Internet-Security.com TCP port 591 details

FileMaker Pro’s Web Companion (FileMaker Pro/Pro Unlimited versions 4–6) commonly used TCP port 591 for its built-in web publishing service; browsers connected to http://host:591/ to access FileMaker databases via CDML.
This was used on classic Mac OS, early Mac OS X, and Windows deployments where FileMaker provided its own HTTP service instead of (or alongside) port 80.
Historically, attackers scanned port 591 for exposed FileMaker Web Companion services; multiple early‑2000s vulnerabilities (e.g., directory traversal and buffer overflows) allowed data disclosure or remote compromise over this port.

TCP 591