SANS ISC: Let's Encrypt Revoking 3 Million Certificates

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SANS ISC InfoSec Forums

Let's Encrypt announced that they will be revoking a large number of certificates today. The revocation is due to an error in how "CAA" records were validated for these certificates.

The "CAA" (Certificate Authority Authorization) DNS record can be used to indicate which certificate authority is allowed to issue certificates for the particular domain or subdomain. For example, for DShield.org the record is:

% dig +short CAA dshield.org
0 iodef "mailto:info@dshield.org"
0 issue "letsencrypt.org"

This will allow "letsencrypt.org" to issue certificates, and the CA can notify info@dshield.org whenever it issues a certificate. 

Let's Encrypt checked the CAA records whenever it validated domain ownership. But for Let's Encrypt, domain ownership validation is good for 30 days. If you requested a new certificate within these 30 days, Let's encrypt did not re-check the CAA record. CAA records are only supposed to be cached for up to 8 hrs.

Let's Encrypt published a list of affected certificates [3]. You can search the list for domains you own. Let's Encrypt also emailed the contact they had for the revoked certificates. They will be revoked today. Just as a precaution, you may want to check your site with SSLLabs or other test sides (good idea to do this from time to time :) ).

[1] https://community.letsencrypt.org/t/revoking-certain-certificates-on-march-4/114864
[2] https://tools.ietf.org/html/rfc8659
[3] https://letsencrypt.org/caaproblem/

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Johannes B. Ullrich, Ph.D. , Dean of Research, SANS Technology Institute
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