Subject: [Acme] security concern for dns challenge

From: YAMAGUCHI Takanori <[email-masked]>
Date: Fri, 8 Jul 2016 10:58:29 +0900

Hi,

When example.com zone and its child zone, www.example.com, are hosted
in the same DNS server, most of DNS server implementations response
authoritative answers to queries for the child zone, even if the
parent does not delegate the child.

Some DNS hosting providers do not confirm whether the owner of the
domain is reasonable or not. A person unconcerned with example.com can
contract www.example.com zone on those providers.

ACME spec says:
> The client constructs the validation domain name by
> prepending the label "_acme-challenge" to the domain name being
> validated, then provisions a TXT record with the digest value under
> that name.  For example, if the domain name being validated is
> "example.com", then the client would provision the following DNS
> record:
>
> _acme-challenge.example.com. 300 IN TXT "gfj9Xq...Rg85nM"

So, if an evil person contracts www.example.com zone on the DNS
provider which hosts example.com and creates _acme-challenge.www.example.com
record, then he can steal the certificate for www.example.com.
If he contracts _acme-challenge.example.com zone, he can get the cert
for example.com itself.

If DNS providers refuse improper zones, this approach doesn't work.
But it is difficult for a third party to confirm it.

Regards.