CVE-2024-21312 - .NET Framework Denial of Service Exploit

In early 2024, Microsoft patched a serious Denial of Service vulnerability in the .NET Framework, tracked as CVE-2024-21312. This flaw could allow attackers to bring down .NET-based applications with specially crafted input. In this post, we’ll break down what happened, show a simple exploit example, and give you a rundown on how to protect your applications.

What Is CVE-2024-21312?

CVE-2024-21312 was publicized as a Denial of Service (DoS) vulnerability affecting several versions of the .NET Framework. In plain terms, an attacker could make your .NET app crash, hang, or become unresponsive without bypassing authentication or executing code.

Microsoft disclosed this issue in their February 2024 Patch Tuesday roundup. The official security advisory is here:
🔗 Microsoft Security Update Guide - CVE-2024-21312

Who Is at Risk?

Any app running on vulnerable versions of the .NET Framework (mostly 4.6-4.8) could be hit, especially if it takes in untrusted input—think APIs, file uploads, deserialization, or web forms.

Technical Details (Simplified!)

The root of the issue is how certain classes within the .NET Framework parse or process external data. Specifically, malicious payloads could trigger infinite loops or excessive resource consumption leading the application to freeze, crash, or eat all the server’s memory.

Let’s look at a hypothetical vulnerable pattern—not real exploit code, but close enough you’ll get the idea.

Example of a Vulnerable .NET Deserialization Pattern

// DO NOT USE: Vulnerable pattern for demonstration

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

public static object BadDeserialize(byte[] data)
{
    var formatter = new BinaryFormatter();
    using (var ms = new MemoryStream(data))
    {
        return formatter.Deserialize(ms); // Dangerous!
    }
}

Why this is risky:
BinaryFormatter has a history of vulnerabilities, and if an attacker supplies a crafted binary blob, the deserializer might enter an infinite loop—or cause a stack overflow—crashing your app.

Here is a pseudo-payload an attacker might use. (For ethical reasons, we won’t publish a real one)

# Python pseudocode to craft a malicious payload (for research ONLY)
payload = b"\x00\x01\x02...malicious bytes designed to break .NET deserialization"
send_to_vulnerable_endpoint(payload)

If received by an API method like BadDeserialize(), this could hang the .NET process.

Desktop apps could freeze, causing users to lose work.

- Background services might eat up CPU/memory on production servers, leading to downtime.

Here is one relevant comment from the community on GitHub:
🔗 GitHub Issue discussing Denial of Service concerns with .NET serialization

How to Tell If You’re Affected

- You’re using .NET Framework 4.6/4.7/4.8 (or related).

Apply the latest Microsoft update ASAP!

🔗 Latest .NET Framework updates

`csharp

// Safe example using System.Text.Json

References

- 🔗 CVE-2024-21312 – NVD Details
- 🔗 Microsoft Security Update Guide
- 🔗 A Guide to .NET Serialization Security

Summary

CVE-2024-21312 shows why you need to keep .NET up to date and never trust untrusted data, especially when it comes to deserialization. Fixes are available—apply them quickly. Always use safe coding patterns, validate inputs, and watch your apps for odd behavior.

Timeline

Published on: 01/09/2024 18:15:55 UTC
Last modified on: 04/11/2024 20:15:17 UTC