BRev Port 139 — NetBIOS session service
Port 139 is SMB over NetBIOS — the way Windows file sharing worked before Windows 2000 introduced SMB directly on 445. It has not gone away: hosts routinely listen on both, older devices and NAS appliances often prefer 139, and Samba enables it by default in many configurations. That dual listening is the practical point, because a firewall rule that blocks 445 alone leaves the same shares reachable.
Historically 139 is also where null sessions lived: an anonymous connection with an empty username and password that, on older Windows, enumerated users, groups, shares and password policy. That is the classic reconnaissance step in every penetration test of a Windows network, and while modern versions restrict it heavily, legacy systems and misconfigured Samba still answer.
The verdict is never, on the same grounds as 445 and with an additional one: it depends on NetBIOS name resolution (UDP 137), which is broadcast-based, spoofable and leaks machine and workgroup names to anyone listening. On a modern network there is nothing 139 provides that 445 does not do better — if nothing needs it, disable NetBIOS over TCP/IP on the adapter and close both.
Port 139 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 139 | Well-known |
| Service | NetBIOS session | File sharing over legacy NetBIOS. (TCP) |
| Category | Infrastructure | the plumbing: DNS, directory, routing, monitoring |
| Exposure | Never | Never expose to the internet |
Should port 139 face the internet? The pre-445 SMB door. Same worm target, older code.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
You blocked 445 and shares are still reachable
Because 139 is still open and serving the same SMB. Any perimeter rule for Windows file sharing has to cover 137/UDP, 138/UDP, 139/TCP and 445/TCP together — the four are one service in practice, and partial rules give a false sense of having closed it.
An old NAS or printer only connects on 139
Common with pre-2000-era protocol support and cheap embedded firmware, and it usually also means SMBv1. Isolate the device on its own VLAN rather than enabling SMBv1 across the network — one appliance is not worth reintroducing the protocol EternalBlue exploited.
Check port 139 yourself
No browser can open a raw TCP socket, so no web page can honestly tell you whether this port is open on a remote host. These are the real commands, generated for port 139 — swap example.com for the host you are actually testing.
Is port 139 reachable on another host?
nc -vz example.com 139"succeeded!" means the TCP handshake completed — the port is open. "Connection refused" is closed; a hang until timeout means filtered (a firewall dropped it silently).
nmap -Pn -p 139 example.comThe only common tool that distinguishes open / closed / filtered. -Pn skips the ping sweep, so it still works against hosts that ignore ICMP.
Test-NetConnection example.com -Port 139Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 139A blank screen or a banner means it connected. "Connection refused" means closed. Quit with Ctrl-] then `quit`.
curl -v --max-time 5 telnet://example.com:139Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 139?
sudo ss -tlnp | grep :139Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :139The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :139The last column is the PID. Match it in Task Manager’s Details tab, or `taskkill /PID <pid> /F`.
No web page can tell you whether a port is open on a remote host
A browser cannot open a raw TCP socket — there is no API for it, by design. So a site that reports “port 3389 is OPEN on 203.0.113.10” is not checking from your browser; it is asking its own server to scan on your behalf. That works, but it means their server sees the host you asked about, scans from their address rather than yours, and answers a question about their network path, not yours.
We don't have a server here and won't pretend to. So this page does three honest things instead:
| Port | Proto | Service | What it is | Exposure | |
|---|---|---|---|---|---|
| 139 | tcp | NetBIOS session | File sharing over legacy NetBIOS.The pre-445 SMB door. Same worm target, older code. | Neverbrowser-blocked |
http://host:port/ and times it. A connection that is established proves a socket is open; an instant refusal usually means nothing is listening; anything else is honestly reported as “can't tell”. Each attempt gives up after 2.5s.nc -vz example.com 443
nmap -Pn -p 443 example.com
telnet example.com 443
curl -v --max-time 5 telnet://example.com:443
This page cannot answer that, and neither can any other page — from your browser. Opening a TCP connection to an arbitrary host and port is not something JavaScript is allowed to do. Every “open port checker” that gives you a green tick is running the scan on their server. Which is fine, as long as you know what you are agreeing to: they learn which host and port you were worried about, and the answer they give you is about the route from their data centre — not from the internet as your users see it.
What to do instead
Run the check from outside your own network. Copy an nmap or nc line from the card above and run it from a machine that is not behind the same router — a phone on mobile data with a terminal app, a cheap VPS, a colleague's laptop. Running it from inside the LAN tests the LAN, not the internet, and will happily say “open” while the world sees nothing.
Then check the thing that is actually blocking it, in this order: the service is bound to0.0.0.0 and not 127.0.0.1 · the host firewall (ufw, firewalld, Windows Defender Firewall) · the cloud security group or the router's port-forward rule · and finally your ISP, which commonly blocks 25, 80 and 445 on residential lines whatever you configure.
What a browser genuinely can verify
One thing: that an HTTPS service answers. If what you actually want to know is “is my site up and how fast is it”, the latency test measures real round trips to a CORS-open URL. If the name doesn't resolve, or resolves somewhere unexpected, that is a DNS problem, not a port problem — and it is the more common cause of “my port is closed” than an actual firewall.
An IP address finds the machine; a port picks which program on it gets the traffic. TCP sets up a connection first — a handshake, ordered delivery, retransmission — which is why the web, email and SSH use it. UDP just sends the packet with no handshake and no guarantee it arrives, which is why DNS, video calls and games use it: late data is worse than lost data.
That difference is why a UDP port is so much harder to test. TCP either completes a handshake or it doesn't, so a tool can tell you. A UDP service that has nothing to say to an unexpected packet stays silent — and silence looks identical to a firewall dropping it.
The three ranges
0–1023Assigned by IANA to core services. On Unix, binding one needs root (or CAP_NET_BIND_SERVICE).1024–49151Registered by vendors for their applications. Any user can bind these — which is why every dev server lives here.49152–65535Never assigned. The OS hands these out as the SOURCE port of your outgoing connections. (Linux actually uses 32768–60999 by default.)Closed is not the same as filtered
Closed means the host answered: the packet arrived, nothing is listening, and you got an immediate refusal (a TCP RST). Filtered means nothing answered at all — a firewall dropped your packet in silence, so you wait for a timeout and learn nothing. The difference matters: closed proves the host is reachable and the port is free; filtered tells you only that something between you and it is discarding traffic.nmap reports the two separately. A browser cannot distinguish them, which is why this page reports a timeout as “can't tell” rather than guessing.
The one habit worth keeping
Bind services to 127.0.0.1 instead of 0.0.0.0 unless something outside the machine genuinely needs them, and reach them over SSH or a VPN. Every database on the never-expose list above ships with that advice, and the leaks all begin the same way: a default bind address, a cloud instance with a public IP, and a scanner that found it within the hour.
The blocked-port list this page uses is the WHATWG Fetch Standard's “bad port” table — the ports every browser refuses to connect to (Chrome calls it ERR_UNSAFE_PORT). Local probing is additionally gated by Chrome's Local Network Access permission, which replaced Private Network Access and ships in Chrome 142.
FAQ
Should I block port 139?
At the perimeter, always — together with 137, 138 and 445. Internally, disable NetBIOS over TCP/IP if nothing legacy needs it; modern Windows file sharing works entirely over 445.
What is a null session?
An anonymous SMB connection with an empty username and password. On older Windows it enumerated users, groups, shares and password policy without any credential — a standard reconnaissance step, and the reason 139 is treated as a leak as well as an attack surface.
Do I need 139 if 445 is open?
Almost never. Modern clients use 445 directly, and 139 exists for legacy NetBIOS-dependent systems. Turning it off narrows the surface with no practical loss on a current network.