> A flat cert.pem file (or similar) can't say "distrust this intermediate".

Why does that matter in this case? Can't this be fully mitigated with next to no collateral damage just by removing the trusted roots that signed them?

The root certs may have signed other intermediaries that are still trusted.

I trust nothing that came from emsign

They aren’t trusted, though. Didn’t Chrome ultimately choose to drop the roots, not just the intermediates, from their root program?

Chrome dropped one of the eMudhra roots. The problem is that the weak eMudhra intermediates were cross-signed by Sectigo. ~~Browsers (at least Chrome) didn’t drop the Sectigo root, instead they store a revokation for the cross-signature.~~ (edit: this is wrong, see josephcsible’s answer below)

Dropping the Sectigo root along with the eMudhra root is the easy and safe solution, but it means that some small percentage of certificates that are fine won’t be accepted.

Wasn't the cross signing root AAA Certificate Services, which modern browsers had already dropped trust of before this incident?

Ah, you’re right. Though unless I’m mistaken again, Safari (but not Chrome and Firefox) still trusts AAA Certificate Services and handles this issue with revokations.

This seems like extremely acceptable collateral damage to me. Sectigo should already be working with their customers to reissue immediately since they are the ones who signed the dumb intermediates. Trust only matters if there’s accountability. If root CAs want to keep their trust, it’s on them to not sign things that are bad. That’s the responsibility that comes with the license to print money that being a root CA gives you.

If I’m an OS vendor I would have been putting them on notice days ago that their root was being dropped and they have X days to reissue certs and deliver a new root cert or otherwise they can just accept that they’re not qualified to be a root anymore.

How should Sectigo have detected the badness?