Your browser can send a request to an API and still refuse to give the response to your page’s JavaScript. That is CORS: a browser-enforced rule for sharing cross-origin responses, controlled by permission headers from the API server. If the request needs a preflight and that check fails, the browser stops before sending the actual request.
What CORS blocks—and what it does not
Browsers restrict scripts from freely reading responses from a different origin under the same-origin policy. An origin is the combination of scheme, host, and port. For example, a page at https://app.example.com and an API at https://api.example.com are cross-origin because their hosts differ; changing only the port or scheme also creates a different origin.
Cross-Origin Resource Sharing (CORS) lets an API server grant selected origins permission to share responses with browser scripts using HTTP response headers. The browser checks those headers. CORS is not a JavaScript permission switch, browser extension, or network firewall, and it is not authentication or authorization.
A CORS message therefore does not by itself tell you whether the API received the request. For a request that does not require preflight, the browser may send it and then withhold its response from JavaScript. A failed preflight, by contrast, prevents the browser from sending the actual request.
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When the browser sends an OPTIONS preflight
Cross-origin fetch() uses CORS by default, but not every request triggers a preflight. A request can require one when its method or manually set headers fall outside the CORS safelist. The browser first sends an OPTIONS request describing the intended method and headers. The API must authorize them before the browser proceeds.
In the Network panel, inspect the preflight request’s Origin, Access-Control-Request-Method, and Access-Control-Request-Headers. Its response needs suitable permissions in Access-Control-Allow-Origin, Access-Control-Allow-Methods, and Access-Control-Allow-Headers. A missing or mismatched permission means the browser will not send the actual request.
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How to diagnose a CORS failure
- Compare origins. Check the page’s address and the API URL, including scheme, host, and port. If any differs, the request is cross-origin.
- Inspect the Network panel. Look for an
OPTIONSrequest. If one appears, check whether it succeeded and whether the actual request followed it. - Compare requested and allowed values. For a preflight, check the requested method and headers against the server’s corresponding allow headers, as well as the allowed origin.
- Check the actual response. Even when the API returns a successful HTTP status, the browser will not share the response with JavaScript unless that response passes the CORS check, including an appropriate
Access-Control-Allow-Origin. - Check credentials separately. If the request uses cookies or other credentials, verify the caller’s credentials setting, the server’s credential and origin headers, and whether browser cookie policy permits the cookie.
Page JavaScript receives only a generic failure rather than detailed CORS diagnostics. MDN notes that “CORS failures result in errors but for security reasons, specifics about the error are not available to JavaScript.” Use the browser developer console and Network panel to identify the failure instead of trying to catch a more informative CORS exception in application code. MDN’s CORS guide explains the browser messages and request flow.
Configure the API for the access you intend
Public resources without credentials
If a resource is intentionally public to browser code on every origin and does not use credentials, the server may return Access-Control-Allow-Origin: *. Apply that permission only to the resources that need it; it does not make an API private or secure.
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Restricted resources
For an API intended for particular sites, validate the incoming Origin against a trusted allowlist and return the matching allowed origin only when it passes validation. Do not blindly copy any caller-supplied Origin into the response. Limit CORS permissions to the methods and headers the resource needs. MDN’s practical CORS guidance covers restricting origins and resources.
Requests that include credentials
Fetch credentials default to same-origin. To request credentials cross-origin, the caller must opt in, for example with credentials: "include". The server must return Access-Control-Allow-Credentials: true and an explicit matching Access-Control-Allow-Origin; * cannot authorize a credentialed response. Preflight requests themselves do not include credentials, but the preflight response must permit credentials for the actual credentialed request to proceed.
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These headers do not override cookie rules. SameSite attributes and browser third-party-cookie policies can still prevent a cookie from being sent. CORS permission and cookie delivery are separate checks. See MDN’s Fetch API guide for the Fetch credentials behavior.
Dynamic origin selection and caching
If the server selects an allowed origin dynamically, include Vary: Origin in the response. It tells intermediary caches that the response can differ depending on the request’s Origin, helping prevent a response authorized for one origin from being reused for another.
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Setting mode: "no-cors" does not grant page JavaScript access to a blocked API response. It produces an opaque response whose headers and body are unavailable to the script, and it restricts which methods and headers the request can use. Fix the server’s CORS response when browser code is meant to read the API result.
CORS does not replace API security
CORS controls whether browser JavaScript can read a cross-origin response; it does not replace authentication, authorization, or CSRF defenses. Some cross-origin requests can still be sent even when the browser withholds their responses. The server must enforce access controls for sensitive operations independently of its CORS policy. The Fetch Standard and MDN’s CORS documentation describe the browser’s response-sharing model.
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