Microsoft Security Advisory CVE-2018-8356: .NET Core Security Feature Bypass Vulnerability

# Microsoft Security Advisory CVE-2018-8356: .NET Core Security Feature Bypass Vulnerability

## Executive summary

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

Microsoft is aware of a security feature bypass vulnerability that exists when .NET Core does not correctly validate certificates. An attacker who successfully exploited this vulnerability could present an expired certificate when challenged.

The update addresses the vulnerability by correcting how .NET Core applications handle certificate validation.

## Discussion

Discussion for this issue can be found at https://github.com/dotnet/wcf/issues/3009

### Mitigation factors

* If an application does not use Windows Communication Foundation you are not affected.

## Affected software

Any .NET Core, or ASP.NET Core based application that uses System.Private.ServiceModel with a version of 4.5.1 or earlier.

Package name | Vulnerable versions | Secure versions
------------ | ---------------- | -------------------------
System.Private.ServiceModel | 4.0.0, 4.1.0, 4.1.1
4.3.0, 4.3.1
4.4.0, 4.4.1, 4.4.2
4.5.0, 4.5.1 | 4.1.3 or later
4.3.3 or later
4.4.4 or later
4.5.3 or later
System.ServiceModel.Duplex | 4.0.0, 4.0.1, 4.0.2
4.3.0, 4.3.1
4.4.0, 4.4.1, 4.4.2
4.5.0, 4.5.1 | 4.0.4 or later
4.3.3 or later
4.443 or later
4.5.3 or later
System.ServiceModel.Http | 4.0.0, 4.0.10, 4.1.0, 4.1.1
4.3.0, 4.3.1
4.4.0, 4.4.1, 4.4.2
4.5.0, 4.5.1 | 4.1.3 or later
4.3.3 or later
4.4.4 or later
4.5.3 or later
System.ServiceModel.NetTcp | 4.0.0, 4.1.0, 4.1.1
4.3.0, 4.3.1
4.4.0, 4.4.1, 4.4.2
4.5.0, 4.5.1 | 4.1.3 or later
4.3.3 or later
4.4.4 or later
4.5.3 or later
System.ServiceModel.Primitives | 4.0.0, 4.1.0, 4.1.1
4.3.0, 4.3.1
4.4.0, 4.4.1, 4.4.2
4.5.0, 4.5.1 | 4.1.2 or later
4.3.3 or later
4.4.4 or later
4.5.3 or later
System.ServiceModel.Security | 4.0.0, 4.0.1, 4.0.2
4.3.0, 4.3.1
4.4.0, 4.4.1, 4.4.2
4.5.0, 4.5.1 | 4.0.4 or later
4.3.3 or later
4.4.4 or later
4.5.3 or later

## Advisory FAQ
### How do I know if I am affected?

.NET Core projects have two types of dependencies: direct and transitive. You must update your projects using the following instructions to address both types of 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, .NET Core 2.0 and Microsoft Visual Studio 2017.

#### Direct dependencies

Direct dependencies are discoverable by examining your `csproj` file or your `project.json` file. They can be fixed by [editing the project file](#direct-dependencies) or using NuGet to update the dependency.

The `System.Private.ServiceModel` package is not meant to be directly depended on and will not appear in your direct dependency list.

#### Transitive dependencies

Transitive dependencies occur when you add a package to your project that in turn relies on another package. For example, if Contoso publishes a package `Contoso.Utility` which, in turn, depends on `Contoso.Internals` and you add the `Contoso.Utility` package to your project now your project has a direct dependency on `Contoso.Utility` and, because `Contoso.Utility` depends 'Contoso.Internals', your application gains a transitive dependency on the `Contoso.Internals` package.

Transitive dependencies are reviewable in three ways, depending on your [project format](#project-format):

* In the Visual Studio Solution Explorer window, which supports searching.
* By examining the `project.assets.json` file contained in the obj directory of your project for `csproj` based projects *OR*.
* By examining the `project.lock.json` file contained in the root directory of your project for `project.json` based projects.

The `project.assets.json` and `project.lock.json` files are the authoritative list of all packages used by your project, containing both direct and transitive dependencies.

#### Fixing direct dependencies in a csproj based project / Visual Studio 2017

Open *projectname.csproj* in your editor. If you're using Visual Studio, right-click the project and choose **Edit projectname.csproj** from the context menu, where projectname is the name of your project. Look for `PackageReference` elements. The following shows an example project file:

```xml

netcoreapp2.0

```

The preceding example has a reference to the [vulnerable package](#affected-software), as seen by the single `PackageReference` element. The name of the package is in the `Include` attribute.
The package version number is in the `Version` attribute. The previous example shows a single direct dependency on `System.ServiceModel.Http` version 4.4.0.

To update the version to the secure package, change the version number to the updated package version as listed on the table [previously](#affected-software).
In this example, update `System.ServiceModel.Http` to the appropriate [fixed package number](#affected-software) for your major version. Save the *csproj* file. The example *csproj* now looks as follows:

```xml

netcoreapp2.0

```

If you're using Visual Studio and you 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're not using Visual Studio, open a command line and change to your project directory. Execute the `dotnet restore` command to restore the updated dependencies.

Now recompile your application. If after recompilation you see a *Dependency conflict warning*, you must update your other direct dependencies to versions that take a dependency on the updated package.

#### Fixing Direct Dependencies in project.json based project / Visual Studio 2015

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

``` json
"dependencies": {
"Microsoft.NETCore.App": {
"version": "1.0.1",
"type": "platform"
},
"Microsoft.AspNetCore.Mvc.Core": "1.0.6",
"System.ServiceModel.Http" : "4.0.0"
}
```

This example has three direct dependencies: `Microsoft.NETCore.App`, `Microsoft.AspNetCore.Mvc.Core` and `System.ServiceModel.Http`.

`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 a vulnerable package, `System.ServiceModel.Http` version 4.4.0.

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 `System.ServiceModel.Http` to 4.4.3.

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

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

```json
"dependencies": {
"Microsoft.NETCore.App": {
"version": "1.0.1",
"type": "platform"
},
"Microsoft.AspNetCore.Mvc.Core": "1.0.6",
"System.ServiceModel.Http ": "4.4.3"
}
```

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.

After you've addressed all of your direct dependencies, you must review your transitive dependencies.

#### Discovering and fixing transitive dependencies

There are two ways to view transitive dependencies. You can either [use Visual Studio’s Solution Explorer](#vs-solution-explorer), or you can review [the `project.assets.json` file](#project-assets-json) or the [the `project.lock.json` file](#project-lock-json).

##### Using Visual Studio Solution Explorer

To use Solution Explorer, open the project in Visual Studio, and then press Ctrl+; to activate the search in Solution Explorer. Search for the [vulnerable package](#affected-software) 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 the previous results, you can see that a reference to `Microsoft.AspNetCore.Mvc.Core` version 1.1.2 is discovered.

Under the Dependencies node is a NuGet node. Under the NuGet node is the list of packages you have directly taken a dependency on and their versions.
In screenshot, 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.
The `Microsoft.AspNetCore.Mvc` package takes a dependency on a version of `Microsoft.AspNetCore.Mvc.ApiExplorer`, that in turn takes a dependency on a version of `Microsoft.AspNetCore.Mvc.Core`.

##### Manually reviewing project.assets.json (csproj/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.
Visual Studio and Visual Studio Code provide JSON friendly editing.

Search the *project.assets.json* file for the [vulnerable package](#affected-software), using the format `packagename/` for each of the package names from the preceding table. If you find the assembly name in your search:

* Examine the line on which they are found, the version number is after the `/`.
* Compare to the [vulnerable versions table](#affected-software).

For example, a search result that shows `System.ServiceModel.Http/4.3.0` is a reference to version 4.3.0 of `System.ServiceModel.Http`.

If your *project.assets.json* file includes references to the [vulnerable package](#affected-software), then you need to fix the transitive dependencies.

If you have not found any reference to any vulnerable packages, this means either

* 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 the [vulnerable package](#affected-software), you must add a direct dependency to the updated package to your *csproj* file to override the transitive dependency.

Open *projectname.csproj* in your editor. If you're using Visual Studio, right-click the project and choose **Edit projectname.csproj** from the context menu, where projectname is the name of your project.
Look for `PackageReference` nodes, for example:

```xml

net461

```

You must add a direct dependency to the updated version of the [vulnerable package](#affected-software) by adding it to the *csproj* file.
You do this by adding a new line to the dependencies section, referencing the fixed version.
For example, if your search showed a transitive reference to a vulnerable `System.ServiceModel.Http` version, you'd add a reference to the [fixed package number](#affected-software).

```xml

net461

```

After you've added the direct dependency reference, save your *csproj* file.

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

##### 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 `System.ServiceModel.Http/4.0.1` is a reference to v4.0.1 of `System.ServiceModel.Http`. 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.

If you have not found any reference to a vulnerable version of `System.ServiceModel.Http` 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:

``` json
"dependencies": {
"Microsoft.NETCore.App": {
"version": "1.0.1",
"type": "platform"
},
"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 `System.ServiceModel.Http` version 1.0.0 you would add a reference to the appropriate fixed version, 1.0.6. Edit the `project.json` file as follows:

``` json
"dependencies": {
"Microsoft.NETCore.App": {
"version": "1.0.1",
"type": "platform"
},
"System.ServiceModel.Http": "4.1.2",
"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.

#### Rebuilding your application

Finally you must rebuild your application, test, and redeploy.

## 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](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. You can ask questions in the 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-2018-8356](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-CVE-2018-8356)

### Revisions

V1.1 (July 23, 2018): Updated package versions, as some assemblies were released unsigned.
V1.0 (July 10, 2018): Advisory published.

_Version 1.1_

_Last Updated 2018-07-23_