I believe LetsEncrypt will issue you a certificate for that subdomain if you can control the server responding at that subdomain. EDIT: Specifically: https://

I believe LetsEncrypt will issue you a certificate for that subdomain if you can control the server responding at that subdomain. EDIT: Specifically: https://letsencrypt.org/docs/challenge-types/ https://letsencrypt.org/docs/challenge-types/ It won't give you a wildcard certificate, but you don't need one for the type of attack we're talking about.