Pierluigi Paganini March 04, 2020

Let’s Encrypt is going to revoke over 3 million certificates today due to a flaw in the software used to verify users and their domains before issuing a certificate.

Let’s Encrypt certificate authority (CA) is going to revoke over 3 million certificates today due to a vulnerability in software used to verify users and their domains before issuing a certificate.

A bug in Let’s Encrypt’s certificate authority (CA) software, dubbed Boulder, caused the correct validation for some certificates.

The bug impacted the way the CAA (Certificate Authority Authorization) specification is implemented by the Boulder.

The CAA security feature allows domain owners to prevent Certificate Authorities (CAs) to issue certificates for their domains.

Domain owners can add a “CAA field” to their domain’s DNS records, this implies that only the CA included in this field can issue a TLS certificate for that domain.

Every CA must check CAA records at most 8 hours before a certificate is issued for a certain domain, but the bug caused a domain on a multi-domain certificate to be checked numerous times rather than all the domains on the certificate being checked at the same time.

This behavior caused certificates to be issued without the proper CAA list for some domains.

“Let’s Encrypt found a bug in our CAA code. Our CA software, Boulder, checks for CAA records at the same time it validates a subscriber’s control of a domain name. Most subscribers issue a certificate immediately after domain control validation, but we consider a validation good for 30 days. That means in some cases we need to check CAA records a second time, just before issuance. Specifically, we have to check CAA within 8 hours prior to issuance (per BRs §3.2.2.8), so any domain name that was validated more than 8 hours ago requires rechecking.” reads the advisory published by Let’s Encrypt.

“The bug: when a certificate request contained N domain names that needed CAA rechecking, Boulder would pick one domain name and check it N times. What this means in practice is that if a subscriber validated a domain name at time X, and the CAA records for that domain at time X allowed Let’s Encrypt issuance, that subscriber would be able to issue a certificate containing that domain name until X+30 days, even if someone later installed CAA records on that domain name that prohibit issuance by Let’s Encrypt,”

Let’s Encrypt is revoking 3,048,289 certificates,  ~116 million certificates (2.6%) are active.

The organization confirmed the bug at 2020-02-29 03:08 UTC, and two minutes later halted issuance. In a couple of hours (05:22 UTC) it fixed the problem and re-enabled issuance.

According to Let’s Encrypt, the bug was likely introduced on 2019-07-25.

The CA has reported the incident via email to the impacted users who must renew their certificates before they can become invalid.

Users can check if their domain is affected by this bug querying the tool at https://checkhost.unboundtest.com/. 

More information about the bug is available here.

Pierluigi Paganini

(SecurityAffairs – hacking, Let’s Encrypt)

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