CVE-2021-1523 in Nexus 9000
Summary
by MITRE • 08/25/2021
A vulnerability in Cisco Nexus 9000 Series Fabric Switches in Application Centric Infrastructure (ACI) Mode could allow an unauthenticated, remote attacker to cause a queue wedge on a leaf switch, which could result in critical control plane traffic to the device being dropped. This could result in one or more leaf switches being removed from the fabric. This vulnerability is due to mishandling of ingress TCP traffic to a specific port. An attacker could exploit this vulnerability by sending a stream of TCP packets to a specific port on a Switched Virtual Interface (SVI) configured on the device. A successful exploit could allow the attacker to cause a specific packet queue to queue network buffers but never process them, leading to an eventual queue wedge. This could cause control plane traffic to be dropped, resulting in a denial of service (DoS) condition where the leaf switches are unavailable. Note: This vulnerability requires a manual intervention to power-cycle the device to recover.
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Analysis
by VulDB Data Team • 08/29/2021
This vulnerability exists within Cisco Nexus 9000 Series Fabric Switches operating in Application Centric Infrastructure mode and represents a significant denial of service risk that can be exploited remotely without authentication. The flaw specifically affects the handling of ingress TCP traffic to a designated port, creating a condition where packets are accepted into a specific queue but never processed, ultimately leading to queue wedging. The vulnerability is classified as a queue wedge condition that can result in complete loss of leaf switch functionality within the ACI fabric, effectively removing critical network infrastructure from service. This represents a serious operational impact as the affected switches become unavailable and cannot process control plane traffic essential for fabric maintenance and operation.
The technical implementation of this vulnerability stems from improper handling of TCP packet streams directed toward a Switched Virtual Interface configured on the device. When an attacker sends a continuous stream of TCP packets to the targeted port, the system accepts these packets into a specific queue but fails to properly process them through the normal packet handling mechanisms. This creates a buffer accumulation scenario where network buffers fill up with unprocessed packets while the queue remains in a wedged state, preventing any further processing of packets destined for that queue. The mechanism operates at the hardware level within the switch's packet processing architecture, specifically affecting how ingress traffic is queued and managed in the switch's memory structures. This behavior aligns with CWE-129, which addresses improper handling of buffer boundaries, and demonstrates how packet queue management flaws can create denial of service conditions.
The operational impact of this vulnerability extends beyond simple service disruption to encompass complete fabric instability and potential network outages. When leaf switches become unavailable due to queue wedging, the entire ACI fabric experiences significant degradation as these switches are removed from the network topology. The control plane traffic that normally manages fabric state, configuration updates, and routing decisions becomes blocked, leading to cascading failures throughout the network infrastructure. Recovery requires manual intervention through device power cycling, which introduces additional operational complexity and potential downtime. This vulnerability particularly affects enterprise networks that rely heavily on ACI for network automation and policy enforcement, as the loss of leaf switch functionality can severely impact application availability and network performance. The requirement for manual intervention also means that organizations may experience extended periods of service disruption while technicians respond to the incident.
Mitigation strategies for this vulnerability should focus on network segmentation and access control measures to prevent unauthorized access to the targeted switch ports. Organizations should implement strict network access controls to limit exposure of Switched Virtual Interface configurations to trusted sources only, while also monitoring for unusual TCP traffic patterns that might indicate exploitation attempts. Network administrators should consider disabling unused SVI interfaces and implementing rate limiting on TCP connections to reduce the attack surface. Cisco has released security advisories and patches that address this specific queue handling issue, and organizations should prioritize applying these updates to their affected devices. Additionally, implementing network monitoring solutions that can detect queue wedge conditions and automatically alert administrators can provide early warning of potential exploitation attempts. The vulnerability demonstrates the importance of robust queue management and buffer handling in network infrastructure devices, and organizations should review their network device configurations to ensure proper handling of ingress traffic patterns that could trigger similar conditions. This issue also highlights the need for comprehensive incident response procedures that include power cycling protocols for affected devices, as well as the importance of maintaining up-to-date security patches across all network infrastructure components.