DVA-C02: How to Fix S3 ListBucket Permission Issue for EC2 Application

Learn how to resolve the S3 ListBucket permission issue for an EC2 application that needs access to files stored in an S3 bucket. Find out the most secure way to grant the necessary permission.

Question

An application that is hosted on an Amazon EC2 instance needs access to files that are stored in an Amazon S3 bucket. The application lists the objects that are stored in the S3 bucket and displays a table to the user. During testing, a developer discovers that the application does not show any objects in the list.

What is the MOST secure way to resolve this issue?

A. Update the IAM instance profile that is attached to the EC2 instance to include the S3:* permission for the S3 bucket.

B. Update the IAM instance profile that is attached to the EC2 instance to include the S3:ListBucket permission for the S3 bucket.

C. Update the developer’s user permissions to include the S3:ListBucket permission for the S3 bucket.

D. Update the S3 bucket policy by including the S3:ListBucket permission and by setting the Principal element to specify the account number of the EC2 instance.

Answer

B. Update the IAM instance profile that is attached to the EC2 instance to include the S3:ListBucket permission for the S3 bucket.

Explanation

The correct answer to the question is B. Update the IAM instance profile that is attached to the EC2 instance to include the S3:ListBucket permission for the S3 bucket.

The reason for this answer is that:

• Option B: Updating the IAM instance profile that is attached to the EC2 instance to include the S3:ListBucket permission for the S3 bucket is the most secure and recommended way to resolve this issue. An IAM instance profile is a container for an IAM role that you can use to pass role information to an EC2 instance when the instance starts. By using an IAM role, you can avoid storing any credentials on the EC2 instance, and instead rely on temporary security tokens that are automatically rotated by AWS. The S3:ListBucket permission allows the EC2 instance to list the objects in the S3 bucket, which is required for the application to display the table to the user. This option follows the principle of least privilege, as it only grants the minimum permission needed for the application to function.
• Option A: Updating the IAM instance profile that is attached to the EC2 instance to include the S3:* permission for the S3 bucket is not a secure way to resolve this issue. The S3:* permission grants full access to all actions and resources related to S3, which is more than what the application needs. This option violates the principle of least privilege, as it grants excessive permissions that could be exploited by malicious actors or compromised applications. This option also increases the risk of accidental data loss or corruption due to human error or misconfiguration.
• Option C: Updating the developer’s user permissions to include the S3:ListBucket permission for the S3 bucket is not a relevant way to resolve this issue. The developer’s user permissions are not used by the application that is hosted on the EC2 instance, unless the developer explicitly provides their credentials to the application, which is not a good practice. The developer’s user permissions only affect their own access to AWS resources through the AWS Management Console or CLI, not through other applications or services. Therefore, this option would not address the root cause of the problem, which is that the EC2 instance does not have permission to list objects in the S3 bucket.
• Option D: Updating the S3 bucket policy by including the S3:ListBucket permission and by setting the Principal element to specify the account number of the EC2 instance is not a correct way to resolve this issue. The Principal element in a bucket policy specifies who can access a resource, such as an AWS account, user, role, or service. However, you cannot use an account number alone as a principal in a bucket policy, as it would grant access to all entities in that account, which is not secure or specific. To grant access to an EC2 instance in a bucket policy, you would need to use its IAM role ARN as a principal, which is more complex and less flexible than using an IAM instance profile. Moreover, using a bucket policy alone would not be sufficient, as you would also need to grant permission for decrypting data encrypted with KMS keys on both the bucket and KMS key policies. Therefore, this option would be more complicated and less secure than option B.

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