Microsoft Security Advisory CVE-2026-21218 | .NET Security Feature Bypass Vulnerability
Date: 2026-02
Affected: nuget/System.Security.Cryptography.Cose, nuget/System.Security.Cryptography.Cose, nuget/System.Security.Cryptography.Cose
Attack vector: other
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-21218
CWE: CWE-166

# 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 ----------


Mitigations:

Architectural layers (MAESTRO):