CVE-2026-96222: Active Directory Federation Services (AD FS) Spoofing Vulnerability
Executive summary
Microsoft is releasing this security advisory to provide information about a vulnerability in System.Security.Cryptography.Xml. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.
KeyInfoX509Data can accept serialized certificate formats containing attacker-controlled chain metadata instead of requiring a plain X.509 certificate, contributing to certificate-forgery and token-spoofing paths.
Announcement
Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/474
CVSS Details
- Version: 3.1 - Severity: High - Score: 7.4 - Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N - Weakness: CWE-295 (Improper Certificate Validation)
Affected Platforms
- Platforms: Windows, Linux, macOS - Architectures: All
<a name="affected-packages"></a>Affected Packages
The vulnerability affects Microsoft .NET projects that use an affected package version listed below.
Package name | Affected version | Patched version ------------ | ---------------- | --------------- System.Security.Cryptography.Xml | >= 8.0.0, <= 8.0.4 | 8.0.5
Advisory FAQ
<a name="how-affected"></a>How do I know if I am affected?
If using a package listed in affected packages, you're exposed to the vulnerability.
<a name="how-fix"></a>How do I fix the issue?
1. To fix the issue please install the latest version of .NET. If you have installed one or more .NET SDKs through Visual Studio, Visual Studio will prompt you to update Visual Studio, which will also update your .NET SDKs. 2. If your application references the vulnerable package, update the package reference to the patched version. You can list the versions you have installed by running the dotnet --info command.
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 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
CVE-2026-96222
Acknowledgements
- Vaisha Bernard with Eye Security
Revisions
V1.0 (10/13/2026): Advisory published.
Other sources
Active Directory Federation Services (AD FS) Spoofing Vulnerability
— Microsoft
Improper certificate validation in Active Directory Federation Services (AD FS) allows an unauthorized attacker to perform spoofing over a network.
— Microsoft
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.8987 AND 3.0.30729.8982 AND 4.7.4150.0Patch KB5126868 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9188 AND 3.0.30729.9171 AND 4.8.9363.0Patch KB5127689 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 4.8.9363.0Patch KB5127691 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9073 AND 3.0.30729.9070 AND 4.7.4150.0Patch KB5127666 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9188 AND 3.0.30729.9171 AND 4.8.4811.0Patch KB5127683 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 10.0.26100.33650Patch KB5126646 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 4.7.4150.0Patch KB5127667 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 4.8.4811.0Patch KB5127684 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 18.10.4 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 4.8.4811.0Patch KB5127686 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9073 AND 3.0.30729.9070 AND 4.8.4811.0Patch KB5127685 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 17.14.42 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9188 AND 3.0.30729.9171Patch KB5127696 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 4.8.4811.0Patch KB5127688 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9188 AND 3.0.30729.9171 AND 4.8.4811.0Patch KB5127687 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 4.7.4150.0Patch KB5127668 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9188 AND 3.0.30729.9171 AND 4.8.10008.0Patch KB5127694 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.8987 AND 3.0.30729.8982Patch KB5127660 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.8987 AND 3.0.30729.8982Patch KB5127664 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9188 AND 3.0.30729.9171 AND 4.8.9363.0Patch KB5127693 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9188 AND 3.0.30729.9171 AND 4.8.9363.0Patch KB5127690 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 2.0.50727.9188 AND 3.0.30729.9171 AND 4.8.9363.0Patch KB5127692 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 10.0.14393.9612Patch KB5126868 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 6.3.9600.23470Patch KB5126836 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 10.0.20348.5761Patch KB5126655 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 10.0.17763.9376Patch KB5126651 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 8.0.5 - Upgrade
Upgrade
nuget/System.Security.Cryptography.Xmlto a version that resolves this vulnerability.Fixed in 8.0.5 - Upgrade
Upgrade
System.Security.Cryptography.Xmlto a version that resolves this vulnerability.Fixed in 8.0.5 - Operational
Recompile and redeploy self-contained applications targeting impacted .NET versions.
- Operational
Restart applications after installing the updated runtime or SDK.
Event History
Frequently Asked Questions
What level of access does an attacker need to exploit this issue?
The vulnerability is exploitable over a network and does not require prior privileges or user interaction. Exploitation has high attack complexity.
What security impact could successful exploitation have?
Successful spoofing can have a high impact on confidentiality and integrity. The available data does not indicate an availability impact.
Which environments should be prioritized for review?
Prioritize systems running AD FS on the listed Microsoft Windows Server versions, as well as environments using the listed Microsoft .NET Framework or Visual Studio 2026 software. The provided data does not identify specific affected builds or configurations.