The fact that any CA can issue a certificate for any domain name is commonly cited as the weakest aspect of the PKI ecosystem. Although CAs want to do the right thing, there are no technical controls that prevent them from doing whatever they chose to do. That’s why we say that the PKI ecosystem is a weak as the weakest link. With hundreds of CAs, there are potentially many weak links.

CAA creates a DNS mechanism that enables domain name owners to whitelist CAs that are allowed to issue certificates for their hostnames. It operates via a new DNS resource record (RR) called CAA (type 257). Owners can restrict certificate issuance by specifying zero or more CAs; if a CA is allowed to issue a certificate, their own hostname will be in the DNS record. For example, this is what someone’s CAA configuration could be (in the zone file):

example.org. CAA 128 issue "letsencrypt.org"