Microsoft Security Advisory CVE-2025-26646: .NET Spoofing Vulnerability

[rbhanda]

Microsoft Security Advisory CVE-2025-26646: .NET Spoofing Vulnerability

Executive summary

Microsoft is releasing this security advisory to provide information about a vulnerability in .NET 9.0.xxx and .NET 8.0.xxx SDK. This advisory also provides guidance on what developers can do to update their applications to address this vulnerability.

A vulnerability exists in .NET SDK or MSBuild applications where external control of file name or path allows an unauthorized attacked to perform spoofing over a network.

Announcement

Announcement for this issue can be found at dotnet/announcements#356

Mitigation factors

Projects which do not utilize the DownloadFile build task are not susceptible to this vulnerability.

Affected software

• Any installation of .NET 9.0.105 SDK, .NET 9.0.203 SDK or earlier.
• Any installation of .NET 8.0.115 SDK, .NET 8.0.311 SDK, .NET 8.0.408 SDK or earlier.

Affected Packages

The vulnerability affects any Microsoft .NET Core project if it uses any of affected packages versions listed below

Package name	Affected version	Patched version
Microsoft.Build.Tasks.Core	>= 15.8.166, <=15.9.20 >=16.0.461, <= 16.11.0 >= 17.0.0, <= 17.8.3 >= 17.9.5, <= 17.10.4 17.11.4 17.12.6 17.13.9	15.9.30 16.11.6 17.8.29 17.10.29 17.12.36 17.13.26 17.14.8

Advisory FAQ

How do I know if I am affected?

If you have a .NET SDK with a version listed, or an affected package listed in affected software or 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 9.0 SDK or .NET 8.0 SDK. If you have installed one or more .NET SDKs through Visual Studio, Visual Studio will prompt you to update Visual Studio, which will also update your .NET SDKs.
2. If your application references the vulnerable package, update the package reference to the patched version.

• You can list the versions you have installed by running the dotnet --info command. You will see output like the following;

.NET SDK:
 Version:           9.0.100
 Commit:            59db016f11
 Workload version:  9.0.100-manifests.3068a692
 MSBuild version:   17.12.7+5b8665660

Runtime Environment:
 OS Name:     Mac OS X
 OS Version:  15.2
 OS Platform: Darwin
 RID:         osx-arm64
 Base Path:   /usr/local/share/dotnet/sdk/9.0.100/

.NET workloads installed:
There are no installed workloads to display.
Configured to use loose manifests when installing new manifests.

Host:
  Version:      9.0.0
  Architecture: arm64
  Commit:       9d5a6a9aa4

.NET SDKs installed:
  9.0.100 [/usr/local/share/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 9.0.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 9.0.0 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

Other architectures found:
  x64   [/usr/local/share/dotnet]
    registered at [/etc/dotnet/install_location_x64]

Environment variables:
  Not set

global.json file:
  Not found

Learn more:
  https://aka.ms/dotnet/info

Download .NET:
  https://aka.ms/dotnet/download

• If you're using .NET 9.0, you should download and install the appropriate SDK: .NET 9.0.300 for Visual Studio 2022 v17.14, .NET 9.0.204 for v17.13, or .NET 9.0.106 for v17.12. Download from https://dotnet.microsoft.com/download/dotnet-core/9.0.

• If you're using .NET 8.0, you should download and install the appropriate SDK: .NET 8.0.409 for Visual Studio 2022 v17.11, .NET 8.0.312 for v17.10, or .NET 8.0.116 for v17.8. Download from https://dotnet.microsoft.com/download/dotnet-core/8.0.

Once you have installed the updated SDK, restart your apps for the update to take effect.

Other Information

Reporting Security Issues

If you have found a potential security issue in .NET 8.0 or .NET 9.0, please email details to [email protected]. Reports may qualify for the Microsoft .NET Core & .NET 5 Bounty. Details of the Microsoft .NET Bounty Program including terms and conditions are at https://aka.ms/corebounty.

Support

You can ask questions about this issue on GitHub in the .NET GitHub organization. The main repos are located at https://github.com/dotnet/aspnetcore. The Announcements repo (https://github.com/dotnet/Announcements) will contain this bulletin as an issue and will include a link to a discussion issue. You 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-2025-26646

Revisions

V1.0 (May 13, 2025): Advisory published.

Version 1.0

Last Updated 2025-05-13

[chrispatrick]

This issue indicates that the latest version of the SDK, e.g. 8.0.409 should have a fix for this. But I'm still seeing vulnerable versions of the package in question in that SDK?

[JanProvaznik]

@chrispatrick I downloaded the sdk 8.0.409, decompiled the Microsoft.Build.Tasks.Core.dll and the fix is present. What makes you say it's not?

[chrispatrick]

I'm seeing version 17.7.2.37605 of Microsoft.Build.Tasks.Core.dll located at DotnetTools/dotnet-format/Microsoft.Build.Tasks.Core.dll
And version 17.11.31.21017 located at FSharp/Microsoft.Build.Tasks.Core.dll

Both in 8.0.409.

Those versions of Microsoft.Build.Tasks.Core.dll are marked as vulnerable.

[JanProvaznik]

This is a bit of an chicken and an egg problem, because the SDK is assembled from different components where it's not trivial to depend on packages that have not been released. It's wrong, thanks for the report. I'm not sure if these are ever loaded and used any scenario of running the DownloadFile task. The dll that's loaded during dotnet build is sdk/$(version)/Microsoft.Build.Tasks.Core.dll which is correct.
I'll bring up if this can be done better @baronfel

[chrispatrick]

OK, if we can get confirmation that the vulnerable part of those packages are not used as part of using the SDK then that's good enough for me, thanks!

[chrispatrick]

Will the same update be applied to the v8 SDK as well do you know?

[chrispatrick]

Or alternatively, could you confirm whether the library at DotnetTools/dotnet-format/Microsoft.Build.Tasks.Core.dll is exploitable at all. Thanks.