What is the MOST secure way to grant the application access to the credentials?
Create an IAM EC2 role for the EC2 instances and grant the role permission to read the Systems Manager parameters
Create an IAM group for the application and grant the group permissions to read the Systems Manager parameters
Create an IAM policy for the application and grant the policy permission to read the Systems Manager parameters
Create an IAM user for the application and grant the user permission to read the Systems Manager parameters
Explanations:
An IAM EC2 role is the most secure way to grant an EC2 instance access to AWS resources. The EC2 instance can assume this role, allowing it to securely access the Systems Manager Parameter Store credentials without needing to hard-code access keys.
An IAM group is used for managing a collection of IAM users, not for granting access to an EC2 instance. It would be unnecessary to create a group for an application running on an EC2 instance.
While an IAM policy could grant permissions, attaching it directly to the application running on the EC2 instance would require additional steps such as creating an IAM user or role, making it less secure than using an IAM EC2 role directly.
Creating an IAM user for the application running on the EC2 instance is not recommended because it requires managing static credentials (access keys), which is less secure than using an IAM EC2 role.
I figure that the answer is:
Create an IAM EC2 role for the EC2 instances and grant the role permission to read the Systems Manager parameters