---
title: "520 Web Server Returned an Unknown Error — what it means and how to fix it"
description: "Cloudflare reached the origin server but got back a response it could not make sense of. What causes an HTTP 520, what to try as a visitor, and what to check if it is your own site."
url: https://atoneplace.net/error/520/
site: "At One Place"
---

# HTTP 520 — Web Server Returned an Unknown Error

**HTTP 520: Web Server Returned an Unknown Error**

Cloudflare reached the origin server but got back a response it could not make sense of.

## What 520 actually means

This is not a standard HTTP status code — it is Cloudflare's, and it appears only on sites behind Cloudflare. It is the catch-all for "the origin answered, but with something empty, malformed or otherwise unexpected": a connection reset mid-response, headers that are too large, or a reply that is not valid HTTP. Because it is a catch-all, 520 is the least specific of Cloudflare's errors and usually needs the origin logs to resolve.

## Common causes

- The origin server reset the TCP connection mid-response
- The origin returned an empty response
- Response headers exceeding Cloudflare's size limit, often from too many cookies
- A malformed response that is not valid HTTP
- The origin's application crashing part-way through generating a response
- A firewall at the origin dropping Cloudflare's connections inconsistently

## If you're just trying to view the page

1. Reload after a moment — 520s are often intermittent
2. Nothing else on your side; this is between Cloudflare and the site's own server
3. Report it to the site owner, including the Cloudflare Ray ID shown on the error page

## If it's your site

1. Check the origin's error logs at the timestamp on the error page; the Ray ID lets you correlate exactly
2. Reduce response header size — a large number of cookies is the most common trigger
3. Confirm the origin is not resetting connections; look for crashes or `RST` packets in a `tcpdump`
4. Whitelist Cloudflare's published IP ranges at the origin firewall
5. Bypass Cloudflare temporarily (test against the origin IP directly) to confirm whether the origin is at fault

## Reference

- **Status code:** 520
- **Reason phrase:** Web Server Returned an Unknown Error
- **Category:** 5xx — Server Error
- **Cacheable by default:** No

## Common questions

### What does HTTP 520 mean?

Cloudflare reached the origin server but got back a response it could not make sense of. This is not a standard HTTP status code — it is Cloudflare's, and it appears only on sites behind Cloudflare. It is the catch-all for "the origin answered, but with something empty, malformed or otherwise unexpected": a connection reset mid-response, headers that are too large, or a reply that is not valid HTTP.

### How do I fix a 520 error?

Reload after a moment — 520s are often intermittent

### Is 520 a client error or a server error?

520 is in the 5xx range, which means server error. The request was acceptable but the server could not fulfil it, so retrying later can succeed.

## Related pages

- [500 Internal Server Error](https://atoneplace.net/error/500/)
- [502 Bad Gateway](https://atoneplace.net/error/502/)
- [503 Service Unavailable](https://atoneplace.net/error/503/)
- [504 Gateway Timeout](https://atoneplace.net/error/504/)
- [524 A Timeout Occurred](https://atoneplace.net/error/524/)
- [521 Web Server Is Down](https://atoneplace.net/error/521/)
- [522 Connection Timed Out](https://atoneplace.net/error/522/)
- [525 SSL Handshake Failed](https://atoneplace.net/error/525/)
- [526 Invalid SSL Certificate](https://atoneplace.net/error/526/)
- [530 Origin DNS Error](https://atoneplace.net/error/530/)
- [501 Not Implemented](https://atoneplace.net/error/501/)
- [511 Network Authentication Required](https://atoneplace.net/error/511/)

## Sources

- Hypertext Transfer Protocol (HTTP) Status Code Registry — IANA (https://www.iana.org/assignments/http-status-codes/)
- RFC 9110: HTTP Semantics — IETF (https://www.rfc-editor.org/rfc/rfc9110.html)