Ssl.com: DCV bypass and issue fake certificates for any MX hostname

What? When was the last time you had to bypass a cert warning because of "CA store wars" (whatever that means)? What examples can you give for public CAs giving certs to "whatever"?

The CAA whitelist is still enforced by the CAs themselves, so a malicious, compromised or buggy CA could ignore it. You still have to monitor CT. CAA mostly does two things:

1. It makes sure that nobody accidentally issues a cert from another CA (giving you better control, avoiding the "an engineer used a different CA" scenario, and meaning that if you see a cert from another CA, you know it's something Very Not Good).

2. It gives you a chance that an attacker able to bypass some but not all controls on a crappy CA won't be able to use that CA to get a cert for your site (if they don't manage to somehow also bypass the CAA check).

I'm not sure whether CAA would have prevented this CA from issuing for this domain. I think it's more likely than not, but not certain, that it would have helped in this case.

Domain owners may find my CAA record generator <https://sslmate.com/caa/> useful, as it can automatically generate a CAA policy that covers all the certificates found in CT logs for your domain. It's not always obvious how to translate from issuer name to CAA domain (due to white labeled intermediates); my tool consults CCADB data to determine the correct CAA domain.

So CAS records are supposed to keep a CA from issuing a certificate if the CAA record exists and doesn't have that CA.

However, this is relying on the CA to properly check the record. If the CA has a bug where it isn't validating properly, they could also fail to check the CAA properly. Also, this doesn't help against a malicious or compromised CA.

I always wonder who/what checks if CAs respect CAA. I know some browsers now check the certificate transparency log, but are there any that check the CAA record against the issuer of the certificate?

It is unlikely that SSL.com would issue a certificate for any major mail host; it would be malpractice for them not to have some kind of exclusion list.

Issuing a Google certificate is a good way to get your whole CA killed.

Or any domain for which you can read an email sent to an inbox. I remember a few years ago an attack where the attacker would read email because a ticket would be created for incoming emails, and he could guess the next ticket ID to read it. A lot of platform that aren't email providers still allow emails in (e.g. GitHub, GitLab). This looks like a rather widely-applicable attack.

edit: I was thinking about this: https://news.ycombinator.com/item?id=41818459

I couldn’t reproduce the attack with a pair of my own domains, so I think it might be even narrower in scope than the initial post suggests. But I suppose we will just have to wait to see what the CA says.

Or potentially one where you could subscribe to a mailing list. Which includes a lot of very important open source software projects.

Even then, use of a DNS CAA record should mitigate this, right?

It's bad, but the WebPKI of the oughts featured CA certificates issued to random big enterprise IT teams that could simply issue arbitrary certificates. We've come a long ways.

Maybe they should have audited the app for basic sanity during a non-holiday weekend.

(Also, Easter is only a holiday in parts of the world.)

> Can we trust them to release full transparent report?

Generally browser vendors take a pretty dim view of CA's not being transparent when bad things happen. Given the seriousness of this issue,i suspect being aggressively transparent is their only hope of saving their business.

I would expect them to be able to report on certificates issued based on this validation method. That's a basic CA capability and other CA incidents often include these kinds of reports.

Depending on what was logged during the validation, it might be tricky to determine if it was abuse or not. If the DNS content wasn't logged, they could pull a live record and report if the current record would support validation or not.

My guess is that use of this method should be low... If you're updating DNS to add a TXT record, you might be more likely to add a direct verification value rather than an email. But that's speculative; I'm not a CA, I've just been a customer of several... IIRC, I've validated domain control by controlling postmaster@ (or the whois address when that was public) or adding direct TXT verification records or ACME http validations.

All such certs should be in transparancy logs, so I think it should be possible for a third party to verify.

>We will provide a preliminary report on or before 2025-04-21.

They will need to most likely do a full mass revocation at this point.

You can see the cert was revoked here https://crt.sh/?id=17926238129

CAA account binding is basically this. Not cryptographically verified but similar idea that the CA confirms you possess a secret (account) before issuing. https://www.rfc-editor.org/rfc/rfc8657

With your solution, we end up with the same problem just one layer down. Browsers have to contain a list of 'trusted' registars, and an attacker only needs to find one buggy registrar that will incorrectly sign for a domain the attacker doesn't own.