What is the MOST secure way to grant the application access to the S3 bucket?
Create an IAM user with the necessary privileges. Generate an access key and embed the key in the code running on the EC2 instances.
Install secure FTP (SFTP) software on the EC2 instances. Use an AWS Lambda function to copy the files from the EC2 instances to Amazon S3 using SFTP.
Create an IAM role with the necessary privileges. Associate the role with the EC2 instances at launch.
Use rsync and cron to set up the transfer of files from the EC2 instances to the S3 bucket. Enable AWS Shield to protect the data.
Explanations:
Embedding an access key in the application code is insecure, as the key can be extracted by anyone with access to the code or instance. This method is not recommended.
Using SFTP and AWS Lambda adds unnecessary complexity and is not the most efficient or secure method for accessing S3 from EC2 instances.
Creating an IAM role with the necessary privileges and associating it with the EC2 instances at launch is the most secure method. This allows EC2 instances to access S3 securely using temporary credentials that are managed by AWS.
Rsync and cron are not ideal for secure, automated file transfers to S3. Enabling AWS Shield does not address the core security requirements for this use case.
As I see it, the answer is:
Create an IAM role with the necessary privileges. Associate the role with the EC2 instances at launch.