the damage caused by this attack vector? My impression is that it is not.
I’m not aware of Gmail being a significant victim here - although it’s possible
they are.
I'm not aware of their taking any significant measure (after the effort, a few
years ago, to reorganize all the different accounts on their different
platforms.) Yes, security has a cost. Why are banks insisting on 2fA?
Endowing signatures with a significant value increases the overall value of
DKIM.
Presumably they already have significant value. That's why this attack works
already.
They’re an identity of a known sender that invests time and resources into
building and managing their reputation. Google? Maybe not. But the email
service providers who do a lot to keep the spammers off their network are a
common victim. These spammers know that they get better delivery if their mail
is signed by the email service provider. The email service provider’s detectors
and defenses are enough to stop the spammer from being able to send through the
ESP. So the spammer sends one email to an account they own and takes a
reputation they’ve already been told they shouldn’t be using.
I guess you refer to the same incident you touched on a few threads ago. Did
it happen more than once to the same ESP? To more ESPs? Cannot (did not) they
configure their DKIM filter to not sign for untrusted prospects?
A DKIM signature is an identity. That identity has a reputation. Attacks that
borrow the identity belonging to senders with good reputation benefit from that
reputation. It’s not about any DKIM signature. It’s not about a random DKIM
signature. It’s about a known entity. Even if Gmail only signed mail from
accountable users, there is still the possibility of spammers posing as
accountable users.
Perhaps they could devise better methods than asking _accountable? (Y/N)_ on a
questionnaire. Linking to bank accounts is an example.
A discernment possibility is to sign differently. RFC 6376 specified an Agent
or User Identifier tag, i=, as a finer grained identity. One having
i=bullshit...@example.com would still be a valid DKIM signature.
Alternatively, could use subdomains, d=bullshit.example.com. How long would it
take receivers to learn it?
The whole idea of a DKIM replay attack is that this is mail that cannot be
directly sent through the infrastructure of the domain owner. That, itself,
implies the domain owners are doing quite a bit to stop the spam from coming
out of their network. If they weren’t doing a good job then replay attacks
wouldn’t be happening - the mail would just be sent over that network directly.
Asking for the domain owners to “stop sending spam” when the whole replay
process indicates they are stopping spam out of the networks they control seems
a bit of a non-starter to me.
The securing activity you describe certainly has non-negligible costs. Then
why do we abhor the cost of classifying users?
Most likely, DMARC was branded as a requirement for email security and DKIM
came as a consequence, without worrying too much about what is being signed.
Replay attacks found the weak point of that paradigm. Don't allow guests into
secured areas of your premises.
Best
Ale
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