Port 139 — NetBIOS Session
Quick reference for TCP/UDP port 139, used by NetBIOS Session. Includes what the service does, security guidance and related ports.
What is port 139?
NetBIOS session service.
What port does NetBIOS Session use?
NetBIOS Session listens on port 139 by default.
Security guidance
High risk — do not expose to the internet
Port 139 (NetBIOS Session) is high-risk because the service it carries is a frequent target for credential attacks, brute force or unpatched-vulnerability scanning. Keep it bound to localhost or your VPN, never forward it directly on a router, and require strong authentication.
Related ports
Frequently asked
What is port 139 used for?+
Port 139 is the standard TCP/UDP port for NetBIOS Session. NetBIOS session service.
What port does NetBIOS Session use?+
NetBIOS Session listens on port 139 by default.
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