Microsoft Windows 11 build 28000.2956 (KB5129194) is an out-of-band security update for Windows 11 version 26H1, released shortly after the September 2026 cumulative security update.

What's new

• Adds protections for CVE-2026-62721, a Windows User-Mode Power Service (UMPS) Elevation of Privilege vulnerability.
• Fixes Remote Desktop Services instability introduced after KB5124012 that could cause RDP connection and sign-in failures or make affected systems unresponsive.
• Fixes an issue affecting Hyper-V and other HCS-managed Linux virtual machines where host folders shared using Plan9 could become unavailable.
• Fixes certain USB Audio Class 1.0 devices that failed when using multichannel features such as 8-channel or 3D audio modes.
• Includes servicing stack improvements from KB5125104 (Build 28000.2950) to improve Windows Update reliability.

Known issues

• Some USB Audio Class 1.0 devices may still fail to start, show Device Manager error Code 10, produce no audio output, or have unavailable volume controls.

Recommendation

Organizations should prioritize deployment of KB5129194 after expedited validation, particularly in environments affected by Remote Desktop Services or Hyper-V Plan9 issues. The update also contains a documented elevation-of-privilege security fix and therefore should not be treated as an optional preview update.