At 16:48 UTC on Tuesday Jan 25, 2022, a third party informed Let’s Encrypt / ISRG that, while examining the Boulder codebase, they had noticed two instances of specification non-compliance in our implementation of the “TLS Using ALPN” validation method (BRs 3.2.2.4.20, RFC 8737). As a result, we have made two changes to the way that our TLS-ALPN-01 challenge validation works.
All active certificates that were issued and validated with the TLS-ALPN-01 challenge before 00:48 UTC on 26 January 2022 when our fix was deployed are considered mis-issued. In compliance with the Let’s Encrypt CP, we have 5-days to revoke and will begin to revoke certificates at 16:00 UTC on 28 January 2022. We estimate <1% of active certificates are affected. Subscribers affected by revocations will receive e-mail notifications if their ACME account contains a valid e-mail address. If you are affected by this revocation and need help renewing your certificate please ask questions in this thread
We will be providing more details about this incident in the next few days.