CORS Cross Origin
Browser — 5 error codes
Reference list of 5 CORS cross origin with meanings, common causes, and related context.
About CORS Cross Origin
CORS (Cross-Origin Resource Sharing) errors occur when a web application attempts to fetch resources from a different origin, and the destination server's HTTP response lacks the necessary security headers to permit the request. Browsers aggressively enforce the Same-Origin Policy by blocking the frontend JavaScript from reading the response if the appropriate Access-Control- headers are missing or mismatched.
It is important to understand that a CORS error is a client-side enforcement block. The request often successfully reaches the server, and the server may fully process it. The failure happens when the browser inspects the response headers and decides it is unsafe to hand the data over to the requesting script. Resolving these errors always requires backend or proxy-level configuration changes.
This collection documents 5 specific cross-origin error conditions. Each entry details the exact console message, identifies the missing or invalid CORS header, and explains the server-side configuration required to satisfy the browser's preflight and response checks.
Showing 5 of 5 error codes
Credentials Not Supported — Wildcard origin '*' cannot be used when credentials mode is 'include'.
Missing Allow-Origin — No 'Access-Control-Allow-Origin' header is present on the requested resource.
Preflight Failed — Response to preflight request doesn't pass access control check.
Header Not Allowed — Request header field is not allowed by Access-Control-Allow-Headers in preflight response.
Multiple Allow-Origin — The 'Access-Control-Allow-Origin' header contains multiple values, but only one is allowed.
Frequently Asked Questions
cURL and backend tools do not enforce the Same-Origin Policy. Web browsers strictly enforce CORS to prevent malicious websites from making unauthorized requests on behalf of the user. The error is a client-side security block.
A preflight request is an HTTP OPTIONS request automatically sent by the browser before a 'complex' request (like a PUT, DELETE, or requests with custom headers). It asks the server if the intended operation is permitted.
No. CORS is enforced by the browser. You cannot write frontend code to force the browser to ignore a missing or mismatched Access-Control-Allow-Origin header. The fix must be implemented on the backend server or proxy.