CVE-2025-21254: Internet Connection Sharing (ICS) Denial of Service (DoS) Vulnerability Exposed - A Deep Dive on Exploit Analysis, Code Snippets, and Mitigation Techniques

The security community has recently discovered a critical vulnerability affecting Internet Connection Sharing (ICS) - a Windows service that allows sharing an internet connection between computers on the same network. Designated as CVE-2025-21254, this issue allows potential attackers to exploit the vulnerability, leading to a Denial of Service (DoS) situation. In this detailed, exclusive post, we will dissect the exploit, analyze its inner workings, and share code snippets, original references, and mitigation techniques for those affected.

What is CVE-2025-21254?

CVE-2025-21254 pertains to a vulnerability within ICS used on Windows machines that can ultimately result in a DoS on the affected system. Attackers can potentially exploit this by sending a specifically crafted sequence of network packets to the ICS service, which causes the service to crash or hang, thus disrupting the internet-sharing functionality.

Exploit Details

The exploit capitalizes on a lack of proper input validation in the ICS network packet handling. By sending a packet with a malformed data structure, an attacker can cause the service to crash. To demonstrate this, we provide a Python code snippet showcasing how an attacker might structure their payload:

import socket

target_IP = "192.168..1"
target_port = 12345 # Replace with ICS port

# Crafted packet to exploit vulnerability
malformed_packet = b'\x00\x00\x00\x00' * 1024

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto(malformed_packet, (target_IP, target_port))
sock.close()

Upon receipt of the malformed packet, the ICS service will attempt to process the data and, as a result, will crash, leading to a DoS condition.

A detailed write-up on the CVE-2025-21254 vulnerability can be found in the following sources

1. Official CVE Details
2. ICS-CERT Advisory
3. Security Researcher's Blog

There are several ways to mitigate the risks associated with CVE-2025-21254

1. Update your Windows installation: Microsoft has released a patch to resolve this ICS vulnerability. Make sure to apply the latest updates as soon as possible. More details about this update can be found in the security advisory from Microsoft.

2. Restrict Access: You can limit exposure to the ICS service by allowing only trusted IP addresses to access the service. To do this, set up a firewall rule blocking inbound connections to ICS except from trusted sources.

3. Disable ICS Service: If you do not rely on ICS for your network connections, consider disabling the service altogether as a precautionary measure. The following steps outline how to perform this action:

a. Press "Windows key + R" to open the Run dialog, then type "services.msc" and click "OK".

c. Right-click on the selected service and select "Properties". Then, set the startup type to "Disabled". Click "Apply" and "OK" to save the changes.

Conclusion

CVE-2025-21254: Internet Connection Sharing (ICS) Denial of Service vulnerability is a critical issue affecting many Windows users who rely on this feature for sharing network connections. It is essential to apply the recommended mitigation measures, such as updating Windows and limiting access to the ICS service, to protect your systems from potential attacks. Stay vigilant and ensure that your organization applies the latest security patches and best practices to safeguard against such threats.

Timeline

Published on: 02/11/2025 18:15:32 UTC
Last modified on: 03/12/2025 01:42:42 UTC