---
title: Error 520 · Cloudflare Support docs
description: This error occurs when the origin server returns an empty, unknown,
  or unexpected response to Cloudflare.
lastUpdated: 2025-11-19T11:10:33.000Z
chatbotDeprioritize: false
source_url:
  html: https://developers.cloudflare.com/support/troubleshooting/http-status-codes/cloudflare-5xx-errors/error-520/
  md: https://developers.cloudflare.com/support/troubleshooting/http-status-codes/cloudflare-5xx-errors/error-520/index.md
---

## Error 520: web server returns an unknown error

This error occurs when the origin server returns an empty, unknown, or unexpected response to Cloudflare.

### Common causes

This error is often triggered by:

* Origin server crashes or misconfigurations.
* Firewalls or security plugins blocking [Cloudflare IPs](https://www.cloudflare.com/ips) at your origin.
* Headers exceeding 128 KB (often due to excessive cookies).
* Empty or malformed responses lacking an HTTP status code or response body.
* Missing response headers or origin web server not returning [proper HTTP error responses](https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml).
* Incorrect HTTP/2 configuration at the origin server.
* Authentication Origin Pull enabled on Cloudflare but the origin is [not configured as expected](https://developers.cloudflare.com/ssl/origin-configuration/authenticated-origin-pull/set-up/zone-level/#2-configure-origin-to-accept-client-certificates).

Note

`520` errors are prevalent with certain PHP applications that crash the origin web server.

### Resolution

Note

As a temporary workaround, you can set the affected DNS record to [DNS-only](https://developers.cloudflare.com/dns/proxy-status/) in the Cloudflare **DNS** app or [temporarily pause Cloudflare](https://developers.cloudflare.com/fundamentals/manage-domains/pause-cloudflare/).

* Contact your hosting provider or site administrator and share the necessary [error details](https://developers.cloudflare.com/support/troubleshooting/http-status-codes/cloudflare-5xx-errors/#required-error-details-for-hosting-provider) to assist with troubleshooting. Request a review of your origin web server error logs for crashes and check for [common causes](https://developers.cloudflare.com/support/troubleshooting/http-status-codes/cloudflare-5xx-errors/error-520/#common-causes) mentioned in the previous section.

* If HTTP/2 is enabled at your origin server, ensure it is correctly set up. Cloudflare connects to servers who announce support of HTTP/2 connections via [ALPN](https://blog.cloudflare.com/introducing-http2). If the origin web server accepts the HTTP/2 connection but then does not respect or support the protocol, an HTTP `520` error will be returned. You can disable the [HTTP/2 to Origin](https://developers.cloudflare.com/speed/optimization/protocol/http2-to-origin/#disable-http2-to-origin) in **Speed** > **Optimization** > **Protocol Optimization** on the Cloudflare dashboard.

* If `520` errors continue after contacting your hosting provider or site administrator, provide the following information to [Cloudflare Support](https://developers.cloudflare.com/support/contacting-cloudflare-support/):

  * Full URL(s) of the resource requested when the error occurred.

  * Cloudflare [**cf-ray**](https://developers.cloudflare.com/fundamentals/reference/cloudflare-ray-id/) from the `520` error message.

  * Output from `http://<YOUR_DOMAIN>/cdn-cgi/trace`.

  * Two [HAR files](https://developers.cloudflare.com/support/troubleshooting/general-troubleshooting/gathering-information-for-troubleshooting-sites/#generate-a-har-file):

    * One with Cloudflare enabled on your website.
    * Another with [Cloudflare temporarily disabled](https://developers.cloudflare.com/fundamentals/manage-domains/pause-cloudflare/).
