520 Web Server Returned an Unknown Error

Cloudflare's catch-all: the origin returned something empty, malformed, or otherwise unintelligible.

Status

HTTP/1.1 520 Web Server Returned an Unknown Error

Details

Category: 5xx
Status: Unofficial (Cloudflare). Not registered with IANA. A server, framework, or CDN convention.
Specification: none, not defined by any RFC
Cacheable: Not cacheable
Response body: Allowed
Retry: Safe to retry

520 is what Cloudflare sends when the origin's response cannot be interpreted: an empty reply, a connection reset mid-response, oversized headers, or a violation of the HTTP framing rules. It is the 5xx equivalent of a shrug.

Because the cause is on the origin side, the useful diagnostic is always the origin's own logs, which Cloudflare cannot see. Requesting the origin directly, bypassing Cloudflare, is the fastest way to confirm.

Common causes

The origin returned an empty response or reset the connection mid-response.
Response headers exceeding Cloudflare's size limits, often from oversized cookies.
An origin crash or process restart during the request.
Headers or framing that violate HTTP, produced by a custom or misconfigured server.

How to fix it

As the client

Retry. 520s are frequently transient.
If you control the site, request the origin address directly to see the real response.

As the server

Check origin error logs for crashes and resets around the timestamps of the 520s.
Shrink response headers, especially Set-Cookie, which is the usual size offender.
Verify the origin speaks valid HTTP, including correct Content-Length and chunked framing.

Examples

GET / HTTP/1.1
Host: www.example.com

HTTP/1.1 520
Server: cloudflare
CF-RAY: 8a1c2f3d4e5f6789-IAD