Security Experts:

Let's Encrypt Will Not Replace 1 Million Bug-Affected Certificates

Free and open certificate authority (CA) Let’s Encrypt has decided that it will not revoke one million of the certificates affected by the recent CAA recheck bug.

The flaw was introduced in July 2019 and resulted in an improper recheck related to subscribers’ control of domain names. Due to the bug, Let’s Encrypt could have issued certificates for domain names even if Certification Authority Authorization (CAA) records installed for that domain within the previous 30 days forbade it.

The CA addressed the issue in its CAA checking code immediately after learning of its existence, on February 28. Several days later, it revealed that approximately 2.6% of its active certificates were impacted, and that it would revoke them all by March 5.

A total of 3,048,289 certificates were supposed to be revoked, but Let’s Encrypt ultimately decided to leave 1 million of them unreplaced at this time.

“We announced the plan to revoke because even though the vast majority of the certificates in question do not pose a security risk, industry rules require that we revoke certificates not issued in full compliance with specific standards,” explains Josh Aas, executive director of ISRG, the entity behind Let’s Encrypt.

Working with subscribers worldwide, Let’s Encrypt was able to replace 1.7 million of the affected certificates in less than 48 hours. However, after determining that it will not be able to replace over 1 million certificates before the previously announced deadline, the CA decided to leave those untouched.

“Rather than potentially break so many sites and cause concern for their visitors, we have determined that it is in the best interest of the health of the Internet for us to not revoke those certificates by the deadline,” Aas notes.

The certificates offered by Let’s Encrypt have a 90-day lifespan, meaning that any certificates that are potentially affected but not revoked will not be in the ecosystem for too long.

Of the affected certificates that were replaced, 445 were treated as high priority, as they had CAA records that forbid issuance by Let’s Encrypt. 1,706,505 more certificates were replaced during the incident period.

Let’s Encrypt will continue to revoke more certificates if it determines that their revocation won’t be disruptive to web users.

“We’d like to thank everyone who helped with the effort. Our focus on automation has allowed us, and our subscribers, to make great progress in a short amount of time. We’ve also learned a lot about how we can do even better in the future,” Aas added.

Related: Bug Forces Let's Encrypt to Revoke 3 Million Certificates

Related: Let's Encrypt Issues Over 1 Billion Certificates

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