Port / TCP
3000Development server (common)
Not an IANA assignment but a de facto default: Node, Next.js, Rails, and Grafana all land here during development.
If it is internet-facing: Port 3000 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Localhost only
Transport
Cleartext
Risk if public
Elevated
Exposure verdict
Port 3000 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 web & proxies ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- Dev servers expose source maps, stack traces, and hot-reload endpoints that were never meant to be public.
- Bind to 127.0.0.1 rather than 0.0.0.0 so a coffee-shop network cannot browse your work in progress.
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 3000/tcpiptables
sudo iptables -A INPUT -p tcp --dport 3000 -j DROPIf you do need it open to everyone
sudo ufw allow 3000/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 = :3000'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:3000 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 3000 <host>A version probe confirms whether Development server (common) is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 3000The quick yes/no when you only need to know whether a path exists through the firewall.