AWS DVA-C02 Drill: Securing S3 Access - Using Pre-Signed URLs vs. Direct 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 DVA-C02 candidates, the confusion often lies in how to securely grant temporary S3 access without exposing credentials or loosening bucket policies. In production, this is about knowing exactly when to generate pre-signed URLs versus direct API access combined with proper IAM roles. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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BrightWave Technologies runs a customer-facing web application hosted on Amazon EC2 instances. The app stores files in a private Amazon S3 bucket that users can download on demand. Recently, BrightWave enabled S3 Block Public Access to enhance security. After doing so, users encountered authorization errors when trying to download files. As the lead developer, you need to update the application so that only authenticated application users can access these S3 objects, while maintaining the strongest security posture.

The Requirement:

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Design the most secure solution that ensures only signed-in users access the private S3 objects without making the bucket or objects publicly accessible.

The Options

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• A) Create an EC2 instance profile and role with an appropriate IAM policy. Attach the role to the EC2 instances.
• B) Create an IAM user with an access key and secret key, deploy these credentials on the EC2 instances.
• C) Modify the application to use the S3 GeneratePresignedUrl API call to generate temporary URLs for the users.
• D) Modify the application to call the S3 GetObject API directly on behalf of each user and return the object stream.
• E) Modify the application to delegate all requests directly to the S3 bucket.

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Correct Answer

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A and C

Quick Insight: The Developer Imperative

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The combination of an instance role (A) with the least-privilege policy and using GeneratePresignedUrl (C) is the best practice for controlling secure, temporary access to privately stored S3 objects without exposing AWS credentials or making the bucket public.

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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Options A and C

The Winning Logic

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• Option A: Assigning an IAM role with the minimum required permissions to the EC2 instances is the best practice for credential management. This avoids storing permanent access keys on instances (which is insecure and against AWS recommendations). The role provides the app running on the EC2 instances limited, auditable access to S3.
• Option C: The application uses the S3 GeneratePresignedUrl API to create a time-limited, signed URL that users can use to download objects directly from S3 without requiring further authentication. This mechanism respects the S3 Block Public Access setting, because the objects remain private and access is strictly controlled via those signed URLs with expiration time baked in.

Together, these ensure users can download objects only if they are authenticated in the app, without exposing credentials or opening bucket access.

The Trap (Distractor Analysis):

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• Why not B? Storing IAM user credentials (access key and secret) directly on the EC2 instance is insecure and violates AWS best practices. It risks key leakage and requires manual rotation.
• Why not D? Having the app download the objects using GetObject and then streaming it back to users increases application latency and resource usage and adds complexity. It’s less performant and less scalable.
• Why not E? Delegating requests directly to the S3 bucket implies some form of public access or loose bucket policies, which conflicts with the requirement to keep the bucket private and S3 Block Public Access enabled.

The Technical Blueprint

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Relevant IAM Role Policy Snippet for EC2 Instance Role

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{
    "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Action": [ "s3:GetObject" ],
        "Resource": [ "arn:aws:s3:::brightwave-app-files/*" ]
      }
    ]
}

Example Code Snippet Using AWS SDK for JavaScript (v3) to Generate Pre-Signed URL

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import { S3Client, GetObjectCommand } from "@aws-sdk/client-s3";
import { getSignedUrl } from "@aws-sdk/s3-request-presigner";

const s3 = new S3Client({ region: "us-east-1" });

async function getPresignedUrl(bucketName, objectKey) {
  const command = new GetObjectCommand({
    Bucket: bucketName,
    Key: objectKey
  });
  const url = await getSignedUrl(s3, command, { expiresIn: 300 }); // 5 min expiry
  return url;
}

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 with Pre-Signed URLs when you see S3 object access for an app backend with private buckets.

Real World

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In production, streaming objects back from the app server (Option D) might be needed for processing or transformation use cases, but it’s less scalable and not recommended purely for access control purposes.

(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.