# WHATPORT(1503)

## NAME

port 1503 — T.120 data conferencing (imtc-mcs) — T.120 conferencing — in practice Microsoft NetMeeting's application sharing, whiteboard, chat, and file transfer.

Protocol: TCP  
IANA registry

## DESCRIPTION

ITU-T T.123 specifies TCP 1503 as the transport port for the T.120 multipoint data conferencing stack, and IANA registers it as "imtc-mcs" to Databeam, one of T.120's authors. The only implementation most people ever met is Microsoft NetMeeting, which used 1503 for program sharing, whiteboard, chat, and file transfer while H.323 audio/video signalling ran on 1720. Microsoft carried the same stack into Windows Live Messenger's sharing features, and both products are long dead. A listener on 1503 today means legacy conferencing software on an old Windows box.

## EXPOSURE

**Do not expose.** T.120 program sharing hands a remote party control of the desktop, with authentication that is per-conference at best — this belongs on no internet-facing host, and the software behind it has been unpatched for over a decade.

## CHECK

```sh
nmap -sV -p 1503 TARGET
```

## SEE ALSO

- spec — [ITU-T Recommendation T.123 — Network-specific data protocol stacks for multimedia conferencing](https://www.itu.int/rec/T-REC-T.123) (itu.int)
- docs — [[MS-SDP]: Application Sharing and Whiteboarding](https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-sdp/b4c65407-d899-4096-b150-d4eb856cac25) (learn.microsoft.com)
- iana — [IANA port registry — 1503](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=1503) (iana.org)

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HTML: <https://whatport.net/1503/> · JSON: <https://whatport.net/1503.json> · full dataset: <https://whatport.net/ports.json>