CORS Checker

The CORS Checker inspects how a web server responds to cross-origin requests by examining the Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, and Access-Control-Allow-Credentials response headers. Cross-Origin Resource Sharing (CORS) is the browser security mechanism that decides whether JavaScript running on one origin is permitted to read responses from a different origin. This tool sends preflight-style and simple requests so you can see exactly which headers a target returns and whether your front-end will be allowed to talk to that API.

Under the hood, the checker performs an OPTIONS preflight request (the same one browsers send before non-simple requests like PUT or requests with custom headers) and a regular request, then surfaces the relevant headers it gets back. It flags risky configurations such as a reflected or wildcard Access-Control-Allow-Origin combined with Access-Control-Allow-Credentials: true, a combination browsers reject and that often signals a misconfigured proxy. Because it reports the raw header values, you can confirm whether a specific origin, method, or header is actually whitelisted.

Common use cases include debugging the classic 'blocked by CORS policy' console error, verifying that an API gateway forwards CORS headers correctly, and auditing third-party endpoints before integrating them into a single-page app. Front-end developers reach for it when a fetch() call works in Postman but fails in the browser, since that gap is almost always a CORS header problem rather than a server error. It also helps confirm that credentialed requests with cookies will be honored.

Practical tips: test with the exact origin your app uses, including the scheme and port, because http://localhost:3000 and http://localhost:3001 are distinct origins to CORS. Remember that a wildcard origin cannot be used alongside credentials, so if you need cookies you must echo back a specific origin. Pair this check with an HTTP Headers tool or a security headers audit to get the full picture of how an endpoint is configured.