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

[rbhanda]

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.

Announcement

Announcement for this issue can be found at dotnet/announcements#178

Mitigation factors

Microsoft has not identified any mitigating factors for this vulnerability.

Affected software

The vulnerable package is 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	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.

How do I know if I am affected?

If you have a runtime or SDK with a version listed in 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:

.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 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 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/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

Revisions

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

Version 1.0

Last Updated 2021-03-09

[omajid]

https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2021-26701#securityUpdates seems a little strange...

• It says the updates for .NET Core 2.1, 3.1 and .NET 5 were released in February
• The link to release notes is broken

[kevbry]

Assuming this affects anyone running asp.net core 2.1 apps on full framework? Will updated dependent packages be released referencing the fixed version?

[johnkors]

$ dotnet --list-runtimes
Microsoft.AspNetCore.App 3.1.3 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.3 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="System.Text.Encodings.Web" Version="4.6.0" />
  </ItemGroup>
</Project>

$ dotnet list package --vulnerable

The following sources were used:
   https://api.nuget.org/v3/index.json

The given project `VulnerableApp` has no vulnerable packages given the current sources

Shouldn't the new .NET 5 SDK feature dotnet list package --vulnerable report these..? Delay?

Or is this the scenario where the runtime hijacks and uses a runtime version of said package instead of what is defined in csproj, and hence not reporting it?

cc @JonDouglas @drewgillies

[drewgillies]

Hi @johnkors --the issue here is that there are no package vulnerabilities registered on GitHub for this CVE: https://github.com/advisories?query=CVE-2021-26701
Until a GitHub vulnerability advisory exists, the NuGet tools (including those included in the dotnet CLI) will have no information to present. But when we have an advisory in place it will surface in the tools.

[I've edited this comment down to something simpler because some of the finer points are still under discussion, but essentially this is why you don't see the advisory in the CLI--because it's not presented in GitHub]

[johnkors]

So basically there is a delay going between these stages:

1. Reported as CVE https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-26701
2. GitHub issue announcement, with affected & fix (like the one here)
3. It appears as a GitHub advisory, shows in CLI

Thanks, @drewgillies !

[drewgillies]

You're welcome! Hopefully we'll have something in place soon.

[miguelcrpinto]

This announcement is a bit confusing... In the executive summary it mentions only ".NET 5.0, .NET Core 3.1, and .NET Core 2.1", then in the "Affected software" you mention "Any .NET 5, .NET Core, or .NET Framework based application".

Why was ".NET Framework" added in the "Affected software" section? Does this mean that .NET Framework applications are also vulnerable or only .Net Core Applications that target the .NET Framework?

[benbhall]

A couple of things might need some clarifying.

1. Affected software: System.Text.Encodings.Web is tied to .NET Core, included out of the box in the runtime with Microsoft.NETCore.App so, the advisory is justified in being released under the headline of for .NET 5 and Core banner. However, it is also being used in .NET Framework applications via the NuGet package. Framework is mentioned once under Affected software but is there a risk of it being missed by not appearing in the headline?

2. Removing vulnerability from applications:

Upgrading your package and redeploying your app should be sufficient to address this vulnerability.

and later on:

To fix the issue, please install the latest version of .NET 5.0, .NET Core 3.1 or .NET Core 2.1.
...
Once you have installed the updated runtime or SDK, restart your apps for the update to take effect.

We could clarify that Framework is fixed via NuGet package update and deploy.

Then Core is just the runtime update and restart? I'm sure there are probably scenarios where Core applications are pulling the NuGet in directly too.

EDIT Apologies for some repetition. Wrote this then went into a meeting without adding comment and missed @miguelcrpinto comment.