Prism Infosec have been following the recent disclosure of a critical Telnet vulnerability affecting the GNU InetUtils Telnetd server, which has refocused attention widely on the risks posed by legacy protocols that continue to exist within modern IT Infrastructure.
Tracked as CVE-2026-24061, the raised flaw allows an unauthenticated attacker the ability to gain root-level access by exploiting how the Telnet service passes environment variables to the system login process.
Despite Telnet being widely regarded as obsolete, security researchers have already observed active exploitation following public disclosure, demonstrating how quickly long-standing weaknesses can be weaponised once they become visible.
Telnet is known to persist across embedded systems, network appliances and Operational Technology (OT), sometimes including components that can support financial services and/or critical infrastructure.
These services can be overlooked during patching and vulnerability management activities, particularly where ownership is unclear or devices are assumed to be isolated. This discovery underlines how legacy protocols can present a disproportionate risk when asset visibility is incomplete and network exposure is not tightly controlled.
Telnet has been around for decades, and I have used it in controlled environments, such as capture the flag exercises and in the past it has allowed me to discover further issues with real-world systems. What this vulnerability demonstrates is that the presence of legacy services can often be underestimated, especially if they are left to lie dormant. When outdated protocols quietly persist in production environments, they become ideal entry points for modern attackers. This is a clear example of how unmanaged technical debt could potentially evolve into a serious operational and regulatory risk.
What can you do?
Organisations should confirm whether Telnet services are present anywhere within their estate, including on embedded devices, legacy infrastructure and third-party managed systems. Where identified, Telnet should be disabled or replaced with secure alternatives such as SSH, supported by strong authentication and restricted network access. Administrative interfaces should only be reachable from explicitly authorised management networks and if they are required to be publicly available, a form of multi-factor authentication should be implemented and required as a minimum.
Asset inventories should be reviewed to ensure they cover appliances and Operational Technology alongside traditional servers. As regulatory focus on operational resilience increases, unmanaged legacy protocols represent a preventable source of material risk.
