.NET Core Elevation of Privilege Vulnerability
CVE-2021-26423GHSA-rh58-r7jh-xhx3
What to do
Vendor fix available. Upgrade each affected package to the fixed version on its release line (microsoft.netcore.app.runtime.mono.linux-arm 5.0.9; microsoft.netcore.app.runtime.win-x86 5.0.9, 3.1.18; microsoft.netcore.app.runtime.mono.llvm.aot.linux-arm64 5.0.9; microsoft.netcore.app.runtime.win-arm64 5.0.9, 3.1.18; microsoft.netcore.app.runtime.mono.linux-arm64 5.0.9; 18 more packages under Affected packages), redeploy, then verify what is actually installed.
This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.
Affected packages
NuGet microsoft.netcore.app.runtime.mono.linux-arm
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.win-x86
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
NuGet microsoft.netcore.app.runtime.mono.llvm.aot.linux-arm64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.win-arm64
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
NuGet microsoft.netcore.app.runtime.mono.linux-arm64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.linux-musl-x64
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
NuGet microsoft.netcore.app.runtime.mono.linux-musl-x64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.linux-arm64
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
NuGet microsoft.netcore.app.runtime.mono.llvm.linux-arm64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.mono.linux-x64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.linux-arm
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
NuGet microsoft.netcore.app.runtime.osx-x64
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
NuGet microsoft.netcore.app.runtime.mono.llvm.linux-x64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.mono.llvm.aot.osx-x64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.linux-x64
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
NuGet microsoft.netcore.app.runtime.mono.llvm.aot.linux-x64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.linux-musl-arm
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.mono.osx-x64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.win-arm
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
NuGet microsoft.netcore.app.runtime.mono.llvm.osx-x64
- Affected
- >= 5.0.0, < 5.0.9
- Fixed in
- 5.0.9
NuGet microsoft.netcore.app.runtime.rhel.6-x64
- Affected
- >= 3.1.0, < 3.1.18
- Fixed in
- 3.1.18
NuGet microsoft.netcore.app.runtime.linux-musl-arm64
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
NuGet microsoft.netcore.app.runtime.win-x64
- Affected
- >= 3.1.0, < 3.1.18>= 5.0.0, < 5.0.9
- Fixed in
- 3.1.18, 5.0.9
Sources
- GitHub Advisory Database and maintainer security advisories on GitHub
GitHub-reviewed advisory GHSA-rh58-r7jh-xhx3 lists affected ranges: NuGet microsoft.netcore.app.runtime.win-x86 >=3.1.0 =5.0.0 =3.1.0 =5.0.0 =3.1.0 =5.0.0 =3.1.0 =5.0.0 =3.1.0 =5.0.0 <5.0.9; NuGet…
VerifiedObserved Structured GitHub Advisory Database record (type=reviewed) from the GitHub REST API.
Severity and exploitation
- Exploitation
- No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
- CVSS
- 7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- EPSS
- CVE-2021-26423: 3.90% estimated exploitation activity in the next 30 days; percentile 90.28 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description
As published by the source; long descriptions are shortened.
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…
Dates and revisions
- Published by the source
- Last modified by the source
- Source last checked
- Revision 1 · created ·
Published from ghsa record GHSA-rh58-r7jh-xhx3
Agent brief
Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.
# Patch Next advisory brief: CVE-2021-26423 Advisory: .NET Core Elevation of Privilege Vulnerability IDs: CVE-2021-26423, GHSA-rh58-r7jh-xhx3 Record: https://patchnext.com/advisories/cve-2021-26423 (revision 1, source checked 26 Oct 2026, 16:58 UTC) Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected. Severity: High (CVSS 7.5) Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV. Vendor response: Vendor fix available; fixed versions by package: microsoft.netcore.app.runtime.mono.linux-arm 5.0.9; microsoft.netcore.app.runtime.win-x86 5.0.9, 3.1.18; microsoft.netcore.app.runtime.mono.llvm.aot.linux-arm64 5.0.9; microsoft.netcore.app.runtime.win-arm64 5.0.9, 3.1.18; microsoft.netcore.app.runtime.mono.linux-arm64 5.0.9; microsoft.netcore.app.runtime.linux-musl-x64 5.0.9, 3.1.18; microsoft.netcore.app.runtime.mono.linux-musl-x64 5.0.9; microsoft.netcore.app.runtime.linux-arm64 5.0.9, 3.1.18; microsoft.netcore.app.runtime.mono.llvm.linux-arm64 5.0.9; microsoft.netcore.app.runtime.mono.linux-x64 5.0.9; microsoft.netcore.app.runtime.linux-arm 5.0.9, 3.1.18; microsoft.netcore.app.runtime.osx-x64 5.0.9, 3.1.18; microsoft.netcore.app.runtime.mono.llvm.linux-x64 5.0.9; microsoft.netcore.app.runtime.mono.llvm.aot.osx-x64 5.0.9; microsoft.netcore.app.runtime.linux-x64 5.0.9, 3.1.18; microsoft.netcore.app.runtime.mono.llvm.aot.linux-x64 5.0.9; microsoft.netcore.app.runtime.linux-musl-arm 5.0.9; microsoft.netcore.app.runtime.mono.osx-x64 5.0.9; microsoft.netcore.app.runtime.win-arm 5.0.9, 3.1.18; microsoft.netcore.app.runtime.mono.llvm.osx-x64 5.0.9; microsoft.netcore.app.runtime.rhel.6-x64 3.1.18; microsoft.netcore.app.runtime.linux-musl-arm64 5.0.9, 3.1.18; microsoft.netcore.app.runtime.win-x64 5.0.9, 3.1.18. A vendor fix does not mean a deployment is fixed. ## Affected components - NuGet microsoft.netcore.app.runtime.mono.linux-arm: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.win-x86: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.llvm.aot.linux-arm64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.win-arm64: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.linux-arm64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.linux-musl-x64: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.linux-musl-x64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.linux-arm64: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.llvm.linux-arm64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.linux-x64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.linux-arm: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 - NuGet microsoft.netcore.app.runtime.osx-x64: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.llvm.linux-x64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.llvm.aot.osx-x64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.linux-x64: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.llvm.aot.linux-x64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.linux-musl-arm: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.osx-x64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.win-arm: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 - NuGet microsoft.netcore.app.runtime.mono.llvm.osx-x64: affected >= 5.0.0, < 5.0.9; fixed in 5.0.9 - NuGet microsoft.netcore.app.runtime.rhel.6-x64: affected >= 3.1.0, < 3.1.18; fixed in 3.1.18 - NuGet microsoft.netcore.app.runtime.linux-musl-arm64: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 - NuGet microsoft.netcore.app.runtime.win-x64: affected >= 3.1.0, < 3.1.18; >= 5.0.0, < 5.0.9; fixed in 3.1.18, 5.0.9 ## Suggested inspection (read-only) - NuGet microsoft.netcore.app.runtime.mono.linux-arm: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.win-x86: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.llvm.aot.linux-arm64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.win-arm64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.linux-arm64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.linux-musl-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.linux-musl-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.linux-arm64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.llvm.linux-arm64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.linux-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.linux-arm: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.osx-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.llvm.linux-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.llvm.aot.osx-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.linux-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.llvm.aot.linux-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.linux-musl-arm: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.osx-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.win-arm: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.mono.llvm.osx-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.rhel.6-x64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.linux-musl-arm64: check the installed version with your package manager. - NuGet microsoft.netcore.app.runtime.win-x64: check the installed version with your package manager. ## Supported action Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version. ## Required regression checks - Run the test suite and a production build. - Exercise the features that depend on microsoft.netcore.app.runtime.mono.linux-arm, microsoft.netcore.app.runtime.win-x86, microsoft.netcore.app.runtime.mono.llvm.aot.linux-arm64, microsoft.netcore.app.runtime.win-arm64, microsoft.netcore.app.runtime.mono.linux-arm64, microsoft.netcore.app.runtime.linux-musl-x64, microsoft.netcore.app.runtime.mono.linux-musl-x64, microsoft.netcore.app.runtime.linux-arm64, microsoft.netcore.app.runtime.mono.llvm.linux-arm64, microsoft.netcore.app.runtime.mono.linux-x64, microsoft.netcore.app.runtime.linux-arm, microsoft.netcore.app.runtime.osx-x64, microsoft.netcore.app.runtime.mono.llvm.linux-x64, microsoft.netcore.app.runtime.mono.llvm.aot.osx-x64, microsoft.netcore.app.runtime.linux-x64, microsoft.netcore.app.runtime.mono.llvm.aot.linux-x64, microsoft.netcore.app.runtime.linux-musl-arm, microsoft.netcore.app.runtime.mono.osx-x64, microsoft.netcore.app.runtime.win-arm, microsoft.netcore.app.runtime.mono.llvm.osx-x64, microsoft.netcore.app.runtime.rhel.6-x64, microsoft.netcore.app.runtime.linux-musl-arm64, microsoft.netcore.app.runtime.win-x64. - Repeat the inspection and confirm that only fixed versions remain installed. ## Rollback considerations - Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward. ## Unresolved questions - Which deployments run an affected version? - Exploitation status can change. Check again before lowering the priority. - Mark the work resolved only after the fixed version is verified in production. ## Sources - Verified: https://github.com/advisories/GHSA-rh58-r7jh-xhx3 (GitHub-reviewed advisory GHSA-rh58-r7jh-xhx3 lists affected ranges: NuGet microsoft.netcore.app.runtime.win-x86 >=3.1.0 =5.0.0 =3.1.0 =5.0.0 =3.1.0 =5.0.0 =3.1.0 =5.0.0 =3.1.0 =5.0.0 <5.0.9; NuGet…) Commands are suggestions: review each one before running it.
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