CVE-1999-0416 in Cisco
Summary
by MITRE
Vulnerability in Cisco 7xx series routers allows a remote attacker to cause a system reload via a TCP connection to the router's TELNET port.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability identified as CVE-1999-0416 represents a critical remote code execution flaw affecting Cisco 7xx series routers that operates through a specific network protocol interaction. This vulnerability resides within the router's handling of TCP connections to the TELNET port, which serves as a legitimate administrative interface for remote router management. The flaw enables an unauthenticated remote attacker to trigger a complete system reload of the affected device, effectively causing a denial of service condition that can disrupt network operations and potentially provide an entry point for further exploitation attempts.
The technical nature of this vulnerability stems from improper input validation and error handling within the router's TCP stack implementation. When a TCP connection is established to the TELNET port, the router fails to properly validate the incoming connection parameters or handle malformed connection requests. This inadequate validation allows a remote attacker to craft specific TCP packets that, when processed by the router's TELNET service, trigger an uncontrolled system reboot. The vulnerability operates at the network protocol level, leveraging the fundamental TCP/IP communication mechanisms that routers use to accept and process remote management connections, making it particularly dangerous as it requires no authentication credentials to exploit.
The operational impact of CVE-1999-0416 extends beyond simple service disruption to encompass potential network stability issues and security implications that can affect entire network infrastructures. When exploited, the vulnerability can cause unexpected router reloads that may interrupt critical network services, particularly in environments where these routers serve as core network infrastructure components. The remote nature of the attack means that adversaries can exploit this vulnerability from anywhere on the internet without requiring physical access or prior authentication, making it a significant threat to network availability. Additionally, the timing of such reloads can coincide with critical network operations, potentially causing cascading failures or creating windows of opportunity for more sophisticated attacks.
Cisco routers in the 7xx series were widely deployed in enterprise and service provider networks during the late 1990s, making this vulnerability particularly concerning from a security perspective. The flaw aligns with CWE-122, which describes improper restriction of operations within a limited context, and demonstrates how network protocol implementations can introduce security weaknesses that affect system stability and availability. From an attack perspective, this vulnerability maps to ATT&CK technique T1203, which covers legitimate credentials and remote access tools, as the TELNET port represents a legitimate administrative interface that can be exploited for unauthorized access. Organizations implementing mitigation strategies should consider network segmentation to isolate affected routers, implement access control lists to restrict TELNET port access, and deploy network monitoring solutions to detect anomalous connection patterns that may indicate exploitation attempts. The vulnerability also underscores the importance of regular firmware updates and proper network security monitoring to prevent unauthorized access to critical network infrastructure components.