On January 9, 2024, Microsoft released an advisory for a new vulnerability: CVE-2024-20685, affecting Azure Private 5G Core. This vulnerability puts 5G network infrastructure at risk for denial-of-service (DoS) attacks, which could bring critical business operations to a standstill.

In this post, I'll explain what CVE-2024-20685 is, show a code snippet that illustrates the issue, walk you through hypothetical exploit details, and share trusted links for more information. If you operate an Azure Private 5G Core environment, this post is for you.

What is CVE-2024-20685?

CVE-2024-20685 is a Denial of Service (DoS) vulnerability. According to Microsoft's advisory, the problem comes from how Azure Private 5G Core handles specially crafted or malformed registration requests from devices (User Equipment, or UE). If an attacker sends a stream of such requests, the core components may stop responding, taking down part or all of the 5G services.

In Microsoft’s own words

> An attacker could use this vulnerability to send crafted packets to Azure Private 5G Core web endpoints, destabilizing the network and causing a denial-of-service.

How Does The Attack Work?

Attackers with access to the network can send a high rate of malformed _subscription management_ or _authentication_ requests. If the Azure 5G Core doesn't validate these properly, it may allocate resources or enter error states, exhausting memory or CPU.

What this means:
A single malicious or compromised device could flood the network control plane, knocking out services for everyone else.

Simple Code Snippet Example

Here’s a Python example showing what a simplified version of the attack might look like. *This is for educational purposes only.*

Suppose the 5G Core has an HTTP-based API for device registration at /api/register. An attacker could write a script to spam this endpoint:

import requests

url = "https://private5gcore.example.com/api/register";
malformed_payload = {
    "device_id": "' OR 1=1; --",  # Intentionally malformed
    "auth_token": "badtoken"
}

for i in range(, 100):
    response = requests.post(url, json=malformed_payload)
    print(f"Request #{i}: Status {response.status_code}")

*If the endpoint does not properly validate input, this kind of flood could quickly exhaust backend resources.*

Preconditions:

The attacker must be able to reach the Azure Private 5G Core layer (e.g., through a connected device or a compromised network element).

1. Probe the network for device registration or subscription management endpoints.

2. Send large numbers of malformed, oversize, or unexpected payloads to these endpoints, causing them to process or log errors.
3. Saturate system resources (CPU, memory, open files/connections) until legitimate device requests are delayed or dropped.

Microsoft’s Patch

The official fix is to apply the security updates released by Microsoft. See here for the latest update:
- CVE-2024-20685 Microsoft Security Update Guide

References

- Microsoft: CVE-2024-20685
- NVD (NIST): CVE-2024-20685 Details
- Microsoft Docs: Azure Private 5G Core Security

Conclusion

CVE-2024-20685 is a reminder that even cloud-managed 5G cores are not immune to denial-of-service attacks. If you’re running Azure Private 5G Core, update immediately and lock down your endpoints. Always validate your inputs, add rate limits, and watch for anomalous traffic. Your network stability depends on it.

*Stay secure and be proactive—DoS attacks are getting smarter every year!*

Timeline

Published on: 04/09/2024 17:15:33 UTC
Last modified on: 04/10/2024 13:24:22 UTC