Old .NET versions become a security liability
As Microsoft continues to clean up old Windows technologies, with WINS being the latest to retire, the company is now sounding the alarm about another legacy problem: outdated .NET runtimes still lingering on servers and PCs. For years, the classic .NET Framework was shipped as part of Windows itself. Modern .NET (formerly .NET Core) works differently.
It’s installed only when an application needs it, and multiple versions can coexist side by side. That flexibility makes development easier and updates faster, but it also leaves room for trouble. If an app relies on a .NET version that’s reached end of support, that runtime stays on the system, quietly becoming a security hole.
Currently, anything before .NET 8 is considered unsupported. And upgrading the runtime alone doesn’t fix the issue, as applications need to be updated to target a newer framework. Until developers ship updated builds, those old runtimes stay active.
Microsoft is urging IT admins to take inventory now. The company recommends scanning each app to see which .NET version it loads with the following commands:
listdlls.exe -d coreclr.dll -accepteula -v
If an app is running on an obsolete runtime, the only real solution is to contact its developer and request an updated version. After the updated app is deployed, admins should re-scan to confirm no outdated runtime is still active, then manually remove old .NET installations.
Microsoft’s stance is blunt: unsupported .NET runtimes don’t receive patches, break compliance rules, and can even trigger alerts in security tools.
