CVE-2026-62899: .NET Security Feature Bypass Vulnerability

Published Aug 6, 2026
·
Updated

.NET Security Feature Bypass Vulnerability

Other sources

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?

1. 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. 2. 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 --info command.

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

CVE-2026-62899

Acknowledgements

Miha Zupan with Microsoft

Revisions

V1.0 (08/11/2026): Advisory published.

— GitHub

System.Net.HttpListener - managed HttpListener incorrectly parses Content-Length header (Linux/.NET Core only)

Affects: 6.0, 8.0, 9.0, 10.0

— Red Hat

Affected Software

44 affected componentsFixes available
Microsoft Visual Studio 2022=17.14
17.14.38
Microsoft .NET 8.0 installed on Mac OS<8.0.30
8.0.30
Microsoft .NET 9.0 installed on Windows<9.0.19
9.0.19
Microsoft .NET 9.0 installed on Mac OS<9.0.19
9.0.19
Microsoft .NET 8.0 installed on Windows<8.0.30
8.0.30
Microsoft .NET 8.0 installed on Linux<8.0.30
8.0.30
Microsoft .NET 9.0 installed on Linux<9.0.19
9.0.19
Microsoft Visual Studio 2026=18.8
18.8.3
Microsoft .NET 10.0 installed on Mac OS<10.0.11
10.0.11
Microsoft .NET 10.0 installed on Linux<10.0.11
10.0.11
nuget/Microsoft.NETCore.App.Runtime.osx-x64>=8.0.0<=8.0.29
8.0.30
nuget/Microsoft.NETCore.App.Runtime.osx-arm64>=8.0.0<=8.0.29
8.0.30
nuget/Microsoft.NETCore.App.Runtime.linux-x64>=8.0.0<=8.0.29
8.0.30
nuget/Microsoft.NETCore.App.Runtime.linux-musl-x64>=8.0.0<=8.0.29
8.0.30
nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm64>=8.0.0<=8.0.29
8.0.30
nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm>=8.0.0<=8.0.29
8.0.30
nuget/Microsoft.NETCore.App.Runtime.linux-arm64>=8.0.0<=8.0.29
8.0.30
nuget/Microsoft.NETCore.App.Runtime.linux-arm>=8.0.0<=8.0.29
8.0.30
nuget/Microsoft.NETCore.App.Runtime.osx-x64>=9.0.0<=9.0.18
9.0.19
nuget/Microsoft.NETCore.App.Runtime.osx-arm64>=9.0.0<=9.0.18
9.0.19
nuget/Microsoft.NETCore.App.Runtime.linux-x64>=9.0.0<=9.0.18
9.0.19
nuget/Microsoft.NETCore.App.Runtime.linux-musl-x64>=9.0.0<=9.0.18
9.0.19
nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm64>=9.0.0<=9.0.18
9.0.19
nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm>=9.0.0<=9.0.18
9.0.19
nuget/Microsoft.NETCore.App.Runtime.linux-arm64>=9.0.0<=9.0.18
9.0.19
nuget/Microsoft.NETCore.App.Runtime.linux-arm>=9.0.0<=9.0.18
9.0.19
nuget/Microsoft.NETCore.App.Runtime.osx-x64>=10.0.0<=10.0.10
10.0.11
nuget/Microsoft.NETCore.App.Runtime.osx-arm64>=10.0.0<=10.0.10
10.0.11
nuget/Microsoft.NETCore.App.Runtime.linux-x64>=10.0.0<=10.0.10
10.0.11
nuget/Microsoft.NETCore.App.Runtime.linux-musl-x64>=10.0.0<=10.0.10
10.0.11
nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm64>=10.0.0<=10.0.10
10.0.11
nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm>=10.0.0<=10.0.10
10.0.11
nuget/Microsoft.NETCore.App.Runtime.linux-arm64>=10.0.0<=10.0.10
10.0.11
nuget/Microsoft.NETCore.App.Runtime.linux-arm>=10.0.0<=10.0.10
10.0.11
Microsoft Visual Studio 2022>=17.14.0<17.14.38
Microsoft Visual Studio 2026>=18.8.0<18.8.3
All of the following
Any of the following
Microsoft .NET>=8.0.0<8.0.30
Microsoft .NET>=9.0.0<9.0.19
Any of the following
Apple macOS
Linux Linux kernel
Microsoft Windows
All of the following
Microsoft .NET>=10.0.0<10.0.11
Any of the following
Apple macOS
Linux Linux kernel

Remediation

Recommended actions to resolve this vulnerability, in priority order.

  1. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 8.0.30Patch KB5122104
  2. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 17.14.38
  3. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 9.0.19Patch KB5122105
  4. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 10.0.11Patch KB5122106
  5. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 18.8.3
  6. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.osx-x64 to a version that resolves this vulnerability.

    Fixed in 8.0.30
  7. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.osx-arm64 to a version that resolves this vulnerability.

    Fixed in 8.0.30
  8. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-x64 to a version that resolves this vulnerability.

    Fixed in 8.0.30
  9. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-musl-x64 to a version that resolves this vulnerability.

    Fixed in 8.0.30
  10. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm64 to a version that resolves this vulnerability.

    Fixed in 8.0.30
  11. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm to a version that resolves this vulnerability.

    Fixed in 8.0.30
  12. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-arm64 to a version that resolves this vulnerability.

    Fixed in 8.0.30
  13. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-arm to a version that resolves this vulnerability.

    Fixed in 8.0.30
  14. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.osx-x64 to a version that resolves this vulnerability.

    Fixed in 9.0.19
  15. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.osx-arm64 to a version that resolves this vulnerability.

    Fixed in 9.0.19
  16. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-x64 to a version that resolves this vulnerability.

    Fixed in 9.0.19
  17. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-musl-x64 to a version that resolves this vulnerability.

    Fixed in 9.0.19
  18. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm64 to a version that resolves this vulnerability.

    Fixed in 9.0.19
  19. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm to a version that resolves this vulnerability.

    Fixed in 9.0.19
  20. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-arm64 to a version that resolves this vulnerability.

    Fixed in 9.0.19
  21. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-arm to a version that resolves this vulnerability.

    Fixed in 9.0.19
  22. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.osx-x64 to a version that resolves this vulnerability.

    Fixed in 10.0.11
  23. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.osx-arm64 to a version that resolves this vulnerability.

    Fixed in 10.0.11
  24. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-x64 to a version that resolves this vulnerability.

    Fixed in 10.0.11
  25. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-musl-x64 to a version that resolves this vulnerability.

    Fixed in 10.0.11
  26. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm64 to a version that resolves this vulnerability.

    Fixed in 10.0.11
  27. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-musl-arm to a version that resolves this vulnerability.

    Fixed in 10.0.11
  28. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-arm64 to a version that resolves this vulnerability.

    Fixed in 10.0.11
  29. Upgrade

    Upgrade nuget/Microsoft.NETCore.App.Runtime.linux-arm to a version that resolves this vulnerability.

    Fixed in 10.0.11
  30. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-arm64 to a version that resolves this vulnerability.

    Fixed in 10.0.11Patch CVE-2026-62899
  31. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-arm64 to a version that resolves this vulnerability.

    Fixed in 8.0.30Patch CVE-2026-62899
  32. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-arm64 to a version that resolves this vulnerability.

    Fixed in 9.0.19Patch CVE-2026-62899
  33. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-arm to a version that resolves this vulnerability.

    Fixed in 10.0.11Patch CVE-2026-62899
  34. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-arm to a version that resolves this vulnerability.

    Fixed in 8.0.30Patch CVE-2026-62899
  35. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-arm to a version that resolves this vulnerability.

    Fixed in 9.0.19Patch CVE-2026-62899
  36. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm64 to a version that resolves this vulnerability.

    Fixed in 10.0.11Patch CVE-2026-62899
  37. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm64 to a version that resolves this vulnerability.

    Fixed in 8.0.30Patch CVE-2026-62899
  38. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm64 to a version that resolves this vulnerability.

    Fixed in 9.0.19Patch CVE-2026-62899
  39. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm to a version that resolves this vulnerability.

    Fixed in 10.0.11Patch CVE-2026-62899
  40. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm to a version that resolves this vulnerability.

    Fixed in 8.0.30Patch CVE-2026-62899
  41. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-musl-arm to a version that resolves this vulnerability.

    Fixed in 9.0.19Patch CVE-2026-62899
  42. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-musl-x64 to a version that resolves this vulnerability.

    Fixed in 10.0.11Patch CVE-2026-62899
  43. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-musl-x64 to a version that resolves this vulnerability.

    Fixed in 8.0.30Patch CVE-2026-62899
  44. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-musl-x64 to a version that resolves this vulnerability.

    Fixed in 9.0.19Patch CVE-2026-62899
  45. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-x64 to a version that resolves this vulnerability.

    Fixed in 10.0.11Patch CVE-2026-62899
  46. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-x64 to a version that resolves this vulnerability.

    Fixed in 8.0.30Patch CVE-2026-62899
  47. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.linux-x64 to a version that resolves this vulnerability.

    Fixed in 9.0.19Patch CVE-2026-62899
  48. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.osx-arm64 to a version that resolves this vulnerability.

    Fixed in 10.0.11Patch CVE-2026-62899
  49. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.osx-arm64 to a version that resolves this vulnerability.

    Fixed in 8.0.30Patch CVE-2026-62899
  50. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.osx-arm64 to a version that resolves this vulnerability.

    Fixed in 9.0.19Patch CVE-2026-62899
  51. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.osx-x64 to a version that resolves this vulnerability.

    Fixed in 10.0.11Patch CVE-2026-62899
  52. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.osx-x64 to a version that resolves this vulnerability.

    Fixed in 8.0.30Patch CVE-2026-62899
  53. Upgrade

    Upgrade Microsoft.NETCore.App.Runtime.osx-x64 to a version that resolves this vulnerability.

    Fixed in 9.0.19Patch CVE-2026-62899
  54. Operational

    After installing the updated runtime or SDK, restart affected apps so the update takes effect.

  55. Operational

    If you deployed self-contained applications targeting any impacted .NET versions, recompile and redeploy those applications.

Event History

Aug 6, 2026
Data Sourced
via Red Hat·08:40 PM
DescriptionSeverityAffected Software
Aug 11, 2026
CVE Published
via Microsoft·02:00 PM
Data Sourced
via Microsoft·02:00 PM
DescriptionSeverityWeakness
Data Sourced
via Microsoft·02:00 PM
Affected Software
Updated
via Microsoft·02:00 PM
Affected Software
Updated
via Microsoft·02:00 PM
Description
CVE Published
via MITRE·05:04 PM
Data Sourced
via MITRE·05:04 PM
DescriptionSeverity
Data Sourced
via NVD·05:18 PM
RemedyDescriptionSeverityWeaknessAffected Software
Advisory Published
via GitHub·06:40 PM
Data Sourced
via GitHub·06:40 PM
DescriptionSeverityWeaknessAffected Software
Aug 19, 2026
Data Sourced
via Debian·07:21 AM
DescriptionAffected Software
Data Sourced
via Launchpad·07:21 AM
Description
Aug 20, 2026
Data Sourced
via Ubuntu·07:20 AM
RemedyDescriptionSeverityAffected Software

Frequently Asked Questions

1

What is the severity of CVE-2026-62899?

CVE-2026-62899 has a severity rating of medium with a score of 5.9.

2

What types of software are affected by CVE-2026-62899?

CVE-2026-62899 affects Microsoft .NET 8.0 and 9.0 on various operating systems, including Windows, Mac OS, and Linux.

3

How does CVE-2026-62899 exploit occur?

CVE-2026-62899 allows an unauthorized attacker to bypass security features through inconsistent interpretation of HTTP requests.

4

What is the impact of CVE-2026-62899 on security?

The impact of CVE-2026-62899 is that it can potentially allow attackers to bypass security mechanisms over a network.

5

How do I fix CVE-2026-62899?

To fix CVE-2026-62899, ensure that you are using the latest security patches and updates provided by Microsoft for the affected .NET versions.

Company

Resources

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