CVE-2023-21808 - .NET and Visual Studio Remote Code Execution Vulnerability Explained (with Exploit Example)

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Introduction

On February 14, 2023, Microsoft published an advisory regarding CVE-2023-21808, a critical vulnerability affecting .NET and Visual Studio. This bug enables remote code execution (RCE), allowing attackers to run their code on vulnerable systems. In this exclusive post, we'll break down how the vulnerability works, show code snippets to explain the exploit, and point out what you can do to protect your environment.

What is CVE-2023-21808?

CVE-2023-21808 is a remote code execution vulnerability in the .NET runtime and Visual Studio IDE. Exploiting this flaw, a remote, unauthenticated attacker can execute arbitrary code on a user's system, potentially leading to full device compromise.

Severity

- CVSS Score: 8.1 - High

How Does the Vulnerability Work?

The vulnerability is triggered when .NET or Visual Studio deserializes malicious data or opens a crafted project or solution file. This could happen with:

Deserializing untrusted data using vulnerable .NET components

If the attacker is able to trick a victim into opening a malicious file with Visual Studio or a .NET application that consumes such data, attacker-supplied code runs with the victim’s privileges.

Key component:  
Problems in deserialization logic in .NET or project handling logic in Visual Studio. If the program processes user-controlled data without proper sanitization or validation, an attacker can supply object graphs that trigger code execution.

Exploit Example

Let's see a simple (simulated) example based on common deserialization pitfalls.

Suppose your .NET code allows users to upload configurations as serialized objects (not recommended)

using System;
using System.IO;
using System.Runtime.Serialization.Formatters.Binary;

public class ConfigLoader
{
    public object LoadConfig(byte[] data)
    {
        BinaryFormatter formatter = new BinaryFormatter();
        using (MemoryStream ms = new MemoryStream(data))
        {
            // UNSAFE! Never do this with untrusted data
            return formatter.Deserialize(ms);
        }
    }
}

If an attacker crafts a malicious payload (for example, using ysoserial.net), such as one that launches Calculator:

ysoserial.exe -f BinaryFormatter -g TypeConfuseDelegate -o raw -c "calc.exe" > exploit_payload.bin

They could upload exploit_payload.bin. When your app runs LoadConfig, the attacker’s code runs – and so does their payload.

> Note: The *actual CVE* in .NET or Visual Studio is in handling certain file types, but the same class of bug: unsafe deserialization or file parsing leading to code execution.

Visual Studio Exploit Scenario

Imagine a developer downloads a .sln or .csproj file from an untrusted source and opens it with Visual Studio. If crafted to exploit CVE-2023-21808, Visual Studio could deserialize objects within the file format, resulting in code execution as soon as the file is opened.

References and Technical Details

- Microsoft Security Advisory for CVE-2023-21808
- NVD – CVE-2023-21808 Detail
- ysoserial.net for gadget chains
- BinaryFormatter considered dangerous – Microsoft Docs

Patch Immediately:

Apply the Microsoft updates to Visual Studio and .NET.

Final Thoughts

CVE-2023-21808 is a stark reminder to always be careful with deserialization and to never trust files from unknown sources. .NET and Visual Studio are powerful, but when flaws like this appear, patching and safe practices are your best lines of defense.

If you use .NET or Visual Studio, patch now — and audit your code for risky deserialization!

*For ongoing updates, check Microsoft’s Security Update Guide and subscribe to their security advisories.*

Timeline

Published on: 02/14/2023 21:15:00 UTC
Last modified on: 02/24/2023 18:56:00 UTC