What causes a CORS preflight request, and how would you configure browser access to an ASP.NET Core API safely?

CORS preflight and browser API access

Explains browser same-origin enforcement, preflight requests, credentials, middleware behavior, and why CORS is not authorization.

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A browser can send some simple cross-origin requests directly, but a non-simple method, header, or content type causes an OPTIONS preflight first. The browser asks whether the origin, method, and headers are allowed. The API returns the appropriate Access-Control-Allow headers, and the browser decides whether the actual request may proceed. I configure an explicit origin allowlist, required methods and headers, and place ASP.NET Core CORS middleware in the documented pipeline position.

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Origins, same-origin policy, simple requests, OPTIONS preflight, allowed methods and headers, credentials, middleware order, and Vary.

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CORS is a browser access rule around cross-origin JavaScript. It is often mistaken for authentication or a server-to-server firewall, which leads to both broken frontends and weak security assumptions.
A browser can send some simple cross-origin requests directly, but a non-simple method, header, or content type causes an OPTIONS preflight first. The browser asks whether the origin, method, and headers are allowed. The API returns the appropriate Access-Control-Allow headers, and the browser decides whether the actual request may proceed. I configure an explicit origin allowlist, required methods and headers, and place ASP.NET Core CORS middleware in the documented pipeline position.
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Credentials make the policy stricter. A credentialed response cannot use a wildcard allowed origin; the server must return the allowed requesting origin and permit credentials intentionally. CORS does not prove identity or permission, so the endpoint still authenticates and authorizes every request. Server-to-server clients are not protected by browser CORS enforcement. I also make sure authentication middleware does not turn preflight into an unrelated login failure before the CORS policy can answer it.

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Using CORS as the endpoint's authorization mechanism.
Allowing any origin together with credentials.
Reflecting the Origin header without checking an allowlist.
Testing only with Postman or curl and assuming the browser will behave the same.

What causes a CORS preflight request, and how would you configure browser access to an ASP.NET Core API safely?

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