BRev Port 3389 — RDP (Remote Desktop)
Port 3389/TCP is the Remote Desktop Protocol: a full interactive Windows session — desktop, clipboard, drive redirection, printers — delivered over one connection. It is genuinely useful, which is why the question “can I just forward 3389 on my router?” gets asked constantly, and why the answer has to be a flat no.
Our reference marks it never, and the evidence is not theoretical. Exposed RDP is the most common initial access vector in ransomware incidents year after year, by two routes: credential stuffing against accounts with no lockout and no MFA, and pre-authentication vulnerabilities in the protocol itself — BlueKeep (CVE-2019-0708) was wormable, and the CVE-2019-1181/1182 family followed. A newly exposed 3389 typically starts receiving login attempts within minutes of the address appearing in a scan feed.
The correct shapes are all “authenticate somewhere else first”: a VPN, so RDP is only reachable once you are on the network; a Remote Desktop Gateway, which fronts RDP with TLS on 443 and enforces policy; or a zero-trust broker. All of them mean 3389 is never exposed. Network Level Authentication should be on regardless — it forces authentication before a session is created, which is what neutered several of the pre-auth bugs.
Port 3389 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 3389 | Registered |
| Service | RDP | Windows Remote Desktop. (TCP) |
| Category | Remote access | shells, desktops and VPNs |
| Exposure | Never | Never expose to the internet |
Should port 3389 face the internet? Never bare — use a VPN or an RD Gateway. This is the single most common ransomware entry point, via credential stuffing and BlueKeep-class bugs.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
“The remote computer requires Network Level Authentication”
The server demands NLA and the client cannot supply it — an old client, a non-domain machine that cannot reach the domain controller to get a ticket, or credentials cached wrong. Fix the client rather than turning NLA off on the server: disabling it is exactly the configuration the pre-auth RDP vulnerabilities needed.
RDP works on the LAN and not through the firewall — and you are about to forward the port
This is the moment to stop. Port-forwarding 3389, or putting the machine in a DMZ, is the configuration behind a large share of ransomware cases. Use a VPN, an RD Gateway on 443, or Windows Admin Center. If a forward is genuinely unavoidable for a short window, restrict the source to a single known IP at the firewall — not a port change, which is trivially discovered.
Check port 3389 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 3389 — swap example.com for the host you are actually testing.
Is port 3389 reachable on another host?
nc -vz example.com 3389"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 3389 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 3389Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 3389A 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:3389Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 3389?
sudo ss -tlnp | grep :3389Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :3389The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :3389The 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 | |
|---|---|---|---|---|---|
| 3389 | tcp | RDP | Windows Remote Desktop.Never bare — use a VPN or an RD Gateway. This is the single most common ransomware entry point, via credential stuffing and BlueKeep-class bugs. | Never |
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
Is it safe to open port 3389 on my router?
No. This is the one exposure verdict in the whole reference that has essentially no safe configuration behind it — the combination of a high-value target, guessable credentials and a history of pre-auth remote code execution makes bare RDP indefensible. Put a VPN or an RD Gateway in front and leave 3389 closed at the perimeter.
Does changing RDP to a different port help?
Barely. It removes untargeted noise, but internet-wide scanners fingerprint the RDP handshake on any port, so exposed RDP is found regardless of the number. It also breaks every client shortcut. Spend the effort on a gateway instead.
What about RDP over UDP 3389?
RDP uses UDP 3389 as a transport accelerator for graphics alongside the TCP connection. It does not change the security picture — the same session, the same authentication — and blocking UDP only degrades performance, so it is not a mitigation.