DigiCert Revocation Incident (CNAME Domain Validation)

That's not the rationale for mandating the underscore prefix. The actual reason is so services that allow users to create DNS records at subdomains (e.g. dynamic DNS services) can block users from registering subdomains starting with an underscore. It serves the same purpose that /.well-known does.

For example, if an attacker requests a certificate for dyndns.example and DigiCert gives them a record without an underscore prefix like da39a3ee5e6b4b0d3255bfef95601890afd80709.dyndns.example, they can register that subdomain with the dynamic DNS provider, publish the required record, and get the certificate for dyndns.example. It doesn't matter how much entropy DigiCert put in the record name.

I definitely commend DigiCert for pledging to revoke the certificates within 24 hours and not having a delayed revocation or trying to language lawyer their way to a 5 day revocation as other CAs have tried. Nevertheless, this post severely minimizes the security impact of their mistake, and provides an excellent example of why CAs should always be required to strictly adhere to the rules and not be permitted to excuse noncompliance based on their own security analysis.

for more background. The short story is that when doing CNAME based validation, they were supposed to put an underscore at the start of the random string for you to add to your DNS records. They still generated sufficiently random strings but didn't include a _ before it which is in violation of the RFC. The rationale is that some sites might do something like give you control of yourusername.example.com and they don't want to make it possible for random users to register the random string and be able to manipulate it. If you don't allow users to generate anything that causes a hostname to appear with a leading underscore, they can't pass the domain validation.

Honestly my opinion is that this should trigger the company being banned by all CAs.

The company in question is Alegeus Technologies LLC: https://www.courtlistener.com/docket/68995396/alegeus-techno...

From basic googling it looks like a healthcare provider, so exactly the kind of company you would want to have shitty IT and security infrastructure. A++ work. Absolutely stellar.

There are no polite words that I can use to accurately convey the depth of my disappointment at this kind of inconsiderate behaviour during a crisis, so I won't say anything more.

This will be interesting.

> Unfortunately, no reviews were done to compare the legacy random value implementations with the random value implementations in the new system for every scenario.

In other words, they didn't do proper testing. At the bottom of the article they suggest they're going to improve it.

Seems more reasonable to me to have a much longer deprecation notice.