Understanding TCP/23: Telnet, Risks, and Modern Alternatives

What TCP/23 Represents

TCP/23 refers to the Telnet service bound to port 23 on servers and networking devices. Telnet is a simple, text-based protocol that enables a user to log into a remote system and run commands. It dates back to the earliest days of networking, when plain, straightforward text communication was enough to get systems under control. When people mention TCP/23, they’re talking about a session gateway for command-line access that, by design, transports data in clear text.

Why Telnet Remains on Port 23 in Some Environments

Despite the emergence of secure alternatives, Telnet still appears on port 23 in certain contexts. Some legacy applications, older network gear, or lab environments rely on Telnet for compatibility or convenience. In many organizations, devices such as routers, switches, or industrial controllers expose a Telnet interface for quick configuration. The absence of encryption makes these sessions relatively easy targets if they are reachable from untrusted networks. That is why port 23 is often discussed in security reviews and network hardening plans.

Security Risks Associated with Port 23

Unencrypted credentials: Telnet transmits usernames and passwords in clear text, which can be captured by anyone monitoring the traffic.
Man-in-the-middle attacks: Without encryption, an attacker on the network path can observe or modify the session.
Credential reuse and brute force: If devices rely on default or weak passwords on port 23, attackers gain access quickly.
Session hijacking: Active Telnet sessions can be intercepted and taken over if traffic is not protected.
Limited authentication controls: Telnet often lacks modern protections, such as robust multi-factor authentication, found in newer protocols.

Practical Use Cases and Risk Assessment

For organizations that still rely on port 23, a careful risk assessment is essential. A typical scenario involves remote management of legacy devices or a lab environment used for testing. Access is frequently restricted to a trusted management network. The risk increases when these devices are exposed to the internet or sit behind standard firewalls that fail to block port 23 by default. In such cases, automated scanners can discover Telnet services, and weak credentials can lead to unauthorized access. In practice, the business value of maintaining port 23 must be weighed against the potential for data exposure and downtime.

Best Practices to Secure or Eliminate TCP/23

Disable Telnet wherever possible: Remove the Telnet service and avoid enabling it on servers and network devices.
Prefer SSH for remote access: SSH provides encrypted communication, stronger authentication, and better session control.
Isolate management traffic: Place devices that still require Telnet behind a dedicated management network or VPN and restrict access with strict firewall rules.
Use strong authentication and access controls: Implement unique credentials, disable default accounts, and consider multi-factor authentication where supported.
Centralize authentication: Use RADIUS or TACACS+ for centralized control of access and monitor login attempts for anomalies.
Enable monitoring and alerting: Set up alerts when port 23 is exposed to untrusted networks or when suspicious login activity occurs.
Keep firmware up to date: If legacy devices must be managed via Telnet, ensure firmware is current and exposure is minimized.

Alternatives and Modern Management Practices

For most environments, SSH has become the standard replacement for Telnet. SSH secures data in transit with strong encryption, supports public-key authentication, and offers features like port forwarding and audit trails. In addition, virtual private networks (VPNs) provide a trusted channel for remote management, reducing the need to expose Telnet on port 23 to any network. Many organizations also rely on management bastion hosts or jump servers to centralize access to critical devices without leaving Telnet exposed.

Beyond SSH, consider using configuration management tools and out-of-band management platforms that centralize administration while limiting direct exposure of any single port. If Telnet remains temporarily enabled for a specific device, ensure it is disabled again after configuration and that access is tightly controlled and monitored. When feasible, implement encrypted alternatives or wrappers that encapsulate plain Telnet commands inside a secure tunnel.

How to Test and Monitor Port 23 Activity

Network discovery: Regularly scan your environment to detect open TCP/23 ports on endpoints and devices.
Access auditing: Log Telnet login events and monitor for repeated failures or odd login times.
Firewall discipline: Block port 23 from untrusted networks and restrict it to management subnets or VPN paths.
Configuration reviews: Periodically review device configurations to ensure Telnet is disabled where it is not required.

Conclusion: The State of TCP/23 Today

Port 23, home to Telnet, serves as a reminder that early networking choices can cast a long shadow. While Telnet made remote command execution possible decades ago, current security standards favor encrypted channels and rigorous access control. Most organizations should view port 23 as a deprecated path and plan a transition to secure alternatives like SSH, reinforced by network segmentation and strong authentication. For teams tasked with legacy devices, a deliberate, well-documented strategy that minimizes exposure remains essential, supported by proactive monitoring and policy enforcement. By staying informed about port 23 activity and prioritizing safer management practices, IT departments can protect critical infrastructure without sacrificing operational capability.