CORS Checker

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Understanding Cross-Origin Resource Sharing (CORS)

Modern web applications frequently separate frontend user interfaces from backend API services. When a browser running a frontend application on one domain requests resources from an API hosted on another domain, the browser enforces the Same-Origin Policy.

Cross-Origin Resource Sharing (CORS) is the HTTP header protocol that lets servers instruct browsers whether cross-origin access is permitted. Without explicit server permission, modern web browsers restrict client-side scripts from reading responses to prevent data theft and session hijacking.

Testing these headers directly in the browser can be tricky because browsers hide error details from script consoles when security policies fail. This CORS checker sends simulated requests directly to your API endpoint, revealing exact preflight outcomes, missing directives, and security configurations.

Core CORS Response Headers Reference

Key HTTP response headers evaluated during cross-origin requests.

CORS Response HeaderDirective PurposeExample Value
Access-Control-Allow-OriginDeclares which client origins can access resource data.https://example.com or *
Access-Control-Allow-MethodsLists permitted HTTP methods for cross-origin requests.GET, POST, PUT, DELETE, OPTIONS
Access-Control-Allow-HeadersLists request headers permitted during preflight negotiation.Content-Type, Authorization
Access-Control-Allow-CredentialsSpecifies if browsers can expose responses when cookies or tokens are passed.true
Access-Control-Max-AgeTells the browser how many seconds to cache preflight responses.86400
Access-Control-Expose-HeadersLists server response headers accessible by frontend client scripts.X-Total-Count, X-Request-ID

Common CORS Errors and Resolutions

Frequent browser console errors and how to fix them on your backend.

Common Browser CORS ErrorUnderlying Technical CauseRecommended Fix
No 'Access-Control-Allow-Origin' header presentThe server did not include any CORS headers in its response.Add Access-Control-Allow-Origin header on the server.
Wildcard origin cannot be used with credentialsServer returned '*' with Access-Control-Allow-Credentials: true.Return the explicit requesting origin instead of wildcard '*'.
Preflight request doesn't pass access control checkThe OPTIONS route returned a non-200/204 status or omitted headers.Ensure the server responds to OPTIONS requests with HTTP 200/204.
Request header field not allowed by Access-Control-Allow-HeadersFrontend sent custom headers omitted from server preflight allowlist.Add the requested header to Access-Control-Allow-Headers.

Why Use Our Online CORS Checker?

Diagnose cross-origin configuration errors quickly and accurately with our lightweight testing engine.

Dual Request Probing

Sends both automated preflight OPTIONS checks and direct HTTP requests to pinpoint exact failure points.

Localhost Simulation

Test localhost origins and custom staging domains without hitting client-side browser origin locks.

Security Risk Detection

Flags illegal combinations like wildcard origins with credentials and missing Vary: Origin headers.

Preflight Latency Tracking

Measures preflight handshake round-trip latency to help optimize API responsiveness.

1-Click Code Remediation

Provides ready-to-paste configuration recipes for Express, Next.js, Nginx, Apache, and FastAPI.

Zero-Setup Debugging

Analyze headers immediately without installing browser extensions or configuring local proxy tunnels.

How to Test CORS Headers Online

Follow four simple steps to audit your API endpoint for cross-origin compliance.

1

Paste your target HTTP or HTTPS API endpoint URL into the primary input field.

2

Select a quick preset (such as localhost:3000) or enter your frontend domain making cross-origin requests.

3

Optionally configure custom HTTP methods, include custom headers, or toggle credentials simulation.

4

Review the preflight status, header directives, security checks, and copy ready-to-use server fixes.

Clear answers to common questions about CORS headers, preflight requests, and API configuration.

CORS Checker FAQ

A CORS checker tests whether an API endpoint returns appropriate Cross-Origin Resource Sharing headers. It simulates cross-origin requests from frontend apps and verifies whether the server permits JavaScript to access the payload.
Postman and cURL are server-side HTTP utilities that bypass browser security sandboxes. Web browsers enforce the Same-Origin Policy to protect user sessions. If the backend omits Access-Control headers, browsers block JavaScript from reading the data.
A preflight request is an automated HTTP OPTIONS request sent by the browser before the actual call. Browsers send preflights when requests include custom headers like Authorization, non-standard content types, or HTTP methods like PUT and DELETE.
No. The W3C specification strictly prohibits using Access-Control-Allow-Origin: * when Access-Control-Allow-Credentials is true. Servers handling authenticated requests must echo the specific requesting origin rather than a wildcard.
When servers dynamically reflect requesting origins, they must include Vary: Origin. This header instructs CDNs and reverse proxies to cache responses separately per origin, preventing cache collisions and cross-origin leaks.
You must configure CORS on the backend API server. Update your middleware or web server configuration to send Access-Control-Allow-Origin with your frontend domain or wildcard.
Our backend runner sets the HTTP Origin header to http://localhost:3000 or your custom development URL. Because tests originate from our testing engine, they bypass browser sandbox restrictions that forbid modifying Origin headers.
A preflight OPTIONS request must return HTTP 200 OK or 204 No Content. Returning error codes like 401, 403, 404, or 500 causes the browser to abort the subsequent call.
By default, browsers only permit JavaScript to inspect a small set of safelisted headers. If your application needs to read custom headers like X-Total-Count, the server must expose them via Access-Control-Expose-Headers.
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Disclaimer
The CORS Checker evaluates public web endpoints using the URL and origin parameters provided. Results reflect headers returned by the target server at the time of testing. We do not store or log your queries.