CVE-2026-62899
MEDIUMCVE-2026-62899 is a medium-severity (CVSS 5.9) CWE-444 vulnerability in Microsoft.NETCore.App.Runtime.linux-arm. O3 Security confirms whether CVE-2026-62899 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Microsoft Security Advisory CVE-2026-62899 – .NET Security Feature Bypass Vulnerability
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-62899.
Real-World Exposure
Microsoft.NETCore.App.Runtime.linux-arm.NETMicrosoft.NETCore.App.Runtime.linux-arm64.NETMicrosoft.NETCore.App.Runtime.linux-musl-arm.NETMicrosoft.NETCore.App.Runtime.linux-musl-arm64.NETMicrosoft.NETCore.App.Runtime.linux-musl-x64.NETMicrosoft.NETCore.App.Runtime.linux-x64.NETMicrosoft.NETCore.App.Runtime.osx-arm64.NETMicrosoft.NETCore.App.Runtime.osx-x64+16 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
Executive summary
Microsoft is releasing this security advisory to provide information about a vulnerability in System.Net.HttpListener. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.
Inconsistent interpretation of http requests ('http request/response smuggling') in .NET allows an unauthorized attacker to bypass a security feature over a network.
Announcement
Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/427
CVSS Details
- Version: 3.1
- Severity: Medium
- Score: 5.9
- Vector:
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N/E:U/RL:O/RC:C - Weakness: CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
Affected Platforms
- Platforms: Linux, macOS
- Architectures: All
<a name="affected-packages"></a>Affected Packages
The vulnerability affects any Microsoft .NET project if it uses any of affected package versions listed below
<a name=".NET 10"></a>.NET 10
| Package name | Affected version | Patched version |
|---|---|---|
| Microsoft.NETCore.App.Runtime.linux-arm | >= 10.0.0, <= 10.0.10 | 10.0.11 |
| Microsoft.NETCore.App.Runtime.linux-arm64 | >= 10.0.0, <= 10.0.10 | 10.0.11 |
| Microsoft.NETCore.App.Runtime.linux-musl-arm | >= 10.0.0, <= 10.0.10 | 10.0.11 |
| Microsoft.NETCore.App.Runtime.linux-musl-arm64 | >= 10.0.0, <= 10.0.10 | 10.0.11 |
| Microsoft.NETCore.App.Runtime.linux-musl-x64 | >= 10.0.0, <= 10.0.10 | 10.0.11 |
| Microsoft.NETCore.App.Runtime.linux-x64 | >= 10.0.0, <= 10.0.10 | 10.0.11 |
| Microsoft.NETCore.App.Runtime.osx-arm64 | >= 10.0.0, <= 10.0.10 | 10.0.11 |
| Microsoft.NETCore.App.Runtime.osx-x64 | >= 10.0.0, <= 10.0.10 | 10.0.11 |
<a name=".NET 9"></a>.NET 9
| Package name | Affected version | Patched version |
|---|---|---|
| Microsoft.NETCore.App.Runtime.linux-arm | >= 9.0.0, <= 9.0.18 | 9.0.19 |
| Microsoft.NETCore.App.Runtime.linux-arm64 | >= 9.0.0, <= 9.0.18 | 9.0.19 |
| Microsoft.NETCore.App.Runtime.linux-musl-arm | >= 9.0.0, <= 9.0.18 | 9.0.19 |
| Microsoft.NETCore.App.Runtime.linux-musl-arm64 | >= 9.0.0, <= 9.0.18 | 9.0.19 |
| Microsoft.NETCore.App.Runtime.linux-musl-x64 | >= 9.0.0, <= 9.0.18 | 9.0.19 |
| Microsoft.NETCore.App.Runtime.linux-x64 | >= 9.0.0, <= 9.0.18 | 9.0.19 |
| Microsoft.NETCore.App.Runtime.osx-arm64 | >= 9.0.0, <= 9.0.18 | 9.0.19 |
| Microsoft.NETCore.App.Runtime.osx-x64 | >= 9.0.0, <= 9.0.18 | 9.0.19 |
<a name=".NET 8"></a>.NET 8
| Package name | Affected version | Patched version |
|---|---|---|
| Microsoft.NETCore.App.Runtime.linux-arm | >= 8.0.0, <= 8.0.29 | 8.0.30 |
| Microsoft.NETCore.App.Runtime.linux-arm64 | >= 8.0.0, <= 8.0.29 | 8.0.30 |
| Microsoft.NETCore.App.Runtime.linux-musl-arm | >= 8.0.0, <= 8.0.29 | 8.0.30 |
| Microsoft.NETCore.App.Runtime.linux-musl-arm64 | >= 8.0.0, <= 8.0.29 | 8.0.30 |
| Microsoft.NETCore.App.Runtime.linux-musl-x64 | >= 8.0.0, <= 8.0.29 | 8.0.30 |
| Microsoft.NETCore.App.Runtime.linux-x64 | >= 8.0.0, <= 8.0.29 | 8.0.30 |
| Microsoft.NETCore.App.Runtime.osx-arm64 | >= 8.0.0, <= 8.0.29 | 8.0.30 |
| Microsoft.NETCore.App.Runtime.osx-x64 | >= 8.0.0, <= 8.0.29 | 8.0.30 |
Advisory FAQ
<a name="how-affected"></a>How do I know if I am affected?
If using a package listed in affected packages, users are exposed to the vulnerability.
<a name="how-fix"></a>How do I fix the issue?
- To fix the issue please install the latest version of .NET. If users have installed one or more .NET SDKs through Visual Studio, Visual Studio will prompt then to update Visual Studio, which will also update their .NET SDKs.
- If a user application references the vulnerable package, update the package reference to the patched version. They can list the versions they have installed by running the
dotnet --infocommand.
Once users have installed the updated runtime or SDK, they should restart their apps for the update to take effect.
Additionally, if they've deployed self-contained applications targeting any of the impacted versions, these applications are also vulnerable and must be recompiled and redeployed.
Other Information
Reporting Security Issues
If users have found a potential security issue in a supported version of .NET, please report it to the Microsoft Security Response Center (MSRC) via the MSRC Researcher Portal. Further information can be found in the MSRC Report an Issue FAQ.
Security reports made through MSRC may qualify for the Microsoft .NET Bounty. Details of the Microsoft .NET Bounty Program including terms and conditions are at https://aka.ms/corebounty.
Support
Users can ask questions about this issue on GitHub in the .NET GitHub organization. The main repos are located at https://github.com/dotnet/runtime. The Announcements repo (https://github.com/dotnet/Announcements) will contain this bulletin as an issue and will include a link to a discussion issue. Users can ask questions in the linked discussion issue.
Disclaimer
The information provided in this advisory is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.
External Links
Acknowledgements
Miha Zupan with Microsoft
Revisions
V1.0 (08/11/2026): Advisory published.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | Microsoft.NETCore.App.Runtime.linux-arm | ≥ 10.0.0&&< 10.0.11 | 10.0.11 |
| .NETNuGet | Microsoft.NETCore.App.Runtime.linux-arm64 | ≥ 10.0.0&&< 10.0.11 | 10.0.11 |
| .NETNuGet | Microsoft.NETCore.App.Runtime.linux-musl-arm | ≥ 10.0.0&&< 10.0.11 | 10.0.11 |
| .NETNuGet | Microsoft.NETCore.App.Runtime.linux-musl-arm64 | ≥ 10.0.0&&< 10.0.11 | 10.0.11 |
| .NETNuGet | Microsoft.NETCore.App.Runtime.linux-musl-x64 | ≥ 10.0.0&&< 10.0.11 | 10.0.11 |
| .NETNuGet | Microsoft.NETCore.App.Runtime.linux-x64 | ≥ 10.0.0&&< 10.0.11 | 10.0.11 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Microsoft.NETCore.App.Runtime.linux-arm. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update Microsoft.NETCore.App.Runtime.linux-arm to 10.0.11 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-62899 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 pinpoints whether CVE-2026-62899 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-62899. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-62899 in your dependencies?
O3 detects CVE-2026-62899 across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.