.NET Core and Visual Studio Denial of Service Vulnerability

published 2021-08-12 5 y ago modified 2026-09-08 2 mo ago also BIT-dotnet-2021-26423 , BIT-dotnet-sdk-2021-26423 , EUVD-2022-7160 , GHSA-rh58-r7jh-xhx3

cvss: 7.5; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 3.9%; EPSS 3.9% chance of exploitation in 30 days, percentile 90%
public exploits: 0; none known

Patch / fix

fixed in 30 distros 2 links 2

nuget Microsoft.NETCore.App.Runtime.win-x86 3.1.18, 5.0.9
nuget Microsoft.NETCore.App.Runtime.win-x64 3.1.18, 5.0.9
nuget Microsoft.NETCore.App.Runtime.win-arm64 3.1.18, 5.0.9
nuget Microsoft.NETCore.App.Runtime.win-arm 3.1.18, 5.0.9
nuget Microsoft.NETCore.App.Runtime.osx-x64 3.1.18, 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, 5.0.9
nuget Microsoft.NETCore.App.Runtime.linux-musl-x64 3.1.18, 5.0.9
nuget Microsoft.NETCore.App.Runtime.linux-musl-arm64 3.1.18, 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, 5.0.9
nuget Microsoft.NETCore.App.Runtime.linux-arm 3.1.18, 5.0.9
bitnami dotnet 5.0.9
bitnami dotnet-sdk 5.0.9
Microsoft .NET 5.0 5.0.9
Microsoft .NET Core 2.1 2.1.30
Microsoft .NET Core 3.1 3.1.18
Microsoft Microsoft Visual Studio 2017 version 15.9 (includes 15.0 - 15.8) 15.9.38
Microsoft Microsoft Visual Studio 2019 version 16.10 (includes 16.0 - 16.9) 16.10.5

red hat arch linux

dotnet RHEL 8 5.0.206-1.el8_4
dotnet5.0 RHEL 8 5.0.206-1.el8_4
dotnet-host RHEL 8 5.0.9-1.el8_4
dotnet-sdk-5.0 RHEL 8 5.0.206-1.el8_4
dotnet-hostfxr-5.0 RHEL 8 5.0.9-1.el8_4
dotnet-runtime-5.0 RHEL 8 5.0.9-1.el8_4
dotnet-templates-5.0 RHEL 8 5.0.206-1.el8_4
aspnetcore-runtime-5.0 RHEL 8 5.0.9-1.el8_4

red hat advisories RHSA-2021:3148 RHSA-2021:3147 RHSA-2021:3142 RHSA-2021:3143

dotnet-sdk Arch Linux 6.0.0.sdk100-1
dotnet-runtime Arch Linux 6.0.0.sdk100-1

arch linux advisories AVG-2277

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-26423 updated 2026-11-07 cvss 7.5

dotnet: ASP.NET Core WebSocket frame processing DoS

Red Hat .NET Core on Red Hat Enterprise Linux ComputeNode (v. 7) rh-dotnet31-dotnet 3.1.118-1.el7_9, rh-dotnet50-dotnet 5.0.206-1.el7_9, rh-dotnet50-dotnet-host 5.0.9-1.el7_9, rh-dotnet31-dotnet-host 3.1.18-1.el7_9, rh-dotnet31-dotnet-sdk-3.1 3.1.118-1.el7_9, rh-dotnet50-dotnet-sdk-5.0 5.0.206-1.el7_9, +16 more Red Hat .NET Core on Red Hat Enterprise Linux Server (v. 7) rh-dotnet31-dotnet 3.1.118-1.el7_9, rh-dotnet50-dotnet 5.0.206-1.el7_9, rh-dotnet50-dotnet-host 5.0.9-1.el7_9, rh-dotnet31-dotnet-host 3.1.18-1.el7_9, rh-dotnet31-dotnet-sdk-3.1 3.1.118-1.el7_9, rh-dotnet50-dotnet-sdk-5.0 5.0.206-1.el7_9, +16 more Red Hat .NET Core on Red Hat Enterprise Linux Workstation (v. 7) rh-dotnet31-dotnet 3.1.118-1.el7_9, rh-dotnet50-dotnet 5.0.206-1.el7_9, rh-dotnet50-dotnet-host 5.0.9-1.el7_9, rh-dotnet31-dotnet-host 3.1.18-1.el7_9, rh-dotnet31-dotnet-sdk-3.1 3.1.118-1.el7_9, rh-dotnet50-dotnet-sdk-5.0 5.0.206-1.el7_9, +16 more
fixed: vendor fix RHSA-2021:3143
Red Hat Enterprise Linux AppStream (v. 8) dotnet 5.0.206-1.el8_4, dotnet-host 5.0.9-1.el8_4, dotnet3.1 3.1.118-1.el8_4, dotnet5.0 5.0.206-1.el8_4, dotnet-sdk-3.1 3.1.118-1.el8_4, dotnet-sdk-5.0 5.0.206-1.el8_4, +15 more
fixed: vendor fix RHSA-2021:3142
Red Hat .NET Core 2.1 on Red Hat Enterprise Linux rh-dotnet21, rh-dotnet21-runtime Red Hat Enterprise Linux 8 dotnet, dotnet-host, dotnet-sdk-2.1, dotnet-runtime-2.1, dotnet-sdk-2.1.5xx, dotnet-host-fxr-2.1
not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation: none
Automatable: yes
Technical impact: partial
Assessed: 17 mo ago (2025-06-04)

Exploit prediction

3.90% chance of exploitation in the next 30 days, higher than 90% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C
GitHub	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD	2.0	5.0	AV:N/AC:L/Au:N/C:N/I:N/A:P
OSV	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem	package	affected	fixed in
nuget	Microsoft.NETCore.App.Runtime.win-x86	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
nuget	Microsoft.NETCore.App.Runtime.win-x64	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
nuget	Microsoft.NETCore.App.Runtime.win-arm64	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
nuget	Microsoft.NETCore.App.Runtime.win-arm	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
nuget	Microsoft.NETCore.App.Runtime.osx-x64	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.osx-x64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.LLVM.osx-x64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-x64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.LLVM.linux-arm64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.osx-x64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-x64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.LLVM.AOT.linux-arm64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.linux-x64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.linux-musl-x64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.linux-arm64	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.Mono.linux-arm	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.linux-x64	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
nuget	Microsoft.NETCore.App.Runtime.linux-musl-x64	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
nuget	Microsoft.NETCore.App.Runtime.linux-musl-arm64	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
nuget	Microsoft.NETCore.App.Runtime.linux-musl-arm	>=5.0.0, <5.0.9	5.0.9
nuget	Microsoft.NETCore.App.Runtime.rhel.6-x64	>=3.1.0, <3.1.18	3.1.18
nuget	Microsoft.NETCore.App.Runtime.linux-arm64	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
nuget	Microsoft.NETCore.App.Runtime.linux-arm	>=3.1.0, <3.1.18 >=5.0.0, <5.0.9	3.1.18 5.0.9
bitnami	dotnet	>=5.0.0, <5.0.9	5.0.9
bitnami	dotnet-sdk	>=5.0.0, <5.0.9	5.0.9

Affected products

Microsoft .NET 5.0
Microsoft .NET Core 2.1
Microsoft .NET Core 3.1
Microsoft Microsoft Visual Studio 2017 version 15.9 (includes 15.0 - 15.8)
Microsoft Microsoft Visual Studio 2019 version 16.10 (includes 16.0 - 16.9)
Microsoft Microsoft Visual Studio 2019 version 16.4 (includes 16.0 - 16.3)
Microsoft Microsoft Visual Studio 2019 version 16.7 (includes 16.0 – 16.6)
Microsoft Microsoft Visual Studio 2019 version 16.9 (includes 16.0 - 16.8)
Microsoft PowerShell Core 7.0
Microsoft PowerShell Core 7.1
Microsoft Visual Studio 2019 for Mac version 8.10

References

msrc.microsoft.com /update-guide/vulnerability/CVE-2021-26423
vendor-advisory, patch, web
portal.msrc.microsoft.com /en-US/security-guidance/advisory/CVE-2021-26423
web, patch, vendor-advisory, advisory
github.com /dotnet/runtime/security/advisories/GHSA-rh58-r7jh-xhx3
web
nvd.nist.gov /vuln/detail/CVE-2021-26423
advisory, web
github.com /dotnet/announcements/issues/194
web

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary