Microsoft Security Advisory CVE-2020-1045 | Microsoft ASP.NET Core Security Feature Bypass Vulnerability

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Microsoft Security Advisory CVE-2020-1045 | Microsoft ASP.NET Core Security Feature Bypass Vulnerability

Executive summary

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

A security feature bypass vulnerability exists in the way Microsoft ASP.NET Core parses encoded cookie names.

The ASP.NET Core cookie parser decodes the cookie name which could allow a malicious attacker to use percent encoded characters to masquerade as a different cookie with different security features.

The security update addresses the vulnerability by fixing the way the ASP.NET Core cookie parser handles encoded names.

Discussion

Discussion for this issue can be found at dotnet/aspnetcore#25701

Mitigation factors

Microsoft has not identified any mitigating factors for this vulnerability.

Affected software

• Any .NET Core 3.1 application running on .NET Core 3.1.7 or lower
• Any .NET Core 2.1 application running on .NET Core 2.1.21 or lower

Please note that .NET Core 3.0 is now out of support and all applications should be updated to 3.1.

How do I know if I am affected?

If you have a runtime or SDK with a version listed in affected software you are exposed to the vulnerability.

How do I fix the issue?

To fix the issue please install the latest version of .NET Core 3.1. 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 will see 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

• You should download and install Runtime 3.1.8 or SDK 3.1.108 (for Visual Studio 2019 v16.4) or 3.1.402 (for Visual Studio 2019 v16.5 or later) from https://dotnet.microsoft.com/download/dotnet-core/3.1

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, 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 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-2020-1045

Revisions

V1.0 (September 8, 2020): Advisory published.

Version 1.0

Last Updated 2020-09-08