Legacy Protocol Crisis: Telnet & SSO Flaws Resurface in Critical Infrastructure Attacks

The cybersecurity landscape is witnessing a troubling resurgence of attacks targeting fundamental but long-deprecated protocols, exposing critical gaps in patch management and legacy system maintenance. Two distinct but thematically linked threats—a critical flaw in GNU Inetutils' telnetd server and the weaponization of unpatched Single Sign-On (SSO) gateways—are being actively exploited in automated campaigns against enterprises and critical infrastructure operators. This dual-pronged offensive underscores a systemic failure to retire insecure technologies and maintain rigorous security hygiene across complex, hybrid environments.

The Telnet Renaissance: A Protocol That Refuses to Die Securely

Telnet, the plaintext remote access protocol largely abandoned in favor of SSH since the early 2000s, remains stubbornly present in operational technology (OT), industrial control systems (ICS), and niche legacy applications. Its persistence has now been weaponized through the discovery of CVE-2025-46873, a critical authentication bypass vulnerability in the telnetd server component of GNU Inetutils. This widely distributed software package is present in many Linux distributions and embedded systems.

The flaw is alarmingly straightforward: by sending a specially crafted sequence of packets during the connection negotiation phase, an unauthenticated remote attacker can completely bypass the login prompt. Successful exploitation grants the attacker a root shell on the vulnerable system, providing immediate, privileged access without requiring credentials. This represents a worst-case scenario for any internet-facing service, but is particularly catastrophic for OT/ICS environments where Telnet is often used for remote maintenance of critical equipment like PLCs, HMIs, and SCADA systems. These systems frequently lack host-based intrusion detection and are rarely included in standard vulnerability scans, making them ideal targets for initial access and lateral movement.

SSO Gateways: The Unpatched Backdoor

Parallel to the Telnet threat, a separate automated attack wave is exploiting a known but unpatched vulnerability in SSO implementations on enterprise firewall and VPN gateways. The flaw, originally disclosed and patched by major vendors in 2023, exists in the SSO authentication relay process. Attackers are leveraging automated scanners to identify internet-facing gateways that missed the patch update. Once identified, they exploit the vulnerability to bypass the SSO mechanism entirely, gaining unauthorized access to the internal network without valid credentials.

This campaign highlights the 'patch gap'—the dangerous period between a patch's release and its actual implementation. For complex network appliances, patching often requires maintenance windows, compatibility testing, and change management approval, leading to delays that attackers are all too willing to exploit. The attacks are not sophisticated in terms of exploit development; they are sophisticated in their operational efficiency, systematically targeting the low-hanging fruit of neglected infrastructure.

Converging Threats and Strategic Implications

The simultaneous exploitation of these vulnerabilities is not coincidental. Both target authentication boundaries—the very gates meant to keep intruders out. Both prey on systemic failures in IT/OT asset management and patch deployment. For threat actors, especially ransomware groups and state-sponsored actors targeting critical infrastructure, these flaws offer a reliable, low-effort method for gaining a foothold.

The strategic implication is clear: the attack surface is expanding backwards into technological history. Security teams focused on cutting-edge threats in cloud and zero-trust architectures must also maintain vigilance over the 'digital archaeology' present in their own networks—the forgotten servers, the 'if it ain't broke, don't touch it' industrial controllers, and the network appliances running on outdated firmware.

Mitigation and Response: A Return to Security Fundamentals

Addressing this crisis requires a back-to-basics approach:

1. Immediate Inventory and Disablement: Conduct an emergency audit to identify all systems running Telnet services, especially on TCP port 23. Where possible, disable and remove Telnetd entirely, replacing it with SSH with key-based authentication and network access controls.
2. Patch Management Overhaul: Verify the patch status of all perimeter security appliances, especially firewalls and VPN gateways with SSO capabilities. The existence of an automated exploit campaign means the vulnerability is in widespread use; prompt patching is no longer preventative but reactive containment.
3. Network Segmentation and Monitoring: Isolate legacy protocols and systems that cannot be immediately retired into tightly controlled network segments. Implement robust network monitoring for anomalous traffic, particularly connection attempts to legacy service ports from unexpected sources.
4. Vendor Accountability: For OT/ICS vendors still incorporating Telnet or other insecure protocols by default, organizations must demand secure alternatives and include protocol retirement clauses in procurement contracts.

Conclusion: The Cost of Technological Debt

The current wave of attacks exploiting Telnet and SSO flaws is a stark reminder that cybersecurity risk is cumulative. Technological debt—the decision to defer upgrading or replacing outdated systems—accrues interest in the form of vulnerability. As these incidents demonstrate, that interest is now being called in by adversaries with automated tools. The community's response must be to finally retire the protocols and practices that have been known liabilities for decades, moving beyond awareness to decisive action. The security of critical infrastructure depends not only on defending against the attacks of tomorrow, but on finally closing the door on the vulnerabilities of the past.