Error 520: Web Server Returned an Unknown Error (Cloudflare)

Error 520 Web Server Returned an Unknown Error — Cloudflare got an empty, unknown or unexpected response from the origin server.

Class

5xx Server Error

Defined in

Unofficial (Cloudflare)

Official

No — Cloudflare-specific

Cacheable by default

No (unless Cache-Control/Expires say so)

Safe to retry

Yes, after a delay (respect Retry-After)

What does error 520 mean?

Error 520 "Web server is returning an unknown error" is a Cloudflare-specific status. Cloudflare reached your origin server, but the origin returned something Cloudflare couldn't interpret: an empty response, a connection reset, or a response with invalid or oversized headers.

It's a catch-all for unexpected origin behaviour, so the fix is always on the origin side.

When is 520 used?

• The origin crashes or resets the connection after Cloudflare connects.
• Response headers exceed Cloudflare's limit (32 KB total headers, often from huge cookies).
• The origin returns an empty reply, a malformed HTTP response, or HTTP/2 errors.
• A firewall or security plugin on the origin kills connections from Cloudflare IPs.

Common causes of error 520

• Application crash or PHP-FPM/Node process dying.
• Very large Set-Cookie or other headers.
• Origin firewall/rate limiting blocking Cloudflare IP ranges.
• Origin web server misconfiguration (e.g. wrong protocol on the port Cloudflare connects to).

How to fix 520 Web Server Returned an Unknown Error

If you're visiting the site

1. Reload in a minute; if it persists, the site owner needs to fix their server. There's nothing on your device causing it.

If you run the site or API (developers)

1. Bypass Cloudflare (curl -svo /dev/null --resolve example.com:443:ORIGIN_IP https://example.com/) to see what the origin really returns.
2. Check origin web-server and application error logs at the timestamps of the 520s.
3. Allow all Cloudflare IP ranges (cloudflare.com/ips) in your firewall and rate limiting.
4. Reduce header size (cookies) below 32 KB; check for responses with no body and no Content-Length.
5. Open a ticket with the Ray ID if the origin looks healthy.

Returning or handling 520: server examples

curl -sv --resolve example.com:443:203.0.113.10 https://example.com/ -o /dev/null
# Look for "Empty reply from server", resets, or huge headers

# Trust Cloudflare as proxy so logs/rate limits see the visitor IP
set_real_ip_from 173.245.48.0/20;   # …add all ranges from cloudflare.com/ips
real_ip_header CF-Connecting-IP;

Example 520 response

HTTP/2 520
server: cloudflare
cf-ray: 8d2f1a3b4c5d6e7f-AMS
content-type: text/html; charset=UTF-8

Related status codes

• 502 Bad GatewayA gateway or proxy got an invalid response from the upstream server.
• 521 Web Server Is DownCloudflare couldn't connect to the origin because it refused the connection.
• 500 Internal Server ErrorThe server hit an unexpected condition that prevented it from fulfilling the request.

Related tools

• HTTP status codes — full list
• HTTP headers reference
• cURL converter
• Mock API — fake REST endpoints with custom status codes
• Webhook tester — inspect incoming requests