Payam Beigi – Software Developer Portfolio

In the vast, interconnected realm of web development, there’s an invisible wall that often stops developers in their tracks: Cross-Origin Resource Sharing (CORS). Designed as a security feature to prevent unwanted cross-origin requests, CORS can pose a significant challenge during the development of web applications that rely on accessing resources from multiple origins.

Understanding the Challenge:
Our team encountered CORS while developing a web application that needed to consume APIs hosted on a different domain. The error messages began to appear: “Access to XMLHttpRequest at ‘http://example-api.com’ from origin ‘http://ourdomain.com’ has been blocked by CORS policy.” This barrier halted our progress, demanding a solution that would enable secure and controlled access to cross-origin resources.

Demystifying CORS:
Before tackling the problem, we ensured that our entire team understood the concept of the same-origin policy and CORS. It was crucial to recognize that CORS is a browser-enforced security feature that restricts how scripts on one origin can interact with resources from another origin. Understanding that the CORS policy is controlled by the server and not the client was a key step in finding a solution.

Developing a Backend Proxy:
Our initial approach to bypassing CORS issues was to implement a server-side proxy. This backend service, built with Node.js and Express, would forward requests from our frontend to the external API. By doing this, we ensured that all API requests originated from the same origin as our web application, effectively avoiding the CORS error.

Setting Up CORS on the Server:
While the proxy solution worked, it was not the most efficient approach for our case. We needed direct browser-to-server communication for performance reasons. We configured the external API server to include CORS headers like Access-Control-Allow-Origin in its responses, specifically allowing our application’s origin or using a wildcard (*) for broader access.

Handling Preflight Requests:
We then tackled the issue of “preflight” requests. Browsers send an HTTP OPTIONS request to the server to determine if the actual request is safe to send. We modified our server configuration to handle these preflight requests correctly, ensuring that appropriate CORS headers were present in the preflight response.

Securely Configuring CORS:
A crucial part of resolving CORS issues was ensuring that the configuration was secure. We avoided the indiscriminate use of wildcards in our CORS header settings and instead specified the exact origins that were allowed. We also implemented strict controls over which HTTP methods and headers were permitted for cross-origin requests.

Frontend Adjustments:
On the frontend, we had to adjust our XMLHttpRequests and fetch API calls to include credentials when making requests to our external API. This involved setting the withCredentials property to true to allow cookies and headers to be included in our cross-origin requests.

Testing and Documentation:
After implementing these solutions, rigorous testing was essential. We tested our application in multiple browsers to ensure that CORS issues were resolved across the board. Additionally, we documented our CORS strategy and configurations to maintain clarity and consistency within our development team and for future reference.

Lessons Learned:
Overcoming CORS was a lesson in both web security and cross-origin resource management. We learned the importance of a deep understanding of web protocols and browser security features. It was also a reminder that backend configurations play a crucial role in frontend development, especially in a world where applications are increasingly decoupled and distributed.

Conclusion:
CORS issues can be a significant stumbling block in web development, but they are not insurmountable. With a combination of server-side configurations, careful planning, and understanding of security principles, we were able to overcome the invisible wall of CORS. Our journey through CORS highlighted the delicate balance between security and functionality in web application development.

Related Tech Stack:

• Node.js and Express (for backend proxy and server-side configurations)
• Webpack Dev Server (for local development and proxying)
• HTTP Headers (Access-Control-Allow-Origin, etc.)
• XMLHttpRequest and Fetch API (for frontend requests)
• Browser DevTools (for testing and debugging CORS issues)