CORS Checker
LiveTest if a URL returns proper CORS headers and diagnose cross-origin issues.
Understanding Cross-Origin Resource Sharing (CORS)
Cross-Origin Resource Sharing (CORS) is a browser security mechanism that controls which web pages can make requests to a different domain than the one serving the page. When a frontend application on app.example.com calls an API on api.example.com, the browser sends a preflight OPTIONS request to check whether the server allows cross-origin access. CORS misconfigurations are among the most common causes of frontend-backend integration failures, producing cryptic browser console errors that block legitimate API calls.
The CORS Checker sends a preflight OPTIONS request to any URL and inspects the response headers for Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, and more. Quickly diagnose why your frontend can't call an API and get actionable fix suggestions.
The Devkitr CORS Checker tests any URL for its CORS policy by examining the response headers the server sends to cross-origin requests. Enter a URL and an origin domain to see the Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, and Access-Control-Allow-Credentials headers returned by the server, instantly revealing whether your frontend can access that API endpoint.
In a typical development workflow, CORS Checker becomes valuable whenever you need to test if a url returns proper cors headers and diagnose cross-origin issues. Whether you are working on a personal side project, maintaining production applications for a company, or collaborating with a distributed team across time zones, having a reliable browser-based validation tool eliminates the need to install desktop software, write one-off scripts, or send data to third-party services that may log or retain your information. Since CORS Checker processes everything locally on your device, your data stays private and your workflow stays uninterrupted — open a browser tab, paste your input, get your result.
Key Features
Preflight Simulation
Sends an OPTIONS request mimicking browser preflight behavior to check CORS headers before actual data requests, just like a real browser would.
Header-by-Header Analysis
Breaks down each CORS header with explanations — showing allowed origins, methods, headers, credentials policy, and max age for preflight caching.
Custom Origin Testing
Specify the requesting origin domain to test whether a specific frontend application is allowed to access the target API.
Wildcard vs Specific Origin Detection
Identifies whether the server uses wildcard (*) or specific origin matching, and warns about security implications of overly permissive configurations.
How to Use CORS Checker
Enter the API URL
Type the full URL of the endpoint you want to test CORS policies for, including the protocol and path.
Set the Origin
Specify the origin domain your frontend application runs on (e.g., https://myapp.com) to test access from that specific origin.
Run the CORS Check
The tool sends preflight and actual requests, then collects all CORS-related response headers from the server.
Review the Policy
Read the header breakdown to understand what the server allows — origins, methods, headers, credentials — and identify misconfiguration.
Use Cases
Diagnosing "CORS Error" Console Messages
When browsers block API calls with CORS errors, test the endpoint to see exactly which CORS headers are missing or misconfigured.
Validating Server CORS Configuration
After setting up CORS middleware on your backend, verify that the correct headers are returned for different origin combinations.
Testing Third-Party API Access
Before integrating a third-party API into your frontend, check if it allows cross-origin access from your domain or requires a backend proxy.
Security Auditing CORS Policies
Review CORS headers on production endpoints to ensure they do not expose APIs to unauthorized origins with overly permissive wildcard policies.
Pro Tips
Use specific origin values (Access-Control-Allow-Origin: https://myapp.com) instead of wildcard (*) for APIs that handle authenticated requests.
Remember that CORS is enforced by browsers only — server-to-server requests, curl, and Postman ignore CORS entirely. CORS protects users, not APIs.
Set Access-Control-Max-Age to cache preflight responses and reduce OPTIONS request overhead — 86400 (24 hours) is a common value.
If using credentials (cookies, auth headers), both Allow-Origin must be specific (not *) and Allow-Credentials must be true — this is a common source of confusion.
Common Pitfalls
Using Access-Control-Allow-Origin: * with credentials
Fix: Browsers reject wildcard origins when credentials are included. Dynamically set Allow-Origin to the requesting origin and add Vary: Origin header instead.
Only configuring CORS for GET requests, forgetting preflight
Fix: POST, PUT, DELETE, and requests with custom headers trigger OPTIONS preflight. Configure your server to respond to OPTIONS with proper CORS headers.
Setting CORS headers in the frontend instead of the backend
Fix: CORS headers must come from the server response, not the client request. The browser checks server response headers — client-side headers have no effect on CORS.
Frequently Asked Questions
QWhat headers does it check?
It checks Access-Control-Allow-Origin, Allow-Methods, Allow-Headers, Allow-Credentials, Expose-Headers, and Max-Age.
QWhy might the test fail?
Some servers block requests from unknown origins. The tool uses a proxy approach to inspect headers that would otherwise be blocked by the browser.
QCan I test with a custom Origin header?
Yes. You can specify a custom Origin value to simulate requests from different domains.
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