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AWS Certificate Manager
User Guide (Version 1.0)

Best Practices

Best practices are recommendations that can help you use AWS Certificate Manager (AWS Certificate Manager) more effectively. The following best practices are based on real-world experience from current ACM customers.

AWS CloudFormation

With AWS CloudFormation you can create a template that describes the AWS resources that you want to use. AWS CloudFormation then provisions and configures those resources for you. AWS CloudFormation can provision resources that are supported by ACM such as Elastic Load Balancing, Amazon CloudFront, and Amazon API Gateway. For more information, see Services Integrated with AWS Certificate Manager.

If you use AWS CloudFormation to quickly create and delete multiple test environments, we recommend that you do not create a separate ACM Certificate for each environment. Doing so will quickly exhaust your certificate limit. For more information, see Limits. Instead, create a wildcard certificate that covers all of the domain names that you are using for testing. For example, if you repeatedly create ACM Certificates for domain names that vary by only a version number, such as <version>.service.example.com, create instead a single wildcard certificate for <*>.service.example.com. Include the wildcard certificate in the template that AWS CloudFormation uses to create your test environment.

Certificate Pinning

Certificate pinning, sometimes known as SSL pinning, is a process that you can use in your application to validate a remote host by associating that host directly with its X.509 certificate or public key instead of with a certificate hierarchy. The application therefore uses pinning to bypass SSL/TLS certificate chain validation. The typical SSL validation process checks signatures throughout the certificate chain from the root certificate authority (CA) certificate through the subordinate CA certificates, if any. It also checks the certificate for the remote host at the bottom of the hierarchy. Your application can instead pin to the certificate for the remote host to say that only that certificate and not the root certificate or any other in the chain is trusted. You can add the remote host's certificate or public key to your application during development. Alternatively, the application can add the certificate or key when it first connects to the host.

Warning

We recommend that your application not pin an ACM Certificate. ACM performs Managed Renewal for ACM's Amazon-Issued Certificates to automatically renew your Amazon-issued SSL/TLS certificates before they expire. To renew a certificate, ACM generates a new public-private key pair. If your application pins the ACM Certificate and the certificate is successfully renewed with a new public key, the application might be unable to connect to your domain.

If you decide to pin a certificate, the following options will not hinder your application from connecting to your domain:

Domain Validation

Before the Amazon certificate authority (CA) can issue a certificate for your site, AWS Certificate Manager (ACM) must verify that you own or control all the domains that you specified in your request. You can perform verification using either email or DNS. For more information, see Use Email to Validate Domain Ownership and Use DNS to Validate Domain Ownership.

Adding or Deleting Domain Names

You cannot add or remove domain names from an existing ACM Certificate. Instead you must request a new certificate with the revised list of domain names. For example, if your certificate has five domain names and you want to add four more, you must request a new certificate with all nine domain names. As with any new certificate, you must validate ownership of all the domain names in the request, including the names that you previously validated for the original certificate.

If you use email validation, you receive up to 8 validation email messages for each domain, at least 1 of which must be acted upon within 72 hours. For example, when you request a certificate with five domain names, you receive up to 40 validation messages, at least 5 of which must be acted upon within 72 hours. As the number of domain names in the certificate request increases, so does the work required to use email to validate domain ownership.

If you use DNS validation instead, you must write one new DNS record to the database for the FQDN you want to validate. ACM sends you the record to create and later queries the database to determine whether the record has been added. Adding the record asserts that you own or control the domain. In the preceding example, if you request a certificate with five domain names, you must create five DNS records. We recommend that you use DNS validation when possible.

Opting Out of Certificate Transparency Logging

Important

Regardless of the actions you take to opt out of certificate transparency logging, your certificate might still be logged by any client or individual that has access to the public or private endpoint to which you bind the certificate. However, the certificate won't contain a signed certificate timestamp (SCT). Only the issuing CA can embed an SCT in a certificate.

Beginning April 30 2018, Google Chrome will stop trusting public SSL/TLS certificates that are not recorded in a certificate transparency log. Therefore, beginning April 24 2018, the Amazon CA will start publishing all new certificates and renewals to at least two public logs. Once a certificate has been logged, it cannot be removed. For more information, see Certificate Transparency Logging.

Logging is performed automatically when you request a certificate or when a certificate is renewed, but you can choose to opt out. Common reasons for doing so include concerns about security and privacy. For example, logging internal host domain names gives potential attackers information about internal networks that would otherwise not be public. In addition, logging could leak the names of new or unreleased products and websites.

To opt out of transparency logging when you are requesting a certificate, use the Options parameter of the request-certificate AWS CLI command or the RequestCertificate API.

If your certificate was issued before April 24 2018 and you want to make sure that it is not logged during renewal, you can call the update-certificate-options command or the UpdateCertificateOptions API to opt out.

Once a certificate has been logged, it cannot be removed from the log. Opting out at that point will have no effect. If you opt out of logging when you request a certificate and then choose later to opt back in, your certificate will not be logged until it is renewed. If you want the certificate to be logged immediately, we recommend that you issue a new one.

Note

You cannot currently use the console to opt out of or in to transparency logging.

The following example shows you how to use the request-certificate command to disable certificate transparency when you request a new certificate.

aws acm request-certificate \ --domain-name www.example.com \ --validation-method DNS \ --options CertificateTransparencyLoggingPreference=DISABLED \ --idempotency-token 184627

The preceding command outputs the ARN of your new certificate.

{ "CertificateArn": "arn:aws:acm:region:account:certificate/12345678-1234-1234-1234-123456789012" }

If you already have a certificate, and you don't want it to be logged when it is renewed, use the update-certificate-options command. This command does not return a value.

aws acm update-certificate-options \ --certificate-arn arn:aws:acm:region:account:\ certificate/12345678-1234-1234-1234-123456789012 \ --options CertificateTransparencyLoggingPreference=DISABLED

Turn on AWS CloudTrail

Turn on CloudTrail logging before you begin using ACM. CloudTrail enables you to monitor your AWS deployments by retrieving a history of AWS API calls for your account, including API calls made via the AWS Management Console, the AWS SDKs, the AWS Command Line Interface, and higher-level AWS services. You can also identify which users and accounts called the ACM APIs, the source IP address the calls were made from, and when the calls occurred. You can integrate CloudTrail into applications using the API, automate trail creation for your organization, check the status of your trails, and control how administrators turn CloudTrail logging on and off. For more information, see Creating a Trail. Go to Using AWS CloudTrail to see example trails for ACM actions.