CORS Header Inspector: Free Online CORS Response Header Checker — How-To Guide

Try it free: CORS Header Inspector

CORS errors are some of the most common — and most confusing — problems web developers hit. Your frontend looks fine, your API responds in tools like curl or Postman, yet the browser blocks every request with a cryptic console message. Nine times out of ten, the culprit is a misconfigured CORS response header. The CORS Header Inspector gives you a fast, free way to review those headers for the three problems that cause most failures: wildcard origins, credential mismatches, and missing preflight support.

What the tool does

The CORS Header Inspector takes the CORS response headers your server sends and runs standards-based checks against them. It flags risky or incorrect configurations — like Access-Control-Allow-Origin: * combined with credentials, or an Access-Control-Allow-Headers list that doesn't cover the headers your client actually sends — and presents the findings in plain language you can act on.

It's a first-pass local tool, not a replacement for your project's CI or infrastructure scanner. Use it for quick reviews, debugging notes, code-review prep, and QA handoffs. It needs no account, works in your browser, and lets you copy or download a text report of the results.

Step-by-step: how to use it

1. Collect your response headers. In Chrome or Firefox DevTools, open the Network tab, trigger the request, click it, and look at the Response Headers section. Copy the headers starting with Access-Control- (and optionally the full set).
2. Paste them into the inspector. Open the CORS Header Inspector and paste your header text into the input area.
3. Run the inspection. Click the run button and review the findings — each one points at a specific header and explains what looks wrong or risky.
4. Fix, re-paste, re-check. Update your server configuration, then repeat with the new headers until the findings come back clean.
5. Copy or download the report. Save the output as documentation for the pull request or share it with whoever owns the server config.

3 practical tips

• Test the failing request's exact headers. CORS behavior depends on the specific endpoint, method, and custom headers involved. Inspect the headers from the real failed request in DevTools, not from a generic OPTIONS probe against a different path.
• Check both the preflight and the actual response. A preflight OPTIONS response can allow a header that the real GET/POST response then omits — the browser rejects the request anyway. Inspect both sets and make sure they agree.
• Treat * plus credentials as an instant fail. Browsers reject Access-Control-Allow-Origin: * whenever Access-Control-Allow-Credentials: true is set. If your app needs cookies or authorization headers, the server must echo the specific origin. The inspector flags this combination immediately.

FAQ

Q: Does the CORS Header Inspector upload my input?

A: No. The normal workflow runs locally in your browser — your headers are never sent to a server.

Q: Can it test my server live?

A: No — it's an inspector, not a live scanner. You paste response headers you've captured (for example, from DevTools) and it analyzes them. For live probing and load testing, use your project's own infrastructure scanner.

Q: What's the difference between this and the CORS Header Generator?

A: The generator builds header values you can drop into your server config; the inspector reviews headers you already have to catch mistakes. Use them together: generate, deploy, then inspect to confirm.

Q: Can I save or share the results?

A: Yes. After running the inspection, copy the output or download it as a text report — handy for code review or a QA handoff.

Privacy note

The CORS Header Inspector runs 100% in your browser. Your pasted headers are processed locally and never leave your device — there's no upload, no account, and nothing stored server-side.

If CORS errors are eating your afternoon, stop guessing: try the CORS Header Inspector free and let it point at the header that's breaking your app.