There's a long StackExchange answer about this: https://security.stackexchange.com/a/158164

But it boils down to there being no practical way for Let's Encrypt to automatically validate that a wildcard certificate is safe to issue.

It's a long answer that completely fails to address the possibility of validating ownership of the domain itself by e.g. adding a TXT record, which the ACME protocol already supports.

The general point is that being able to control the parent domain doesn't necessarily mean you control all possible subdomains as well. You need to prove ownership, not just control. Here's the relevant bit from the SO answer:

> If I have ownership of the parent domain example.com then I can freely create and control anything as a subdomain, at any level I choose. Note that here "ownership" is distinct from "control", which is what is validated by the ACME protocol.

Probably their agreements with their partner CAs. Given that those partners sell wildcards themselves...

Their "Let’s Encrypt Authority X3" intermediate is signed by their own root (ISRG Root X1). See https://letsencrypt.org/certificates/.

Subdomains were killed by SEO a long time ago (afaik, Google does not transfer domain PageRank credit to subdomains), so this is not limited solely by the cost of wildcard certs.

But this is orthogonal to the issue of LetsEncrypt not delivering wildcard certs.

They cost like $199 or less. It's an anoying tax, but they do offer a lot of options, so they're often worth it.

Way less than that. I've got a wildcard SSL cert for my domain for $60, although that was an add-on to the domain itself and hosting, purchased from the provider of the latter.

How would they prove that you own every subdomain?

I'm not sure I understand the question. If you own something.com then you automatically own any possible subdomains.

The Let's Encrypt process is about validating control of the content on a domain, not about OWNERSHIP of the domain. To get a cert, you just have to be able to update a file at a Let's Encrypt specified location on the domain. This is only proving that you are in control of the website for that specific domain, not that you are in control of the DNS for the entire domain and all subdomains.

Of course if I own a domain, I own all the subdomains. However, being in control of the site served at port 80 for a domain does not mean I own it.

But the ACME protocol, the automation underpinning Let's Encrypt, supports validation via a DNS challenge (adding a specific TXT record to the domain). Would it not be possible to issue wildcards if-and-only-if a DNS challenge succeeds?

I think you're right.