AWS DVA-C02 Drill: IAM Permissions - Least Privilege for S3 Access

Jeff’s Note

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Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.

For AWS DVA-C02 candidates, the confusion often lies in understanding the narrow scope of permissions needed for specific API calls. In production, this is about knowing exactly which IAM permissions correspond to required SDK operations (e.g., ListBucket vs. general S3 access). Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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BrightApps Inc. is developing a web application hosted on an Amazon EC2 instance that needs to display a list of files stored in an Amazon S3 bucket for its users. The application uses AWS SDK calls to retrieve the list of objects in the bucket. However, during testing, the development team notices that the application fetches no objects and the listing remains empty.

The Requirement:

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Determine the MOST secure and minimal IAM permission update required to enable the EC2-hosted application to successfully list the objects in the specific S3 bucket.

The Options

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• A) Update the IAM instance profile attached to the EC2 instance to include the s3:* permissions for the S3 bucket.
• B) Update the IAM instance profile attached to the EC2 instance to include only the s3:ListBucket permission for the S3 bucket.
• C) Update the individual developer’s IAM user permissions to include the s3:ListBucket permission for the S3 bucket.
• D) Update the S3 bucket policy to include the s3:ListBucket permission and set the Principal to the EC2 instance’s account number.

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leave a comment:

Correct Answer

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B

Quick Insight: The Developer’s Least Privilege Imperative

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• AWS SDK’s ListObjects call requires the s3:ListBucket permission on the bucket resource.
• Granting excessive permissions like s3:* (Option A) violates the Principle of Least Privilege.
• Modifying developer’s user permissions (Option C) is irrelevant as the app runs on EC2, not under the developer’s identity.
• S3 bucket policies (Option D) can restrict, but fundamentally the application IAM role needs the right permission.

Content Locked: The Expert Analysis

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You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?

The Expert’s Analysis

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Correct Answer

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Option B

The Winning Logic

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The AWS SDK method that lists objects in an S3 bucket requires the s3:ListBucket permission granted on the bucket resource. This permission allows the caller—here the EC2 instance role—to perform the ListObjects or ListObjectsV2 API action.

• The application runs with the IAM instance profile attached to the EC2 instance, so permissions need to be granted there.
• Granting s3:* (all S3 permissions) is overly broad and violates best practices for least privilege.
• Updating the developer’s personal IAM user permissions is irrelevant since the code runs on EC2, not under the developer user.
• A bucket policy can allow or deny access, but the primary permission check is done via the calling identity’s IAM permissions. Simply adding the principal with s3:ListBucket in the bucket policy alone does not grant permissions unless the caller’s IAM role also allows it.

The Trap (Distractor Analysis):

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• Why not A? Granting s3:* is a broad wildcard that gives the application full access to S3 actions, which opens security risk and violates the Principle of Least Privilege.
• Why not C? The developer’s user permissions control what the developer can do in the AWS console or CLI but do not affect the EC2 instance’s role permissions.
• Why not D? Bucket policies supplement but don’t replace IAM role permissions; the EC2 instance profile must still permit s3:ListBucket actions for the API call to succeed.

The Technical Blueprint

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{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::example-app-files-bucket"
    }
  ]
}

This is a minimal IAM policy snippet to attach to the EC2 instance profile to enable listing objects in the bucket.

The Comparative Analysis

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Real-World Application (Practitioner Insight)

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Exam Rule

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“For listing objects in an S3 bucket, always grant s3:ListBucket permission scoped to the bucket resource on the executing IAM role.”

Real World

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“In real applications, developers often want to combine ListBucket with GetObject for reading files. But for listing only, avoid broader permissions for safety and easier compliance.”

(CTA) Stop Guessing, Start Mastering

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Disclaimer

This is a study note based on simulated scenarios for the AWS DVA-C02 exam.