On Thu, Oct 10, 2019 at 11:32 PM Ryan Sleevi via dev-security-policy <
dev-security-policy@lists.mozilla.org> wrote:
> On Thu, Oct 10, 2019 at 11:42 PM Jeremy Rowley via dev-security-policy <
> dev-security-policy@lists.mozilla.org> wrote:
>
> > Question, is there any prohibition against demonstration of domain
> control
> > being delegated to a third party or even the CA itself? I don't think so,
> > but figured we've discussed differences in interpretation a lot lately so
> > wanted to see if people agreed.
> >
>
> Huge thanks for bringing this up early :) Definitely the example for all
> CAs :)
>
Thank you for the response, insight, and feedback here! Really, really
helpful in clarifying some of the open questions/discussions we've been
having.
>
> That said, you probably could have worded better, "demonstration of domain
> control being delegated to a third party" is... probably not the best way
> of framing ;)
>
>
> > I mean, the obvious issue is the customer.com domain would need to want
> > to delegate this domain.com. But if you had a pretty non-technical
> person
> > operating the DNS, they could set it to the domain.com name and leave
> > their DNS settings forever.
> >
> > This looks allowed under the BRs, but should it be? Or is it like key
> > escrow - okay if a reseller does it (but frowned upon). Totally not cool
> if
> > the CA does it.
> >
>
> This is definitely a good way of looking at it!
>
> I'm familiar with at least one CA using an approach like this - Amazon
> Trust Services, with
> https://docs.aws.amazon.com/acm/latest/userguide/gs-acm-validate-dns.html
> .
> There's subtlety here, though, which is why I like how you drew the
> comparison to key escrow.
>
> On first glance, it seems Amazon Trust Services is issuing certificates to
> Amazon Web Services, both (through a complex structure) Affiliates, on
> behalf of the user. And so it might seem that the user is pointing their
> DNS to the CA, and that's how it's working.
>
> However, when you look through the CCADB disclosures, along with the
> certificates actually issued, it's clearer that the user is pointing those
> records to Amazon Web Services, but the actual issuing CA is DigiCert (as
> disclosed at https://www.amazontrust.com/repository/ ). It's also the case
> that, if you look at how things are setup, the domain holder isn't the one
> applying for the certificate - it's Amazon Web Services applying for the
> certificate (i.e. they are the Applicant, not the domain owner), and AWS
> configuring their own DNS records to contain a random value, to demonstrate
> to the CA (DigiCert) that AWS, the Applicant, is authorized for the domain.
>
> It's a really elegant solution, but it turns out it's not a "CA
> self-dealing" kind of thing, which would have been useful as an example if
> it was already allowed.
>
> Now let's discuss how this idea could all go wrong, because it teases out a
> little why the question of who is Applicant is relevant and important to
> understanding.
>
> Let's imagine that I setup the following (and I'm going to butcher this
> pseudo-code, so bear with me)
> _validation.sleevi.example 3600 IN CNAME .ca.example
> .ca.example 1 IN CNAME .random.ca.example
> .random.ca.example 1 IN TXT "{the CA challenge goes here}"
>
> Now, whenever the CA receives a request to validate "sleevi.example", they
> map that to a domain-id. They then update the CNAME, and the TXT record, to
> match the CA-defined challenge value. Then, they issue a TXT record lookup
> for _validation.sleevi.example, get the challenge value back, and huzzah,
> the cert is authorized!
>
> But there's a small problem with this. Who was the Applicant requesting
> sleevi.example? From the CA's perspective, how does it distinguish me, the
> legit domain holder, from 'evil hacker'? In the Amazon case, AWS is the
> Applicant, and AWS is the one doing this fancy DNS stuff, so it's clear -
> only AWS can do this, and they're who the user delegated to.
>
> However, if the "party doing this domain dance" is != "the applicant", then
> a lot can go wrong here, because they might do the dance for two different
> applicants. The easiest example would be if the CA, for every CSR it
> received, regardless of the Applicant, it updated the for
> sleevi.example - at that point, anyone in the world could get a cert for my
> domain from that CA, and that would not be the good result.
>
Apologies, but this isn't entirely clear to me. I'm guessing (hoping) my
misunderstanding centers around a difference between the Applicant fully
delegating DNS to the CA vs the Applicant only configuring a single CNAME
record? If the Applicant has configured
_validation.sleevi.example 3600 IN CNAME .ca.example
then the CA wouldn't be able to use any other value to complete
the full lookup to include the TXT record, without the Applicant directly
changing the CNAME value.
However, if the CA is fully managing this DNS, and therefore able to
independently reconfigure:
_validati
