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CVE-2026-66822

Microsoft Security Advisory CVE-2026-66822 - .NET and Visual Studio Information Disclosure Vulnerability

Published Oct 13, 2026 · updated Oct 13, 2026 · Microsoft .NET 10.0

## Executive summary Microsoft is releasing this security advisory to provide information about a vulnerability in System.Net.Http. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability. SocketsHttpHandler can retain a header across an HTTP redirect and leak basic proxy credentials to the destination server. ## Announcement Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/448 ## CVSS Details - **Version:** 3.1 - **Severity:** Moderate - **Score:** 5.9 - **Vector:** - **Weakness:** CWE-201 (Insertion of Sensitive Information Into Sent Data) ## Affected Platforms - **Platforms:** Windows, Linux, macOS - **Architectures:** All ## Affected Packages The vulnerability affects Microsoft .NET projects that use an affected package version listed below. Package name | Affected version | Patched version ------------ | ---------------- | --------------- [Microsoft.NETCore.App.Runtime.linux-arm](https://www.nuget.org/packages/Microsoft.NETCore.App.Runtime.linux-arm) | >= 9.0.0, = 9.0.0, = 9.0.0, = 9.0.0, = 9.0.0, = 9.0.0, = 9.0.0, = 9.0.0, = 9.0.0, = 9.0.0, = 9.0.0, = 9.0.0, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 11.0.0-preview.1, = 10.0.0, = 10.0.0, = 10.0.0, = 10.0.0, = 10.0.0, = 10.0.0, = 10.0.0, = 10.0.0, = 10.0.0, = 10.0.0, = 10.0.0, = 10.0.0, = 8.0.0, = 8.0.0, = 8.0.0, = 8.0.0, = 8.0.0, = 8.0.0, = 8.0.0, = 8.0.0, = 8.0.0, = 8.0.0, = 8.0.0, = 8.0.0, How do I know if I am affected? If using a package listed in [affected packages](#affected-packages), you're exposed to the vulnerability. ### How do I fix the issue? 1. To fix the issue please install the latest version of .NET. If you have installed one or more .NET SDKs through Visual Studio, Visual Studio will prompt you to update Visual

Timeline

Detection, updates, and resolution for this record.

  1. Oct 13, 2026 · Catalog

    Detected

    ## Executive summary Microsoft is releasing this security advisory to provide information about a vulnerability in System.Net.Http. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability. SocketsHttpHandler can retain a header across an HTTP redirect and leak basic proxy credentials to the destination server. ## Announcement Announc

  2. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.linux-arm fixed in 9.0.21; Microsoft.NETCore.App.Runtime.linux-arm64 fixed in 9.0.21; Microsoft.NETCore.App.Runtime.linux-musl-arm fixed in 9.0.21; Microsoft.NETCore.App.Runtime.linux-musl-arm64 fixed in 9.0.21

  3. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.linux-arm 9.0.0 → 9.0.21

  4. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.linux-arm64 9.0.0 → 9.0.21

  5. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.linux-musl-arm 9.0.0 → 9.0.21

  6. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.linux-musl-arm64 9.0.0 → 9.0.21

  7. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.linux-musl-x64 9.0.0 → 9.0.21

  8. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.linux-x64 9.0.0 → 9.0.21

  9. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.osx-arm64 9.0.0 → 9.0.21

  10. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.osx-x64 9.0.0 → 9.0.21

  11. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.win-arm64 9.0.0 → 9.0.21

  12. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.win-x64 9.0.0 → 9.0.21

  13. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.win-x86 9.0.0 → 9.0.21

  14. Oct 13, 2026 · Catalog

    Fix available

    NuGet Microsoft.NETCore.App.Runtime.linux-arm 11.0.0-preview.1 → 11.0.0-rc.2

  15. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.linux-arm fixed in 9.0.21

  16. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.linux-arm64 fixed in 9.0.21

  17. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.linux-musl-arm fixed in 9.0.21

  18. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.linux-musl-arm64 fixed in 9.0.21

  19. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.linux-musl-x64 fixed in 9.0.21

  20. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.linux-x64 fixed in 9.0.21

  21. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.osx-arm64 fixed in 9.0.21

  22. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.osx-x64 fixed in 9.0.21

  23. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.win-arm64 fixed in 9.0.21

  24. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.win-x64 fixed in 9.0.21

  25. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.win-x86 fixed in 9.0.21

  26. Oct 13, 2026 · GitHub Advisory Database

    Fix available

    Microsoft.NETCore.App.Runtime.linux-arm fixed in 11.0.0-rc.2

  27. Oct 13, 2026 · Catalog

    Updated

    ## Executive summary Microsoft is releasing this security advisory to provide information about a vulnerability in System.Net.Http. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability. SocketsHttpHandler can retain a header across an HTTP redirect and leak basic proxy credentials to the destination server. ## Announcement Announc

Affected

  • NuGet Microsoft.NETCore.App.Runtime.linux-arm11.0.0-preview.1 → 11.0.0-rc.2
  • NuGet Microsoft.NETCore.App.Runtime.linux-arm9.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.linux-arm649.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.linux-musl-arm9.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.linux-musl-arm649.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.linux-musl-x649.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.linux-x649.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.osx-arm649.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.osx-x649.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.win-arm649.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.win-x649.0.0 → 9.0.21
  • NuGet Microsoft.NETCore.App.Runtime.win-x869.0.0 → 9.0.21
  • nuget microsoft.netcore.app.runtime.linux-armversion unspecified
  • nuget microsoft.netcore.app.runtime.linux-arm64version unspecified
  • nuget microsoft.netcore.app.runtime.linux-musl-armversion unspecified
  • nuget microsoft.netcore.app.runtime.linux-musl-arm64version unspecified
  • nuget microsoft.netcore.app.runtime.linux-musl-x64version unspecified
  • nuget microsoft.netcore.app.runtime.linux-x64version unspecified
  • nuget microsoft.netcore.app.runtime.osx-arm64version unspecified
  • nuget microsoft.netcore.app.runtime.osx-x64version unspecified
  • nuget microsoft.netcore.app.runtime.win-arm64version unspecified
  • nuget microsoft.netcore.app.runtime.win-x64version unspecified
  • nuget microsoft.netcore.app.runtime.win-x86version unspecified
  • vendor Microsoft .NET 10.0version unspecified

Sources

  • CVE.orgCVE-2026-66822

    Insertion of sensitive information into sent data in .NET allows an unauthorized attacker to disclose information over a network.

    cve.org
  • FIRST EPSSCVE-2026-66822

    30-day exploitation probability 0.7%

    first.org
  • GitHub Advisory DatabaseGHSA-wxxw-5cvc-53j7

    GitHub Advisory Database record mirrored by OSV.

    github.com
  • GitLab Advisory DatabaseCVE-2026-66822

    Microsoft is releasing this security advisory to provide information about a vulnerability in System.Net.Http. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability. SocketsHttpHandler can retain a Proxy-Authorization header across an HTTP redirect and leak basic proxy credentials to the destination server.

    advisories.gitlab.com
  • OSV.devGHSA-wxxw-5cvc-53j7

    Microsoft Security Advisory CVE-2026-66822 - .NET and Visual Studio Information Disclosure Vulnerability

    osv.dev

References