Port / UDP
1812RADIUS authentication
RADIUS authenticates network access requests from Wi-Fi controllers, VPN concentrators, and switches.
If it is internet-facing: Port 1812 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Trusted network only
Transport
Encryption optional
Risk if public
Elevated
Exposure verdict
Port 1812 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
Encryption is available but negotiated or configured rather than guaranteed. A client that does not insist on it will silently fall back to plaintext.
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 directory & auth ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- Only the password attribute is obscured, with a shared-secret MD5 construction; the rest of the packet is cleartext.
- RadSec (TCP 2083) wraps the protocol in TLS where the path is not trusted.
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 1812 proto udpiptables
sudo iptables -A INPUT -p udp -s 10.0.0.0/8 --dport 1812 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 1812 -j DROPIf you do need it open to everyone
sudo ufw allow 1812/udpWrite 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 -ulpn 'sport = :1812'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 -iUDP:1812macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 1812 <host>A version probe confirms whether RADIUS authentication is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 1812The quick yes/no when you only need to know whether a path exists through the firewall.