Port / TCP
2376Docker API (TLS)
The Docker remote API secured with mutual TLS client certificates.
If it is internet-facing: Port 2376 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 2376 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.
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
1 note- Certificate verification must be mutual: without client certs, TLS only hides an equally total level of access.
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 2376 proto tcpiptables
sudo iptables -A INPUT -p tcp -s 10.0.0.0/8 --dport 2376 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 2376 -j DROPIf you do need it open to everyone
sudo ufw allow 2376/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 = :2376'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:2376 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 2376 <host>A version probe confirms whether Docker API (TLS) is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 2376The quick yes/no when you only need to know whether a path exists through the firewall.