GHSA-qvhc-9v3j-5rfw
Microsoft Security Advisory CVE-2026-21218 | .NET Security Feature Bypass Vulnerability
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
System.Security.Cryptography.Cose.NETSystem.Security.Cryptography.Cose.NETSystem.Security.Cryptography.CoseReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.
Description
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 Portal. Further information can be found in the MSRC Report an Issue FAQ.
Security reports made through MSRC may qualify for the Microsoft .NET 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. 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
Acknowledgements
vcsjones with GitHub
Revisions
V1.0 (February 10, 2026): Advisory published.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | System.Security.Cryptography.Cose | ≥ 8.0.0&&< 8.0.2 | 8.0.2 |
| .NETNuGet | System.Security.Cryptography.Cose | ≥ 9.0.0&&< 9.0.13 | 9.0.13 |
| .NETNuGet | System.Security.Cryptography.Cose | ≥ 10.0.0&&< 10.0.3 | 10.0.3 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for System.Security.Cryptography.Cose. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update System.Security.Cryptography.Cose to 8.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-qvhc-9v3j-5rfw is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-qvhc-9v3j-5rfw is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-qvhc-9v3j-5rfw. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-qvhc-9v3j-5rfw in your dependencies?
O3 detects GHSA-qvhc-9v3j-5rfw across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.