CORS Checker — Cross-Origin Resource Sharing Test

Browsers enforce the same-origin policy by default. Without CORS headers, fetch and XMLHttpRequest from https://app.example.com cannot read responses from https://api.example.com even when the user is authenticated. CORS is a server opt-in mechanism, not a substitute for authentication.

Server-side curl requests ignore CORS — only browser contexts enforce it. That is why APIs appear to work in backend tests but fail in frontend consoles with CORS errors.

Non-simple methods and custom headers trigger a preflight OPTIONS request before the actual call. Our checker simulates this preflight with Origin and Access-Control-Request-Method headers. If OPTIONS fails or omits required Allow headers, browsers block the subsequent GET or POST.

Some servers respond correctly to GET but not OPTIONS — a common misconfiguration when only GET routes are wired at the gateway.

A specific origin like https://app.example.com is safest for credentialed requests. Wildcard * allows any origin but cannot be combined with Access-Control-Allow-Credentials: true. Reflecting the request Origin without validation effectively allows any site — treat reflection as equivalent to wildcard unless you validate against an allowlist server-side.

Our results show the exact Allow-Origin string returned for your test Origin so you can confirm intentional restriction.

When Allow-Credentials is true, browsers include cookies on cross-origin requests only if Allow-Origin is a specific match, not *. Frontend code must set credentials: 'include' in fetch options.

Misconfigured credentialed CORS is a frequent source of session leakage — verify Allow-Origin never reflects arbitrary attacker origins on authenticated endpoints.

Allow-Methods lists permitted HTTP verbs for cross-origin calls. Allow-Headers lists custom request headers clients may send. Missing entries cause preflight failure even when Allow-Origin is correct.

Review both fields when debugging PATCH or DELETE failures from browser clients.

Supply your production frontend origin to confirm it is explicitly allowed. Test a deliberately untrusted origin to confirm it is rejected — absence of Allow-Origin or a mismatch means the browser will block read access.

Staging and production APIs may differ — run separate checks per environment.

CORS protects browser users from malicious sites reading API responses. It does not stop direct API abuse via curl or other non-browser clients. Always authenticate and authorize on the server regardless of CORS settings.

Public read-only APIs may legitimately use permissive CORS. Private data APIs should use strict origin allowlists.

Duplicate conflicting CORS headers from API gateway and application layer. OPTIONS not routed to the application on serverless platforms. Allow-Origin set only on error responses. CDN caching OPTIONS responses with wrong Origin for all clients.

Fix by centralizing CORS middleware, ensuring OPTIONS reaches the same handler as other methods, and disabling inappropriate caching of preflight responses.

Debugging browser console CORS errors after deploying a new frontend domain. Validating third-party API integrations before production cutover. Security review of public APIs that return sensitive JSON.

Document Allow-Origin decisions in runbooks alongside API authentication requirements.

We send one preflight from our server infrastructure. Some APIs geo-block or require API keys on all requests — those may return errors unrelated to CORS. IP allowlists on your API may block our probe.

Complex CORS with dynamic origin validation may require testing from an actual browser session on your frontend origin.