Comodo fixes bug that led to issue of banned digital certificates

The company issued new certs for internal hosts, which is now banned by the CAB Forum

Comodo said Monday it fixed a bug that led to the issuance of some now-banned digital certificates. Other CAs might have the same problem, too.

Under new rules from the CA/Browser Forum (CAB) that came into force on Nov. 1, certification authorities (CAs) are not supposed to issue new SSL/TLS (Secure Sockets Layer/Transport Layer Security) certificates for internal host names.

Comodo had been preparing for the rule change, but a "subtle bug" was introduced in its issuing system on Oct. 30, wrote Rob Stradling, senior research and development scientist, in a post on the CAB Forum.

"Despite our code review and QA processes, this bug still made it into production code," Stradling wrote.

The result was that eight certificates ended up being issued which shouldn't have, and those certificates have now been revoked, he wrote.

Other CAs may have had the same problem. Stradling wrote that "we found non-compliant certificates issued by quite a number of other CAs, but I'll document these in another post."

The reason why CAs aren't supposed to issue SSL/TLS certificates for internal hosts is to prevent man-in-the-middle attacks.

Companies and organizations have traditionally bought SSL/TLS certificates for servers or devices with internal host names that can't be seen from the public Internet.

Those certificates are used to authenticate the machines that are talking to each other. But since organizations aren't CAs themselves, they've had to buy those certificates from CAs.

While CAs validate request for digital certificates for public domains to ensure the right entity is requesting one, they can't do that for internal hosts.

That makes it possible for an attacker to obtain a digital certificate for a server with a generic name such as "," and then use it in an attack to monitor encrypted data traffic of another organization.

By October 2016, CAs are supposed to revoke certificates for internal hosts if those certificates have not yet expired.

Stradling wrote that a hot fix was distributed about two hours after Comodo discovered the problem.

"We regret that our implementation of this important and long-trialed policy change fell below the standards that are expected of us and that we expect of ourselves," Stradling wrote.

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