Port 139: NetBIOS Session
Port 139 carries Windows file sharing over the older NetBIOS layer.
What runs on port 139
Before Windows 2000, SMB file sharing ran over NetBIOS sessions on TCP 139. Modern systems use 445 directly but keep 139 for compatibility with old clients. Samba on Linux and NAS devices listens on both.
On a home network
Windows PCs, NAS boxes, routers with shared USB storage and some media players may listen on 139. It usually appears alongside 445.
If port 139 is reachable from the internet
139 should never be reachable from the internet. It exposes the same file-sharing service as 445, including its logins and share lists, and was a classic route for worms and data theft in the early 2000s. Many ISPs block it.
How to check whether it is open on your network
A port scan of your NAS or PCs will show 139 next to 445 if legacy sharing is on. Many NAS settings let you disable SMB1 and NetBIOS. Confirm no router rule forwards it.
To find out whether port 139 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.