Port / TCP
9090Prometheus
The Prometheus server's web UI and query API for scraped metrics.
If it is internet-facing: Port 9090 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Trusted network only
Transport
Cleartext
Risk if public
Elevated
Exposure verdict
Port 9090 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 containers & platform ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- Prometheus has no built-in authentication; anything reachable can run PromQL against every series it holds.
- The targets and configuration pages describe your whole infrastructure inventory.
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 9090 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 9090 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 9090 -j DROPIf you do need it open to everyone
sudo ufw allow 9090/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 = :9090'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:9090 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 9090 <host>A version probe confirms whether Prometheus is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 9090The quick yes/no when you only need to know whether a path exists through the firewall.