[ryandv]

The signing keys used by the Certificate Authority to assert that the client (leaf) certificate is authentic through cryptographic signing differ from the private keys used to secure communication with the host(s) referenced in the x509 CN/SAN fields.

[wakawaka28]

I know that. At issue is the fact that the signing keys can be used to sign a MITM key. If there were multiple signatures on the original key, it would (or could) be a lot harder to MITM (presumably). Do you trust any CA enough to never be involved in this kind of scandal? Certainly government CA's and corporate CA's MITM people all the time.

Edit: I'm gonna be rate limited, but let me just say now that Certificate Transparency sounds interesting. I need to look into that more, but it amounts to a 3rd party certificate verification service. Now, we have to figure out how to connect to that service securely lol... Thanks, you've given me something to go read about.

[coffee--]

This is where Certificate Transparency -- and it being mandatory for browser trust -- comes in to save the day.