Windows has long had a complicated relationship with file system security, especially when it comes to following links (like shortcuts and symbolic links). CVE-2025-27727 is the latest example where an attacker with access to a machine can trick the Windows Installer into following a symbolic link, ultimately letting them execute code with elevated privileges. This post explains the issue, walks you through an example exploit, provides code snippets, and links to original references, using simple and clear language.
What is CVE-2025-27727?
CVE-2025-27727 is a security vulnerability in the Windows Installer component. When Windows Installer is updating or installing some software, it often cleans up or replaces files. If an attacker can replace a legitimate file or directory with a symlink (symbolic link) to a sensitive location, the Installer may follow the link _without checking_, allowing the attacker to make changes outside their permissions.
Link Following 101
A symbolic link or symlink is a kind of file that points to another file or folder. If a program doesn't check carefully, it could operate not on the intended file, but on whatever the symlink points to.
The Windows Installer (msiexec.exe and related tools) often runs as SYSTEM. If it’s tricked into writing to or deleting files via a symlink, an attacker can overwrite whatever they want—even system files.
Replace it with a symlink to a protected file (like a SYSTEM DLL or configuration).
3. When the Installer runs, it unknowingly follows the link and makes changes to the target file as SYSTEM.
4. Attacker leverages the change for privilege escalation (for instance, by replacing a service executable or dropping a malicious DLL in a protected path).
Example Exploit: Proof of Concept (PoC)
Let's walk through a simplified example using PowerShell and command-line tools on Windows.
Warning: This is for educational purposes only. Don't attack systems you do not own.
Scenario: The Windows Installer will delete a log file during uninstall. The attacker replaces the log file with a symlink pointing to C:\Windows\System32\drivers\etc\hosts.
Step 1: Find a target file
Suppose the Installer removes C:\ProgramData\MyApp\install.log when uninstalling.
Delete the legit log file
Remove-Item "C:\ProgramData\MyApp\install.log"
Create a symlink to hosts file
cmd /c mklink "C:\ProgramData\MyApp\install.log" "C:\Windows\System32\drivers\etc\hosts"
Step 3: Trigger the Installer
Uninstall the app in a way that uses Windows Installer (for example, from "Add or Remove Programs" or via msiexec /x {ProductCode}).
Step 4: Check the result
If vulnerable, the hosts file might be deleted or modified as SYSTEM — something a regular user shouldn't be able to do.
Real-World Impact
- Gain SYSTEM privileges: Attacker could replace a service binary and reboot for SYSTEM code execution.
Monitor Installer activity: Tools like Process Monitor (Procmon) can help.
- Apply Security Updates: Microsoft releases patches for such vulnerabilities as soon as possible—keep your system up to date.
References and Further Reading
- Microsoft Security Response Center (MSRC)
- Original NVD Entry for CVE-2025-27727
- Windows Symbolic Links Documentation
Summary
CVE-2025-27727 shows how something as simple as not checking a file path after following a link can lead to full system compromise. If you manage Windows systems, ensure you’re patched, educate your users, and keep an eye out for symlink tricks. Attackers love when security takes shortcuts!
Timeline
Published on: 04/08/2025 18:16:00 UTC
Last modified on: 05/06/2025 17:03:42 UTC