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

TCP port 23 is best known as the default channel for Telnet, a protocol that dates back to the early days of networking. Telnet allows a user to establish a text-based session with a remote device, typically for administration or troubleshooting. However, Telnet and its associated port 23 have fallen out of favor in modern networks because the data they send is unencrypted. As a result, traffic can be read, credentials can be stolen, and attackers can maneuver within a network if Telnet services are exposed. This article explores the fundamentals of TCP port 23, the risks it poses, and the practical steps organizations and individuals can take to reduce exposure while maintaining legitimate remote access where necessary.

What is TCP Port 23 and Telnet?

TCP port 23 is the network endpoint used by Telnet to establish a management session. Telnet itself is a simple, text-based protocol that transmits commands, responses, and user credentials in plain text. Because nothing in Telnet is encrypted, anyone who can observe the network traffic between a client and a Telnet server can read the exact commands typed by the user, along with any passwords entered during login. While Telnet can be convenient for quick remote access to devices such as routers, switches, or legacy equipment, this convenience comes with a cost: weak protection for sensitive information.

In practical terms, a device listening on TCP port 23 is offering a remote shell. This makes it possible for an administrator to manage the device from someone’s desk or from a centralized management system. Yet the same capability also makes port 23 an attractive target for attackers who survey networks for unencrypted management services. Because Telnet does not provide confidentiality or integrity guarantees, it is rarely appropriate for modern security environments unless it is isolated and tightly controlled.

Why TCP Port 23 Is a Growing Concern

The main concern with TCP port 23 is the lack of encryption. When credentials and configuration data move over Telnet, they are exposed to eavesdropping, packet capture, and man-in-the-middle attacks. This risk increases when devices with Telnet enabled are connected directly to the internet or placed on poorly segmented networks. In many organizations, legacy devices still rely on Telnet because they were designed years ago and lack modern remote management alternatives. The result is a fragile balance between legacy compatibility and modern security standards.

Beyond encryption, there is also the issue of default credentials and weak authentication. Telnet sessions often rely on simple username/password configurations, which can be reused across devices or easily guessed in the face of automated attack tools. When attackers gain access to a Telnet session, they can issue commands, extract configuration details, and sometimes pivot to other parts of the network. For these reasons, exposing TCP port 23 to untrusted networks should be avoided whenever possible.

Threats and Vulnerabilities Linked to Port 23

Understanding the types of threats that target TCP port 23 helps in prioritizing defense efforts. The most common risks include:

Plaintext credentials and data: Telnet transmits user names and passwords in an readable form, making it easy for anyone listening to capture sensitive information.
Credential stuffing and brute-force attempts: Automated tools can try numerous username/password combinations against Telnet services, increasing the chance of unauthorized access.
Non-repudiation gaps: Without robust logging and tamper-evident records, it can be difficult to prove who performed certain actions in a Telnet session.
Lateral movement: Once an attacker gains Telnet access to one device, they may use that foothold to reach other systems within the same network segment.
Outdated or misconfigured devices: Many embedded devices, industrial controllers, and older servers still expose port 23 with weak or no security controls.

Because Telnet does not provide integrity protection, attackers can tamper with the session or inject commands in transit under certain circumstances. Combined with weak authentication and exposed networks, these factors create a compelling case for rethinking how remote management is performed on systems that listen on port 23.

Assessing Exposure: How to Know If Port 23 Is a Risk

Effective risk management starts with visibility. Organizations should:

Inventory devices: Identify all devices that expose TCP port 23 on internal networks and the internet.
Evaluate usage: Determine whether Telnet is actively used for management and if there are modern alternatives in place.
Check authentication: Review login methods for Telnet, including whether strong, unique credentials are enforced.
Assess network segmentation: Ensure that any remaining Telnet services are isolated from untrusted networks and limited to a controlled management network.
Review logging and monitoring: Verify that Telnet activity is logged and monitored for unusual login attempts or commands.

For teams responsible for security posture, conducting a controlled audit and applying a risk-based prioritization helps decide whether to retire Telnet entirely or to reign in its exposure with strict controls. While high-fidelity scanners can detect open port 23, interpretation should focus on the context—which devices are involved, what data might be exposed, and how access is limited.

Best Practices: Reducing Risk Without Disrupting Operations

When a network still relies on Telnet, or when legacy devices must be managed, several best practices can minimize risk while preserving essential functionality:

Disable Telnet from the internet: If a device must be reachable remotely, ensure that port 23 is not exposed directly to the internet. Use a VPN or jump host to provide controlled access.
Segment management networks: Place Telnet-enabled devices on isolated, well-monitored segments that have strict firewall rules and access controls.
Replace or upgrade devices: Where possible, upgrade to devices that support SSH or modern remote management protocols with encryption and strong authentication.
Use SSH instead of Telnet: SSH (typically on port 22) provides encryption, integrity, and strong authentication, reducing the risk of credential theft and session hijacking.
Enforce strong authentication: If Telnet must exist, require complex, unique credentials and consider multi-factor authentication where feasible, though note that MFA support for Telnet is uncommon and often limited.
Apply access controls: Use ACLs, firewall rules, and device-specific whitelisting to restrict Telnet access to trusted hosts and management workstations only.
Audit and monitor: Enable centralized logging, monitor for abnormal Telnet login patterns, and implement alerting to detect potential compromises early.
Document change management: Maintain records of any changes to Telnet configurations, including which devices still rely on it and why it remains in use.

Shadowy or ad-hoc deployments of Telnet can be equally risky. Governance, risk, and compliance programs often require a clear sunset plan for port 23, with a concrete timeline for removing Telnet in favor of secure protocols wherever possible.

Alternatives: What to Use Instead of Telnet

The recommended path for most organizations is to replace Telnet with secure remote access technologies:

SSH (Secure Shell): The standard replacement for Telnet in most environments. SSH provides encryption, integrity, and strong user authentication, making remote administration far safer.
VPN-based access: For devices that require remote reach, a VPN can create a secure tunnel to a management network before any remote session begins.
Zero-trust and jump hosts: Implement access through a controlled intermediary host that enforces authentication and auditing before connecting to target devices.
Modern management protocols: Where available, use vendor-specific secure management interfaces and protocols that support encryption and modern authentication mechanisms.

Transition planning should consider device compatibility, downtime, and testing. In many cases, a phased migration, starting with the most exposed devices, yields the best balance between security and continuity.

Operational and Compliance Considerations

From an operational perspective, retiring port 23 can reduce attack surface and simplify security monitoring. Compliance frameworks that require data protection, integrity, and confidentiality often favor encryption-enabled management channels. Maintaining out-of-date, plaintext management services is typically incompatible with modern security standards. Documented risk assessments, asset inventories, and interdepartmental coordination support a smoother migration path away from Telnet.

IoT deployments add another layer of complexity. Many IoT devices still rely on Telnet due to legacy constraints or vendor limitations. In such cases, isolating these devices, applying strict network access controls, and prioritizing vendor updates or replacements can reduce risk without impacting essential device management.

Conclusion: Proactive Management of TCP Port 23

TCP port 23 and Telnet represent a relic of early networking that remains visible in some environments today. While they can offer a straightforward path to remote device management, the lack of encryption and the tendency for weak authentication make Telnet a high-risk choice for modern security programs. By increasing visibility, applying strict access controls, and prioritizing secure alternatives such as SSH and VPNs, organizations can dramatically reduce the dangers associated with port 23. A thoughtful, phased approach—starting with inventory, then hardening or retiring Telnet—provides a practical route to improved security without sacrificing essential management capabilities.

In the end, the goal is not to fear TCP port 23 itself but to manage the exposure responsibly. With deliberate policy, updated devices, and strong cryptographic channels, Telnet can become a historical footnote rather than a lingering vulnerability in today’s network architectures.