Port / TCP
6000X11
The X Window System display server; additional displays occupy 6001 and upward.
If it is internet-facing: Finding 6000 open to the internet is a finding in its own right — it warrants an explanation, not just a rule.
Exposure
Localhost only
Transport
Cleartext
Risk if public
High
Exposure verdict
Port 6000 should be bound to 127.0.0.1. Nothing outside the host needs to reach it, and binding to 0.0.0.0 is the single change that turns it into an incident.
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- An X server that accepts remote connections lets a client read every keystroke and capture the screen.
- Use SSH X11 forwarding, which tunnels the display rather than opening the port.
Recommended firewall rule
Deny at the edge and bind to loopback
A firewall rule is the second line here. Fix the bind address in the service configuration so the listener never leaves the host.
ufw
sudo ufw deny 6000/tcpiptables
sudo iptables -A INPUT -p tcp --dport 6000 -j DROPIf you do need it open to everyone
sudo ufw allow 6000/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 = :6000'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:6000 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 6000 <host>A version probe confirms whether X11 is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 6000The quick yes/no when you only need to know whether a path exists through the firewall.
The safer path
port 22Move to port 22 — SSH / SFTP
SSH / SFTP does the same job with the session protected from the first byte, so there is no plaintext phase for an on-path attacker to read or strip. Migrating is usually a client configuration change plus a firewall swap.