Your API Works in cURL. Why Does the Browser Reject It?

The issue described in the report revolves around an API working correctly when accessed via cURL but failing in a browser due to Cross-Origin Resource Sharing (CORS) restrictions. The discrepancy arises because cURL sends requests directly, bypassing browser security protocols like CORS. To troubleshoot this, one should compare the requests made by cURL and the browser, with a focus on the preflight request (OPTIONS) that browsers send to check if the actual request is allowed.

This involves using tools like the browser's Network panel to reproduce the failure and inspect headers. If the preflight fails, the intended request may never reach the server handler. The report suggests creating a small incident note capturing the page origin, destination URL, intended method, requested headers, and the first failing status, as well as determining which layer (application, gateway, or reverse proxy) produced the failure.

Additionally, it's important to test the actual response after a successful preflight, as the response needs to include appropriate CORS headers. When dealing with cookie-based cross-origin requests, ensure the server allows credentials and returns the specific allowed origin, as browser cookie policies still apply. The report also advises against switching to the "no-cors" mode, as it produces an opaque response that cannot be read by the code.

A concise reproduction package containing the working request, preflight probe, and browser failure details should be kept for future reference.

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