CORS Concepts New to CORS? Follow the learning path. The guides below start with the basics and continue into practical debugging. What is CORS? Understand why an API can work in curl but fail in a browser, and how a server grants cross-origin response access. What is an origin? Compare origins precisely, understand localhost mismatches, and separate same-origin checks from cookie same-site rules. Same-origin policy Learn what the browser isolates and why blocking response access does not prevent every cross-origin request. Simple requests Predict preflight from the method and request headers. Credentials alone do not trigger it; JSON and Authorization commonly do. Preflight requests Follow JSON POST and PUT requests from the browser’s permission probe to the actual API response, including authentication and caching. CORS headers Know which CORS headers belong on OPTIONS, the actual response, or the browser’s request, with concrete examples. Credentials & cookies Separate fetch credentials mode, CORS permission, cookie eligibility and explicit Authorization headers. Debugging CORS Use a repeatable browser-first workflow to identify the failing response, responsible layer, smallest safe fix and verification. Postman vs browser Compare server connectivity with browser CORS permission, preflight, headers, redirects and cookie rules. The no-cors trap Understand opaque responses and why removing a console error does not make an API response readable. Readable API errors Apply origin permission to intended readable error responses and distinguish authentication failure from CORS failure. Proxies & caching Choose one CORS policy owner, return a single origin permission, and prevent caches from mixing responses across origins. CORS vs CSRF Understand why a blocked response does not prevent a forged state-changing request and how cookie sessions need dedicated CSRF defenses. Configuration risks Distinguish unsafe credentialed origin reflection from legitimate public API permission, and test origin boundaries.
