Certificate transparency doesn't prevent misissuance, it only makes detection easier after the fact. Someone still needs to be monitoring CT and revoke the cert. I actually believe most HTTP stacks on Android don't even check cert revocations by default.

I'm not too sure what the detection process is like, but being found to sign fraudulent certificates results in your CA being untrusted and is the end of your business. So it's not going to be done lightly even if there isn't automated systems to catch it instantly (which there likely are at least for major websites)

The detection process basically boils down to 'server admins need to check CT themselves'. A CA also doesn't have to be malicious; a non-CA malicious actor could also exploit a vulnerability in the verification process of an honest CA. Depending on the severity of the situation that's unlikely to get them removed from the root stores.

Interesting example: last year Cloudflare found out that a CA had been (incorrectly) issuing certs for 1.1.1.1. They only found out 1.5 years after the first cert had been issued. The CA didn't do it with malicious intent, and as far as I know they're still in business. https://blog.cloudflare.com/unauthorized-issuance-of-certifi...

I don't believe it's supposed to proactively check the logs as that would inevitably break in the presence of properly configured MITM middleboxes which are present on many (most?) corporate networks.

The point of the logs as I understand it is to surface events involving official CAs after the fact.

Clients are supposed to check. For example, Apple requires a varying number of SCTs in order for Safari to trust server certificates. https://support.apple.com/en-us/103214

And yes, it does break MITM use cases, for example on Chrome: https://httptoolkit.com/blog/chrome-android-certificate-tran...

So how does that work with middleboxes? Corporate isn't about to forgo egress security (nor should they).

I don't currently MITM my LAN but my general attitude is that if something won't accept my own root certificate from the store then it's broken, disrespecting my rights, and I want nothing to do with it. Trust decisions are up to me, not some third party.

Corporate managed machines can control the software running on the computer to do anything. I'm not sure the details, but chrome certainly can support corporate MITM. There's likely some setting you have to configure first.

The default should be to reject certificates which aren't being logged, and if you as a user or corporation have a reason to use private certificates, you just configure your computer to do that. Which fully protects against the risk of normal CAs signing fraudulent certificates.

Corporate machines would have the proper certs pushed to them for the MITM box to work though - would that affect this ?

The entire point of transparency logs is to detect a cert issued by a different root CA despite both being trusted. The corporate MITM cert won't be present in the logs by design.