What is the MOST secure way to allow CloudFormation to access the Lambda code in the S3 bucket?

Explanations:

This option grants the CloudFormation service role the necessary permissions to list and retrieve objects from the S3 bucket. By adding a bucket policy that specifies the accounts allowed to access the bucket, it ensures that only those accounts can deploy the Lambda function while maintaining the principle of least privilege.

While granting the CloudFormation service role S3 GetObject permission is necessary, using a bucket policy with the principal of “*” exposes the bucket to all AWS accounts, which significantly reduces security. This approach does not restrict access to only the specified accounts, making it less secure.

Using a service-based link to grant permissions directly to the Lambda function may allow access, but it does not address the CloudFormation service role’s need to access the S3 bucket. This option does not provide a comprehensive solution for securing access across multiple accounts for CloudFormation deployments.

Although using a service-based link grants the Lambda function GetObject permission, allowing access to all resources (resource of “*”) can lead to unintended access to other objects in the bucket. This approach lacks specificity and security as it does not restrict access to the appropriate accounts or resources.