7.5
High
Microsoft.NETCore.App.Runtime.win-x86

CVE-2021-26423

CVE-2021-26423 is a high-severity security vulnerability in Microsoft.NETCore.App.Runtime.win-x86 (nuget), affecting versions >= 3.1.0, < 3.1.18. It is fixed in 3.1.18, 5.0.9.

Key facts
◎
CVSS score
7.5
High
↗
Attack vector
Network
☷
Issuing authority
GitHub Advisory Database
□
Affected package
Microsoft.NETCore.App.Runtime.win-x86
✓
Fixed in
3.1.18, 5.0.9
☼
Disclosed
2021

Summary

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

Impact

Severity and exposure

CVE-2021-26423 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment.

A fixed version is available (3.1.18, 5.0.9). Upgrading removes the vulnerable code path.

Affected versions

nuget

  • Microsoft.NETCore.App.Runtime.win-x86 (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.win-x86 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.win-x64 (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.win-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.win-arm64 (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.win-arm64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.win-arm (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.win-arm (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.osx-x64 (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.osx-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.osx-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.osx-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-arm64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.osx-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-arm64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.linux-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.linux-musl-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.linux-arm64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.linux-arm (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.linux-x64 (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.linux-musl-x64 (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-musl-x64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.linux-musl-arm64 (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-musl-arm64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.linux-musl-arm (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.rhel.6-x64 (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-arm64 (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-arm64 (>= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.linux-arm (>= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-arm (>= 5.0.0, < 5.0.9)

Security releases

  • Microsoft.NETCore.App.Runtime.win-x86 → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.win-x86 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.win-x64 → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.win-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.win-arm64 → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.win-arm64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.win-arm → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.win-arm → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.osx-x64 → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.osx-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.osx-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.osx-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-arm64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.osx-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-arm64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.linux-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.linux-musl-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.linux-arm64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.Mono.linux-arm → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-x64 → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-musl-x64 → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-musl-x64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-musl-arm64 → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-musl-arm64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-musl-arm → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.rhel.6-x64 → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-arm64 → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-arm64 → 5.0.9 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-arm → 3.1.18 (nuget)
  • Microsoft.NETCore.App.Runtime.linux-arm → 5.0.9 (nuget)
Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter instead of chasing every advisory.

Kodem's runtime-powered SCA identifies whether CVE-2021-26423 is reachable in your applications. Explore open-source security for your team.

See if CVE-2021-26423 is reachable in your applications. Get a demo

Remediation advice

Upgrade the following packages to resolve this vulnerability:

  • Upgrade Microsoft.NETCore.App.Runtime.win-x86 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-x86 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-arm64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-arm to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-arm to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.osx-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.osx-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.osx-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.osx-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.osx-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.linux-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.linux-musl-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.linux-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.linux-arm to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.rhel.6-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-arm64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-arm to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-arm to 5.0.9 or later

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently asked questions about CVE-2021-26423

What is CVE-2021-26423?

CVE-2021-26423 is a high-severity security vulnerability in Microsoft.NETCore.App.Runtime.win-x86 (nuget), affecting versions >= 3.1.0, < 3.1.18. It is fixed in 3.1.18, 5.0.9.

How severe is CVE-2021-26423?

CVE-2021-26423 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Which packages are affected by CVE-2021-26423?
  • Microsoft.NETCore.App.Runtime.win-x86 (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.win-x64 (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.win-arm64 (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.win-arm (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.osx-x64 (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.Mono.osx-x64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.osx-x64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-x64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-arm64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.osx-x64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-x64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-arm64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.linux-x64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.linux-musl-x64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.linux-arm64 (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.Mono.linux-arm (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.linux-x64 (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-musl-x64 (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-musl-arm64 (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-musl-arm (nuget) (versions >= 5.0.0, < 5.0.9)
  • Microsoft.NETCore.App.Runtime.rhel.6-x64 (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-arm64 (nuget) (versions >= 3.1.0, < 3.1.18)
  • Microsoft.NETCore.App.Runtime.linux-arm (nuget) (versions >= 3.1.0, < 3.1.18)
Is there a fix for CVE-2021-26423?

Yes. CVE-2021-26423 is fixed in 3.1.18, 5.0.9. Upgrade to this version or later.

Is CVE-2021-26423 exploitable, and should I be worried?

Whether CVE-2021-26423 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo

What actually determines whether CVE-2021-26423 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

How do I fix CVE-2021-26423?
  • Upgrade Microsoft.NETCore.App.Runtime.win-x86 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-x86 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-arm64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-arm to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.win-arm to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.osx-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.osx-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.osx-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.osx-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.osx-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.linux-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.linux-musl-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.linux-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.Mono.linux-arm to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-x64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.rhel.6-x64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-arm64 to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-arm64 to 5.0.9 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-arm to 3.1.18 or later
  • Upgrade Microsoft.NETCore.App.Runtime.linux-arm to 5.0.9 or later

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