Lets Encrypt announced mass revoking of certain certificates on March 4 2020 - here is a quick check if you are affected

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Let's Encrypt announced yesterday (on March 3rd 2020) that it will have to revoke certain certificates due to a CAA rechecking bug. According to LE, 2.6% of all issues certificates are effected. In bigger numbers: over 3 million certificates (3'048'289 to be precise) are affected and need to be re-issued.

The revocations will start in a couple of hours from now on and by tomorrow, March 5th at 3am UTC, the deadline is reached.

Which certificates are affected?

Let's Encrypt mentioned, that the affected subscribers were informed. However it is possible to create LE certificates without subscribing using the --register-unsafely-without-email parameter on the certbot command. This is most likely widely used in automated setups.

To actively identify whether or not a certificate is affected, one can submit a curl request on checkhost.unboundtest.com:

# curl -XPOST -d "fqdn=www.infiniroot.com" https://checkhost.unboundtest.com/checkhost
The certificate currently available on www.infiniroot.com is OK. It is not one of the certificates affected by the Let's Encrypt CAA rechecking problem. Its serial number is 03d59a6c6f46679a4403003093c111f54b27

Handy one liner for (central) reverse proxies

A common way to use Let's Encrypt certificates is to have them installed and issued on central reverse proxies. This makes it easy to obtain an overview of all certificates (using certbot certificates) hence creating a loop to iterate through each domain and check whether the certificate is affected by the revocation:

# for i in $(certbot certificates |grep Domains|awk '{ print $2}'); do curl -XPOST -d "fqdn=$i" https://checkhost.unboundtest.com/checkhost; done

Thanks to my colleague Pascal for this handy one liner!

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