Understanding Same-Origin Policy and CORS: Preflight Requests and Security Implications

The author, a senior architect, introduces the topic by noting that POST requests are common in front‑end development and often appear in interview questions.

Same‑origin policy is described as a browser security mechanism that restricts how a document or script from one origin can interact with resources from another origin, preventing attacks such as XSS, SQL injection, CSRF, and others.

Two URLs are considered same‑origin when they share the same protocol, host, and port. The article lists the definitions of protocol, host, and port.

The policy manifests in three areas: DOM access restrictions, web‑data (XHR/Fetch) restrictions, and network communication restrictions.

CORS (Cross‑Origin Resource Sharing) is explained as a way to relax the same‑origin policy under controlled conditions. The browser sends a preflight OPTIONS request for non‑simple requests, including headers such as Access‑Control‑Request‑Method and Access‑Control‑Request‑Headers . The server responds with Access‑Control‑Allow‑Origin , Access‑Control‑Allow‑Headers , Access‑Control‑Allow‑Methods , and optional Access‑Control‑Max‑Age .

Simple requests are defined (GET, HEAD, POST only; limited headers; no custom headers; no ReadableStream, etc.) and do not trigger a preflight.

The article also warns against using the wildcard “*” for Access‑Control‑Allow‑Origin or other headers when credentials are involved, recommending explicit domain specifications.

Finally, the author summarizes that preflight requests are automatically issued by browsers to ensure security before actual CORS requests, and that proper header configuration is essential to protect user data.

In addition to the technical discussion, the article contains promotional material inviting readers to join a ChatGPT community, offering paid subscriptions, exclusive accounts, and various benefits.