Preflight Triggers

• Any header except: Accept, Accept-Language, Content-Language, Content-Type (with restrictions)
• Content-Type other than: application/x-www-form-urlencoded, multipart/form-data, text/plain
• Methods: PUT, DELETE, PATCH, or any custom method
• ReadableStream in request body
• Event listeners on XMLHttpRequest.upload
• One trigger = preflight; simple requests skip OPTIONS entirely

Credentials Mode

• Access-Control-Allow-Origin: * incompatible with credentials—must specify exact origin
• Access-Control-Allow-Credentials: true required for cookies/auth headers
• Fetch: credentials: 'include'; XHR: withCredentials = true
• Without credentials mode, cookies not sent even to same origin for cross-origin requests

Wildcard Limitations

• * doesn't match subdomains—*.example.com is invalid, not a pattern
• Can't use * with credentials—specify origin dynamically from request
• Access-Control-Allow-Headers: * works in most browsers but not all—list explicitly for compatibility
• Access-Control-Expose-Headers: * same issue—list headers you need to expose

Origin Validation

• Check Origin header against allowlist—don't reflect blindly (security risk)
• Regex matching pitfall: example.com matches evilexample.com—anchor the pattern
• null origin: sandboxed iframes, file:// URLs—usually reject, never allow as trusted
• Missing Origin header: same-origin or non-browser client—handle explicitly

Vary Header (Critical)

• Always include Vary: Origin when response depends on origin—even if you allow only one
• Without Vary: CDN/proxy caches response for one origin, serves to others—breaks CORS
• Add Vary: Access-Control-Request-Headers, Access-Control-Request-Method for preflight caching correctness

Exposed Headers

• By default, JS can only read: Cache-Control, Content-Language, Content-Type, Expires, Last-Modified, Pragma
• Custom headers invisible to JS unless listed in Access-Control-Expose-Headers
• X-Request-ID, X-RateLimit-*, etc. need explicit exposure—common oversight

Preflight Caching

• Access-Control-Max-Age: 86400 caches preflight for 24h—reduces OPTIONS traffic significantly
• Chrome caps at 2 hours; Firefox at 24 hours—values above are silently reduced
• Cached per origin + URL + request characteristics—not globally
• Set to 0 or omit during development—caching hides config changes

Debugging

• CORS error in browser = request reached server and came back—check server logs
• Preflight failure: server must return 2xx with CORS headers on OPTIONS—404/500 = failure
• Opaque response in fetch: mode: 'no-cors' succeeds but response is empty—usually not what you want
• Network tab shows CORS errors; Console shows which header is missing

Common Server Mistakes

• Only setting CORS headers on main handler, not OPTIONS—preflight fails
• Setting headers after error response—CORS headers missing on 4xx/5xx breaks error handling
• Proxy stripping headers—verify headers reach client, not just that server sets them
• Access-Control-Allow-Origin: "*", "https://example.com"—must be single value, not list

Security

• Don't reflect Origin header blindly—validate against allowlist first
• Private Network Access: Chrome requires Access-Control-Allow-Private-Network: true for localhost access from public web
• CORS doesn't prevent request from being sent—just blocks response reading; server still processes it
• Sensitive endpoints: don't rely on CORS alone; use authentication + CSRF tokens