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Known update issues

KB5121391 – known issues and fixes

There is no data from the Microsoft Security Update Guide for KB5121391 – for example for an optional preview update or an update from another vendor. Affected according to our entries: Microsoft Windows Server (AD FS). Known issues with this update: 1 dated change.

0Open
0Workaround
0Fixed
1Dated change

← All known issues

Known issues with KB5121391

In effectDated change

AD FS: DKM container ACL hardening now enforced with October 2026 update (CVE-2026-56155)

With the October 13, 2026 Windows security update, Microsoft moved the ACL (access control list) hardening of the AD FS Distributed Key Manager (DKM) container into Enforcement mode. It addresses the elevation of privilege vulnerability CVE-2026-56155. On supported Windows Server versions, AD FS remediates insecure ACLs automatically unless administrators opted out. Windows Server 2012 and 2012 R2 require manual remediation. The change is live.

Microsoft · Windows Server (AD FS)

Where the information comes from

We continuously read the vendors’ official sources, community reports and trade media and summarise them in our own words, with a link to the source:

  • Microsoft: Windows release health (Windows 10/11, Server 2016 to 2025), Windows message center, known issues of Microsoft 365 Apps (Outlook, Excel, Word, PowerPoint), Edge, Intune, Autopilot, Windows 365 and Entra Connect, Microsoft KB articles with timelines, Exchange and Windows IT Pro blogs, optionally Microsoft 365 service health and message center
  • other vendors: Veeam KB, Citrix (known issues), Ubuntu and Debian (regressions), Broadcom (VMware), Fortinet, Cisco, Mozilla, Let’s Encrypt
  • community (title, short excerpt and link only, without the names of the authors): Microsoft Q&A, Microsoft Tech Community (discussions), Veeam R&D Forums, Eleven Forum
  • targeted web research with AI (Claude), e.g. on the updates of the month – this also finds discussions on Reddit and in forums; such entries are labelled as community reports until the vendor confirms them
  • early warning from trade media (title and link only): Born’s Tech and Windows World, BleepingComputer, AskWoody, Windows Latest, Windows Central, gHacks, The Windows Club, Deskmodder, Dr. Windows, WindowsArea, Frankys Web, heise online

Summary and translation with AI (Claude by Anthropic) – without any data of visitors.

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