Preparing for AWS Certificate Manager (ACM) Support of Certificate Transparency

Update from April 24, 2018: On April 24, 2018, we updated ACM to publish certificates to CT logs on issuance and on renewal, unless you disable Certificate Transparency logging. No action from you is required if you want ACM to publish your certificates to Certificate Transparency logs, which will avoid Google Chrome displaying error messages for your certificates.

Update from March 27, 2018: On March 27, 2018, we updated ACM APIs so that you can disable Certificate Transparency logging on a per-certificate basis.

Starting April 30, 2018, Google Chrome will require all publicly trusted certificates issued after this date to be logged in at least two Certificate Transparency logs. This means that any certificate issued that is not logged will result in an error message in Google Chrome. Beginning April 24, 2018, Amazon will log all new and renewed certificates in at least two public logs unless you disable Certificate Transparency logging.

Without Certificate Transparency, it can be difficult for a domain owner to know if an unexpected certificate was issued for their domain. Under the current system, no record is kept of certificates being issued, and domain owners do not have a reliable way to identify rogue certificates.

To address this situation, Certificate Transparency creates a cryptographically secure log of each certificate issued. Domain owners can search the log to identify unexpected certificates, whether issued by mistake or malice. Domain owners can also identify Certificate Authorities (CAs) that are improperly issuing certificates. In this blog post, I explain more about Certificate Transparency and tell you how to prepare for it.

How does Certificate Transparency work?

When a CA issues a publicly trusted certificate, the CA must submit the certificate to one or more Certificate Transparency log servers. The Certificate Transparency log server responds with a signed certificate timestamp (SCT) that confirms the log server will add the certificate to the list of known certificates. The SCT is then embedded in the certificate and delivered automatically to a browser. The SCT is like a receipt that proves the certificate was published into the Certificate Transparency log. Starting April 30, Google Chrome will require an SCT as proof that the certificate was published to a Certificate Transparency log in order to trust the certificate without displaying an error message.

What is Amazon doing to support Certificate Transparency?

Certificate Transparency is a good practice. It enables AWS customers to be more confident that an unauthorized certificate hasn’t been issued by a CA. Beginning on April 24, 2018, Amazon will log all new and renewed certificates in at least two Certificate Transparency logs unless you disable Certificate Transparency logging.

We recognize that there can be times when our customers do not want to log certificates. For example, if you are building a website for an unreleased product and have registered the subdomain, newproduct.example.com, requesting a logged certificate for your domain will make it publicly known that the new product is coming. Certificate Transparency logging also can expose server hostnames that you want to keep private. Hostnames such as payments.example.com can reveal the purpose of a server and provide attackers with information about your private network. These logs do not contain the private key for your certificate. For these reasons, on March 27, 2018 we updated ACM APIs so that you can disable Certificate Transparency logging on a per-certificate basis using the ACM APIs or with the AWS CLI. Doing so will lead to errors in Google Chrome, which may be preferable to exposing the information.

Please refer to ACM documentation for specifics on how to opt out of Certificate Transparency logging.

Conclusion

Beginning April 24, 2018, ACM will begin logging all new and renewed certificates by default. If you don’t want a certificate to be logged, you’ll be able to opt out using the AWS API or CLI. However, for Google Chrome to trust the certificate, all issued or imported certificates must have the SCT information embedded in them by April 30, 2018.

If you have comments about this blog post, submit them in the “Comments” section below. If you have questions, start a new thread in the ACM forum.

– Jonathan

Interested in AWS Security news? Follow the AWS Security Blog on Twitter.