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Introduction:
Cybersecurity researchers have uncovered a critical initial access vector targeting the Microsoft Copilot web installer, leveraging a classic yet highly effective DLL hijacking technique. This attack exploits the implicit trust placed in Microsoft-signed binaries, allowing threat actors to execute malicious code by simply placing a specially crafted DLL file alongside the legitimate installer. For security professionals and IT administrators, understanding this vulnerability is crucial, as it bypasses traditional security defenses by abusing the very components that systems are designed to trust.
Learning Objectives:
- Understand the mechanics of DLL hijacking and how it applies to trusted Microsoft binaries.
- Learn to identify vulnerable applications using Windows Process Monitor (ProcMon).
- Gain practical knowledge of creating proof-of-concept DLLs for security testing.
- Develop mitigation strategies to protect endpoints against sideloading attacks.
- Analyze the broader implications of supply chain trust in software installation processes.
You Should Know:
1. Anatomy of the Copilot Installer DLL Hijack
The vulnerability resides in how the Microsoft Copilot web installer dynamically loads libraries. When executed, the installer searches for specific DLL files in a predefined order, starting with its own directory before moving to system paths. Lawrence Amer highlighted that the installer attempts to load several DLLs that do not exist in its directory or the expected search path. A threat actor can exploit this by placing a malicious DLL with one of these missing names in the same directory as the installer. Upon execution, the installer loads the attacker’s DLL instead of failing gracefully, leading to arbitrary code execution with the privileges of the user running the installer.
Step‑by‑step guide explaining what this does and how to use it (for defensive testing):
To verify if an application is vulnerable to this attack, security professionals can use Sysinternals Process Monitor (ProcMon) to filter for “NAME NOT FOUND” results.
1. Download and run ProcMon with administrator privileges.
- Set a filter: `Process Name`
is`CopilotInstaller.exe` (or the target executable name) thenInclude. - Add another filter: `Result`
is`NAME NOT FOUND` thenInclude. - Execute the installer and observe the log. Look for DLL files (e.g.,
version.dll,winhttp.dll,msvcp140.dll) that the process attempts to load from its own directory but cannot find. - Any DLL listed in this failed lookup is a potential hijacking point. An attacker would compile a malicious DLL with the same export functions as the legitimate one to ensure the installer runs correctly while executing malicious payloads in the background.
2. Crafting a Proof-of-Concept DLL for Security Testing
For educational and defensive research purposes, understanding how a basic hijacking DLL works is essential. The goal is to create a DLL that executes a benign payload (like launching notepad.exe) to demonstrate execution without causing harm.
Step‑by‑step guide explaining what this does and how to use it (Linux cross-compilation using MinGW):
On a Linux machine, you can use MinGW to compile a DLL for Windows testing.
1. Create a C file, e.g., `payload.c`:
include <windows.h>
BOOL APIENTRY DllMain(HMODULE hModule, DWORD ul_reason_for_call, LPVOID lpReserved) {
switch (ul_reason_for_call) {
case DLL_PROCESS_ATTACH:
// Create a process silently as a proof-of-concept
WinExec("calc.exe", SW_HIDE); // Or "notepad.exe" for less suspicion
break;
case DLL_THREAD_ATTACH:
case DLL_THREAD_DETACH:
case DLL_PROCESS_DETACH:
break;
}
return TRUE;
}
2. Compile using MinGW: `x86_64-w64-mingw32-gcc -shared -o version.dll payload.c -luser32`
3. Place the resulting `version.dll` in the same directory as the vulnerable CopilotInstaller.exe.
4. When a user double-clicks the installer, `calc.exe` will execute, demonstrating successful DLL hijacking. Note: For a real attack, the DLL would contain shellcode to establish C2 communication or drop further malware.
- Exploring the Affected Search Order and Trusted Binaries
Windows follows a specific search order when loading DLLs. By default, the standard search order begins with the directory from which the application is loaded. This behavior, while intended for flexibility, becomes a security risk when combined with trusted binaries. The attacker does not need to compromise any system files; they simply need to trick the user into downloading a zip file containing the legitimate installer alongside the malicious DLL. This is often delivered via phishing emails, compromised websites, or direct messages.
The key to the attack’s effectiveness is that the parent executable (CopilotInstaller.exe) is signed by Microsoft. Endpoint Detection and Response (EDR) solutions often whitelist or apply less scrutiny to signed Microsoft processes. Therefore, the malicious DLL is loaded into a trusted process context, potentially allowing it to evade detection, disable security software, or perform privileged operations.
4. Windows Command Line Verification and Hunting
IT administrators can proactively hunt for signs of DLL sideloading in their environments using PowerShell. The following script searches for instances where a process loads a DLL from a non-standard path that should typically be loaded from System32.
Step‑by‑step guide explaining what this does and how to use it (PowerShell Hunting):
Run this PowerShell script as an administrator to check running processes for potential DLL hijacking indicators.
Get all processes with their loaded modules
Get-Process | ForEach-Object {
$process = $_
try {
$<em>.Modules | ForEach-Object {
Check if the DLL path is not in System32 or SysWOW64 but the process is from a trusted location
if ($</em>.FileName -notlike "C:\Windows\System32" -and $<em>.FileName -notlike "C:\Windows\SysWOW64") {
if ($</em>.FileName -like ".dll") {
[bash]@{
ProcessName = $process.ProcessName
ProcessId = $process.Id
DLLPath = $_.FileName
}
}
}
}
}
catch {
Access denied errors are common; skip
}
} | Export-Csv "DLL_Hijack_Potential.csv" -NoTypeInformation
This script is a starting point for investigation. Analysts should review the output for any DLLs loaded from temporary folders, user download directories, or other suspicious locations by a trusted Microsoft process.
5. Mitigation and Hardening Against DLL Sideloading
Defending against this attack requires a multi-layered approach. Traditional antivirus may not detect the malicious DLL if it is new or uses obfuscation.
– Application Control: Implement application whitelisting solutions like Windows Defender Application Control (WDAC) or AppLocker to prevent unauthorized binaries and DLLs from executing, especially from user-writable paths.
– Safe DLL Search Mode: While enabling `SafeDllSearchMode` via registry can change the search order to prioritize system directories, this is a system-wide setting that may impact application compatibility. It moves the user’s current directory later in the search order but does not eliminate the threat entirely if the attacker’s DLL is in the application’s directory.
– User Education: Train users to be wary of downloading software from unofficial sources or opening zip files from untrusted emails, even if the primary executable appears legitimate.
– Vendor Accountability: Report vulnerabilities to vendors like Microsoft. In this case, the vendor should update their installer to load DLLs securely using absolute paths or by calling `SetDllDirectory(“”)` to remove the current directory from the search path.
What Undercode Say:
- Key Takeaway 1: The implicit trust in Microsoft-signed binaries is a double-edged sword; attackers exploit this trust to sideload malicious code, bypassing security layers that focus solely on unsigned executables.
- Key Takeaway 2: DLL hijacking remains a prevalent and effective initial access vector because it abuses legitimate application functionality. Prevention requires shifting from a “trust by signature” model to a “trust by behavior and origin” model, using application control and monitoring for anomalous process behavior.
This discovery by @DarkWebInformer and highlighted by Lawrence Amer serves as a stark reminder that the software supply chain extends to the installation process itself. As attackers continue to refine their tradecraft, defenders must scrutinize every step of software execution, not just the final payload. The Copilot installer vulnerability is a clear call to action for both software developers to adopt secure coding practices and for security teams to harden their endpoints against such sideloading techniques.
Prediction:
This trend of exploiting trusted, signed binaries for initial access will intensify over the next 12-24 months. As EDR solutions become more adept at detecting anomalous PowerShell and script-based attacks, threat actors will increasingly turn to “living-off-the-land” binary (LOLBin) and DLL sideloading techniques. We can expect to see a rise in “bring your own vulnerable driver” (BYOVD) tactics combined with DLL hijacking, allowing attackers to load kernel-level components. Consequently, Microsoft and other major vendors will be forced to implement more rigorous auditing of their installers and update mechanisms, potentially leading to a new wave of patches for legacy software. Security operations centers will need to invest in behavioral analysis tools that can baseline normal DLL load patterns for trusted executables to detect these subtle anomalies.
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IT/Security Reporter URL:
Reported By: Lawrence Amer – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
