Port / TCP
990FTPS control
Implicit-TLS FTP control channel: the session is encrypted from connect, unlike the AUTH TLS upgrade on port 21.
If it is internet-facing: Port 990 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Behind a gateway
Transport
Encrypted by default
Risk if public
Elevated
Exposure verdict
Port 990 should sit behind a reverse proxy, VPN, or source allow-list rather than being forwarded at the perimeter. Publishing it directly puts an administrative or internal interface on the open internet.
The session is protected from the first byte, so there is no plaintext phase for an on-path attacker to strip or read.
Assigned by IANA through a review process. On Unix systems a process needs root or the CAP_NET_BIND_SERVICE capability to bind here, which is why containers so often remap these to a higher port.
Grouped with the other file transfer & shares ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
1 note- Firewalls cannot inspect the encrypted control channel to open data ports, so passive ranges must be configured by hand.
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 990 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 990 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 990 -j DROPIf you do need it open to everyone
sudo ufw allow 990/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 = :990'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:990 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 990 <host>A version probe confirms whether FTPS control is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 990The quick yes/no when you only need to know whether a path exists through the firewall.