CORS Header Checker
Free online CORS header checker and analyzer. Validate Access-Control headers, diagnose cross-origin issues, and verify preflight configuration.
Analysis
What Is a CORS Header Checker?
Cross-Origin Resource Sharing (CORS) is a browser security mechanism that controls how web pages on one domain can request resources from a different domain. When a frontend application at https://myapp.com tries to fetch data from https://api.example.com, the browser checks the server's CORS headers to determine whether the request is allowed. A CORS Header Checker analyzes the response headers from an API server and evaluates whether they correctly permit cross-origin requests from your specified origin, HTTP method, and custom headers. It identifies common configuration mistakes such as missing Access-Control-Allow-Origin headers, wildcard origins combined with credentials, and disallowed methods or headers.
How to Use the CORS Header Checker
- Enter your frontend application's origin (e.g., 'https://myapp.com') in the Request Origin field.
- Select the HTTP method your application uses to make the cross-origin request.
- Paste the response headers from your API server into the Response Headers textarea. Enter one header per line in 'Key: Value' format.
- Review the analysis results below. Green checkmarks indicate passing checks, yellow warnings suggest potential issues, and red crosses indicate blocking problems.
Common Use Cases
- Debugging CORS Errors — When your browser console shows 'Access to fetch has been blocked by CORS policy', paste the response headers here to quickly identify what's missing or misconfigured.
- Server Configuration Validation — Before deploying API changes, verify that your CORS headers correctly allow the expected origins, methods, and custom headers without being overly permissive.
- Security Review — Audit your API's CORS configuration to ensure it follows security best practices — no wildcard origins with credentials, appropriate preflight caching, and minimal allowed headers.
- Teaching & Learning CORS — Understand how different CORS header combinations affect browser behavior by experimenting with various configurations and seeing the analysis results in real-time.