[ZoneZealot]

Parent comment is making a point that it might have been possible for an attacker to avoid discovery via certificate transparency logs, because anyone 'with a wildcard' could pull off the attack, which is not correct.

I'm pointing out that a wildcard at the apex of your domain (which is what basically everyone means when saying 'a wildcard'), would not work for this attack. Instead if you were to perform the attack using a wildcard certificate, it would need to be issued for '*.asus.com.example.com.' - which would certainly be obvious in certificate transparency logs.

[smileybarry]

Can you still publicly apply for a “*.*.mydomain.com” certificate? IIRC a wildcard cert starting with “*.*.” allows you to chain 2+ names with that cert, I think? (E.g.: “*.*.example.com” cert would match “hello.world.and.hi.com.example.com”)

[ZoneZealot]

With public CAs, you can only apply for a wildcard at a single label. You can't have nested wildcards.

RFC6125 limits wildcards to a left-most label (6.4.3. paragraph 2): https://www.rfc-editor.org/rfc/rfc6125.html#section-6.4.3

I don't know of any CA that allows for wildcard characters within the label, other than when the whole label is a wildcard, but it is possible under that RFC.

The CA/Browser Forum's baseline requirements dictates how any publicly trusted CA should operate, and it defines a wildcard certificate in section 1.6.1 (page 26) here https://cabforum.org/working-groups/server/baseline-requirem...

> Wildcard Certificate: A Certificate containing at least one Wildcard Domain Name in the Subject Alternative Names in the Certificate.

> Wildcard Domain Name: A string starting with “*.” (U+002A ASTERISK, U+002E FULL STOP) immediately followed by a Fully‐Qualified Domain Name.

Now of course with your own internal CA, you have complete free reign to issue certificates - as long as they comply with the technical requirements of your software (i.e. webserver and client).

Also note that a cert issued as '..example.com.' would only match 'hi.com.example.com.', not an additional three labels.