CORS Error Troubleshooting

Match CORS error text with headers in your browser to pinpoint where the request gets blocked. No input is sent to a server.

Status

Runs in your browser. No input is sent to a server. Accuracy improves when you paste both error text and headers.

How to use

Paste console error text (CORS policy...) and request/response headers, then click “Troubleshoot”. Works with blank-line separated or merged input.

Notes (this tool)

  • This is a first-pass diagnostic. For final confirmation, compare OPTIONS and actual requests in browser Network tab.
  • Mask sensitive values such as Authorization or cookies before pasting.

About this page

What does this tool do?

Correlates CORS console errors from Chrome/Edge/Firefox with request/response headers and suggests likely root causes.

Acts as a symptom-first hub to quickly split preflight failures, origin mismatches, and credentials + wildcard mistakes.

Common symptoms

  • “No 'Access-Control-Allow-Origin' header” appears
  • “preflight request doesn't pass access control check”
  • “Request header field ... is not allowed by Access-Control-Allow-Headers”
  • “Access-Control-Allow-Origin must not be * when credentials flag is true”
  • “Redirect is not allowed for a preflight request” / “CORS request did not succeed”

Debugging workflow (recommended)

  • Paste full console error text and request/response headers
  • Check BLOCKED factors first in the diagnosis section
  • Verify preflight conditions (method/headers/status)
  • Cross-check Origin, Allow-Origin, Credentials, and Vary
  • Deep-dive individual values with related tools

Cases that are not actually CORS

  • DNS/TLS/certificate errors or mixed content (https page calling http)
  • OPTIONS blocked by API gateway, WAF, or CDN
  • Server-to-server calls are outside browser CORS enforcement

Recommendations (practical)

  • With credentials, return a concrete Access-Control-Allow-Origin (never *)
  • When Origin is dynamic, add Vary: Origin
  • Implement OPTIONS preflight route returning 2xx + required Allow-* headers
  • Explicitly include Authorization and other custom headers in Access-Control-Allow-Headers
  • CORS Diagnostic
  • CORS Response Inspect
  • Origin Allowlist Check
  • Host/Authority/Origin Inspect
  • Request Headers Parser
  • Response Headers Parser

What this tool does

  • Correlate CORS error strings with headers
  • Extract preflight failure hints (method/header/status)
  • Detect origin mismatch, credentials misconfig, and missing Vary
  • Suggest next checks and related tools

Operational notes

  • Final behavior can vary by browser implementation. Reproduce in multiple browsers.
  • Diagnosis depends on pasted data. Capture both OPTIONS and actual requests from Network tab.
  • Mask sensitive tokens before pasting.

Referenced specs

  • Fetch Standard (CORS protocol)
  • RFC 6454 (The Web Origin Concept)
  • MDN: CORS / CORS errors
  • Private Network Access (PNA)

FAQ

How do I tell if preflight is failing?

If Access-Control-Request-Method/Headers are present and OPTIONS is non-2xx, Allow-* is missing, or the error explicitly mentions preflight, preflight failure is likely.

Is Access-Control-Allow-Origin: * always bad?

It can be valid for public APIs, but it is invalid with credentials by spec. For authenticated endpoints, return concrete origins.

Why does it work server-side but fail only in browser?

CORS is a browser security restriction. It is usually not enforced on backend-to-backend calls, so failures appear only in browser flows.

References

  1. Fetch Standard: CORS protocol
  2. RFC 6454
  3. MDN: CORS
  4. MDN: CORS errors
  5. Private Network Access

Page-specific case studies

Start from concrete browser error messages to isolate CORS misconfiguration quickly.

  • For "No Access-Control-Allow-Origin", suspect missing headers in response path.
  • For "preflight did not succeed", inspect OPTIONS route and auth handling.
  • For credentials errors, validate Allow-Origin and Allow-Credentials combination.

Page-specific implementation checklist

  • Maintain runbooks mapped to common browser error messages.
  • Allow OPTIONS through both API gateway and app server.
  • Make server logs traceable by request-id for CORS failures.
  • Retest the exact scenario on real browsers after fixes.

These links are generated from site_map rules in recommended diagnostic order.

  1. CORS Diagnostic — Diagnose CORS decisions by comparing Origin and Allow-*
  2. CORS Response Inspect — Parse Access-Control-Allow-* headers to audit CORS responses
  3. Origin Allowlist Check — Match Origin values against an allowlist
  4. Host/Authority/Origin Inspect — Cross-check Host/:authority/Origin/Referer for mismatches
  5. CORS Checklist — Provide a step-by-step CORS verification checklist
  6. How to Diagnose CORS Preflight Failures — Fix preflight failures by validating OPTIONS responses, Allow-* directives, and origin rules in order
  7. How to choose CORS tools — Map preflight failures, origin mismatches, and credential conflicts to the right checks

CORS

Compare Origin and Allow-* headers to audit CORS decisions