Port / TCP
3389RDP
Remote Desktop Protocol provides graphical remote sessions on Windows hosts.
If it is internet-facing: Finding 3389 open to the internet is a finding in its own right — it warrants an explanation, not just a rule.
Exposure
Trusted network only
Transport
Encrypted by default
Risk if public
High
Exposure verdict
Port 3389 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
The session is protected from the first byte, so there is no plaintext phase for an on-path attacker to strip or read.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other remote access ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- Internet-exposed RDP is the single most common initial access vector in ransomware incidents.
- Require Network Level Authentication, and place the service behind a VPN or an RD Gateway rather than a port forward.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 3389 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 3389 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 3389 -j DROPIf you do need it open to everyone
sudo ufw allow 3389/tcpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -tlpn 'sport = :3389'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iTCP:3389 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 3389 <host>A version probe confirms whether RDP is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 3389The quick yes/no when you only need to know whether a path exists through the firewall.
The safer path
no port changeChange the configuration, not the port
Publish RDP through a VPN or Remote Desktop Gateway; never forward 3389 at the perimeter.
The fix here lives in the service configuration rather than in a different port assignment.