[jakecraige]

It’s not assuming MITM or that the attacker can upload the signature to the site.

The attack is that the attacker can reuse the already uploaded signature in a way that allows them to get certificates issued under their account instead of the initial owner.

This blog is a little confusing about that since it does read like they are supposed to upload their own sig with the graphic used.

This post and linked IETF report is a little more clear: https://www.agwa.name/blog/2015/12

[baby]

BTW this is not yet in the book, so if you have any suggestion to make the explanation or the diagrams clearer I will take them :)