Understanding Port 23: Telnet’s Role, Risks, and Modern Alternatives

Port 23 is best known as the default TCP port used by Telnet. For many years, Telnet provided a straightforward way to log into remote machines and manage them from the command line. Today, the combination of a simple protocol and the lack of built-in encryption makes port 23 a topic of concern for security teams, network administrators, and IT leaders. This article explains what port 23 means, why Telnet remains in use in some environments, and how to approach securing or migrating away from it when appropriate.

What is Port 23?

Port 23 is a well-established entry point defined by the Internet Assigned Numbers Authority (IANA) for Telnet. In networking terms, a port is a logical endpoint for network connections, and port 23 signifies the service Telnet offers. Telnet itself is a protocol that facilitates bidirectional, text-based communication between a client and a server. When a user or administrator connects to a device via port 23, the session is typically established using Telnet, allowing remote command execution and configuration tasks.

The Telnet Protocol and How It Works

Telnet operates over TCP, which provides reliable data delivery. The Telnet session is text-oriented, and credentials, commands, and responses flow in plaintext. This means anyone who can intercept the traffic—whether on an insecure Wi-Fi network, through a compromised router, or via a malware-infected host—could potentially read usernames, passwords, and sensitive configuration details. The simplicity of Telnet—no strong encryption, minimal authentication, and straightforward command negotiation—made it appealing in the early days of networking. As networks grew and the value of secure remote management became clear, the inherent weaknesses of port 23 and Telnet became increasingly problematic.

Common Uses of Port 23 Today

Managing legacy devices: Some routers, switches, old servers, and industrial equipment still rely on Telnet for configuration because their hardware or firmware predates secure alternatives.
Testing and lab environments: In controlled training or testing setups, Telnet on port 23 can be useful for quick demonstrations or simulating remote access scenarios.
Legacy application interoperability: Certain older applications or integrations expect Telnet-based administration and continue to operate in environments where updates are impractical.

Security Risks Involving Port 23

The primary risk associated with port 23 and Telnet is the lack of encryption. Plaintext credentials can be captured by packet sniffers on the same network segment, making it easier for attackers to gain unauthorized access. In addition to credential exposure, Telnet sessions are vulnerable to eavesdropping, tampering, and replay attacks if not isolated within a trusted network. Misconfigured access controls or exposed Telnet services can broaden an attack surface, especially when devices are reachable from the internet. Given these risks, many organizations treat port 23 as a high-priority security concern and pursue mitigation strategies accordingly.

When Is Port 23 Still Useful?

There are scenarios where port 23 and Telnet might be acceptable or necessary. In air-gapped networks—networks without direct internet connectivity—Telnet can be deployed with strict controls and monitoring without exposing credentials over external networks. For some legacy equipment that cannot be upgraded, Telnet might remain the only feasible method to perform essential maintenance. In these cases, organizations typically implement compensating controls, such as isolating devices behind firewalls, restricting access to a minimum set of administrative hosts, and enabling robust logging and audit trails.

Modern Alternatives to Port 23 and Telnet

The mainstream and recommended alternative for remote management is SSH (Secure Shell), usually operating over port 22. SSH provides encrypted communications, secure authentication, and strong integrity checks, dramatically reducing the risk of credential theft and eavesdropping. In most modern networks, SSH has supplanted Telnet for remote access to servers, network devices, and management consoles. Other secure management options include:

Using SSH with key-based authentication rather than passwords
Enabling encrypted web interfaces (HTTPS) for devices that offer a web-based admin panel
Implementing VPNs or zero-trust access models to minimize exposure of administrative interfaces
Employing secure remote access gateways and bastion hosts to control entry points

Best Practices to Secure Remote Management

Disable Telnet on devices whenever possible and remove port 23 exposure to the broader network.
Replace Telnet with SSH for all remote command-line access; ensure SSH is configured with strong algorithms and key-based authentication.
Change default ports where feasible and implement port-knocking or access control lists to limit who can attempt to connect.
Enforce strict user authentication, restrict privileged accounts, and enable multi-factor authentication where supported.
Segment networks to keep management interfaces on separate, secured subnets and apply the principle of least privilege.
Log, monitor, and alert on all SSH and Telnet activity; review access logs regularly for unusual patterns.
Regularly patch firmware and software on devices that expose remote management services to reduce exploitable weaknesses.

Migration Strategies for Port 23-Dependent Environments

For organizations with devices that still rely on port 23, a thoughtful migration plan is essential. Start by inventorying all devices that accept Telnet connections, categorize them by risk, and determine which devices can be updated, replaced, or reconfigured to support SSH. Develop a phased timeline that prioritizes critical or exposed systems, ensure rollbacks, and test configurations in a controlled environment before broad deployment. If upgrading is not immediately possible, implement strict network segmentation, disable public exposure, and apply robust monitoring to detect unauthorized access attempts. Document policies and provide staff with training on secure remote management practices to reduce the chance of human error opening doors to attackers.

Conclusion

Port 23 has played a foundational role in remote administration, but the security landscape has evolved. Telnet’s association with port 23 and plaintext communication makes it ill-suited for untrusted networks or internet-facing management. While there are legitimate, limited-use cases for port 23 in tightly controlled environments, the prevailing guidance is to migrate to secure alternatives such as SSH and to harden any remaining access with layered protections. By prioritizing encryption, strong authentication, network segmentation, and vigilant monitoring, organizations can maintain effective remote management without compromising security. In short, port 23 should be a legacy concern rather than a daily practice, with a clear path toward safer, modern methods.