What should be done to ensure optimal security?
Apply an S3 bucket policy to allow access from all EC2 instances.
Create an IAM user and create a script to inject the credentials on boot.
Create and assign an IAM role for Amazon S3 access to the EC2 instance.
Embed an AWS credentials file for an IAM user inside the Amazon Machine Image (AMI).
Explanations:
Applying a bucket policy to allow access from all EC2 instances is not optimal for security. It opens the S3 bucket to all instances, which is a security risk. Instead, access should be restricted to only the EC2 instance that needs it.
Creating an IAM user and injecting credentials on boot is not ideal. It involves handling sensitive credentials, which increases the risk of exposure. IAM roles for EC2 instances are a more secure approach.
Creating and assigning an IAM role for Amazon S3 access to the EC2 instance is the most secure and optimal solution. The role can be scoped to specific permissions, and no credentials need to be manually managed on the instance.
Embedding AWS credentials inside the AMI increases the risk of credential exposure. If the AMI is shared or compromised, the credentials could be accessed. Using IAM roles is a more secure approach.
I draft that the answer is:
Create and assign an IAM role for Amazon S3 access to the EC2 instance.