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If a browser reports a CORS error only in production, inspect the production request and its responses before changing policy. A failed OPTIONS preflight, missing credential permission, or a response cached for the wrong origin can each explain the mismatch—but the error message alone does not identify the cause. Use the browser console and Network panel to find which transaction failed.
Why can CORS work locally but fail in production?
CORS is a browser-enforced response-sharing mechanism. A server sends headers that tell the browser whether code from a different origin may read a response. When those headers do not satisfy the browser, JavaScript commonly gets a generic network-style failure; the browser console gives more specific diagnostics. As MDN puts it, “The only way to determine what specifically went wrong is to look at the browser’s console for details.” MDN’s CORS guide explains the mechanism.
Local and production requests can differ in origin, method, request headers, credentials mode, redirects, and the systems handling the request. A local success therefore does not establish that production returns the same headers or follows the same path. Record the requesting page’s full origin (scheme, host, and port), target URL, method, requested headers, and whether the client includes credentials. Note whether all users are affected or only some; these observations help narrow the investigation without assuming a particular infrastructure cause.
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How to find which CORS transaction failed
- Reproduce in the failing production browser context. Open developer tools before reproducing. In Console, note the browser’s specific CORS diagnostic. In Network, preserve the log if navigation or redirects could otherwise hide earlier requests.
- Look for an
OPTIONSrequest. If present, inspect its status, any redirect, and response headers. A preflight is a separate browser request, made when the actual request uses a method, header, or content type outside the CORS safelist. A failed preflight can prevent the actual request from being sent. - Check the preflight’s requested permissions. The request may include
Access-Control-Request-MethodandAccess-Control-Request-Headers. The response must allow the requesting origin, the requested method, and the requested headers. Compare the names and values with what the browser actually sent rather than relying on local configuration. - Inspect the actual request and response separately. A successful preflight does not mean the actual response is readable. If the actual request was sent, check its status and CORS headers too. Error responses can also fail to carry the headers the browser needs.
- Compare what production returns for different origins. If the server selects an explicit
Access-Control-Allow-Originvalue based on the incomingOrigin, confirm the response also includesVary: Origin. Inspect responses from the production path; a cache or intermediary is a possibility to test, not a cause to presume. - Retest after a narrow server-side change. Use a fresh browser context and inspect the new responses. Browser preflight caching can delay a new preflight, so distinguish a cached permission result from the headers on the actual response.
Why might an OPTIONS preflight fail?
Preflight is an OPTIONS transaction that asks whether the browser may send a later cross-origin request. It is not the actual application request. The browser identifies the intended method with Access-Control-Request-Method and may identify non-safelisted headers with Access-Control-Request-Headers. The server’s preflight response needs to permit those items and the page origin.
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Credentials are not sent on the preflight itself. When the eventual request uses credentials, however, the preflight response must allow credentials before the browser sends that request. Check whether the server, middleware, or infrastructure handles OPTIONS differently from the actual method: a route may support the actual endpoint but reject or redirect the preflight. Some browsers do not follow redirects after preflighted requests, so inspect redirects rather than assuming they are harmless. MDN documents preflight behavior and redirect considerations.
How do credentials change the required CORS headers?
For a request that does not use credentials, an endpoint may allow any origin with Access-Control-Allow-Origin: *. For a credentialed request, such as one that needs cookies, the server must return the specific permitted origin—not a wildcard—and Access-Control-Allow-Credentials: true. That header’s valid value is true; it is not a list, and false is not a valid substitute. If credentials are not needed, omit the header and do not request credentials from the client.
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Keep the allowlist limited to origins that need access. Reflecting any supplied Origin or broadly enabling credentialed access can expose data to unauthorized origins and increase CSRF risk. CORS permission also does not override browser cookie policy: third-party-cookie restrictions may still prevent cookies from being sent or accepted even when the CORS response headers are correct. Treat cookie behavior and CORS permission as separate checks. MDN’s credentialed-request guidance describes these requirements; OWASP’s CSRF overview explains the separate security concern.
Can a cached preflight or CDN response hide a CORS fix?
There are two different caches to consider. Access-Control-Max-Age tells the browser how long it may reuse a preflight permission result. It does not cache the actual response headers in a shared HTTP cache. MDN documents a five-second default for preflight caching, a Firefox cap of 24 hours (86,400 seconds), and a Chromium cap of two hours (7,200 seconds) starting with Chromium 76. These are documented browser behaviors, not a guarantee that every browser uses the full advertised duration or that an intermediary cache behaves the same way. See MDN’s Access-Control-Max-Age reference.
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Separately, when the server chooses an explicit allowed origin dynamically, Vary: Origin tells HTTP caches that the response varies according to the request’s Origin. Without it, a shared cache may not distinguish responses intended for different origins. Its absence is a cache-correctness issue to investigate, not proof that a CDN served stale headers. Compare responses through the production route and, where possible, across requesting origins. A browser’s cached preflight result likewise does not prove that the actual response is configured correctly.
What should the server return?
| Request case | Relevant response behavior |
|---|---|
| Public request without credentials | Access-Control-Allow-Origin: * may be used if the endpoint is genuinely public to any origin. Do not request credentials for this case. |
| Request using credentials | Return the explicitly permitted origin and Access-Control-Allow-Credentials: true; a wildcard origin is not valid for credentialed access. |
| Preflight for a non-safelisted method or headers | The OPTIONS response must permit the requesting origin, requested method, and requested headers. If the actual request will use credentials, the preflight response must allow credentials. |
| Origin selected dynamically | Return the permitted explicit origin and include Vary: Origin on the relevant response so HTTP caches can distinguish origin variants. |
These headers grant browser access to responses; they are not a reason to allow more origins, methods, or headers than the application needs. For the normative header details, consult the MDN CORS guide.
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Why can a CORS error remain after changing headers?
- The browser has a usable preflight result cached. Check the preflight age and retest in a fresh browser context; do not assume a configuration edit immediately forces a new
OPTIONSrequest. - The actual response is still wrong. Confirm its headers independently. Fixing the preflight does not make the actual response readable by itself.
- An error or redirect follows a different path. Inspect status, redirects, and whether error responses or middleware-generated responses include the needed headers.
- The response varies by origin. Verify the explicit origin and
Vary: Origin, then compare what the production path returns for the relevant origins. - Cookie policy blocks credentials. Correct CORS headers do not guarantee that third-party cookies will be accepted or sent.
Should you use no-cors or simplify the request?
Use a simpler request only if the application can genuinely use a CORS-safelisted method, headers, and content type. Do not change request semantics just to avoid a preflight. The no-cors mode is not a general fix: it produces an opaque response that JavaScript cannot read. The normal correction is to configure the server to allow the specific origin and request the application requires. MDN’s CORS guide describes the opaque-response limitation.
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