Port 3389: RDP
Port 3389 is Remote Desktop Protocol, used to control a Windows PC's screen over the network.
What runs on port 3389
Windows Remote Desktop listens on TCP 3389, and newer versions also use UDP 3389 for better performance. It lets you log in to a Windows machine and use its desktop remotely. Only Pro, Enterprise and Server editions of Windows can accept incoming Remote Desktop connections.
On a home network
A Windows Pro PC with Remote Desktop turned on listens on 3389. Most home devices do not. Small offices sometimes forward 3389 so staff can reach an office PC from home.
If port 3389 is reachable from the internet
RDP should not be reachable from the internet. It is one of the most common ways ransomware gangs get into small businesses, either by brute forcing weak passwords or by using stolen logins. In 2019 the BlueKeep flaw let attackers take over unpatched older Windows systems through RDP without any password. Put RDP behind a VPN or a gateway with multi-factor login.
How to check whether it is open on your network
Check your router for port forwarding rules pointing at 3389, or at a PC on some other outside port. RECON's Exposure Check tests whether RDP answers from outside, and a LAN port scan finds which PCs have Remote Desktop on.
To find out whether port 3389 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.