CVE-2000-0268 in IOS
Summary
by MITRE
Cisco IOS 11.x and 12.x allows remote attackers to cause a denial of service by sending the ENVIRON option to the Telnet daemon before it is ready to accept it, which causes the system to reboot.
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Analysis
by VulDB Data Team • 04/21/2026
The vulnerability described in CVE-2000-0268 represents a critical denial of service flaw within Cisco IOS versions 11.x and 12.x operating systems. This weakness specifically targets the Telnet daemon implementation and exploits a race condition in the handling of the ENVIRON option during the Telnet connection establishment process. The vulnerability stems from insufficient input validation and improper state management within the network operating system's Telnet service implementation.
The technical flaw occurs when remote attackers send the ENVIRON option to the Telnet daemon before the system has fully initialized and is ready to process such options. This premature option transmission triggers an unhandled exception within the IOS kernel, causing the system to enter an unstable state that ultimately results in an automatic reboot. The vulnerability is particularly dangerous because it requires no authentication or privileged access, making it accessible to any remote attacker with network connectivity to the affected device.
From an operational impact perspective, this vulnerability poses significant risks to network infrastructure availability and reliability. Organizations relying on Cisco routers and switches running affected IOS versions face potential service disruption that could affect critical network operations, especially in environments where continuous uptime is essential. The automatic reboot behavior means that network administrators have no opportunity to intervene or recover the system manually, potentially leading to extended service outages that can cascade across dependent network services and applications.
The vulnerability aligns with CWE-121, which describes heap-based buffer overflow conditions, and represents a form of improper input validation that leads to system instability. From an ATT&CK framework perspective, this vulnerability maps to T1499.004, which covers network denial of service attacks, and T1566.001, covering spearphishing via social engineering, though the latter is more indirect in its exploitation pathway. The attack vector requires only network access to the target device, making it particularly concerning for publicly accessible network equipment.
Mitigation strategies for this vulnerability include applying the appropriate Cisco IOS patches and updates that address the Telnet daemon handling of the ENVIRON option. Network administrators should also implement access controls to limit Telnet access to trusted networks and consider disabling Telnet services entirely in favor of more secure protocols such as SSH. Additionally, network segmentation and monitoring should be implemented to detect and respond to unusual reboot patterns that might indicate exploitation attempts. Regular vulnerability assessments and security audits should be conducted to identify and remediate similar race condition vulnerabilities in network infrastructure components.