LetsEncrypt – Expiration Notification Service Has Ended

Certificates are still a pain in the butt. One of the most cumbersome aspects of the web.

Especially domain wide certs which need DNS auth.

DNS auth would be okish if it was simply tied to a txt entry in the DNS and valid as long as the txt entry is there. Why does LetsEncrypt expire the cert while the acme DNS entry is still there? Which attack vector does this prevent?

Also, why not support file based auth in .well-known/acme-challenge/... for domain wide certs? Which attack vector does that prevent?

> DNS auth would be okish if it was simply tied to a txt entry in the DNS and valid as long as the txt entry is there. Why does LetsEncrypt expire the cert while the acme DNS entry is still there? Which attack vector does this prevent?

An attacker should not gain the ability to persistently issue certificates because they have one-time access to DNS. A non-technical user may not notice that the record has been added.

> Also, why not support file based auth in .well-known/acme-challenge/... for domain wide certs? Which attack vector does that prevent?

Control over a subdomain (or even control over the root-level domain) does not and should not allow certificate issuance for arbitrary subdomains. Consider the case where the root level domain is hosted with a marketing agency that may not follow security best practices. If their web server is compromised, the attacker should not be able to issue certificates for the secure internal web applications hosted on subdomains.

> An attacker should not gain the ability to > persistently issue certificates because they > have one-time access to DNS.

They wouldn't. As soon as the owner of the domain removes the TXT entry that ability would be gone.

Of course - but that requires the owner to know they were attacked, know the attacker added a TXT verification, potentially overcome fear of deleting it breaking something unexpected, etc.

If the owner does not find out that someone got control of their DNS server, the attacker can do anything with the domain anyhow. Including issuing certs.