520 Web Server Returned an Unknown Error

What it means

HTTP 520 is a non-standard status used by Cloudflare when the origin server returns something Cloudflare cannot make sense of — an empty reply, an unexpected response, a connection reset, or a violation of HTTP that does not fit a more specific 52x code. It is essentially Cloudflare's way of saying the origin behaved in a way it could not interpret.

When it happens

It happens when the origin server crashes mid-response, sends malformed headers, drops the connection, or returns an empty answer. Large headers or cookies and origin-side application crashes are common triggers.

How to fix it

• Check the origin server's error and access logs for crashes or malformed responses around the time of the error.
• Look for oversized response headers or cookies that may break the connection.
• Confirm the origin is up and responding correctly when requested directly, bypassing Cloudflare.
• Review server resource limits and application errors that could cause the origin to fail mid-response.

SEO impact

Damaging if sustained. To a crawler a 520 looks like a server error, so affected pages will not be indexed and repeated occurrences can slow crawling. Fix the origin promptly.