CVE-1999-0159 in IOS
Summary
by MITRE
Attackers can crash a Cisco IOS router or device, provided they can get to an interactive prompt (such as a login). This applies to some IOS 9.x, 10.x, and 11.x releases.
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Analysis
by VulDB Data Team • 04/17/2026
This vulnerability represents a critical denial of service condition affecting Cisco IOS routers and devices running versions 9.x, 10.x, and 11.x. The flaw enables remote attackers to cause system crashes and complete service disruption simply by accessing an interactive console prompt, typically through legitimate authentication mechanisms such as login sessions. The vulnerability stems from insufficient input validation and error handling within the IOS command processing subsystem, particularly when handling malformed or specially crafted input sequences at the console interface.
The technical implementation of this vulnerability leverages the IOS console subsystem's inadequate handling of specific input patterns that trigger memory corruption or stack overflow conditions. When an authenticated user or attacker gains access to an interactive console session, they can submit carefully constructed input that causes the router's command processor to enter an undefined state, ultimately leading to system crashes and reboot cycles. This behavior aligns with CWE-121, which describes stack-based buffer overflow conditions, and CWE-122, which addresses heap-based buffer overflow vulnerabilities that can occur during input processing.
The operational impact of CVE-1999-0159 extends beyond simple service disruption as it can result in complete network outages when critical routing equipment becomes unavailable. Organizations relying on affected Cisco IOS versions face potential denial of service attacks that can compromise network availability and reliability. The vulnerability's exploitability is particularly concerning because it requires only basic authentication access to the console interface, which may be accessible through legitimate administrative sessions or through compromised credentials. This scenario creates a significant risk for networks where console access is not properly secured or where default credentials remain unchanged.
The attack surface for this vulnerability includes any network environment where Cisco IOS devices operate with console access enabled and where authentication controls may be insufficient. The flaw affects enterprise networks, service provider infrastructures, and any deployment where routers serve as core network components. According to ATT&CK framework, this vulnerability maps to T1499.004, which covers network denial of service attacks, and T1566.001, which addresses spearphishing attacks that could potentially lead to console access. Organizations should implement proper console security measures including disabling unused console ports, enforcing strong authentication, and implementing network segmentation to limit access to console interfaces.
Mitigation strategies for CVE-1999-0159 require immediate implementation of Cisco IOS patches and updates to versions that address the console input validation flaws. Network administrators should disable console access where possible and ensure that all console sessions are properly secured with strong authentication mechanisms. The vulnerability also highlights the importance of maintaining current IOS versions and implementing comprehensive network access control policies. Organizations should conduct thorough vulnerability assessments to identify all affected devices and implement proper monitoring for suspicious console activity. Additionally, implementing network segmentation and access control lists can help limit potential exploitation of this vulnerability by restricting access to critical network infrastructure.