How to use AWS Certificate Manager to enforce certificate issuance controls

AWS Certificate Manager (ACM) lets you provision, manage, and deploy public and private Transport Layer Security (TLS) certificates for use with AWS services and your internal connected resources. You probably have many users, applications, or accounts that request and use TLS certificates as part of your public key infrastructure (PKI); which means you might also need to enforce specific PKI enterprise controls, such as the types of certificates that can be issued or the validation method used. You can now use AWS Identity and Access Management (IAM) condition context keys to define granular rules around certificate issuance from ACM and help ensure your users are issuing or requesting TLS certificates in accordance with your organizational guidelines.

In this blog post, we provide an overview of the new IAM condition keys available with ACM. We also discuss some example use cases for these condition keys, including example IAM policies. Lastly, we highlight some recommended practices for logging and monitoring certificate issuance across your organization using AWS CloudTrail because you might want to provide PKI administrators a centralized view of certificate activities. Combining preventative controls, like the new IAM condition keys for ACM, with detective controls and comprehensive activity logging can help you meet your organizational requirements for properly issuing and using certificates.

This blog post assumes you have a basic understanding of IAM policies. If you’re new to using identity policies in AWS, see the IAM documentation for more information.

Using IAM condition context keys with ACM to enforce certificate issuance guidelines across your organization

Let’s take a closer look at IAM condition keys to better understand how to use these controls to enforce certificate guidelines. The condition block in an IAM policy is an optional policy element that lets you specify certain conditions for when a policy will be in effect. For instance, you might use a policy condition to specify that no one can delete an Amazon Simple Storage Service (Amazon S3) bucket except for your system administrator IAM role. In this case, the condition element of the policy translates to the exception in the previous sentence: all identities are denied the ability to delete S3 buckets except under the condition that the role is your administrator IAM role. We will highlight some useful examples for certificate issuance later in the post.

When used with ACM, IAM condition keys can now be used to help meet enterprise standards for how certificates are issued in your organization. For example, your security team might restrict the use of RSA certificates, preferring ECDSA certificates. You might want application teams to exclusively use DNS domain validation when they request certificates from ACM, enabling fully managed certificate renewals with little to no action required on your part. Using these condition keys in identity policies or service control policies (SCPs) provide ACM users more control over who can issue certificates with certain configurations. You can now create condition keys to define certificate issuance guardrails around the following:

• Certificate validation method — Allow or deny a specific validation type (such as email validation).
• Certificate key algorithm — Allow or deny use of certain key algorithms (such as RSA) for certificates issued with ACM.
• Certificate transparency (CT) logging — Deny users from disabling CT logging during certificate requests.
• Domain names — Allow or deny authorized accounts and users to request certificates for specific domains, including wildcard domains. This can be used to help prevent the use of wildcard certificates or to set granular rules around which teams can request certificates for which domains.
• Certificate authority — Allow or deny use of specific certificate authorities in AWS Private Certificate Authority for certificate requests from ACM.

Before this release, you didn’t always have a proactive way to prevent users from issuing certificates that weren’t aligned with your organization’s policies and best practices. You could reactively monitor certificate issuance behavior across your accounts using AWS CloudTrail, but you couldn’t use an IAM policy to prevent the use of email validation, for example. With the new policy conditions, your enterprise and network administrators gain more control over how certificates are issued and better visibility into inadvertent violations of these controls.

Using service control policies and identity-based policies

Before we showcase some example policies, let’s examine service control policies, or SCPs. SCPs are a type of policy that you can use with AWS Organizations to manage permissions across your enterprise. SCPs offer central control over the maximum available permissions for accounts in your organization, and SCPs can help ensure your accounts stay aligned with your organization’s access control guidelines. You can find more information in Getting started with AWS Organizations.

Let’s assume you want to allow only DNS validated certificates, not email validated certificates, across your entire enterprise. You could create identity-based policies in all your accounts to deny the use of email validated certificates, but creating an SCP that denies the use of email validation across every account in your enterprise would be much more efficient and effective. However, if you only want to prevent a single IAM role in one of your accounts from issuing email validated certificates, an identity-based policy attached to that role would be the simplest, most granular method.

It’s important to note that no permissions are granted by an SCP. An SCP sets limits on the actions that you can delegate to the IAM users and roles in the affected accounts. You must still attach identity-based policies to IAM users or roles to actually grant permissions. The effective permissions are the logical intersection between what is allowed by the SCP and what is allowed by the identity-based and resource-based policies. In the next section, we examine some example policies and how you can use the intersection of SCPs and identity-based policies to enforce enterprise controls around certificates.

Certificate governance use cases and policy examples

Let’s look at some example use cases for certificate governance, and how you might implement them using the new policy condition keys. We’ve selected a few common use cases, but you can find more policy examples in the ACM documentation.

Example 1: Policy to prevent issuance of email validated certificates

Certificates requested from ACM using email validation require manual action by the domain owner to renew the certificates. This could lead to an outage for your applications if the person receiving the email to validate the domain leaves your organization — or is otherwise unable to validate your domain ownership — and the certificate expires without being renewed.

We recommend using DNS validation, which doesn’t require action on your part to automatically renew a public certificate requested from ACM. The following SCP example demonstrates how to help prevent the issuance of email validated certificates, except for a specific IAM role. This IAM role could be used by application teams who cannot use DNS validation and are given an exception.

Note that this policy will only apply to new certificate requests. ACM managed certificate renewals for certificates that were originally issued using email validation won’t be affected by this policy.

{
    "Version":"2012-10-17",
    "Statement":{
        "Effect":"Deny",
        "Action":"acm:RequestCertificate",
        "Resource":"*",
        "Condition":{
            "StringLike" : {
                "acm:ValidationMethod":"EMAIL"
            },
            "ArnNotLike": {
                "aws:PrincipalArn": [ "arn:aws:iam::123456789012:role/AllowedEmailValidation"]
            }
        }
    }
}

Example 2: Policy to prevent issuance of a wildcard certificate

A wildcard certificate contains a wildcard (*) in the domain name field, and can be used to secure multiple sub-domains of a given domain. For instance, *.example.com could be used for mail.example.com, hr.example.com, and dev.example.com. You might use wildcard certificates to reduce your operational complexity, because you can use the same certificate to protect multiple sites on multiple resources (for example, web servers). However, this also means the wildcard certificates have a larger impact radius, because a compromised wildcard certificate could affect each of the subdomains and resources where it’s used. The US National Security Agency warned about the use of wildcard certificates in 2021.

Therefore, you might want to limit the use of wildcard certificates in your organization. Here’s an example SCP showing how to help prevent the issuance of wildcard certificates using condition keys with ACM:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "DenyWildCards",
      "Effect": "Deny",
      "Action": [
        "acm:RequestCertificate"
      ],
      "Resource": [
        "*"
      ],
      "Condition": {
        "ForAnyValue:StringLike": {
          "acm:DomainNames": [
            "${*}.*"
          ]
        }
      }
    }
  ]
}

Notice that in this example, we’re denying a request for a certificate where the leftmost character of the domain name is a wildcard. In the condition section, ForAnyValue means that if a value in the request matches at least one value in the list, the condition will apply. As acm:DomainNames is a multi-value field, we need to specify whether at least one of the provided values needs to match (ForAnyValue), or all the values must match (ForAllValues), for the condition to be evaluated as true. You can read more about multi-value context keys in the IAM documentation.

Example 3: Allow application teams to request certificates for their FQDN but not others

Consider a scenario where you have multiple application teams, and each application team has their own domain names for their workloads. You might want to only allow application teams to request certificates for their own fully qualified domain name (FQDN). In this example SCP, we’re denying requests for a certificate with the FQDN app1.example.com, unless the request is made by one of the two IAM roles in the condition element. Let’s assume these are the roles used for staging and building the relevant application in production, and the roles should have access to request certificates for the domain.

Multiple conditions in the same block must be evaluated as true for the effect to be applied. In this case, that means denying the request. In the first statement, the request must contain the domain app1.example.com for the first part to evaluate to true. If the identity making the request is not one of the two listed roles, then the condition is evaluated as true, and the request will be denied. The request will not be denied (that is, it will be allowed) if the domain name of the certificate is not app1.example.com or if the role making the request is one of the roles listed in the ArnNotLike section of the condition element. The same applies for the second statement pertaining to application team 2.

Keep in mind that each of these application team roles would still need an identity policy with the appropriate ACM permissions attached to request a certificate from ACM. This policy would be implemented as an SCP and would help prevent application teams from giving themselves the ability to request certificates for domains that they don’t control, even if they created an identity policy allowing them to do so.

{
    "Version":"2012-10-17",
    "Statement":[
    {
        "Sid": "AppTeam1",    
        "Effect":"Deny",
        "Action":"acm:RequestCertificate",
        "Resource":"*",      
        "Condition": {
        "ForAnyValue:StringLike": {
          "acm:DomainNames": "app1.example.com"
        },
        "ArnNotLike": {
          "aws:PrincipalARN": [
            "arn:aws:iam::account:role/AppTeam1Staging",
            "arn:aws:iam::account:role/AppTeam1Prod" ]
        }
      }
   },
   {
        "Sid": "AppTeam2",    
        "Effect":"Deny",
        "Action":"acm:RequestCertificate",
        "Resource":"*",      
        "Condition": {
        "ForAnyValue:StringLike": {
          "acm:DomainNames": "app2.example.com"
        },
        "ArnNotLike": {
          "aws:PrincipalARN": [
            "arn:aws:iam::account:role/AppTeam2Staging",
            "arn:aws:iam::account:role/AppTeam2Prod"]
        }
      }
   }
 ] 
}

Example 4: Policy to prevent issuing certificates with certain key algorithms

You might want to allow or restrict a certain certificate key algorithm. For example, allowing the use of ECDSA certificates but restricting RSA certificates from being issued. See this blog post for more information on the differences between ECDSA and RSA certificates, and how to evaluate which type to use for your workload. Here’s an example SCP showing how to deny requests for a certificate that uses one of the supported RSA key lengths.

{
    "Version":"2012-10-17",
    "Statement":{
        "Effect":"Deny",
        "Action":"acm:RequestCertificate",
        "Resource":"*",
        "Condition":{
            "StringLike" : {
                "acm:KeyAlgorithm":"RSA*"
            }
        }
    }
}  

Notice that we’re using a wildcard after RSA to restrict use of RSA certificates, regardless of the key length (for example, 2048, 4096, and so on).

Creating detective controls for better visibility into certificate issuance across your organization

While you can use IAM policy condition keys as a preventative control, you might also want to implement detective controls to better understand certificate issuance across your organization. Combining these preventative and detective controls helps you establish a comprehensive set of enterprise controls for certificate governance. For instance, imagine you use an SCP to deny all attempts to issue a certificate using email validation. You will have CloudTrail logs for RequestCertificate API calls that are denied by this policy, and can use these events to notify the appropriate application team that they should be using DNS validation.

You’re probably familiar with the access denied error message received when AWS explicitly or implicitly denies an authorization request. The following is an example of the error received when a certificate request is denied by an SCP:

If you use AWS Organizations, you can have a consolidated view of the CloudTrail events for certificate issuance using ACM by creating an organization trail. Please refer to the CloudTrail documentation for more information on security best practices in CloudTrail. Using Amazon EventBridge, you can simplify certificate lifecycle management by using event-driven workflows to notify or automatically act on expiring TLS certificates. Learn about the example use cases for the event types supported by ACM in this Security Blog post.

Conclusion

In this blog post, we discussed the new IAM policy conditions available for use with ACM. We also demonstrated some example use cases and policies where you might use these conditions to provide more granular control on the issuance of certificates across your enterprise. We also briefly covered SCPs, identity-based policies, and how you can get better visibility into certificate governance using services like AWS CloudTrail and Amazon EventBridge. See the AWS Certificate Manager documentation to learn more about using policy conditions with ACM, and then get started issuing certificates with AWS Certificate Manager.

 
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