Retiring DigiCert Nessie2023
Chris Thompson
On August 6, 2023, multiple SCTs were found to be issued by DigiCert Nessie2023 log but not incorporated by the MMD. This issue was independently detected by Chrome’s SCT Auditing infrastructure (reported to ct-policy@) and Cloudflare’s monitoring (reported directly to DigiCert and also to ct-policy@), and additional affected SCTs were shared by Andrew Ayer on ct-policy@. The affected SCTs that we are aware of were issued between 19:00 to 19:30 GMT on August 1, 2023. However, we are unable to fully enumerate all potentially affected SCTs.
Because of the failure to include submitted certificates in the log, we unfortunately have to retire Nessie2023.
Effective August 25, 2023, the DigiCert Nessie2023 Log (https://nessie2023.ct.digicert.com/log/) will transition to Retired, with the last ‘Qualified’ SCT having a timestamp no later than 1692921600, or 2023-08-25T00:00:00Z in ISO 8601 format.
After August 25, 2023, SCTs from DigiCert Nessie2023 Log will no longer count towards the CT Compliance requirements stating that at least one SCT must come from a CT Log that was Qualified, Usable, or ReadOnly at time of check. As such, after this point, it is no longer appropriate to serve SCTs from this Log in the TLS handshake, in OCSP responses, or embedded in certificates issued on-or-after 2023-08-25T00:00:00Z.
Embedded SCTs dated prior to 2023-08-25T00:00:00Z will still satisfy CT Compliance requirements that permit SCTs to come from CT Logs that are Qualified, Usable, ReadOnly, or Retired at time of check; however, at least one other SCT must come from a non-Retired CT Log in order for the certificate to successfully validate in Chrome.
What does this mean for site operators?
If you are delivering SCTs embedded in the certificate, this should require no action on your part. All previously-issued certificates containing SCTs from these logs that complied with the Chrome CT Policy will continue to do so.
If you are currently serving SCTs via stapled OCSP, then your CA must take appropriate action to update their OCSP pipeline to ensure they are embedding a policy-satisfying set of SCTs. Once done, you must refresh the OCSP response stapled to the connections. Alternatively, you may choose to provide a policy-satisfying set of SCTs via another mechanism outlined in the Chrome CT Policy.
If you are currently delivering SCTs via a TLS extension, SCTs issued by these logs will no longer contribute towards CT Compliance. As such, you will need to update the SCTs included in the handshake to comply with the Chrome CT Policy in order to satisfy the requirements for SCTs delivered via TLS.
What does this mean for CAs?
If you are embedding SCTs in your certificates, SCTs from these logs for newly-issued certificates will no longer meet CT Compliance requirements stating that at least one SCT must come from a CT Log that was Qualified, Usable, or ReadOnly at time of check. To ensure that newly-issued certificates will be CT-compliant, you should update your CA’s CT log configuration to remove these logs while also ensuring that you are still logging to a policy-satisfying set of CT logs after the removal.
While it is not required by policy, CAs with existing certificates embedding SCTs from these logs may wish to proactively reissue affected certificates to increase the resilience of these certificates to possible future log incidents.
Finally, if you are embedding SCTs from these logs in your OCSP responses, you must issue new OCSP responses that replace these SCTs with new ones from a set of CT logs that satisfies the Chrome’s CT Policy. Your customers must then begin to serve these new responses or provide a policy-satisfying set of SCTs via another mechanism before the effective date above.
What does this mean for Log Operators?
If you are operating a CT Log not listed in the above Retirement announcement, you do not need to take any action.