Port 139: NetBIOS session
Legacy Windows file sharing over NetBIOS.
- Port
- 139
- Protocol
- TCP
- Service
- NetBIOS session
- Exposure
- Do not expose to the internet
What is port 139 used for?
Old SMB file sharing.
Is it safe to expose port 139 to the internet?
Never expose to the internet.
If port 139 seems closed or unreachable
- Check that the NetBIOS session service is running and listening on port 139, not only on 127.0.0.1.
- Check the firewall on the host itself (Windows Defender Firewall, iptables, nftables, ufw) for a rule that allows TCP 139.
- If the host is behind a router, confirm the forwarding rule points to the correct internal address and that your internet provider does not use carrier-grade NAT.
- Test from outside your own network. A port can be open locally and blocked at the edge, or the other way round.
Using port 139 safely
Treat port 139 as an internal-only service. Reach it through a VPN or an SSH tunnel, restrict it by source address, and keep the software that listens on it patched.
Check whether port 139 is open on your host
The check connects from our server, so it shows what the internet can reach.
Open the port checker