Cross-Origin Resource Sharing, explained

Understand the error.
Fix the right layer.

A CORS message tells you the browser could not grant your frontend access to a response. Follow the evidence in the request and response headers to find out why.

Fix a CORS error

Match the browser message to its cause, safe fix, and verification steps.

Find your error →

Test response headers

Check an API or analyze pasted headers. Separate CORS permission from HTTP status.

Open CORS Tester →

Learn how CORS works

Start with origins, then requests, preflight, credentials, and debugging.

Follow the learning path →

Start with what you can observe

Missing Access-Control-Allow-Origin: inspect the actual response, including error responses.
Failed preflight: inspect the OPTIONS status and permissions.
Origin mismatch: compare scheme, host, and port exactly.
Credentials rejected: check the exact origin and credentials permission.

Configure your server

Use the Express, Django, and Spring guides when you control the API. Use the FAQ for quick answers and the quiz to check your understanding.