Port 23 / TCP
Telnet
Port 23 – Telnet Protocol Specification
Technical analysis, security advisories, vulnerability mitigation, and firewall syntax for TCP/UDP port 23 (Telnet).
Port Number
23
Transport Protocol
TCP
Port Range Type
Well-Known (0-1023)
Default Status
Restricted / LAN
Service Description & Architectural Context
Legacy unencrypted terminal emulation communication protocol.
IANA / RFC Designation:
Port 23 is assigned to Telnet under the Internet Assigned Numbers Authority (IANA) service name registry. In enterprise infrastructure and homelab setups, services listening on this port must follow the principle of least privilege.
Security Analysis & Attack Surface Assessment
CRITICAL: Transmits credentials in cleartext. Deprecated in favor of SSH (22).
Hardening Recommendations:
- Do not expose port 23 to public WAN interfaces unless authenticated TLS or VPN wrapping is strictly enforced.
- Enforce rate-limiting using Fail2ban, CrowdSec, or cloud edge DDoS protection.
- Audit active listening sockets locally:
ss -tulpn | grep :23ornetstat -ano | findstr :23.
Firewall Rule Snippets for Port 23
UFW (Ubuntu / Debian Linux)
sudo ufw allow 23/tcp
iptables (Standard Linux Kernel)
sudo iptables -A INPUT -p tcp --dport 23 -j ACCEPT
Windows PowerShell (Defender Advanced Firewall)
New-NetFirewallRule -DisplayName "Allow Port 23" -Direction Inbound -LocalPort 23 -Protocol TCP -Action Allow