How severe is CVE-2021-1723?

CVE-2021-1723 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-1723?

CVE-2021-1723 affects the following packages: Microsoft.AspNetCore.Server.Kestrel.Core (NuGet), Microsoft.AspNetCore.App.Runtime.linux-arm (NuGet), Microsoft.AspNetCore.App.Runtime.linux-arm (NuGet), Microsoft.AspNetCore.App.Runtime.linux-arm64 (NuGet), Microsoft.AspNetCore.App.Runtime.linux-arm64 (NuGet), Microsoft.AspNetCore.App.Runtime.linux-musl-arm64 (NuGet), Microsoft.AspNetCore.App.Runtime.linux-musl-arm64 (NuGet), Microsoft.AspNetCore.App.Runtime.linux-musl-x64 (NuGet), and 14 more. Ecosystems affected: NuGet.

How do I fix CVE-2021-1723?

Update Microsoft.AspNetCore.Server.Kestrel.Core to 2.1.25 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-1723 is resolved across your whole dependency graph.

How do I detect CVE-2021-1723 in my NuGet dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Microsoft.AspNetCore.Server.Kestrel.Core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-1723 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-1723 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-1723 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-1723?

CVE-2021-1723 has an EPSS (Exploit Prediction Scoring System) score of 5.3%, placing it in the 93th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

When was CVE-2021-1723 published, and has it been updated?

CVE-2021-1723 was published on January 12, 2021 and was last updated on October 24, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

.NET

.NET NuGet

Not in CISA KEV

HIGH severity

CVE-2021-1723 — Microsoft.AspNetCore.Serv…

HIGH

CVE-2021-1723 is a high-severity (CVSS 7.5) vulnerability in Microsoft.AspNetCore.Server.Kestrel.Core. A fix is available for Microsoft.AspNetCore.Server.Kestrel.Core — see the affected versions and patch details below.

ASP.NET Core and Visual Studio Denial of Service Vulnerability

Also known asBIT-aspnet-core-2021-1723GHSA-242j-2gm6-5rwx

Published

Jan 12, 2021

Updated

Oct 24, 2026

Affected

22 pkgs

Patched

22 / 22

Exploits

None indexed

Exploitation data as of Nov 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

5.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs93th percentile — riskier than 93% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

22 pkgs affected

.NETMicrosoft.AspNetCore.Server.Kestrel.Core.NETMicrosoft.AspNetCore.App.Runtime.linux-arm.NETMicrosoft.AspNetCore.App.Runtime.linux-arm.NETMicrosoft.AspNetCore.App.Runtime.linux-arm64.NETMicrosoft.AspNetCore.App.Runtime.linux-arm64.NETMicrosoft.AspNetCore.App.Runtime.linux-musl-arm64.NETMicrosoft.AspNetCore.App.Runtime.linux-musl-arm64.NETMicrosoft.AspNetCore.App.Runtime.linux-musl-x64+14 more

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.

Description

A denial-of-service vulnerability exists in the way Kestrel parses HTTP/2 requests. The security update addresses the vulnerability by fixing the way the Kestrel parses HTTP/2 requests. Users are advised to upgrade.

Affected Packages

22 total 22 fixed

Ecosystem	Package	Vulnerable range	Fix
.NETNuGet	`Microsoft.AspNetCore.Server.Kestrel.Core`	all versions	2.1.25`dotnet add package Microsoft.AspNetCore.Server.Kestrel.Core --version 2.1.25`
.NETNuGet	`Microsoft.AspNetCore.App.Runtime.linux-arm`	≥ 3.1.0&&< 3.1.11	3.1.11`dotnet add package Microsoft.AspNetCore.App.Runtime.linux-arm --version 3.1.11`
.NETNuGet	`Microsoft.AspNetCore.App.Runtime.linux-arm`	≥ 5.0.0&&< 5.0.2	5.0.2`dotnet add package Microsoft.AspNetCore.App.Runtime.linux-arm --version 5.0.2`
.NETNuGet	`Microsoft.AspNetCore.App.Runtime.linux-arm64`	≥ 3.1.0&&< 3.1.11	3.1.11`dotnet add package Microsoft.AspNetCore.App.Runtime.linux-arm64 --version 3.1.11`
.NETNuGet	`Microsoft.AspNetCore.App.Runtime.linux-arm64`	≥ 5.0.0&&< 5.0.2	5.0.2`dotnet add package Microsoft.AspNetCore.App.Runtime.linux-arm64 --version 5.0.2`
.NETNuGet	`Microsoft.AspNetCore.App.Runtime.linux-musl-arm64`	≥ 3.1.0&&< 3.1.11	3.1.11`dotnet add package Microsoft.AspNetCore.App.Runtime.linux-musl-arm64 --version 3.1.11`

Affected Products

3 products · 5 configurations

fedorafedoraproject

2 versions

3233

Application

asp.net coremicrosoft

≥ 5.0 && ≤ 5.0.1

range

Application

visual studio 2019microsoft

≥ 16.0 && ≤ 16.8

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Microsoft.AspNetCore.Server.Kestrel.Core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update Microsoft.AspNetCore.Server.Kestrel.Core to 2.1.25 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-1723 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

Product	Fixed in	Advisory
.NET Core on Red Hat Enterprise Linux	`rh-dotnet31-dotnet-0:3.1.111-1.el7_9`	RHSA-2021:0114
.NET Core on Red Hat Enterprise Linux	`rh-dotnet50-dotnet-0:5.0.102-1.el7_9`	RHSA-2021:0096
Red Hat Enterprise Linux 8	`dotnet5.0-0:5.0.102-2.el8_3`	RHSA-2021:0094
Red Hat Enterprise Linux 8	`dotnet3.1-0:3.1.111-1.el8_3`	RHSA-2021:0095

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-1723 in your dependencies?

Find it across NuGet, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

7.5/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Raw vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Weaponization

No Nuclei template, Metasploit module or public proof-of-concept found.

Timeline

Disclosed

Jan 12, 2021

Last updated

Oct 24, 2026

Identifiers

CVE-2021-1723

GHSA-242j-2gm6-5rwx

BIT-aspnet-core-2021-1723

References

Advisoryportal.msrc.microsoft.com · CVE 2021 1723 Advisorynvd.nist.gov · CVE 2021 1723 Webmsrc.microsoft.com · CVE 2021 1723 Webgithub.com Weblists.fedoraproject.org · 3L27CGRVEWUPELNJOGTCW6GLEDBECB4B Weblists.fedoraproject.org · RRXHERXW4KR5WCP76UDW5PC7GX3YQLUW

Related vulnerabilities

Also affects Microsoft.AspNetCore.Server.Kestrel.Core

This CVE is high severity; 3 of 5 other CVEs affecting Microsoft.AspNetCore.Server.Kestrel.Core are rated as high or higher (top: critical).

CVE-2025-55315CVE-2019-0564CVE-2018-0787GHSA-3m2r-q8x3-xmf7GHSA-cgpw-2gph-2r9g

Also affects Microsoft.AspNetCore.App.Runtime.linux-arm

This CVE is high severity; 2 of 3 other CVEs affecting Microsoft.AspNetCore.App.Runtime.linux-arm are rated as high or higher (top: high).

CVE-2026-70183CVE-2026-70196CVE-2026-96236

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedOct 24, 2026

CVE-2021-1723 — Microsoft.AspNetCore.Serv…

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Fixing This On Your OS

Frequently Asked Questions

Is CVE-2021-1723 in your dependencies?