Error Code Portal
CloudflareCloudflare HTTP errorsError code

521

Error 521: web server is down

Alternative forms: Error 521

What does this code mean?

Code 521 · An issue reported by Cloudflare HTTP errors.

Official explanation · Original text

Error 521 occurs when the origin web server refuses connections from Cloudflare. Security solutions at your origin may block legitimate connections from certain Cloudflare IP addresses. The two most common causes of 521 errors are:

Troubleshooting and diagnostic links

First check that the document's product, version, and environment match your situation.

Source and terms of use

Error 521: web server is down ↗

Creator: Cloudflare, Inc. and contributors
License: Creative Commons Attribution 4.0 International
Checked on: 2026-11-12

Modifications: Selected introductory official description; Markdown stripped; original Korean product context added. Direct official error page retains the complete cause and resolution instructions.

Source details · Original licenses and notices