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 :) 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 <DOMAIN-ID>.ca.example <DOMAIN-ID>.ca.example 1 IN CNAME <RNDVALUE>.random.ca.example <RNDVALUE>.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 <DOMAIN-ID> 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. This would suggest that we don't want the CA doing it, because the CA is not the Applicant, and the goal of 3.2.2.4 is to make sure the Applicant can demonstrate control. Another way of looking at this is imagining the following? - Do we think it's allowed by the BRs for the domain operator to set their MX to be the CA, so the CA can auto-answer their own emails sent under 3.2.2.4.4.? - Do we think it's allowed by the BRs for the domain operator to give the CA FTP/SSH/file upload access to /.well-known/pki-validation, so that the CA can place the answer file on the server, and then request it as the CA, under 3.2.2.4.6? - Do we think it's allowed by the BRs for the domain owner to set an email of domain-id@ca.example using 3.2.2.4.13 / 3.2.2.4.14, and allowing the CA to self-acknowledge that e-mail? I seem to recall that there is a provision in the BRs that would prohibit this, at least in that none of the above (in the CA-controlled example) are demonstrating the Applicant themselves has control. And we certainly know from the above example that it could go quite poorly if naively implemented, so that's probably the right thing. That said, the solution Amazon uses here might be a useful solution to allowing it. Rather than <DOMAIN-ID>, AWS uses something like <ACCOUNT-UNIQUE-ID>, so that _validation.sleevi.example points to <SLEEVI-ACCOUNT-UNIQUE-ID>.ca.example. In that setup, only the <SLEEVI-ACCOUNT-UNIQUE-ID> can get new certs for sleevi.example, and any other accounts would fail, because _validation.sleevi.example wouldn't be pointing to <EVIL-HACKER-ACCOUNT-UNIQUE-ID>.ca.example, which is the one the CA would update. But if we're going to go that route, we probably would minimally want to codify that as the expectation, similar to the intent of 3.2.2.4.12 of making sure it's actually the same person. I realize that I gave this long answer that didn't clearly answer the crux of your question: do the BRs forbid it? The recap, with all this context, is: I seem to recall that, in the case where it was not the Applicant making the change, it was/is forbidden. But I can see good reason to make it allowed, if we can get the safety guards correct. _______________________________________________ dev-security-policy mailing list dev-security-policy@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-security-policy
