.NET Core Elevation of Privilege Vulnerability

HIGH 7.5CVE-2021-26423 · BIT-dotnet-2021-26423 · BIT-dotnet-sdk-2021-26423 · Published 12/08/2021 · updated 10/08/2026

Package	Affected versions	Patched versions
Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-arm64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-x64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.osx-x64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-arm64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-x64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.LLVM.osx-x64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.linux-arm NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.linux-arm64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.linux-musl-x64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.linux-x64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.Mono.osx-x64 NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.linux-arm NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
Microsoft.NETCore.App.Runtime.linux-arm64 NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
Microsoft.NETCore.App.Runtime.linux-musl-arm NuGet	`≥ 5.0.0, < 5.0.9`	`5.0.9`
Microsoft.NETCore.App.Runtime.linux-musl-arm64 NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
Microsoft.NETCore.App.Runtime.linux-musl-x64 NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
Microsoft.NETCore.App.Runtime.linux-x64 NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
Microsoft.NETCore.App.Runtime.osx-x64 NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
Microsoft.NETCore.App.Runtime.rhel.6-x64 NuGet	`≥ 3.1.0, < 3.1.18`	`3.1.18`
Microsoft.NETCore.App.Runtime.win-arm NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
Microsoft.NETCore.App.Runtime.win-arm64 NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
Microsoft.NETCore.App.Runtime.win-x64 NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
Microsoft.NETCore.App.Runtime.win-x86 NuGet	`≥ 3.1.0, < 3.1.18≥ 5.0.0, < 5.0.9`	`3.1.185.0.9`
.net microsoft	`≥ 5.0, ≤ 5.0.8`	No fix yet
.net core microsoft	`≥ 2.1, ≤ 2.1.28≥ 3.1, ≤ 3.1.17`	No fix yet
powershell core microsoft	`≥ 7.0, < 7.0.7≥ 7.1, < 7.1.4`	`7.0.77.1.4`
visual studio 2017 microsoft	`≥ 15.0, ≤ 15.9`	No fix yet
visual studio 2019 microsoft	`= 8.10≥ 16.0, ≤ 16.10`	No fix yet

Description

Microsoft is releasing this security advisory to provide information about a vulnerability in .NET 5.0 and .NET Core 3.1. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.

A denial of service vulnerability exists in .NET 5.0, .NET Core 3.1 and .NET Core 2.1 where .NET (Core) server applications providing WebSocket endpoints could be tricked into endlessly looping while trying to read a single WebSocket frame.

Patches

If you're using .NET 5.0, you should download and install Runtime 5.0.9 or SDK 5.0.206 (for Visual Studio 2019 v16.8) or SDK 5.0.303 (for Visual Studio 2019 V16.10) from https://dotnet.microsoft.com/download/dotnet-core/5.0.
If you're using .NET Core 3.1, you should download and install Runtime 3.1.18 or SDK 3.1.118 (for Visual Studio 2019 v16.4) or 3.1.412 (for Visual Studio 2019 v16.7 or later) from https://dotnet.microsoft.com/download/dotnet-core/3.1.
If you're using .NET Core 2.1, you should download and install Runtime 2.1.29 or SDK 2.1.525 (for Visual Studio 2019 v15.9) or 2.1.817 from https://dotnet.microsoft.com/download/dotnet-core/2.1.

Other Details

Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/194
An Issue for this can be found at https://github.com/dotnet/runtime/issues/57175
MSRC details for this can be found at https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2021-26423

NVD description

.NET Core and Visual Studio Denial of Service Vulnerability

References

Running one of these products? Scan your site to see whether it exposes a vulnerable version.

Scan my site free