CVE-2025-21204 focuses on a recently discovered security weakness in Microsoft’s Windows Update mechanism. This flaw, known as an “improper link resolution before file access” (or “link following”), lets any user with local access trick Windows Update into running files under higher privileges. This post breaks down exactly what’s happening, how it works, and shows you some simple code to understand the vulnerability.

Link following is when a program tries to open a file, but the "file" is actually a special shortcut pointing somewhere else—a symbolic link or "symlink". If the program doesn’t check carefully, it might open or change a file it shouldn’t.

Imagine you ask someone to fetch a folder from the desk and put it in a safe. If they blindly follow a note that says "the folder is under the plant"—not knowing you replaced it with a secret folder—they might put your private notes in the safe without realizing.

Where Does the Vulnerability Live?

In this case, the problem lives in Windows Update Stack, a set of Windows processes that handle updating your system. These processes often run as System, the most powerful local account in Windows.

When Windows Update unpacks and moves files, it doesn’t always check if the files are in fact *symlinks* pointing elsewhere. An attacker with normal privileges can plant symlinks in places Windows Update will access, tricking it into overwriting or modifying files they couldn’t normally touch.

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3. Attacker creates a symlink inside this folder (or one it has access to) that points to a sensitive system file. For example, the attacker points:

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4. Windows Update runs as SYSTEM and tries to write to "important.txt", but instead writes (or deletes, or changes permissions) on the real system file.

Let’s see a PowerShell trick using the built-in cmd tool

# Example: Create a symlink named "trickfile.txt" that points to the system hosts file.
$symlinkPath = "C:\Windows\SoftwareDistribution\Download\trickfile.txt"
$targetPath  = "C:\Windows\System32\drivers\etc\hosts"

cmd /c mklink $symlinkPath $targetPath

Or, using Python (requires "Developer Mode" in Windows 10+)

import os
symlink = r"C:\Windows\SoftwareDistribution\Download\trickfile.txt"
target  = r"C:\Windows\System32\drivers\etc\hosts"

os.symlink(target, symlink)

Note: You’ll sometimes need permission to create symlinks, and Developer Mode must be enabled for non-admins.

Real-World Use Case

Suppose you want to escalate privileges. You could overwrite a benign system executable with your own code, replacing the target binary via the symlink just as Windows Update runs. Next time the binary runs (via a scheduled task or by a service), it runs as SYSTEM—your code, full privilege.

Warning: Don’t try this on production or personal machines! Messing with system files can break your install. This info is for educational and security professional use only.

How To Defend

- Patch Immediately: Microsoft Security Update for CVE-2025-21204

References and More Reading

- Microsoft Security Advisory: CVE-2025-21204
- Mitre CVE Entry for CVE-2025-21204
- Windows Symlink Abuse Research (blog)
- PowerShell mklink reference
- Microsoft Developer: Symlinks

In Conclusion

CVE-2025-21204 spotlights a simple—but serious—oversight: never trust links without checking where they lead, especially when you’re running with system power. If you’re a sysadmin or security pro, patch your systems and regularly check for suspicious activity—even in trusted system folders.

Timeline

Published on: 04/08/2025 18:15:45 UTC
Last modified on: 04/30/2025 17:14:02 UTC