139
Windows file sharing over NetBIOS (legacy SMB)
NetBIOS Session Service / SMB over NetBIOS
What is port 139?
Port 139 carries SMB (file and printer sharing) over the NetBIOS session layer, used by older Windows systems. It has been superseded by port 445, which runs SMB directly over TCP/IP. Extremely dangerous when exposed to the internet — it was a primary propagation vector for the WannaCry ransomware outbreak. This port must always be blocked at the internet perimeter.
Quick checks
$
nc -zv example.com 139
$
nmap -p 139 example.com
$
telnet example.com 139