Cross-Origin Resource Sharing (CORS)

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Cross-Origin Resource Sharing (CORS) is a security protocol implemented by web browsers that controls how web pages in one domain can request and interact with resources hosted on another domain. This mechanism helps protect users from malicious websites while enabling legitimate cross-origin data sharing.

How CORS Works

When a web application makes a request to a different origin (domain, protocol, or port), the browser sends a preflight request to the server hosting the resource. This preflight request uses the HTTP OPTIONS method and includes headers specifying the intended request method, headers, and origin. The server then responds with headers indicating whether the actual request is allowed.

Key CORS Headers

Request Headers

Origin: Indicates where the request originates from
Access-Control-Request-Method: Specifies the HTTP method for the actual request
Access-Control-Request-Headers: Lists any custom headers the actual request will include

Response Headers

Access-Control-Allow-Origin: Specifies which origins can access the resource
Access-Control-Allow-Methods: Lists permitted HTTP methods
Access-Control-Allow-Headers: Indicates which headers can be used
Access-Control-Max-Age: Defines how long the preflight response can be cached

Common CORS Scenarios

Simple Requests

Some requests don’t trigger a preflight check. These “simple requests” must meet specific criteria:

Use GET, HEAD, or POST methods
Only include standard headers
Use standard content types

Preflighted Requests

More complex requests require a preflight check, including those that:

Use methods other than GET, HEAD, or POST
Include custom headers
Use non-standard content types

CORS Best Practices

Security Considerations

Never use Access-Control-Allow-Origin: * in production for sensitive resources
Implement proper authentication and authorization
Validate and sanitize all cross-origin requests
Use HTTPS for all cross-origin communications

Performance Optimization

Set appropriate Access-Control-Max-Age values to reduce preflight requests
Minimize the number of cross-origin requests
Consider using same-origin solutions when possible

Understanding and properly implementing CORS is crucial for modern web development, as it balances security requirements with the need for cross-origin resource sharing in today’s interconnected web applications.