Port 139: NetBIOS-SSN
Port 139 is the NetBIOS Session Service, used by older Windows file sharing. Modern SMB uses port 445 directly, and port 139 is often disabled.
| Port number | 139 |
|---|---|
| Service | NetBIOS Session Service |
| Protocol | TCP |
| Category | Windows |
| Default encryption | No |
How do I check whether port 139 is open?
Port 139 carries TCP, so a connection either completes its handshake or it does not, which makes the check definitive.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | nc -vz example.com 139 | A TCP handshake either completes or it does not, so netcat gives a definitive answer in one line. |
| Same check on Windows | Test-NetConnection example.com -Port 139 | Built into PowerShell. TcpTestSucceeded in the output is the answer; telnet is not installed by default on modern Windows. |
| What is listening locally? | ss -tlnp | grep ':139 ' | Lists the process bound to port 139 on Linux. On macOS use lsof -nP -iTCP:139 -sTCP:LISTEN, on Windows netstat -ano | findstr :139 |
Replace example.com with the host you are testing. A blocked port and a port with nothing listening on it look identical from the outside, so if a service should be running, check locally before blaming the firewall.
Frequently asked questions
What is port 139 used for?
Port 139 is the NetBIOS Session Service, used by older Windows file sharing. Modern SMB uses port 445 directly, and port 139 is often disabled.
Is port 139 TCP or UDP?
Port 139 (NetBIOS-SSN) uses TCP.
Is port 139 secure?
Port 139 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 139 on my firewall?
Only if you specifically need NetBIOS-SSN. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.