Mar 27, 2026 - For details about updated CVE-2025-53521 (BIG-IP APM vulnerability), refer to K000156741.

Forum Discussion

Philippe_Page_2's avatar
Philippe_Page_2
Icon for Cirrus rankCirrus
Jan 21, 2019

Error 521: Web server is down - Cloudflare generated error

I have an virtual server created with many websites hosted in it. It has one public IP assigned to it and is NATed to a private IP. The websites are accessible via HTTP and HTTPS. However one website...
application delivery
big-ip
cloudflare
LTM
security
JG's avatar
JG
Icon for Cumulonimbus rankCumulonimbus
Jan 22, 2019

"521" is a Cloudflare-specific error code, issued when its reverse proxy server's connection to the origin content server was refused. You might want to investigate the issue on the origin content server itself.