Port 139 · NetBIOS Session

Port 139 is NetBIOS Session, TCP.

NetBIOS session service — legacy SMB over NetBIOS. Modern Windows uses SMB directly on 445.

Palo Alto

Create the service object for this port, then allow the App-ID on it. Restricting the rule to the port's service object keeps the rule working on a non-default port and stops the App-ID matching on other ports. Replace the placeholders with your own rule name and zones.

set service tcp-139 protocol tcp port 139

set rulebase security rules <rule-name> from <from-zone> to <to-zone> source any destination any application netbios-ssn service tcp-139 action allow

Risk and exposure

Rated medium risk. Restrict this port to the sources that need it and keep the service patched; avoid exposing it to the whole internet unless the service is meant to be public.

Check it

• Linux ss -tlnp | grep :139
• PowerShell Test-NetConnection <host> -Port 139
• nmap nmap -p 139 <host>

Related ports

• 445 SMB
• 42 WINS
• 137 NetBIOS Name Service
• 138 NetBIOS Datagram
• 19 Chargen
• 23 Telnet

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