Today we’re speaking with Rupert Marais, our in-house Security Specialist, whose expertise in endpoint and device security is especially relevant given recent events. A critical, decade-old vulnerability in a common Telnet server has suddenly surfaced, impacting hundreds of thousands of devices from industrial controllers to office printers. This flaw isn’t complex; it’s a simple authentication bypass that grants attackers immediate control, turning a long-forgotten protocol into a clear and present danger for networks across every sector.
A critical authentication bypass flaw in GNU InetUtils, CVE-2026-24061, remained undiscovered for over a decade. Why do simple yet powerful bugs like this persist in widely-used open-source tools, and what does its easy exploitability mean for defenders now that it’s public?
It’s astonishing, but not entirely surprising. Bugs like this often hide in plain sight within foundational, open-source utilities that everyone assumes are stable and secure. InetUtils has been around for ages, and the focus for security researchers often shifts to newer, more complex technologies. This particular flaw, introduced back in 2015, is a simple argument injection. It’s the kind of thing that gets missed in code reviews when people aren’t specifically looking for it. Now that it’s public, its simplicity is its most dangerous feature. As one penetration tester, Shivam Bathla, put it, he was blown away by how easy it was to exploit. For defenders, this means the race is on against attackers who need virtually no skill to gain complete control of a device.
With around 800,000 telnet instances exposed globally, often on IoT and OT devices, what are the biggest hurdles for security teams in inventorying these vulnerable systems? Could you detail a practical, step-by-step approach for discovering which printers, PLCs, or controllers on their network are affected?
The single biggest hurdle is the “out of sight, out of mind” nature of these devices. We’re talking about printers, VoIP phones, and even critical operational technology like building automation controllers and PLCs that were installed years ago and have been quietly doing their jobs ever since. Security teams often lack a complete, up-to-date inventory of every connected device, especially in complex manufacturing or healthcare environments. The first practical step is active network scanning to identify any device listening on the telnet port. Once you have a list, the real work begins: you must correlate those IP addresses with physical devices. Step two involves interrogating those devices to identify their manufacturer, model, and firmware version. Finally, the most challenging step is the supply chain investigation: contacting vendors to confirm if their product uses the vulnerable GNU InetUtils component, a process that, as Forescout’s research has shown, can take years.
Recent data indicates a concerning trend: Telnet usage has increased across every industry, even growing from 2% to 10% on government networks. What operational factors are driving organizations back to this insecure protocol, and what are the immediate consequences of this regression in security posture?
This trend is deeply troubling, and it’s driven by a reliance on legacy and embedded systems. As organizations expand their networks, particularly in sectors like government and manufacturing, they’re often deploying devices with older, embedded operating systems that were designed with Telnet as the primary means of remote administration. The encrypted alternative, SSH, is seeing a decline in use on these devices, likely because it’s not supported or is more complex to configure on older hardware. The immediate consequence is a massive expansion of the attack surface with an inherently insecure protocol. Every command, every password is sent in plaintext, making eavesdropping trivial. This regression means that even before this new vulnerability, these organizations were creating a perfect environment for attackers to move laterally and compromise sensitive systems.
This vulnerability highlights a deep supply chain challenge, as organizations may not know which of their devices contain the flawed component. Beyond waiting for vendor patches, what proactive containment strategies and compensating controls should security teams implement today to mitigate their immediate risk?
Waiting for vendors is not a viable strategy; it’s a recipe for disaster. The most immediate and effective control is network segmentation. Identify every device running Telnet and isolate it. High-risk devices, like PLCs in a manufacturing plant, should be placed in a separate network zone with strict firewall rules that only allow access from trusted administrative workstations. You absolutely must ensure none of these devices are exposed directly to the internet. Another critical control is to disable the telnetd server wherever possible. If it must remain active for operational reasons, then restrict network access to its port to only trusted IP addresses. These actions won’t fix the underlying flaw, but they create crucial barriers that can stop an attacker in their tracks.
Telnet was reportedly the 10th most attacked protocol last year, primarily through brute-forcing. How will a simple authentication bypass vulnerability like this change threat actor tactics? Please elaborate on the new attack scenarios you anticipate and the types of targets they will likely prioritize.
This vulnerability completely changes the game. Previously, attacking Telnet was a noisy, effort-intensive process of brute-forcing usernames and passwords, which could trigger security alerts. Now, attackers can bypass authentication instantly and silently with a single, specially crafted command. We will see a shift from brute-force scripts to targeted, automated exploits that simply scan for vulnerable instances and take them over. I anticipate attackers will prioritize targets that offer the most leverage. For example, compromising a building’s automation controller could allow them to disrupt HVAC systems in a data center or a hospital. Taking over network equipment like routers and switches could enable them to intercept or redirect traffic across the entire organization. This isn’t about just compromising one device; it’s about using these forgotten, insecure endpoints as a silent gateway deep into the corporate network.
Do you have any advice for our readers?
My advice is to treat this as a wake-up call. The era of connecting a device to the network and forgetting about it is over. You need to assume that every device, no matter how insignificant it seems, can be a potential entry point for an attacker. Develop a robust asset inventory program so you know exactly what is on your network. Vigorously pursue network segmentation to contain breaches when they inevitably happen. And finally, challenge the status quo. If a protocol like Telnet is active, ask why. In almost every case, it shouldn’t be running at all, and your organization’s security depends on turning it off for good.
