CVE-2023-20089: Cisco NX-OS LLDP DoS Vulnerability

CVE-2023-20089 Overview

A memory leak vulnerability exists in the Link Layer Discovery Protocol (LLDP) feature for Cisco Nexus 9000 Series Fabric Switches operating in Application Centric Infrastructure (ACI) Mode. This vulnerability allows an unauthenticated, adjacent attacker to cause a memory leak that could result in an unexpected device reload, leading to a denial of service condition.

Critical Impact

Successful exploitation causes memory exhaustion leading to device reload, disrupting network fabric operations in ACI environments.

Affected Products

• Cisco NX-OS versions 15.2(1g) through 16.0(1j)
• Cisco Nexus 9000 Series Fabric Switches in ACI Mode (including 9200, 9300, 9400, 9500, and 9800 series)
• Cisco Nexus 9000V virtual switch

Discovery Timeline

• February 23, 2023 - CVE-2023-20089 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-20089

Vulnerability Analysis

This vulnerability stems from improper error handling in the LLDP packet parsing mechanism within Cisco NX-OS software. When the affected device receives malformed LLDP packets on a directly connected interface, the parsing routine fails to properly release allocated memory during error conditions. This creates a memory leak that accumulates over time as the attacker continues sending crafted packets.

The vulnerability is classified under CWE-401 (Missing Release of Memory after Effective Lifetime) and CWE-789 (Memory Allocation with Excessive Size Value). The attack requires the adversary to be Layer 2 adjacent to the target device, meaning they must be in the same broadcast domain. Transit traffic cannot be used to exploit this vulnerability—the malicious LLDP packets must be directed at an interface on the affected switch.

Root Cause

The root cause is incorrect error checking when parsing ingress LLDP packets. When the LLDP processing function encounters malformed or crafted packets, it allocates memory for packet processing but fails to properly deallocate that memory when errors occur during parsing. This improper memory management results in memory resources being consumed without release, eventually exhausting available system memory.

Attack Vector

An attacker must be positioned within the same Layer 2 broadcast domain as the target Cisco Nexus 9000 switch. From this adjacent network position, the attacker sends a continuous stream of specially crafted LLDP packets to a directly connected interface on the affected device. As the switch processes these malformed packets, memory is allocated but never freed due to the error handling flaw.

Over time, this memory leak accumulates until system memory is exhausted. When memory resources are depleted, the switch experiences an unexpected reload, causing a denial of service condition. The attack does not require authentication and can be sustained indefinitely as long as the attacker maintains network adjacency.

Detection Methods for CVE-2023-20089

Indicators of Compromise

• Gradual increase in memory utilization on Nexus 9000 switches in ACI mode without corresponding workload increase
• Unexpected switch reloads with memory exhaustion noted in crash logs
• Abnormally high volume of LLDP packets observed on switch interfaces
• System logs indicating LLDP processing errors or memory allocation failures

Detection Strategies

• Monitor switch memory utilization metrics for sustained upward trends using SNMP or streaming telemetry
• Configure syslog alerting for LLDP-related error messages and memory threshold warnings
• Implement network traffic analysis to detect anomalous LLDP packet volumes on switch-facing ports
• Review system crash reports for memory-related reload events

Monitoring Recommendations

• Establish baseline memory usage patterns for Nexus 9000 switches and alert on deviations
• Enable LLDP packet counters and monitor for unusual spikes in received LLDP frames
• Configure automated alerts when memory utilization exceeds 80% capacity
• Implement periodic health checks that verify LLDP subsystem stability

How to Mitigate CVE-2023-20089

Immediate Actions Required

• Apply the latest Cisco NX-OS software update as referenced in the Cisco Security Advisory
• Disable LLDP on interfaces where the protocol is not required to reduce attack surface
• Implement Layer 2 access controls to restrict which devices can send traffic to switch interfaces
• Review network segmentation to limit potential attacker adjacency to critical ACI fabric switches

Patch Information

Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory cisco-sa-aci-lldp-dos-ySCNZOpX to determine the appropriate fixed software release for their specific hardware and current software version. The advisory provides detailed version-specific remediation guidance for all affected NX-OS releases.

Workarounds

• Disable LLDP globally or on specific interfaces where it is not operationally required
• Implement port security and 802.1X authentication to restrict unauthorized devices from connecting to switch ports
• Deploy private VLANs or other Layer 2 isolation mechanisms to limit broadcast domain access
• Use infrastructure ACLs where possible to restrict LLDP traffic to trusted network management systems

# Disable LLDP on specific interfaces (NX-OS CLI)
switch# configure terminal
switch(config)# interface ethernet 1/1
switch(config-if)# no lldp transmit
switch(config-if)# no lldp receive
switch(config-if)# exit

# Disable LLDP globally if not required
switch(config)# no feature lldp