Port / TCP
5900VNC
VNC shares a graphical desktop over RFB; additional displays listen on 5901, 5902, and upward.
If it is internet-facing: Finding 5900 open to the internet is a finding in its own right — it warrants an explanation, not just a rule.
Exposure
Trusted network only
Transport
Cleartext
Risk if public
High
Exposure verdict
Port 5900 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
Traffic crosses the network readable and modifiable. Anything sensitive on this port needs a tunnel, a VPN, or a different protocol.
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- The classic VNC authentication scheme is DES-based with an eight-character password limit, and many servers offer no authentication at all.
- Screenshots of exposed VNC desktops — including unlocked workstations — are indexed by public scanners.
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 5900 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 5900 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 5900 -j DROPIf you do need it open to everyone
sudo ufw allow 5900/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 = :5900'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:5900 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 5900 <host>A version probe confirms whether VNC is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 5900The 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
Tunnel VNC through SSH (-L 5900:localhost:5900) instead of exposing the display port.
The fix here lives in the service configuration rather than in a different port assignment.