Definition
Cross-Origin Resource Sharing (CORS) is a security feature implemented by web browsers that allows or restricts web pages from making requests to a domain different from the one that served the original web page. It manages web application interactions with resources located outside their domain.How It Works
- 1Origin Request: A web page sends a request to a different domain (cross-origin request).
- 2Preflight Request: For certain requests, the browser first sends a preflight request to the server to check if the cross-origin request is allowed.
- 3Server Response: The server responds with headers indicating whether the request is permitted.
- 4Access Control: If the server allows, the browser proceeds with the actual request; otherwise, it's blocked.
Key Characteristics
- Access-Control-Allow-Origin: Specifies which origins can access the resource.
- Preflight Requests: Necessary for HTTP methods like POST, PUT, DELETE.
- Security Headers: Includes headers like Access-Control-Allow-Methods and Access-Control-Allow-Headers.
Comparison
| Feature | CORS | Same-Origin Policy |
|---|---|---|
| Purpose | Allows controlled cross-origin requests | Restricts requests to same origin |
| Use Case | APIs, cross-domain requests | Secure web pages |
| Flexibility | Configurable by server response | Strict, no flexibility |
Real-World Example
When a dashboard built with Plotly needs to fetch data from an API on another server, CORS policies determine if the request is allowed. If the API server has configured CORS headers to accept requests from the dashboard's domain, the data exchange proceeds smoothly.Best Practices
- Configure the server to only allow trusted origins.
- Use precise methods and headers in CORS configurations to minimize exposure.
- Regularly review and update CORS policies as your application evolves.
Common Misconceptions
- CORS is a security feature for servers. It's actually a browser feature, though it relies on server configuration.
- All cross-origin requests require preflight. Only certain requests, like those using non-simple methods, need preflight.
- CORS is optional. Browsers enforce it for security, not just as a best practice.