Understanding port 23: What It Is Used For, Risks, and How to Secure It

Port 23 is widely recognized as the default network channel for Telnet, a protocol that has powered remote command-line access since the early days of networking. Telnet enables an administrator or operator to log in to a device and issue commands as if they were physically present at the console. While Telnet and the port it uses are simple and familiar, they come with a big caveat: the data transmitted during a session, including usernames and passwords, travels in clear text. This makes port 23 a focal point for modern security assessments and a frequent topic of debate in network teams that balance legacy practicality with current best practices.

To understand why port 23 matters today, it helps to differentiate between a feature that once offered convenience and a risk profile that can expose sensitive information. In many organizations, Telnet is not the primary access method for day-to-day operations, yet it persists on certain devices or in older environments where upgrading or reconfiguring management interfaces is difficult. The goal of this article is not to demonize a protocol, but to explain its role, the associated risks, and the practical steps teams can take to reduce exposure while maintaining essential access where it is truly necessary.

What is port 23 and what is it used for?

Port 23 is the network port assigned to Telnet. When a device opens this port, it is listening for remote login attempts, typically over a TCP connection. The Telnet protocol provides a simple, text-based session where commands are sent in plaintext and responses are returned unencrypted. This simplicity is part of why Telnet emerged early in networking history, but it is also why port 23 is often flagged by security tools and best-practice guides today. In many environments, you will encounter Telnet on legacy equipment, or in specific lab or development scenarios where administrators need rapid, scriptable access without configuring SSH keys or advanced authentication mechanisms.

Historically, port 23 was used to manage network devices such as routers and switches, as well as servers and printers that offered remote management capabilities. In some cases, industrial control systems or older enterprise gear rely on Telnet interfaces for configuration tasks. The prevalence of port 23 in such contexts is a reminder of how network infrastructure evolves—some components simply lag behind the shift toward encrypted management channels. In practice, port 23 is a beacon of convenience that must be weighed against potential eavesdropping risks and credential theft if it remains reachable from untrusted networks.

Why port 23 is risky and what to watch out for

The core risk associated with port 23 is the absence of encryption. When a Telnet session is established, usernames, passwords, commands, and output traverse the network as plain text. Anyone capable of intercepting the traffic—whether a malicious actor on the same Wi-Fi, a compromised router, or a misconfigured firewall—can read or reuse that data. This makes port 23 a common target for attackers who scan for exposed Telnet services and attempt to gain unauthorized access through credential reuse, weak passwords, or default accounts.

Beyond eavesdropping, there are other security considerations. Telnet does not provide strong authentication by default, and session integrity is limited compared to modern protocols. If an attacker can insert themselves into a Telnet session or capture credentials, they may be able to move laterally within a network or exfiltrate sensitive information. Because of these factors, many security frameworks recommend disabling Telnet wherever possible and replacing it with more secure alternatives. The presence of port 23 on devices that face the internet or a broad corporate network is often a red flag for security teams to review configurations, patch levels, and access controls.

Best practices for securing or replacing port 23

• Audit exposure and disable Telnet where it is not essential. Start with a device-by-device inventory to identify which systems offer Telnet on port 23 and whether remote access is actually required. If possible, disable Telnet completely on critical devices and in public-facing segments.
• Use SSH or other encrypted methods instead of Telnet. SSH (usually on port 22) provides encryption and more robust authentication. Where feasible, migrate management access to SSH, and consider enabling key-based authentication and disabling password-based login.
• If Telnet must remain for legitimate reasons, isolate and protect it. Place Telnet access behind a dedicated, tightly controlled network segment (management VLAN), enforce strong ACLs to limit who can reach port 23, and require VPN access or jump hosts for remote sessions.
• Enforce strong authentication and logging. When Telnet is in use, enable rigorous logging, monitor login attempts, and ensure that credentials are unique and rotated. Even with Telnet, you should avoid default accounts and shared credentials.
• Keep firmware and software updated. Regularly update devices to mitigate vulnerabilities that might be exploited in Telnet-enabled services. Security patches can reduce the attack surface even if Telnet remains enabled on isolated segments.
• Perform periodic scans and reviews. Schedule routine network scans to detect open port 23 on devices, review firewall rules, and verify that Telnet is not exposed to the internet or to untrusted networks.

During audits, teams often find that port 23 is open on several devices, even in relatively modern environments. That finding should trigger a risk-based discussion about whether the exposure is justified, what compensating controls are in place, and what a realistic migration path looks like. It is often possible to retire Telnet in favor of more secure alternatives without sacrificing the operational needs of administrators who rely on remote management tools.

Alternatives and secure configurations for remote management

Across most organizations, the recommended path is clear: replace Telnet with encrypted options. SSH remains the gold standard for secure remote access, offering strong encryption, secure authentication, and a flexible feature set that supports non-interactive scripting, agent forwarding, and robust key management. When possible, disable port 23 entirely in favor of SSH on devices that support it. In environments where Telnet remains part of the workflow, consider wrapping access in an encrypted channel such as a VPN or a dedicated management tunnel to ensure the credentials and commands do not traverse the open internet in clear text.

Other secure management approaches include modern remote administration protocols that leverage TLS or dedicated management interfaces with strong authentication and role-based access controls. For some devices, you may also configure access through jump hosts or bastion servers, which centralize authentication and enable tighter monitoring and auditing of who accessed what, when, and from where. The key idea is to reduce the time any user spends on an unencrypted session and to minimize the number of devices reachable via port 23 from untrusted networks.

Practical steps for organizations to reduce risk

1. Conduct an asset inventory to identify every device that exposes Telnet or uses port 23 for management.
2. Run a network scan to determine if port 23 is open on endpoints that should not expose it publicly.
3. Create a plan to retire Telnet on non-essential devices and to migrate critical devices to SSH or other secure protocols.
4. Implement access controls that restrict who can reach port 23, using firewall rules and VLAN segmentation to isolate management traffic.
5. Deploy centralized authentication and logging for all remote management activity, with alerts for unusual login attempts.
6. Establish a clear upgrade path and timeline for legacy devices, including contingencies for devices that cannot be upgraded immediately.

In practice, many organizations find it effective to pair network segmentation with continuous monitoring. By limiting where Telnet is allowed to operate and by collecting logs from devices that still use port 23, security teams gain visibility and can respond quickly to suspicious activity. Even when Telnet remains temporarily, the combination of restricted access, encryption for adjacent channels, and rigorous auditing helps reduce the risk profile associated with port 23.

Conclusion

Port 23 remains a recognizable part of network management history, but it also embodies a clear trade-off: simplicity versus security. While Telnet and its default port can be convenient in controlled environments, the potential for credential theft and data exposure makes proactive management essential. By auditing exposure, migrating to encrypted alternatives like SSH, and applying strict access controls, organizations can preserve operational capabilities while strengthening their security posture. In the end, the prudent path is to minimize reliance on port 23, or at least to ensure that any remaining usage is tightly protected and well monitored. And for responsible operators, ongoing vigilance on port 23 will help keep networks safer as technology evolves and new management tools emerge.