Understanding the Interaction Between AWS Secrets Manager and KMS in CloudFormation

When deploying applications or infrastructure using AWS CloudFormation, it's crucial to manage secrets such as passwords and API keys securely. AWS Secrets Manager and AWS Key Management Service (KMS) are often used together to achieve this. However, the interaction between these services and how they are utilized in CloudFormation can be a bit complex, especially in terms of permissions and key management.

Defining Secrets in CloudFormation

AWS Secrets Manager secrets can be created via CloudFormation using the resource type:

AWS::SecretsManager::Secret

The initial creation of a secret is usually done using the CreateSecret API, which requires the kms:GenerateDataKey permission. This permission is needed for the service to generate a key and then encrypt the secret. However, the tricky part arises when updating the secret.

Updating Secrets in CloudFormation

When a CloudFormation stack is updated, the service uses the UpdateSecret API. Additionally, it requires the kms:Decrypt permission. This means that the secret needs to be decryptable by all KMS keys in the AWS account. This can pose a risk if the secret itself is sensitive and should not be accessible without specific authorization.

The Potential Risk

The key phrase here is 'decrypt to all keys', which essentially means the secret can be decrypted by any KMS key in the account. This could expose the secret to unauthorized access. For example, if a KMS key is inadvertently given too broad a scope, the secret could be decrypted by anyone with the appropriate IAM permissions.

Best Practices and Solutions

To mitigate these risks, consider the following best practices:

Use IAM Policies Correctly: Ensure that IAM policies only allow access to specific KMS keys that are relevant to the application or service that needs access to the secret. Implement Strict Key Policies: Define strict key policies on KMS keys to ensure that only trusted entities can use them. Enforce Least Privilege: Follow the principle of least privilege when configuring IAM roles and policies that interact with KMS keys. Regular Audits: Regularly audit and review IAM roles and KMS key permissions to ensure no unintended access exists.

Security Considerations and Documentation

For more detailed information, refer to the official AWS documentation. This provides a comprehensive guide on how AWS Secrets Manager uses AWS KMS to encrypt and manage secrets securely. It also offers insights into the APIs and permissions required for different actions.

By carefully managing permissions and key policies, you can ensure that AWS Secrets Manager and KMS work seamlessly in your CloudFormation deployments while maintaining a high level of security to protect your sensitive information.

Conclusion

While AWS Secrets Manager and KMS offer powerful tools for managing secrets in a secure and efficient manner, it's crucial to understand their interactions within CloudFormation. By following best practices and carefully configuring permissions, you can maximize the benefits of these services while minimizing security risks.

Refer to the official AWS documentation for detailed information and best practices. Always keep your permissions up-to-date and regularly review them to ensure compliance with security policies.