GitHub Security Advisory: GHSA-qvhc-9v3j-5rfw
Package: System.Security.Cryptography.Cose (NUGET)
Severity: HIGH (CVSS 0.0)
CWE: CWE-166: Improper Handling of Missing Special Element
Vulnerable versions: >= 9.0.0, <= 9.0.12
Patched in: 9.0.13

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

Details:
# 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](https://www.nuget.org/packages/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](https://www.nuget.org/packages/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](https://www.nuget.org/packages/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](#affected-software) or [affected packages](#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