Microsoft Security Advisory CVE-2021-26701 | .NET Core Remote Code Execution Vulnerability

# Microsoft Security Advisory CVE-2021-26701 | .NET Core Remote Code Execution Vulnerability

## Executive summary

Microsoft is releasing this security advisory to provide information about a vulnerability in .NET 5.0, .NET Core 3.1, and .NET Core 2.1. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.

A remote code execution vulnerability exists in .NET 5 and .NET Core due to how text encoding is performed.

## Discussion

Discussion for this issue can be found at https://github.com/dotnet/runtime/issues/49377

## Mitigation factors

Microsoft has not identified any mitigating factors for this vulnerability.

## Affected software

The vulnerable package is [System.Text.Encodings.Web](https://www.nuget.org/packages/System.Text.Encodings.Web/) . Upgrading your package and redeploying your app should be sufficient to address this vulnerability.

Vulnerable package versions:

Any .NET 5, .NET Core, or .NET Framework based application that uses the System.Text.Encodings.Web package with a vulnerable version listed below.

Package Name| Vulnerable Versions| Secure Versions
:----- | :----: | -----:
[System.Text.Encodings.Web](https://www.nuget.org/packages/System.Text.Encodings.Web/)| 4.0.0 - 4.5.0
4.6.0-4.7.1
5.0.0 | 4.5.1
4.7.2
5.0.1

Please validate that each of the .NET versions you are using is in support. Security updates are only provided for [supported .NET versions](https://github.com/dotnet/core/blob/main/releases.md).

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

If you have a runtime or SDK with a version listed in [affected software](#affected-software), you're exposed to the vulnerability.

### How do I fix the issue?

To fix the issue, please install the latest version of .NET 5.0, .NET Core 3.1 or .NET Core 2.1. If you have installed one or more .NET Core SDKs through Visual Studio, Visual Studio will prompt you to update Visual Studio, which will also update your .NET Core SDKs.

You can list the versions you have installed by running the `dotnet --info` command. You should see an output like the following:

```txt
.NET Core SDK (reflecting any global.json):
Version: 3.1.100
Commit: cd82f021f4

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18363
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.1.100\

Host (useful for support):
Version: 3.1.0
Commit: 65f04fb6db

.NET Core SDKs installed:
3.1.100 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download
```
* If you're using .NET 5.0, you should download and install Runtime 5.0.4 or SDK 5.0.104 (for Visual Studio 2019 v16.8) from https://dotnet.microsoft.com/download/dotnet-core/5.0.

* If you're using .NET Core 3.1, you should download and install Runtime 3.1.13 or SDK 3.1.113 (for Visual Studio 2019 v16.4) or 3.1.407 (for Visual Studio 2019 v16.5 or later) from https://dotnet.microsoft.com/download/dotnet-core/3.1.

* If you're using .NET Core 2.1, you should download and install Runtime 2.1.26 or SDK 2.1.522 (for Visual Studio 2019 v15.9) or 2.1.814 from https://dotnet.microsoft.com/download/dotnet-core/2.1.

.NET 5.0, .NET Core 3.1 and .NET Core 2.1 updates are also available from Microsoft Update. To access this either type "Check for updates" in your Windows search, or open Settings, choose Update & Security and then click Check for Updates.

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

Additionally, if you've deployed [self-contained applications](https://docs.microsoft.com/dotnet/core/deploying/#self-contained-deployments-scd) targeting any of the impacted versions, these applications are also vulnerable and must be recompiled and redeployed.

## Other Information

### Reporting Security Issues

If you have found a potential security issue in .NET Core or .NET 5, please email details to secure@microsoft.com. Reports may qualify for the Microsoft .NET Core & .NET 5 Bounty. Details of the Microsoft .NET Bounty Program including terms and conditions are at .

### 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/runtime and https://github.com/dotnet/aspnet/. 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-2021-26701](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-26701)

### Revisions

V1.0 (March 09, 2021): Advisory published.

_Version 1.0_

_Last Updated 2021-03-09_