Plex Media Server's web UI and API — someone on the network is running a media library.
Plex Media Server binds TCP 32400 for its web app (/web), its HTTP API, and all media streaming; the port is formally registered to Plex Inc. with IANA. It appears on scans of NAS boxes, home servers, and desktops, and Plex's remote-access feature deliberately maps it through the router via UPnP or a manual forward. An unauthenticated GET /identity returns the server's machineIdentifier and version, which is how you fingerprint one.
Expose with care
Plex is designed for internet exposure with account-based auth and relays, but it has shipped pre-auth vulnerabilities (CVE-2020-5741 among them), so expose it only on a current version with remote access managed by Plex rather than a hand-rolled forward.
$ curl -s -m 3 http://TARGET:32400/identitySeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.