Microsoft Issues Emergency Update Fix for Windows Server SSO Authentication Bug

Microsoft has issued an out-of-band emergency update fix to patch an authentication issue that was caused by the November 9^th cumulative update for Windows Server. The bug affects Windows Server 2008 SP2 through to Windows Server 2019.

The November 9^th Patch Tuesday cumulative update (CU) for Windows Server causes a problem that can cause authentication failures on Active Directory (AD) domain controllers (DC). Microsoft says about the new out-of-band update:

Addresses a known issue that might cause authentication failures related to Kerberos tickets you acquired from Service for User to Self (S4U2self). This issue occurs after you install the November 9, 2021 security updates on domain controllers (DC) that are running Windows Server.

The bug caused by the Patch Tuesday update can prevent end users from signing into services or apps that are configured to use single sign-on (SSO) in AD or hybrid Azure Active Directory (AAD) environments.

Errors on systems impacted by the SSO Windows Server authentication bug

Microsoft says that organizations might see the following issues on systems affected by the bug:

• Event Viewer might show Microsoft-Windows-Kerberos-Key-Distribution-Center Event 18 logged in the System event log
• Error 0x8009030c with text Web Application Proxy encountered an unexpected is logged in the Azure AD Application Proxy event log in Microsoft-AAD Application Proxy Connector event 12027
• Network traces contain the following signature similar to the following:
  • 7281 24:44 (644) 10.11.2.12 .contoso.com KerberosV5 KerberosV5:TGS Request Realm: CONTOSO.COM Sname: http/xxxxx-xxx.contoso.com
  • 7282 7290 (0) . CONTOSO.COM

Kerberos extension S4U2self impacted by November 2021 Patch Tuesday cumulative update

S4U2self is a Kerberos protocol extension for AD that lets a service get a Kerberos service ticket for itself on behalf of a user. Using S4U2self, the user is identified to the Key Distribution Center (KDC) with the user’s name and realm, or alternatively the user’s certificate.

Service for User (S4U) can be used in different scenarios, like running Windows Task Scheduler tasks under a user account without storing the user’s password. Or allowing an external web server to get a Kerberos service ticket on a user’s behalf.

Here is a list of the updates that you can download for each affected version of Windows Server:

How to install the S4U2self authentication fix for Windows Server domain controllers

Microsoft isn’t making this out-of-band fix available via Windows Update. But you can download the updates manually from the Microsoft Update Catalog using the links above and then manually important each update into Windows Server Update Services (WSUS).