How to Fix Cloudflare Error 520 (Web Server Returned an Unknown Error)
By Ava Collins
· About Zone Desire
Quick answer
Cloudflare error 520 means the website's origin server sent an empty, unknown or unexpected response to Cloudflare. Visitors should wait and retry; site owners should check server logs, firewall rules blocking Cloudflare IPs and header size.
What does “520 Web Server Returned an Unknown Error” mean?
520 is a Cloudflare-specific code, not an official HTTP status.
What causes it?
- Origin server crashed or reset the connection
- Firewall blocking Cloudflare IP addresses
- Headers or cookies too large
- Empty response from app
Steps to try
Stop when the issue is resolved.
- Visitors: reload after a few minutes.
- Site owners: check the origin server is running.
- Allow Cloudflare IP ranges in the server firewall.
- Check error logs for crashes at the same time.
- Reduce very large cookies or headers.
Need more support? Check the official help: MDN Web Docs: HTTP response status codes
FAQs
Is 520 Cloudflare's fault?
Usually the origin server is the cause, not Cloudflare.
What are 521 and 522?
521 means the origin refused the connection; 522 means it timed out.