Critical Telnet Server Flaw Exposes Forgotten Attack Surface
Summary
A critical authentication bypass in the GNU InetUtils telnetd server (CVE-2026-24061) allows attackers to bypass authentication by injecting an “-f root” value into the USER environment variable. The bug was introduced in 2015 and fixed in InetUtils 2.8, but many devices remain unpatched. CISA added the flaw to its Known Exploited Vulnerability catalogue, and the Shadowserver Foundation estimates around 800,000 telnet instances are exposed on the public Internet. The protocol is commonly still present on legacy and IoT/OT devices (printers, networking kit, VoIP, building controllers), and Forescout notes telnet usage has increased across industries even as SSH adoption fell. Recommended mitigations include disabling telnetd, applying vendor patches when available, restricting access to trusted clients, and segmenting high-risk devices.
Key Points
- Critical auth bypass in GNU InetUtils telnetd (CVE-2026-24061) allows remote root access via a simple argument injection using the USER environment variable.
- The vulnerability dates back to 2015 and was patched in InetUtils 2.8, but many devices are likely unpatched or unpatchable.
- Shadowserver estimates ~800,000 telnet instances are exposed on the Internet, creating a large, neglected attack surface.
- Telnet use is rising in legacy IoT/OT devices and across industries (notably government); Forescout reports 4% of monitored devices still use telnet.
- Organisations face identification and patching challenges due to supply-chain issues and long vendor patch cycles.
- Immediate mitigation: do not run telnetd if possible; otherwise restrict port access, disable exposed telnet services, or use a custom login wrapper to block the “-f” parameter; network segmentation for high-risk devices is advised.
Why should I read this?
Look, if you manage networks, OT or IoT devices — this is the kind of old-school mess that’ll ruin your week. It’s easy to exploit, widespread, and hands attackers root access. Read it so you know what to turn off, patch or isolate first.
Author style
Punchy: this isn’t a dry CVE note — it’s a blunt warning about an ancient bug living in modern kit. If you care about the security of connected devices, the article spells out who’s vulnerable and what immediate steps to take. Read the detail and act.
Context and Relevance
The flaw highlights persistent risks from legacy protocols and supply-chain exposure. As telnet resurges in some sectors while secure alternatives decline, organisations must confront inventory gaps, vendor delays and long-lived embedded systems. This ties into broader trends in IoT/OT security: increasing device exposure, slow patching, and the need for segmentation and stricter access controls.