AWS DVA-C02 Drill: S3 CORS Configuration - Resolving Browser Resource Blocking

Jeff’s Note

#

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 correctly configuring client-side access to shared static assets. In production, this is about knowing exactly how browsers enforce cross-origin restrictions and how to configure S3 to satisfy those policies without breaking user experience. Let’s drill down.

The Certification Drill (Simulated Question)

#

Scenario

#

FinBay Technologies manages multiple customer-facing single-page web applications (SPAs) hosted on Amazon S3 buckets. Each SPA is used by a separate customer segment but shares a common library of JavaScript frameworks and web fonts. To optimize management and reduce duplication, a lead developer extracts all these common static assets and puts them in a dedicated “shared-assets” S3 bucket. Each SPA accesses these common assets at runtime via URLs pointing to the shared-assets bucket.

However, during integration testing, browsers block the loading of these JavaScript files and fonts due to security restrictions.

The Requirement:

#

What change should the lead developer implement to allow these separate SPA web applications to load the common JavaScript files and web fonts from the shared S3 bucket without being blocked by browsers?

The Options

#

• A) Create four access points that allow access to the central S3 bucket. Assign an access point to each web application bucket.
• B) Create a bucket policy that allows access to the central S3 bucket. Attach the bucket policy to the central S3 bucket.
• C) Create a cross-origin resource sharing (CORS) configuration that allows access to the central S3 bucket. Add the CORS configuration to the central S3 bucket.
• D) Create a Content-MD5 header that provides a message integrity check for the central S3 bucket. Insert the Content-MD5 header for each web application request.

Google adsense

#

leave a comment:

Correct Answer

#

C

Quick Insight: The DVA-C02 Imperative

#

• For developers, the core challenge here is resolving browser-enforced cross-origin restrictions, which block shared static assets unless S3 explicitly signals permission via CORS headers. Simply opening bucket access or adding integrity checks will not satisfy this browser policy.

Content Locked: The Expert Analysis

#

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

The Expert’s Analysis

#

Correct Answer

#

Option C

The Winning Logic

#

Browsers enforce the Same-Origin Policy, which restricts scripts and fonts from loading resources hosted on different domains unless the server hosting those resources explicitly opts in via Cross-Origin Resource Sharing (CORS).

• When multiple SPAs on different origins (or subdomains) request shared JavaScript and fonts from a centralized S3 bucket, the browser expects the S3 response to include CORS headers allowing these cross-origin requests.
• S3 buckets have configurable CORS policies that specify allowed HTTP methods, allowed origins, headers, and exposed headers.
• By creating a proper CORS configuration on the shared-assets bucket, the developer instructs browsers to allow these assets to load across domains safely.
• A bucket policy (Option B) controls authorization to buckets but does not affect browser security policies.
• Access points (Option A) relate to access control within VPCs and permissions, not browser CORS enforcement.
• The Content-MD5 header (Option D) relates to data integrity but does not influence browser cross-origin restrictions.

The Trap (Distractor Analysis)

#

• Why not A? Access points manage network/permission access but do nothing about browser security policies on cross-origin resource sharing.
• Why not B? Bucket policies impact AWS permissions but do not add the necessary HTTP headers required by browsers to allow cross-domain requests.
• Why not D? Content-MD5 is useful for integrity checks in API requests but irrelevant to browser CORS violations.

The Technical Blueprint

#

{
  "CORSRules": [
    {
      "AllowedHeaders": ["*"],
      "AllowedMethods": ["GET", "HEAD"],
      "AllowedOrigins": ["https://app1.finbay.com", "https://app2.finbay.com", "https://app3.finbay.com"],
      "ExposeHeaders": [],
      "MaxAgeSeconds": 3000
    }
  ]
}

This JSON snippet is the kind of CORS configuration you add to the shared-assets S3 bucket to permit cross-origin requests from the SPAs.

The Comparative Analysis

#

Real-World Application (Practitioner Insight)

#

Exam Rule

#

For the exam, always pick S3 CORS configuration when you see client-side resource sharing across multiple domains.

Real World

#

In production, improper or missing CORS configurations cause frustrating frontend issues that are often misdiagnosed as permission or CDN problems. Developers should use the browser developer console to confirm CORS errors and fix bucket CORS policies accordingly.

(CTA) Stop Guessing, Start Mastering

#

Disclaimer

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