CVE-2026-20033 in NX-OS
Summary
by MITRE • 02/25/2026
A vulnerability in Cisco Nexus 9000 Series Fabric Switches in ACI mode could allow an unauthenticated, adjacent attacker to cause a denial of service (DoS) condition on an affected device.
This vulnerability is due to insufficient validation when processing specific Ethernet frames. An attacker could exploit this vulnerability by sending a crafted Ethernet frame to the management interface of an affected device. A successful exploit could allow the attacker to cause the device to reload unexpectedly, resulting in a DoS condition. Note: Only the out-of-band (OOB) management interface is affected.
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Analysis
by VulDB Data Team • 02/28/2026
The vulnerability identified as CVE-2026-20033 represents a critical denial of service weakness in Cisco Nexus 9000 Series Fabric Switches operating in ACI mode. This flaw specifically targets the device's management interface processing capabilities, creating a pathway for unauthorized disruption of network operations. The vulnerability exists within the frame validation mechanisms that govern how the switch handles incoming Ethernet traffic, particularly when processed through the out-of-band management interface. Security researchers have identified that this weakness stems from inadequate input validation procedures that fail to properly examine the structure and content of specific Ethernet frames before processing them within the management context. The affected devices operate under ACI mode, which introduces additional complexity to the management interface processing that makes this vulnerability particularly concerning for enterprise network environments relying on these switches for critical infrastructure operations.
The technical exploitation of this vulnerability requires an adjacent attacker who can physically access the network segment containing the affected switch. This adjacency requirement reduces the attack surface but does not eliminate the threat, as physical access to network infrastructure remains a realistic concern in many enterprise environments. The attacker must craft a specific Ethernet frame that triggers the validation bypass, causing the switch to enter an unexpected state that ultimately results in system reload. This behavior constitutes a classic denial of service attack vector where the attacker leverages the device's own processing logic against itself. The vulnerability's impact is significant because management interfaces are critical for device operation and network administration, making any disruption to these interfaces potentially catastrophic for network availability and operational continuity. The out-of-band management interface is specifically targeted, which means that the attack does not require network-level privileges or complex routing configurations, but rather relies on direct physical access to the switch's management segment.
The operational impact of this vulnerability extends beyond simple service interruption to potentially compromise network stability and administrative capabilities. When the switch experiences an unexpected reload, all network services dependent on that device become unavailable until the system recovers and reinitializes. This disruption affects not only the immediate network segment but can cascade through the broader ACI fabric, potentially causing larger-scale network instability. Network administrators lose access to the device during the reload process, which prevents them from performing critical maintenance, monitoring, or troubleshooting activities. The vulnerability particularly affects enterprise data centers and cloud environments where ACI mode is commonly deployed, as these environments typically rely heavily on the stability and availability of their core switching infrastructure. Organizations using Cisco Nexus 9000 Series switches in ACI mode must consider this vulnerability as a high-priority threat that could be exploited to cause significant operational disruption, especially in environments where physical security controls are insufficient to prevent unauthorized access to network infrastructure.
Mitigation strategies for this vulnerability should focus on both physical and network-level security controls to prevent unauthorized access to the affected management interfaces. Organizations should implement strict physical access controls around network infrastructure, ensuring that only authorized personnel can access the segments containing the vulnerable switches. Network segmentation and access control lists should be configured to limit access to the out-of-band management interface to trusted network segments only. The implementation of network access control solutions and monitoring systems can help detect unusual traffic patterns that might indicate exploitation attempts. Cisco has released software updates addressing this vulnerability, and organizations should prioritize applying these patches to affected devices. Additionally, network administrators should consider implementing monitoring solutions that can detect unexpected device reloads or abnormal management interface behavior. The vulnerability aligns with CWE-20, which addresses improper input validation, and represents a specific instance of how insufficient validation can lead to system instability and denial of service conditions. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and denial of service, with the attack requiring physical access as a prerequisite for exploitation. Regular security assessments and vulnerability scanning should include verification of the patched status for this specific vulnerability, as the attack surface remains limited but the potential impact significant for enterprise networks relying on these critical switching platforms.