HTTP 520 Cloudflare — Web Server Returned an Unknown Error

Overview

Cloudflare error 520 means the origin web server returned an unexpected or empty response that Cloudflare could not process.

Key Details

• 520 is a Cloudflare-specific status code, not part of standard HTTP
• Cloudflare received a connection but got an invalid or empty response
• The origin server established TCP connection but sent no HTTP response
• Common with misconfigured origin servers or applications crashing on request
• Different from 522 (connection timeout) and 524 (timeout on response)

Common Causes

• Origin server crashed or sent a malformed response
• Origin server PHP/application error returning empty response
• Cloudflare IP addresses not whitelisted on origin server firewall
• Response headers from origin exceed Cloudflare's 32 KB header buffer
• Origin server closing the connection before sending a complete response

Steps

1. 1Check origin server logs (Apache/nginx error log) for crashes or errors at the time of the 520
2. 2Whitelist all Cloudflare IP ranges on the origin server firewall (cloudflare.com/ips)
3. 3Reduce response header size if exceeding 32 KB (large Set-Cookie headers are common)
4. 4Test origin directly (bypass Cloudflare) to see if the server responds correctly
5. 5Check if the origin application is crashing: PHP, Node, Python error logs

Tags

More in 5xx Server Error

Frequently Asked Questions