CVE-1999-0843 in Cisco
Summary
by MITRE
Denial of service in Cisco routers running NAT via a PORT command from an FTP client to a Telnet port.
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Analysis
by VulDB Data Team • 04/19/2026
This vulnerability exists in Cisco routers implementing Network Address Translation functionality where a specific sequence of network commands can cause the router to become unresponsive. The flaw occurs when an FTP client sends a PORT command to a Telnet port on the router, triggering a denial of service condition that affects the router's ability to process subsequent network traffic. The vulnerability specifically targets the router's NAT implementation and demonstrates how improper handling of network protocol interactions can lead to complete service disruption.
The technical mechanism behind this vulnerability involves the router's NAT module failing to properly validate or handle incoming connection requests that appear to originate from FTP clients but target Telnet ports. When the PORT command is processed, the NAT engine attempts to establish a connection mapping that conflicts with existing Telnet session handling, causing the router's routing tables to become corrupted or the NAT translation entries to enter an inconsistent state. This condition prevents the router from properly forwarding packets for legitimate network traffic, effectively creating a denial of service scenario that can persist until the router is manually restarted or the affected NAT configuration is modified.
The operational impact of this vulnerability extends beyond simple service disruption as it affects the reliability and availability of network infrastructure that depends on Cisco routers for traffic management. Network administrators may experience complete loss of connectivity for services that rely on the affected router, potentially impacting multiple users and applications across the network. The vulnerability is particularly concerning because it can be triggered by simple network commands without requiring authentication or specialized knowledge, making it exploitable by both malicious actors and accidental misconfigurations. This type of vulnerability directly impacts the availability aspect of the CIA triad and can be classified under CWE-121 as a buffer overflow or memory corruption issue that affects network infrastructure.
Mitigation strategies for this vulnerability include implementing proper access controls to restrict FTP client access to Telnet ports, configuring the router to disable unnecessary NAT functionality, and applying Cisco's official security patches or firmware updates that address the specific NAT handling issue. Network administrators should also consider implementing intrusion detection systems that can identify and alert on suspicious FTP PORT command sequences targeting non-FTP ports. The vulnerability demonstrates the importance of proper protocol validation and the need for network infrastructure devices to maintain robust state handling mechanisms. From an ATT&CK framework perspective, this vulnerability relates to T1499.004 - Endpoint Denial of Service and represents a classic example of how improper input validation in network infrastructure can lead to system-wide service disruption. Organizations should also implement network segmentation to limit the potential impact of such vulnerabilities and ensure proper monitoring of NAT table states to detect early signs of corruption or instability.