AWS DVA-C02 Drill: IAM Roles for EC2 - Secure API Calls with Least Privilege

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 managing credentials securely on compute resources and understanding the role of IAM roles vs. IAM users. In production, this is about knowing exactly how to implement the principle of least privilege without embedding static credentials. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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TechValley Apps is designing a backend service that runs on Amazon EC2 instances and needs to access multiple Amazon S3 buckets for reading and writing application data. The development team wants to ensure secure API communication from the EC2 hosts to S3 without the overhead and security risks of managing long-lived AWS access keys embedded in the application code. The security policy enforces the principle of least privilege.

The Requirement:

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Implement a solution allowing the EC2 instances to securely call the S3 APIs with permissions scoped only to necessary buckets and APIs—without manually managing credentials.

The Options

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• A) Create an IAM user with programmatic access keys. Attach an IAM policy granting s3:* permissions. Distribute the keys to the EC2 instances.
• B) Attach an IAM role to the EC2 instances with an inline IAM policy allowing s3:ListBucket and s3:*Object permissions scoped to the required buckets.
• C) Attach the AWS managed policy AmazonS3FullAccess to an IAM role assigned to the EC2 instances.
• D) Configure S3 bucket policies granting s3:ListBucket and s3:*Object permissions directly to the EC2 instance.

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Correct Answer

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B

Quick Insight: The Developer’s Security Imperative

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Leveraging IAM roles attached directly to EC2 instances is the recommended method to eliminate static credentials while strictly enforcing least privilege. Granting only the minimum necessary S3 actions on specific buckets aligns with security best practices and makes the authentication process seamless and manageable.

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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Assigning an IAM role directly to the EC2 instance is the most secure and AWS-recommended practice for granting permissions to an application running on EC2. The role’s temporary credentials are automatically rotated and injected via Instance Metadata Service (IMDS), eliminating the need to store or rotate access keys manually. Further, applying an inline policy scoped to only s3:ListBucket and s3:*Object on the specific buckets strictly adheres to the principle of least privilege, ensuring the application gets exactly the permissions it needs, no more, no less.

The Trap (Distractor Analysis):

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• Why not A?
  Embedding IAM user access keys on EC2 instances is a much less secure practice because keys can be leaked or compromised, and key rotation requires manual intervention. This violates the principle of least privilege and good security hygiene.

• Why not C?
  Using the managed policy AmazonS3FullAccess grants overly broad permissions to all S3 buckets and actions, violating the least privilege principle and increasing risk.

• Why not D?
  S3 bucket policies apply permission to AWS principals (IAM users, roles, or accounts), not directly to an EC2 instance resource. You cannot grant permissions to an EC2 instance itself via bucket policy without an IAM principal. This option misunderstands the model of resource-based vs. identity-based policies.

The Technical Blueprint

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# Example CLI snippet to create IAM role and attach a scoped inline policy
aws iam create-role --role-name EC2S3AccessRole --assume-role-policy-document file://trust-policy.json

aws iam put-role-policy --role-name EC2S3AccessRole --policy-name ScopedS3AccessPolicy --policy-document file://s3-scope-policy.json

# Attach the role to the EC2 instance via AWS Console or CLI during instance launch (or later)

Example trust policy (trust-policy.json):

{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Allow",
    "Principal": { "Service": "ec2.amazonaws.com" },
    "Action": "sts:AssumeRole"
  }]
}

Example inline policy (s3-scope-policy.json):

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:ListBucket"
      ],
      "Resource": [
        "arn:aws:s3:::example-bucket-1",
        "arn:aws:s3:::example-bucket-2"
      ]
    },
    {
      "Effect": "Allow",
      "Action": [
        "s3:GetObject",
        "s3:PutObject",
        "s3:DeleteObject"
      ],
      "Resource": [
        "arn:aws:s3:::example-bucket-1/*",
        "arn:aws:s3:::example-bucket-2/*"
      ]
    }
  ]
}

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 pick IAM Roles for EC2 when you see secure API calls without managing static credentials.

Real World

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In production, we avoid embedding keys wherever possible. Roles attached to compute resources enable scalable, secure, and auditable access to AWS APIs — reducing attack surface and easing credential rotation.

(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.