CORS Checker

Instantly test Cross-Origin Resource Sharing (CORS) headers for any public endpoint. Our backend performs the request and reveals full response headers, even if the server doesn't support CORS. Identify misconfigurations, verify Access-Control-Allow-Origin, and get actionable insights to fix CORS errors.

Enter a full URL (include https:// or http://). The request is made from our server, so CORS restrictions do not apply.
Test examples: ? JSONPlaceholder (CORS enabled) ? GitHub API (CORS wildcard) ? httpbin.org (Echo CORS) ? Example.com (likely no CORS) ? SpaceX API (GraphQL)
Privacy-first analysis: URLs are sent to our server only to perform the CORS check; no data is stored.

Understanding Cross-Origin Resource Sharing (CORS)

CORS (Cross-Origin Resource Sharing) is a critical security mechanism implemented by web browsers that controls how web applications running at one origin (domain, protocol, port) can request resources from a different origin. Without proper CORS headers, browsers block cross-origin requests, preventing malicious websites from reading sensitive data. However, for legitimate API integrations, developers must configure servers to return the correct Access-Control-Allow-Origin header.

? How this tool works:

Unlike browser‑side CORS checkers, our tool uses a server‑side PHP script that makes an HTTP request to the target URL. Because the request originates from our server, the browser’s CORS policy does not interfere. We then display all response headers, including any Access-Control-* headers, along with the HTTP status and final URL. This approach reveals the exact CORS configuration of the target server.

Why CORS Matters for Modern Web Development

  • Security by default: Same-origin policy protects users from CSRF and data leaks.
  • API interoperability: Public APIs rely on CORS to allow third-party apps to fetch data.
  • Microservices & frontend-backend separation: Frontend apps (React, Vue, Angular) often run on different ports or domains — CORS enables safe communication.
  • Debugging tool: Quickly diagnose why your fetch() calls fail with cryptic “No 'Access-Control-Allow-Origin'” errors.

Anatomy of CORS Headers

Header Purpose Example
Access-Control-Allow-Origin Specifies which origins can access the resource. Use * for public APIs or specific origin. https://your-app.com or *
Access-Control-Allow-Methods Lists HTTP methods allowed (GET, POST, PUT, etc.) during preflight. GET, POST, PUT, DELETE
Access-Control-Allow-Headers Indicates which custom headers can be used in the actual request. Content-Type, Authorization, X-API-Key
Access-Control-Max-Age Time in seconds that preflight response can be cached. 86400
Access-Control-Expose-Headers Allows JavaScript to read specific headers from the response. X-Total-Count, Custom-Header
Real‑world scenario: Fixing CORS in a React + Express App

A development team deployed a React frontend on http://localhost:3000 and an Express API on http://localhost:5000. All API calls failed with CORS errors. Using this CORS checker they discovered the API server was not sending Access-Control-Allow-Origin. The fix: installing the cors middleware and configuring origin: true for development. The proper headers instantly resolved the issue, enabling seamless local development and later production deployment with domain restrictions.

Server-Side Configuration Best Practices

Node.js (Express)
const cors = require('cors');
app.use(cors({
  origin: 'https://yourdomain.com',
  methods: ['GET','POST'],
  credentials: true
}));
Nginx
add_header 'Access-Control-Allow-Origin' 'https://example.com';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
Flask (Python)
from flask_cors import CORS
CORS(app, origins=["https://trusted.com"])
Apache .htaccess
Header set Access-Control-Allow-Origin "https://client.com"

Common CORS Misconceptions & Pitfalls

  • Myth: "CORS is a server-side security feature" — Actually, CORS is enforced by browsers; the server merely provides headers.
  • Misconfig: Using Access-Control-Allow-Origin: * with credentials (withCredentials: true) is forbidden by browsers — must specify explicit origin.
  • Preflight ignorance: Non‑simple requests (e.g., custom headers, PUT) trigger an OPTIONS preflight. Many servers handle only GET and miss OPTIONS → CORS failure.
  • Localhost nuance: Different ports are distinct origins — CORS headers must reflect exact origin including port.

Step-by-Step: Using This CORS Checker

  1. Paste or type the full endpoint URL (must include http:// or https://).
  2. Click "Check CORS Headers" — our server sends a GET request to the target.
  3. Review the result: CORS status, all Access-Control-* headers, and actionable recommendations.
  4. Use the example presets to compare CORS behavior across known public APIs.
  5. Copy the report for team collaboration or documentation.

Trusted by developers and security engineers — The CORS analyzer implements the W3C CORS specification (Fetch Standard) and follows guidelines from MDN Web Docs. Our server‑side methodology uses cURL, ensuring accurate header retrieval. Reviewed by GetZenQuery Tech team, last updated April 2026.

Frequently Asked Questions

If the server does not return any Access-Control-Allow-Origin header or returns one that does not match your current origin (or is not '*'), the browser blocks the request and prevents reading any headers. Our tool captures this scenario and informs you that CORS is not enabled or that the origin is mismatched.

The current version performs a simple GET request (no preflight). For endpoints that require preflight (e.g., methods other than GET or custom headers), we analyze the CORS headers returned during the GET. However, we provide insights about expected preflight behavior and recommend checking with server logs or using tools like curl to inspect OPTIONS responses.

It means the target server did not respond with appropriate CORS headers, or the headers were present but did not allow your origin (the page’s origin). In such cases, a browser would block the request. The error message often includes details about missing ACAO header.

Our tool focuses on GET requests because CORS inspection for POST often requires preflight. However, the core CORS headers (Allow-Origin, Allow-Methods) are returned regardless. For complex scenarios, use the displayed headers to infer if the server supports your intended method. We are planning an advanced mode soon.

Since the check executes live requests from our server, results reflect real network conditions and server responses. However, some servers may behave differently based on the Origin header sent. Our tool sends the current window.location.origin as the origin, which is typical for frontend apps. Accuracy is high for public endpoints.
References & Further Reading: MDN CORS Documentation • Fetch Standard (CORS protocol) • W3C CORS Specification