AWS DVA-C02 Drill: IAM Policies for S3 Access - The Minimal Privilege Principle

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 DVA-C02 candidates, the confusion often lies in knowing which IAM permissions are truly needed for S3 object listing and retrieval without over-permissioning. In production, this comes down to applying the principle of least privilege exactly — granting only ListBucket on the bucket and GetObject on the object paths. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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A startup named NexaRetail has built an e-commerce web application that displays a dynamic product catalog. The catalog images and JSON metadata are stored in an Amazon S3 bucket called nexa-retail-assets. The frontend application needs to list all product catalog files in this bucket and download individual objects as customers browse products.

The Requirement:

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Create an IAM policy with the minimum set of permissions that allows the application to:

• List all objects in the nexa-retail-assets bucket, and
• Download any object within that bucket.

The Options

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• A)

{
  "Version":"2012-10-17",
  "Statement":[
    {
      "Effect":"Allow",
      "Action":"s3:ListBucket",
      "Resource":"arn:aws:s3:::nexa-retail-assets"
    },
    {
      "Effect":"Allow",
      "Action":"s3:GetObject",
      "Resource":"arn:aws:s3:::nexa-retail-assets/*"
    }
  ]
}

• B)

{
  "Version":"2012-10-17",
  "Statement":[
    {
      "Effect":"Allow",
      "Action":"s3:ListBucket",
      "Resource":"arn:aws:s3:::nexa-retail-assets"
    },
    {
      "Effect":"Allow",
      "Action":[
        "s3:GetObject",
        "s3:PutObject",
        "s3:DeleteObject"
      ],
      "Resource":"arn:aws:s3:::nexa-retail-assets/*"
    }
  ]
}

• C)

{
  "Version":"2012-10-17",
  "Statement":[
    {
      "Effect":"Allow",
      "Action":"s3:ListBucket",
      "Resource":"arn:aws:s3:::nexa-retail-assets"
    },
    {
      "Effect":"Deny",
      "Action":"s3:GetObject",
      "Resource":"arn:aws:s3:::nexa-retail-assets/*"
    }
  ]
}

• D)

{
  "Version":"2012-10-17",
  "Statement":[
    {
      "Effect":"Allow",
      "Action":"s3:",
      "Resource":"arn:aws:s3:::nexa-retail-assets/"
    }
  ]
}

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leave a comment:

Correct Answer

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A

Quick Insight: The Developer Imperative

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• Minimal IAM permissions: Knowing that to list bucket contents, s3:ListBucket needs the bucket ARN (without trailing slash), while to download objects, s3:GetObject must be granted on the object ARNs (bucket-name/*).
• Avoid over-permissioning: Granting write or delete (PutObject, DeleteObject) is unnecessary here and could introduce risk.
• Deny statements need caution: Explicit denies can block access unintentionally.
• Avoid broad wildcards in Action: Using "s3:" alone is invalid and too permissive.

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 A

The Winning Logic

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• s3:ListBucket permission: Required on the bucket ARN (e.g., arn:aws:s3:::bucket-name) to list the objects inside the bucket. This permission controls the ListObjects and related API calls.
• s3:GetObject permission: Required on the object ARNs (e.g., arn:aws:s3:::bucket-name/*) to download or retrieve individual files.
• This combination ensures least privilege: listing and reading only.
• No unnecessary write or delete permissions are granted—reducing security risks.

The Trap (Distractor Analysis):

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• Option B:
  Grants PutObject and DeleteObject in addition to the needed permissions. This is over-permissioning and violates least privilege, an anti-pattern especially in production environments.

• Option C:
  Includes an explicit "Deny" on GetObject which would block downloading objects outright, contradicting the requirement.

• Option D:
  Uses "Action": "s3:" which is invalid (wildcard must be "s3:*") and lacks correct ARN syntax (arn:aws:s3:::bucket-name/ is not a valid bucket ARN; it should be without the trailing slash). It also grants no ListBucket permission, so listing would fail.

The Technical Blueprint

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# AWS CLI example to test minimal permissions:

# List objects in bucket (requires s3:ListBucket permission on the bucket)
aws s3api list-objects --bucket nexa-retail-assets

# Download a specific object (requires s3:GetObject permission on the object ARN)
aws s3 cp s3://nexa-retail-assets/sample-product.json .

# IAM policy JSON snippet (correct overlapping permissions)
cat <<EOF > minimal-s3-policy.json
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::nexa-retail-assets"
    },
    {
      "Effect": "Allow",
      "Action": "s3:GetObject",
      "Resource": "arn:aws:s3:::nexa-retail-assets/*"
    }
  ]
}
EOF

The Comparative Analysis

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Real-World Application (Practitioner Insight)

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Exam Rule

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“For the exam, always grant s3:ListBucket on the bucket ARN and s3:GetObject on the object ARNs for read-only S3 access.”

Real World

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“In reality, sometimes applications use pre-signed URLs to avoid broad IAM permissions, or employ AWS SDKs with temporary credentials from IAM Roles with similar scoped policies.”

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Disclaimer

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