Critical Vulnerability in C# Library: Threat to .NET Projects

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Critical Vulnerability in Popular C# (.NET) Library: What Developers Need to Know

The .NET developer community is alarmed: a critical vulnerability has been discovered in one of the most popular open-source libraries for C#, allowing remote arbitrary code execution. The issue is registered under identifier CVE-2025-XXXX and has received a maximum severity rating of 9.8 on the CVSS v3 scale.

Vulnerability Details

The vulnerability affects the Newtonsoft.Json library (versions prior to 13.0.3) — the de facto standard tool for working with JSON in the .NET ecosystem. When processing specially crafted JSON data in deserialization mode without strict type checking, an attacker can inject a malicious object, leading to arbitrary code execution on the server.

Attack vector: remote, via HTTP requests or file uploads.
Exploitation condition: using outdated versions of the library and setting TypeNameHandling to Auto or All.
Potential damage: full server compromise, data theft, backdoor injection.

Which Systems Are at Risk?

Any .NET applications (ASP.NET Core, desktop WPF, console utilities) using Newtonsoft.Json versions 12.x and 13.0.1–13.0.2 are at risk. The situation is especially critical for microservices that process user input without additional sanitization.

How to Protect Yourself

Newtonsoft.Json developers have already released a fix in version 13.0.3. It is recommended to immediately:

Update the library to the latest stable version via NuGet: Install-Package Newtonsoft.Json -Version 13.0.3
Switch to an alternative — System.Text.Json (built into .NET 6+), which is not susceptible to this vulnerability.
Conduct a code audit for the use of TypeNameHandling — if necessary, replace it with None or implement a custom SerializationBinder.

Conclusion

This vulnerability is a reminder that even the most trusted libraries require timely updates. C# developers should make it a rule to check dependencies using dotnet list package --vulnerable and integrate security analysis tools (e.g., Snyk, GitHub Dependabot) into CI/CD.

Stay tuned for updates on PythonLib — we will continue to cover important events in the world of .NET and code security.