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// Directory & authentication

Port 1812: RADIUS

RADIUS authentication for Wi-Fi, VPN and network devices (1813 for accounting).

1812

How RADIUS works

Directory & authentication
userdirectoryAccess-RequestAccept userdirectoryAccess-RequestAccept

The client proves who it is on port 1812 and gets an answer (or a ticket) it can use with other services. It travels in clear text: anyone on the path can read it, passwords included.

Transport
UDP · TCP
Exposure risk
5/10 moderate if reachable from the Internet
Encryption
Clear textreadable on the network
IANA status
Assigned1 registered service
Range
Registered (1024-49151)
Category
Directory & authentication
01 Security

Is port 1812 dangerous?

Common attacks and weaknesses

  • Blast-RADIUS (CVE-2024-3596) forging responses with MD5.
  • Weak shared secrets.

How to secure it

  • Use RADIUS over TLS (RadSec) or require Message-Authenticator; use long shared secrets.
02 Registry

IANA registry entries for port 1812

ServicePortTransportDescriptionReferenceUpdated
radius 1812 TCP, UDP RADIUS RFC 2865 -

// source: IANA service name and port number registry

03 Check it

Which program uses port 1812, and is it reachable?

Linux

sudo ss -ltnup 'sport = :1812'      # process listening on 1812
sudo lsof -nP -i :1812

macOS

sudo lsof -nP -i :1812

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 1812 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 1812
netstat -ano | findstr :1812

From another machine

nmap -p 1812 -sV <host>
sudo nmap -sU -p 1812 <host>
nc -zv <host> 1812

Block it in the firewall

sudo ufw deny 1812/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 1812" -Direction Inbound -LocalPort 1812 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 1812 used for?

RADIUS authentication for Wi-Fi, VPN and network devices (1813 for accounting).

Is port 1812 TCP or UDP?

Port 1812 is registered for UDP and TCP. In practice the service mostly uses UDP.

Is it safe to open port 1812?

It is rated 5/10 (moderate) when exposed to the Internet. Blast-RADIUS (CVE-2024-3596) forging responses with MD5. Use RADIUS over TLS (RadSec) or require Message-Authenticator; use long shared secrets.

How do I check if port 1812 is open?

On Linux run "ss -ltnup 'sport = :1812'", on macOS "lsof -nP -i :1812", and on Windows "netstat -ano | findstr :1812" to see the local process. From another machine, "nmap -p 1812 <host>" shows whether it is reachable.

05 Related
:389 LDAP tcp · udp · high risk :636 LDAPS tcp · udp · moderate risk :88 Kerberos tcp · udp · high risk :3268 Global Catalog (LDAP) tcp · udp · high risk :31 MSG Authentication tcp · udp :49 Login Host Protocol (TACACS) tcp · udp :56 XNS Authentication tcp · udp :113 Authentication Service tcp · udp

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