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Windows Server 2025 Windows Server 2025 Security update
Patch Tuesday Zero-day

KB5050009

KB5050009: Cumulative update for Windows Server, version 2025, January 2025. Exploited Hyper-V flaws patched; USB audio issues reported

In short

Fixes 126 vulnerabilities, including three exploited Hyper-V elevation-of-privilege flaws. It also expands the vulnerable-driver blocklist; USB audio might stop working after installation.

Does it affect you? Applies to: Devices running Windows Server 2025, including systems receiving the x64 update package. The update installs build 26100.2894.

Build
26100.2894
Security
126 fixed · 9 critical · 3 exploited
Restart
Required

Microsoft publishes separate notes for this KB for Windows 11.

What changes

Improvements 2 changes

Windows Kernel Vulnerable Driver Blocklist file

Verified in package
  • More at-risk drivers are added to the blocklist to help protect against attacks that use vulnerable drivers.1
    Adds drivers at risk for Bring Your Own Vulnerable Driver (BYOVD) attacks to DriverSiPolicy.p7b.

Servicing stack

  • The component that installs Windows updates receives quality improvements.2
    Includes servicing stack update KB5050387, build 26100.2890.

Known issues

  1. Resolved Affects: Organizations and IT administrators

    Devices that have certain Citrix components installed might be unable to complete installation of the January 2025 Windows security update.

    Devices with Citrix Session Recording Agent 2411 might fail to complete the January 2025 security update installation and revert changes. The issue is resolved in Session Recording Agent version 2503 and newer; home users are not expected to be affected.3

    Resolution Resolved

    Microsoft has resolved this issue in a subsequent update. No manual workaround is required after updating.

  2. Resolved Affects: Everyone Resolved by KB5051987

    After installing this security update, you might experience issues with USB audio devices.

    USB audio devices might stop working after installation, especially when the audio setup uses a USB 1.0 audio driver based DAC. Device Manager might show Code 10. KB5051987 addresses the issue.4

    Resolution Fixed in KB5051987

    Install KB5051987 (or a later cumulative update) on affected devices. No manual workaround is required after updating.

Technical details

Data sheet · Security · Downloads · Components Click to collapse or expand

Data sheet

Released
2025-01-14
Classification
Security Updates
Type
Cumulative update
Restart
Required
Replaces
KB5048667, KB5046617, KB5043080, KB5044284
Replaced by
KB5051987
VersionBuildChannel
2025 26100.2894 General availability

Security

126 vulnerabilities fixed
3 exploited 9 critical 117 important
VulnerabilitySeverityCVSS
CVE-2025-21333 Exploited
Windows Hyper-V NT Kernel Integration VSP Elevation of Privilege Vulnerability
 Important 7.8
CVE-2025-21334 Exploited
Windows Hyper-V NT Kernel Integration VSP Elevation of Privilege Vulnerability
 Important 7.8
CVE-2025-21335 Exploited
Windows Hyper-V NT Kernel Integration VSP Elevation of Privilege Vulnerability
 Important 7.8
CVE-2025-21298
Windows OLE Remote Code Execution Vulnerability
 Critical 9.8
CVE-2025-21307
Windows Reliable Multicast Transport Driver (RMCAST) Remote Code Execution Vulnerability
 Critical 9.8
CVE-2025-21311
Windows NTLM V1 Elevation of Privilege Vulnerability
 Critical 9.8
CVE-2024-49120
Windows Remote Desktop Services Remote Code Execution Vulnerability
 Critical 8.1
CVE-2025-21294
Microsoft Digest Authentication Remote Code Execution Vulnerability
 Critical 8.1
CVE-2025-21295
SPNEGO Extended Negotiation (NEGOEX) Security Mechanism Remote Code Execution Vulnerability
 Critical 8.1
CVE-2025-21297
Windows Remote Desktop Services Remote Code Execution Vulnerability
 Critical 8.1
CVE-2025-21309
Windows Remote Desktop Services Remote Code Execution Vulnerability
 Critical 8.1
CVE-2025-21296
BranchCache Remote Code Execution Vulnerability
 Critical 7.5
CVE-2025-21275
Windows App Package Installer Elevation of Privilege Vulnerability
 Important 7.8
CVE-2025-21308
Windows Themes Spoofing Vulnerability
 Important 6.5

The table lists the 14 exploited, publicly disclosed or critical vulnerabilities. See all 126 vulnerabilities →

Updated components

1,660 files changed since the previous update for the same versions.

Top changed componentsFiles
Secure Boot17
File Explorer and shell16
Security and sign-in15
Networking14
Search14
Graphics and display13
Kernel11
Storage and file systems10
Printing9
Remote Desktop9
Emoji and input6
Hyper-V and virtualization5

Verified in package: changed files confirm Windows Kernel Vulnerable Driver Blocklist file.

Sources

Updated
Cited passages from Microsoft's official notes (4)
  1. 1. This update adds to the list of drivers that are at risk for Bring Your Own Vulnerable Driver (BYOVD) attacks. support.microsoft.com
  2. 2. This update makes quality improvements to the servicing stack, which is the component that installs Windows updates. support.microsoft.com
  3. 3. The issue has been resolved in Citrix Session Recording Agent version 2503, released on April 28, 2025, and newer versions. support.microsoft.com
  4. 4. This issue is addressed in KB5051987. support.microsoft.com
Checked automatically: 8 of 8 checks passed, 4 quoted passages · compared with Microsoft's page on Oct 20, 2026
  • Quotes found in the official pages: passed
  • Numbers found in the sources: passed
  • Vulnerabilities from the dossier only: passed
  • Links to allowed domains only: passed
  • Every official change covered: passed
  • Readable plain text: passed
  • Changes confirmed by the file list: passed
  • Sources agree: passed

A second AI read the text against the sources: correctness 5/5, clarity 4/5, completeness 5/5. How we work

Published by The Update Bureau. Written with AI from Microsoft's own documents, checked by rules and a second AI. How we work · Report a mistake