Browser

Access to XMLHttpRequest blocked by CORS policy

CORS Policy Blocked

The browser blocked a cross-origin request because the server didn't send appropriate CORS headers.

Common Causes

Server not sending Access-Control-Allow-Origin
Preflight (OPTIONS) not handled
Credentials mode without Allow-Credentials
Wrong origin in Allow-Origin

Fixes

1. Add CORS headers on server

Access-Control-Allow-Origin: https://yourdomain.com
Access-Control-Allow-Methods: GET, POST, OPTIONS

2. Handle preflight

if (req.method === 'OPTIONS') return res.sendStatus(204);

3. Use a proxy in development
```
// next.config.js: rewrites to backend
```

Still not fixed?

• Search the exact error message on Stack Overflow or GitHub Issues
• Check the official docs for the language or tool
• Ensure your versions (Node, Python, npm, Docker) are up to date
• Try in a minimal reproduction (new project, single file)

Related Errors

Address Already in Use

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