There’s a lot going on in this blog. Interestingly, the core mechanism at play here is the http-01 challenge validations which they state is fetched by the CA over HTTPS. This is particularly amusing when you consider that http-01 is explicitly NOT HTTPS (it’s HTTP), and this is actually the entire reason there’s a different code path to take.
The modern web requires secure (HTTPS) context for many things to work, so it’s commonplace to do so “HTTPS enforcement”; all requests are forcibly upgraded to HTTPS. However, you can’t do that to the CA when it’s performing a http-01 challenge validation. This necessitates a “well known” URL route be used for challenges so that they can very deliberately take a different code path that doesn’t enforce HTTPS (and be routed differently).
This is true of basically every ACME client used for http-01 challenges, not just cloudflare. So while they’ve unfortunately missed the mark on correctly explaining the mechanism at play here, I hope that I succeeded in making it a bit more clear. Other implementations are, of course, similarly exploitable.