← All Status Codes
521
Web Server Is Down
☁️ Cloudflare
Critical Risk
📖 What Is HTTP 521?
Cloudflare-specific. The origin web server refused the connection. Cloudflare cannot reach the origin at all.
🛡️ Security Implications
Origin may be under DDoS attack causing it to refuse connections, or the web server process has crashed.
🔍 Common Causes
Origin web server not running, firewall blocking Cloudflare IPs, server crashed, port 443/80 not listening.
🔧 How to Fix
Verify origin web server is running. Whitelist Cloudflare IP ranges in origin firewall. Check if the server process crashed.
🖥️ How to Check
curl -I -o /dev/null -w "%{http_code}" https://example.com