Microsoft Security Advisory CVE-2018-0765: .NET Core Denial Of Service Vulnerability

# Microsoft Security Advisory CVE-2018-0765: .NET Core Denial Of Service Vulnerability

## Executive summary

Microsoft is releasing this security advisory to provide information about a vulnerability in .NET Core and .NET native version 2.0. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.

Microsoft is aware of a denial of service vulnerability that exists when .NET Framework and .NET Core improperly process XML documents. An attacker who successfully exploited this vulnerability could cause a denial of service against a .NET Framework, .NET Core, or .NET native application.

The update addresses the vulnerability by correcting how .NET Framework, .NET Core, and .NET native applications handle XML document processing.

If your application is an ASP.NET Core application, developers are also advised to update to [ASP.NET Core 2.0.8](https://www.nuget.org/packages/Microsoft.AspNetCore.All/2.0.8).

## Discussion

Discussion for this issue can be found at https://github.com/dotnet/corefx/issues/29578

### Mitigation factors

* If an application does not process signed XML, it is not affected by this vulnerability.
* If your application targets .NET Core 1.x or .NET native 1.x, it is not affected as the vulnerable package is not available for these platforms.

## Affected software

Any .NET Core, .NET native, or ASP.NET Core based application that uses System.Security.Cryptography.Xml with a version of 4.4.1 or earlier.

Package name | Vulnerable versions | Secure versions
------------ | ---------------- | -------------------------
System.Security.Cryptography.Xml | ≤4.4.1 | 4.4.2 or later

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

.NET Core has two types of dependencies: direct and transitive. Direct dependencies are dependencies where you specifically add a package to your project, transitive dependencies occur when you add a package to your project that in turn relies on another package.

For example, the `Microsoft.AspNetCore.Mvc` package depends on the `Microsoft.AspNetCore.Mvc.Core` package. When you add a dependency on `Microsoft.AspNetCore.Mvc` in your project, you're taking a transitive dependency on `Microsoft.AspNetCore.Mvc.Core`.

Any application that has a direct or transitive dependency on the [affected package](#affected-software) can be exposed to the vulnerability if it does not meet any of the [mitigation factors](#mitigation-factors).

### How do I fix the issue?
---

If you're targeting ASP.NET Core 2.0 and using the `Microsoft.AspNetCore.All` metapackage:

* Update its version number to [2.0.8](https://www.nuget.org/packages/Microsoft.AspNetCore.All/2.0.8) to pull in updated packages that update their transitive dependencies to the fixed version of `System.Security.Cryptography.Xml`.

---

.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.

#### Direct dependencies

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

#### 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 two ways:

* 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.

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

#### Fixing direct dependencies

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.Security.Cryptography.Xml` version 4.4.1.

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.Security.Cryptography.Xml` to a [fixed package number](#affected-software). 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.

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).

##### Using Visual Studio Solution Explorer

To use Solution Explorer, open the project in Visual Studio 2017, 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

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.Security.Cryptography.Xml/4.4.0` is a reference to version 4.4.0 of `System.Security.Cryptography.Xml`.
If your *project.assets.json* file includes references to the [vulnerable package](#affected-software), then you need to fix the transitive dependencies.

##### Fixing 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.Security.Cryptography.Xml` 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.

#### Rebuilding your application

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

---

If you're targeting .NET native for a Universal Windows Platform (UWP) application, you should update your dependencies and republish your application.

If you submit an application to the Windows Store with an outdated reference, the store will update the reference automatically, which has the potential for compatibility issues. Thus, we recommend updating your application directly and testing its functionality before resubmitting.

---

## 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-2018-0787](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-CVE-2018-0765)

### Revisions

V1.0 (May 8, 2018): Advisory published.

_Version 1.0_

_Last Updated 2018-05-08_