CVE-2021-1723 — Microsoft.AspNetCore.Serv…
HIGHCVE-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
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-1723 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
Microsoft.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 moreReal-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | Microsoft.AspNetCore.Server.Kestrel.Core | all versions | 2.1.25dotnet add package Microsoft.AspNetCore.Server.Kestrel.Core --version 2.1.25 |
| .NETNuGet | Microsoft.AspNetCore.App.Runtime.linux-arm | ≥ 3.1.0&&< 3.1.11 | 3.1.11dotnet 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 | 5.0.2dotnet 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 | 3.1.11dotnet 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 | 5.0.2dotnet 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 | 3.1.11dotnet add package Microsoft.AspNetCore.App.Runtime.linux-musl-arm64 --version 3.1.11 |
Detection & mitigation playbook
Open-source dependencyDetect
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-1723 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-1723. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
| 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
Is CVE-2021-1723 in your dependencies?
O3 Security finds CVE-2021-1723 across NuGet dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.