Unregistered, but the number is LDAP's 389 plus a 50000 offset, which is how unprivileged directory servers are usually run.
There is no IANA assignment for 50389; RFC 6335 puts it in the Dynamic/Private range. What the number does encode is a common convention: a directory server running as a non-root user cannot bind 389, so deployments shift the standard LDAP pair by a fixed offset — the same trick ApacheDS ships by default with 10389 and 10636. An open 50389 alongside 50636 is a strong hint of an offset LDAP/LDAPS pair, but it is a hint, not an assignment, and an LDAP search is the way to confirm it.
Do not expose
If it is LDAP, it is a cleartext directory service carrying simple-bind credentials and your whole identity tree; keep it internal and use the TLS port.
$ ldapsearch -x -H ldap://TARGET:50389 -s base -b '' namingContextsSeen 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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