The most secure and recommended method for an application running on an Amazon EC2 instance to access other AWS services is to use an IAM role. By attaching an IAM role to the EC2 instance, the application can acquire temporary security credentials automatically through the instance metadata service. This avoids the need to store long-lived credentials, such as an access key and secret key, within the application or on the instance itself. Furthermore, the policy attached to the role should grant only the specific permissions required (least privilege), which in this case is access to Amazon S3. This approach aligns with AWS security best practices by using temporary credentials and enforcing the principle of least privilege.

A. Granting administrative access violates the principle of least privilege. The role should only have the permissions necessary to interact with Amazon S3, not manage the entire AWS account.

B. Using an IAM user with long-lived access keys is a significant security risk. It also violates the principle of least privilege by granting administrative access.

D. While this option correctly scopes permissions to S3, it relies on storing long-lived IAM user access keys on the instance, which is a security anti-pattern and less secure than using IAM roles.

1. AWS IAM User Guide

"IAM roles for Amazon EC2": "We strongly recommend that you use IAM roles with your applications. They provide temporary credentials that your application can use to sign requests

and you don't have to manage long-term credentials. Using roles is more secure than sharing access keys..."

2. AWS Whitepaper

"AWS Security Best Practices" (August 2023): Under the "Manage identities for workloads and applications" section (p. 19)

it states

"Use temporary credentials with IAM roles instead of long-term credentials like access keys. For workloads that run on AWS compute services

such as Amazon EC2

you can use IAM roles to provide temporary credentials to your workloads."

3. AWS IAM User Guide

"Security best practices in IAM": Under the "Grant least privilege" section

it advises

"When you set permissions with IAM policies

grant only the permissions required to perform a task. You do this by defining the actions that can be taken on specific resources under specific conditions..." This directly contradicts options A and B which grant administrative access.