CVE-2026-21218

Improper handling of missing special element in .NET allows an unauthorized attacker to perform spoofing over a network.

Microsoft Security Advisory CVE-2026-21218 | .NET Security Feature Bypass Vulnerability

<a name="executive-summary"></a>Executive summary

Microsoft is releasing this security advisory to provide information about a vulnerability in .NET 8.0, .NET 9.0, and .NET 10.0. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.

An attacker could exploit this vulnerability by crafting a malicious payload that bypasses the security checks in the affected System.Security.Cryptography.Cose versions, potentially leading to unauthorized access or data manipulation.

Announcement

Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/380

<a name="mitigation-factors"></a>Mitigation factors

If your application does not use System.Security.Cryptography.Cose it is not affected. By default, no .NET applications reference this component.

<a name="affected-packages"></a>Affected Packages

The vulnerability affects any Microsoft .NET project if it uses any of affected packages versions listed below

<a name=".NET 10"></a>.NET 10

Package name | Affected version | Patched version ------------ | ---------------- | ------------------------- System.Security.Cryptography.Cose | >= 10.0.0, <= 10.0.2 | 10.0.3

<a name=".NET 9"></a>.NET 9

Package name | Affected version | Patched version ------------ | ---------------- | ------------------------- System.Security.Cryptography.Cose | >= 9.0.0, <= 9.0.12 | 9.0.13

<a name=".NET 8"></a>.NET 8

Package name | Affected version | Patched version ------------ | ---------------- | ------------------------- System.Security.Cryptography.Cose | >= 8.0.0, <= 8.0.1 | 8.0.2

Advisory FAQ

<a name="how-affected"></a>How do I know if I am affected?

If an affected package listed in affected software or affected packages, you're exposed to the vulnerability.

<a name="how-fix"></a>How do I fix the issue?

To update the Using the System.Security.Cryptography.Cose NuGet package, use one of the following methods:

NuGet Package Manager UI in Visual Studio:

• Open your project in Visual Studio.
• Right-click on your project in Solution Explorer and select "Manage NuGet Packages..." or navigate to "Project > Manage NuGet Packages".
• In the NuGet Package Manager window, select the "Updates" tab. This tab lists packages with available updates from your configured package sources.
• Select the package(s) you wish to update. You can choose a specific version from the dropdown or update to the latest available version.
• Click the "Update" button.

Using the NuGet Package Manager Console in Visual Studio:

• Open your project in Visual Studio.
• Navigate to "Tools > NuGet Package Manager > Package Manager Console".
• To update a specific package to its latest version, use the following Update-Package command:

Update-Package -Id System.Security.Cryptography.Cose

Using the .NET CLI (Command Line Interface):

• Open a terminal or command prompt in your project's directory.
• To update a specific package to its latest version, use the following add package command:

dotnet add package System.Security.Cryptography.Cose

Once you have updated the nuget package reference you must recompile and deploy your application. Additionally we recommend you update your runtime and/or SDKs, but it is not necessary to patch the vulnerability.

Other Information

Reporting Security Issues

If you have found a potential security issue in a supported version of .NET, please report it to the Microsoft Security Response Center (MSRC) via the [MSRC Researcher…

Improper handling of missing special element in .NET allows an unauthorized attacker to perform spoofing over a network.

Improper handling of missing special element in .NET allows an unauthorized attacker to perform spoofing over a network.

Improper handling of missing special element in .NET allows an unauthorized attacker to perform spoofing over a network.

Kevin Jones discovered that the System.Security.Cryptography.Cose component in .NET did not properly handle certain missing special elements in input data. An attacker could possibly use this issue to bypass security checks and gain unauthorized access or perform data manipulation.

USN 8025-1 fixed a vulnerability in .NET. This update provides the corresponding fix for Ubuntu 24.04 LTS.

Original advisory details:

Kevin Jones discovered that the System.Security.Cryptography.Cose component in .NET did not properly handle certain missing special elements in input data. An attacker could possibly use this issue to bypass security checks and gain unauthorized access or perform data manipulation.