This page hosts the list of affected serial numbers and a hostname checking utility for the incident reported at https://community.letsencrypt.org/t/2020-02-29-caa-rechecking-bug/114591.
We have sent notification emails to affected subscribers who have registered an email address. If you need to re-subscribe to email notifications, please see this document. You may not have received an email if you did not provide an email address while registering your ACME account. You may also be missing an email due to email delivery issues or spam filtering. If you are unsure whether your hostname is affected, use the checking tools below. The email will have the following subject line:
ACTION REQUIRED: Renew these Let's Encrypt certificates by March 4
The file caa-rechecking-incident-affected-serials.txt.gz contains a list of all affected certs, sorted by account ID. Each entry has the following syntax:
serial 0362...8fdb 131 b8dc...ed3f names: [....] missing CAA checking results for <name> at 2019-12-03 21:08:34.826011675 +0000 UTC
The first chunk is the serial (03..), next is the account id (131), next is a hash of the names on the cert (you can disregard), then the list of names on the cert. The date is when the affected cert was issued.
You can download this file and look up your account id(s) (if you received an email, your account ids were in that email). For each certificate associated you will need to renew and replace it, unless you have renewed it more recently than the date listed. For instance, if you know your ACME client always renews certificates when they are 30 days from expiration, you can safely ignore any entries with a date earlier than 2020-01-02.
If you want to double check whether a given hostname still needs its certificate replaced, you can use the tool at https://checkhost.unboundtest.com/
$ curl -XPOST -d 'fqdn=letsencrypt.org' https://checkhost.unboundtest.com/checkhost The certificate currently available on letsencrypt.org is OK. It is not one of the certificates affected by the Let's Encrypt CAA rechecking problem. Its serial number is 03a1c95bdaa36a8268327f2253cbd3ba243
If you have a large list of domains you need to check, this tool will be more effective.