Microsoft Security Advisory CVE-2017-8700: CORS bypass can enable Information Disclosure

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Microsoft Security Advisory CVE-2017-8700 CORS Bypass can enable Information Disclosure

Executive Summary

Microsoft is releasing this security advisory to provide information about a vulnerability in public ASP.NET Core 1.0 and 1.1. This advisory also provides guidance on what developers can do to update their applications correctly.

Microsoft is aware of a security vulnerability in some public versions of ASP.NET Core where Cross-Origin Resource Sharing (CORS) can be bypassed, leading to information disclosure.

Discussion

Please use aspnet/Mvc#7054 for discussion of this advisory.

Mitigation Factors

ASP.NET Core applications using version 2.0.0 or higher are not vulnerable.

Affected Software

The vulnerabilities affect any Microsoft ASP.NET Core project if it uses the following affected package versions.

Package name	Package versions	Fixed package versions
Microsoft.AspNetCore.Mvc.Core	1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5 1.1.0, 1.1.1, 1.1.2 ,1.1.3, 1.1.4	1.0.6 1.1.6
Microsoft.AspNetCore.Mvc.Cors	1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5 1.1.0, 1.1.1, 1.1.2 ,1.1.3, 1.1.4	1.0.6 1.1.6

Advisory FAQ

How do I know if I am affected?

.NET Core and ASP.NET Core have two types of dependencies: direct and transitive. If your project has a direct or transitive dependency on any of the packages and versions listed above, you are affected.
Note: As part of patching ASP.NET Core MVC we update every Microsoft.AspNetCore.Mvc.* package. If, for example, you have a dependency on Microsoft.AspNetCore.Mvc you should update to the appropriate version first (1.0.x should be updated to 1.0.6, 1.1.x should be updated to 1.1.6), and it will also update any other vulnerable Microsoft.AspNetCore.Mvc dependency.

NET Core Project formats

.NET Core has two different project file formats, depending on what software created the project.

1. project.json is the format used in .NET Core 1.0 and Microsoft Visual Studio 2015.
2. csproj is the format used in .NET Core 1.1 and Microsoft Visual Studio 2017.

You must ensure you follow the correct update instructions for your project type.

Direct Dependencies

Direct dependencies are dependencies where you specifically add a package to your project. For example, if you add the Microsoft.AspNetCore.Mvc package to your project then you have taken a direct dependency on Microsoft.AspNetCore.Mvc.

Direct dependencies are discoverable by reviewing your project.json or csproj file.

Transitive Dependencies

Transitive dependencies occur when you add a package to your project that in turn relies on another package. For example, if you add the Microsoft.AspNetCore.Mvc package to your project it depends on the Microsoft.AspNetCore.Mvc.Core package (among others). Your project has a direct dependency on Microsoft.AspNetCore.Mvc and a transitive dependency on the Microsoft.AspNetCore.Mvc.Core package.
Transitive dependencies are reviewable in the Visual Studio Solution Explorer window, which supports searching, or by reviewing the project.lock.json file contained in the root directory of your project for project.json projects or the project.assets.json file contained in the obj directory of your project for csproj projects. These files are the authoritative list of all packages used by your project, containing both direct and transitive dependencies.

How do I fix my affected application?

You will need to fix both direct dependencies and review and fix any transitive dependencies. The affected packages and versions in the previous “Affected Software” section include each vulnerable package, the vulnerable versions, and the patched versions

---

Note: If you are using ASP.NET Core MVC in your projects you should first update the Microsoft.AspNetCore.Mvc version to the fixed version in the table above. If you are currently using version 1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4 or 1.0.5 you should update your package version to 1.0.6. If you are using version 1.1.0, 1.1.1, 1.1.2, 1.1.3 or 1.1.4 you should update your package version to 1.1.6. This will update every MVC package to the fixed versions.

---

Fixing Direct Dependencies – project.json/VS2015

Open your project.json file in your editor. Look for the dependencies section. Below is an example dependencies section:

    "dependencies": {
      "Microsoft.NETCore.App": {
        "version": "1.0.1",
        "type": "platform"
      },
     "Microsoft.AspNetCore.Server.Kestrel": "1.0.1",
     "Microsoft.AspNetCore.Mvc.Core ": "1.0.1",
    }

This example has three direct dependencies: Microsoft.NETCore.App, Microsoft.AspNetCore.Server.Kestrel and Microsoft.AspNetCore.Mvc.Core.

Microsoft.NetCore.App is the platform the application targets, you should ignore this. The other packages expose their version to the right of the package name. In our example, our non-platform packages are version 1.0.1.

Review your direct dependencies for any instance of the packages and versions listed above. In the example above, there is a direct dependency on the vulnerable package, Microsoft.AspNetCore.Mvc.Core version 1.0.1.

To update to the fixed package, change the version number to be the appropriate package for your release. In the example, this would be updating Microsoft.AspNetCore.Mvc.Core to 1.0.6.

After updating the vulnerable package versions, save your project.json file.

The dependencies section in our example project.json would now look as follows:

  "dependencies": {
    "Microsoft.NETCore.App": {
      "version": "1.0.1",
      "type": "platform"
    },
    "Microsoft.AspNetCore.Server.Kestrel": "1.0.1",
    "Microsoft.AspNetCore.Mvc.Core": "1.0.6",     
  }

If you are using Visual Studio and save your updated project.json file, Visual Studio will restore the new package version. You can see the restore results by opening the Output Window (Ctrl+Alt+O) and changing the Show output from drop-down list to Package Manager.

If you are not using Visual Studio, open a command line and change to your project directory. Execute the dotnet restore command to restore your new dependency.

After you have addressed all of your direct dependencies, you must also review your transitive dependencies.

Fixing Direct Dependencies – csproj/VS2017

Open your projectname.csproj file in your editor, or right click the project in Visual Studio 2017 and choose Edit projectname.csproj from the content menu, where projectname is the name of your project. Look for PackageReference nodes. The following shows an example project file:

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp1.1</TargetFramework>
  </PropertyGroup>
  <PropertyGroup>
    <PackageTargetFallback>$(PackageTargetFallback);portable-net45+win8+wp8+wpa81;</PackageTargetFallback>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Core" Version="1.1.3" />
  </ItemGroup>
  <ItemGroup>
    <DotNetCliToolReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Tools" Version="1.0.0 " />
  </ItemGroup>
</Project>

The example has a single direct dependency, as seen by the PackageReference element. The name of the package is in the Include attribute, and the package version number is in the Version attribute that is exposed to the right of the package name. The example shows a single package Microsoft.AspNetCore.Mvc.Core version 1.1.6.

Review your PackageReference elements for any instance of the packages and versions listed above. In the example above, there is a direct dependency on the vulnerable package, Microsoft.AspNetCore.Mvc.Core version 1.1.3.

To update to the fixed package, change the version number to the appropriate package for your release. In the example, this would be updating Microsoft.AspNetCore.Mvc.Cors to 1.1.6.

After updating the vulnerable package version, save your csproj file. The example csproj would now look as follows:

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp1.1</TargetFramework>
  </PropertyGroup>
  <PropertyGroup>
    <PackageTargetFallback>$(PackageTargetFallback);portable-net45+win8+wp8+wpa81;</PackageTargetFallback>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Core" Version="1.1.6" />
  </ItemGroup>
  <ItemGroup>
    <DotNetCliToolReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Tools" Version="1.0.0 " />
  </ItemGroup>
</Project>

If you are using Visual Studio and save your updated csproj file, Visual Studio will restore the new package version. You can see the restore results by opening the Output Window (Ctrl+Alt+O) and changing the Show output from drop-down list to Package Manager.

If you are not using Visual Studio open a command line and change to your project directory. Execute the dotnet restore command to restore your new dependency.

After updating your direct dependencies

Recompile your application.

If after recompilation you see a Dependency conflict warning, you must update your other direct dependencies to the appropriate version.

For example if your project refers to Microsoft.AspNetCore.Mvc.Cors with a version number of 1.0.1 when you update your Microsoft.AspNetCore.Mvc package to 1.0.6, compilation will throw:

NU1012 Dependency conflict. Microsoft.AspNetCore.Mvc.Core 1.0.6 expected Microsoft.AspNetCore.Mvc.Cors >= 1.0.6 but received 1.0.1

To fix this, edit the version for the expected package to be the version expected by updating your csproj or project.json in the same way that you used to update the vulnerable package versions.

After you have addressed all of your direct dependencies, you must also review your transitive dependencies.

Reviewing Transitive Dependencies

There are two ways to view transitive dependencies. You can either use Visual Studio’s Solution Explorer, or you can review your project.lock.json (project.json/VS2015) or project.assets.json (csproj/VS2017) file.

Using Visual Studio Solution Explorer (VS2015)

If you want to use Visual Studio 2015, open your project in Visual Studio 2015 and then press Ctrl+; to activate the search in Solution Explorer. Search for each of the vulnerable package names and make a note of the version numbers of any results you find.

For example, searching for Microsoft.AspNetCore.Mvc.Core in an example project that contains a reference to Microsoft.AspNetCore.Mvc shows the following results in Visual Studio 2015.

The search results appear as a tree. In these results, you can see we have found references to Microsoft.AspNetCore.Mvc, version 1.0.1, ` vulnerable version.

The first entry under the References heading refers to the target framework your application is using. This will be .NETCoreApp, .NETStandard or .NET-Framework-vX.Y.Z (where X.Y.Z is an actual version number) depending on how you configured your application. Under your target framework will be the list of packages you have directly taken a dependency on. In this example, the application takes a dependency on Microsoft.AspNetCore.Mvc. Microsoft.AspNetCore.Mvc in turn has leaf nodes that list its dependencies and their versions. In this case the Microsoft.AspNetCore.Mvc package takes a dependency on a vulnerable version of Microsoft.AspNetCore.Mvc.Core and numerous other packages.

Manually reviewing project.lock.json (project.json/VS2015)

Open the project.lock.json file in your editor. We suggest you use an editor that understands json and allows you to collapse and expand nodes to review this file; both Visual Studio and Visual Studio Code provide this functionality.

If you are using Visual Studio the project.lock.json file is “under” the project.json file. Click the right pointing triangle, ▷, to the left of the project.json file to expand the solution tree to expose the project.lock.json file. The following image shows a project with the project.json file expanded to show the project.lock.json file.

Search the project.lock.json file for the vulnerable packages, using the format packagename/, using each of the package names from the table above. If you find any vulnerable assembly name in your search examine the line on which they are found, the version number is after the / and compare to the vulnerable versions table above. For example a search result that shows Microsoft.AspNetCore.Mvc.Cors/1.0.1 is a reference to v1.0.1 of Microsoft.AspNetCore.Mvc.Cors. If your project.lock.json file includes references to any of the package versions shown above then you will need to fix the transitive dependencies.

Fixing transitive dependencies (project.json/VS2015)

If you have not found any reference to a vulnerable version of Microsoft.AspNetCore.Mvc.Cors this means none of your direct dependencies depend on any vulnerable packages or you have already fixed the problem by updating the direct dependencies.

If your transitive dependency review found references to any of the vulnerable packages you must add a direct dependency to the updated package to your project.json file to override the transitive dependency. Open your project.json and find the dependencies section. For example:

  "dependencies": {
    "Microsoft.NETCore.App": {
      "version": "1.0.1",
      "type": "platform"
    },
    "Microsoft.AspNetCore.Server.Kestrel": "1.1.0",
    "Microsoft.AspNetCore.Mvc": "1.0.0"
  }

For each of the vulnerable packages your search returned you must add a direct dependency to the updated version by adding it to the project.json file. You do this by adding a new line to the dependencies section, referring the fixed version. For example, if your search showed a transitive reference to the vulnerable Microsoft.AspNetCore.Mvc.Cors version 1.0.0 you would add a reference to the appropriate fixed version, 1.0.6. Edit the project.json file as follows:

  "dependencies": {
    "Microsoft.NETCore.App": {
      "version": "1.0.1",
      "type": "platform"
    },
    "Microsoft.AspNetCore.Mvc.Cors": "1.0.6",
    "Microsoft.AspNetCore.Server.Kestrel": "1.1.0",
    "Microsoft.AspNetCore.Mvc": "1.0.0"
  }

After you have added direct dependencies to the fixed packages, save your project.json file.

If you are using Visual Studio save your updated project.json file and Visual Studio will restore the new package versions. You can see the restore results by opening the Output Window (Ctrl+Alt+O) and changing the Show output from drop-down list to Package Manager.

If you are not using Visual Studio open a command line and change to your project directory. Execute the dotnet restore command to restore your new dependencies.

Using Visual Studio Solution Explorer (VS2017)

If you want to use Solution Explorer, open your project in Visual Studio 2017, and then press Ctrl+; to activate the search in Solution Explorer. Search for each of the vulnerable package names and make a note of the version numbers of any results you find.

For example, searching for Microsoft.AspNetCore.Mvc.Core in an example project that contains a package that takes a dependency on Microsoft.AspNetCore.Mvc shows the following results in Visual Studio 2017.

The search results appear as a tree. In these results, you can see we have found references to Microsoft.AspNetCore.Mvc.Core version 1.1.2.

Under the Dependencies node will be a NuGet node. Under the NuGet node will be the list of packages you have directly taken a dependency on and their versions. In this example, the application takes a direct dependency on Microsoft.AspNetCore.Mvc. Microsoft.AspNetCore.Mvc in turn has leaf nodes that list its dependencies and their versions. In the example the Microsoft.AspNetCore.Mvc package takes a dependency on a version of Microsoft.AspNetCore.Mvc.ApiExplorer which in turn takes a dependency on a vulnerable version of Microsoft.AspNetCore.Mvc.Core.

Manually reviewing project.assets.json (VS2017)

Open the project.assets.json file from your project’s obj directory in your editor. We suggest you use an editor that understands json and allows you to collapse and expand nodes to review this file; both Visual Studio and Visual Studio Code provide this functionality.

Search the project.assets.json file for each of the vulnerable packages, using the format packagename/ using the package name from the table above. If you find the assembly name in your search examine the line on which they are found, the version number is after the / and compare to the vulnerable versions table above. For example a search result that shows Microsoft.AspNetCore.Mvc.Cors/1.1.0 is a reference to v1.1.0 of Microsoft.AspNetCore.Mvc.Cors. If your project.assets.json file includes references to any of the vulnerable packages shown above then you will need to fix the transitive dependencies.

If you have not found any reference to any vulnerable packages this means none of your direct dependencies depend on any vulnerable packages or you have already fixed the problem by updating the direct dependencies.

If your transitive dependency review found references to any of the vulnerable packages you must add a direct dependency to the updated package to your csproj file to override the transitive dependency. Open your projectname.csproj file in your editor, or right click on the project in Visual Studio 2017 and choose Edit projectname.csproj from the content menu, where projectname is the name of your project. Look for PackageReference nodes, for example:

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp1.1</TargetFramework>
  </PropertyGroup>
  <PropertyGroup>
    <PackageTargetFallback>$(PackageTargetFallback);portable-net45+win8+wp8+wpa81;</PackageTargetFallback>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore" Version="1.1.1" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc" Version="1.1.3" />
  </ItemGroup>
  <ItemGroup>
    <DotNetCliToolReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Tools" Version="1.0.0" />
  </ItemGroup>
</Project>

For each of the vulnerable packages your search returned you must add a direct dependency to the updated version by adding it to the csproj file. You do this by adding a new line to the dependencies section, referring the fixed version. For example, if your search showed a transitive reference to the vulnerable Microsoft.AspNetCore.Mvc.Cors, version 1.1.4 you would add a reference to the appropriate fixed version, 1.1.6.

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp1.1</TargetFramework>
  </PropertyGroup>
  <PropertyGroup>
    <PackageTargetFallback>$(PackageTargetFallback);portable-net45+win8+wp8+wpa81;</PackageTargetFallback>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Cors" Version="1.1.6" />
    <PackageReference Include="Microsoft.AspNetCore" Version="1.1.1" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc" Version="1.1.3" />
  </ItemGroup>
  <ItemGroup>
    <DotNetCliToolReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Tools" Version="1.0.0" />
  </ItemGroup>

After you have added the direct dependency reference, save your csproj file.

If you are using Visual Studio, save your updated csproj file and Visual Studio will restore the new package versions. You can see the restore results by opening the Output Window (Ctrl+Alt+O) and changing the Show output from drop-down list to Package Manager.

If you are not using Visual Studio, open a command line and change to your project directory. Execute the dotnet restore command to restore your new dependencies.

Rebuilding your application

Finally rebuild your application, test as you would do normally and redeploy using your favored deployment mechanism.

Other Information

Reporting Security Issues

If you have found a potential security issue in .NET Core, please email details to secure@microsoft.com. Reports may qualify for the .NET Core Bug Bounty. Details of the .NET Core Bug Bounty including Terms and Conditions are at https://aka.ms/corebounty.

Support

You can ask questions about this issue on GitHub in the .NET Core or ASP.NET Core organizations. These are located at https://github.com/dotnet/ and https://github.com/aspnet/. The Announcements repo for each product (https://github.com/dotnet/Announcements and https://github.com/aspnet/Announcements) will contain this bulletin as an issue and will include a link to a discussion issue where you can ask questions.

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-2017-8700

Revisions

V1.1 (Dec 13, 2017): Updated 1.1.5 to 1.1.6 to fix a packaging issue (see aspnet/Mvc#7070)
V1.0 (Nov 14, 2017): Advisory published.

Version 1.1
Last Updated 2017-12-13