Skip to content
Building REST API Microservices

CORS

1. Introduction

CORS (Cross-Origin Resource Sharing) is a security feature implemented by web browsers to control how web applications interact with resources from different origins. In the context of REST APIs, CORS determines whether a client from one origin can make requests to a server located at another origin. Understanding CORS is crucial for API developers as it directly affects the accessibility and security of your APIs when consumed by web applications.

This chapter provides an in-depth overview of CORS, how it works, common challenges developers face, and best practices for configuring CORS in your REST APIs.

2. What is CORS?

CORS is a security protocol implemented by web browsers to prevent unauthorized access to resources on a server from a different origin. An origin is defined by the combination of a scheme (protocol), hostname, and port (e.g., https://example.com:443).

Without CORS, web pages running in one domain would be unable to make requests to another domain, limiting data sharing and interaction between different web applications. CORS provides a controlled way to overcome this restriction, allowing safe and secure cross-origin requests.

3. How CORS Works

CORS uses HTTP headers to communicate between the client and server, controlling which resources can be requested by whom and under what circumstances. Here’s a step-by-step look at how CORS functions during a cross-origin request:

  1. Simple Requests

    • Simple requests are HTTP requests using methods like GET, POST (with specific content types like application/x-www-form-urlencoded, multipart/form-data, or text/plain), or HEAD that do not involve custom headers or credentials.

    • For these requests, the browser automatically sends the request to the server. If the server allows the origin, it responds with appropriate CORS headers, such as Access-Control-Allow-Origin.

  2. Preflight Requests

    • For more complex operations, such as requests using methods like PUT, DELETE, or POST with content types like application/json, or when custom headers or credentials are included, the browser sends a preflight request before the actual request.

    • The preflight request uses the OPTIONS method to check with the server whether the intended action is safe and allowed.

    • The server responds with CORS headers that specify allowed methods, headers, origins, and other parameters.

  3. Response Headers

    • If the CORS headers in the server response indicate that the request is permitted, the browser proceeds with the actual request.

    • If the headers do not allow the request, the browser blocks the request and displays an error, preventing the client from accessing the response.

4. Key CORS Headers

  1. Access-Control-Allow-Origin

    • Specifies which origins are permitted to access the resource. It can be set to a specific origin (e.g., https://example.com), or * to allow all origins.

    • Example:

      Access-Control-Allow-Origin: https://example.com

  2. Access-Control-Allow-Methods

    • Lists the HTTP methods allowed when accessing the resource during actual requests.

    • Example:

      Access-Control-Allow-Methods: GET, POST, PUT, DELETE

  3. Access-Control-Allow-Headers

    • Specifies which headers are allowed in the actual request. Custom headers or specific headers must be explicitly listed.

    • Example:

      Access-Control-Allow-Headers: Content-Type, Authorization

  4. Access-Control-Allow-Credentials

    • Indicates whether the browser should include credentials, such as cookies, authorization headers, or TLS client certificates, with the request. This header must be set to true to enable credentialed requests.

    • Example:

      Access-Control-Allow-Credentials: true

  5. Access-Control-Max-Age

    • Defines the maximum duration (in seconds) that the results of a preflight request can be cached, reducing the need for repeated preflight requests.

    • Example:

      Access-Control-Max-Age: 600

  6. Access-Control-Expose-Headers

    • Lists headers that can be exposed as part of the response, allowing the client to access those headers.

    • Example:

      Access-Control-Expose-Headers: X-My-Custom-Header

5. Common CORS Issues and Solutions

  1. CORS Policy Errors

    • Problem: The browser blocks requests due to missing or incorrect CORS headers, often leading to “CORS policy error” messages.

    • Solution: Ensure the server’s response includes the correct CORS headers for the request type. Use tools like curl or browser developer tools to inspect headers and debug issues.

  2. Preflight Request Fails

    • Problem: Preflight requests using the OPTIONS method fail, preventing the actual request from executing.

    • Solution: Ensure the server correctly handles OPTIONS requests and responds with the necessary CORS headers, including Access-Control-Allow-Methods.

  3. Credentialed Requests Blocked

    • Problem: Requests with credentials (e.g., cookies, authorization headers) are blocked due to missing Access-Control-Allow-Credentials: true.

    • Solution: Add the Access-Control-Allow-Credentials header and ensure Access-Control-Allow-Origin is not set to * when allowing credentials.

  4. Incorrect Allowed Methods or Headers

    • Problem: Requests are blocked due to missing allowed methods or headers in the CORS configuration.

    • Solution: Update the Access-Control-Allow-Methods and Access-Control-Allow-Headers headers to include all methods and headers needed by the client.

6. Best Practices for Configuring CORS

  1. Restrict Origins: Avoid using * in Access-Control-Allow-Origin for public APIs. Instead, specify allowed origins to enhance security.

  2. Limit Methods and Headers: Only allow necessary HTTP methods and headers to minimize exposure and reduce the potential attack surface.

  3. Enable Credentials Sparingly: Allow credentials only when necessary and ensure proper authentication and authorization checks are in place.

  4. Cache Preflight Responses: Use Access-Control-Max-Age to reduce the frequency of preflight requests, improving performance for clients that repeatedly access the same endpoints.

  5. Test with Multiple Clients: Test your CORS configuration with various clients (e.g., browsers, mobile apps) to ensure compatibility and identify any configuration gaps.

  6. Monitor and Adjust: Continuously monitor CORS-related errors and adjust configurations as needed to accommodate new clients or changes in the API.

7. Conclusion

CORS plays a crucial role in modern web security, balancing the need for cross-origin resource access with the protection of data. By understanding how CORS works, configuring it correctly, and adhering to best practices, developers can ensure their APIs are both accessible and secure, providing a seamless experience for users and applications alike.

Let me know if you need further elaboration or any additional information!