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Windows Server 2019 Windows Server 2019 Security update
Patch Tuesday Zero-day

KB5066586

KB5066586: Cumulative update for Windows Server, version 2019, October 2025. Exploited Remote Access Connection Manager flaw addressed

In short

Addresses an actively exploited elevation-of-privilege flaw in Remote Access Connection Manager and fixes several input and remote-management problems. It also changes File Explorer previews for downloaded files.

Does it affect you? Applies to: Devices running Windows Server 2019, all editions.

Build
17763.7919
Security
98 fixed · 2 critical · 3 exploited
Restart
Required

Microsoft publishes separate notes for this KB for Windows 10.

What changes

Fixes 4 changes

Input and Composition

2 items
  • The Chinese input method displays private Unicode characters correctly and meets the GB18030 standard.1
    Fixes incorrect rendering of private Unicode characters in the Chinese Input Method Editor (IME).
  • Some text fields no longer show paired characters as empty boxes when the field reaches its length limit.2
    Fixes surrogate pairs appearing as empty boxes in USER32 Edit controls when text fields reach their length limit.

Windows Remote Management (WinRM)

  • PowerShell Remoting and WinRM commands no longer time out after 600 seconds.3
    Fixes command timeouts after 600 seconds in PowerShell Remoting and WinRM.

Stability issue

  • The update addresses rare cases where devices became unresponsive in specific scenarios after the May 2025 security update and later updates.4

Improvements 3 changes

Cryptography

  • RSA-based smart card certificates must use the newer certificate storage method. Smart card authentication might be affected by this change.5
    Enforces Key Storage Provider (KSP) instead of Cryptographic Service Provider (CSP) for RSA-based smart card certificates. The page directs users with resulting authentication problems to Windows Release Health for resolution steps.

File Explorer

  • File Explorer automatically disables previews for files downloaded from the internet, helping prevent a vulnerability when previewing potentially unsafe files.6

Servicing stack

  • The component that installs Windows updates receives quality improvements intended to make the update process more reliable.7
    The cumulative update includes servicing stack update KB5066585, version 17763.7912.

Removal 1 change

Fax modem driver

For: devices using fax modem hardware dependent on ltmdm64.sys

  • Fax modem hardware that depends on the ltmdm64.sys driver no longer works in Windows.8
    The update removes the ltmdm64.sys driver.

Technical details

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

Data sheet

Released
2025-10-14
Classification
Security Updates
Type
Cumulative update
Restart
Required
Replaces
KB5065428, KB5066187, KB5063877, KB5062557
+7 earlier KB5060531, KB5061978, KB5058392, KB5059091, KB5058922, KB5050008, KB5037425
Replaced by
KB5070883
VersionBuildChannel
2019 17763.7919 General availability

Security

98 vulnerabilities fixed
3 exploited 2 critical 96 important
VulnerabilitySeverityCVSS
CVE-2025-24990 Exploited
Windows Agere Modem Driver Elevation of Privilege Vulnerability
 Important 7.8
CVE-2025-59230 Exploited
Windows Remote Access Connection Manager Elevation of Privilege Vulnerability
 Important 7.8
CVE-2025-47827 Exploited
MITRE CVE-2025-47827: Secure Boot bypass in IGEL OS before 11
 Important 4.6
CVE-2025-49708
Microsoft Graphics Component Elevation of Privilege Vulnerability
 Critical 9.9
CVE-2016-9535
MITRE CVE-2016-9535: LibTIFF Heap Buffer Overflow Vulnerability
 Critical 4.0
CVE-2025-24052
Windows Agere Modem Driver Elevation of Privilege Vulnerability
 Important 7.8

The table lists the 6 exploited, publicly disclosed or critical vulnerabilities. See all 98 vulnerabilities →

Downloads (Microsoft Update Catalog)

x64 2019
710.3 MB
windows10.0-kb5066586-x64.msu Main update · KB5066586 SHA-1: 062fcc3b…75eb18

Updated components

583 files changed since the previous update for the same versions.

Top changed componentsFiles
Graphics and display9
Hyper-V and virtualization9
Networking8
Secure Boot5
Kernel4
Search4
Storage and file systems4
Bluetooth3
File Explorer and shell3
Printing3
Remote Desktop3
Security and sign-in3

Sources

Updated
Cited passages from Microsoft's official notes (8)
  1. 1. Fixed: An issue with the Chinese Input Method Editor (IME). Private Unicode characters were shown incorrectly and did not meet GB18030 standard. support.microsoft.com
  2. 2. Fixed: An issue that affects USER32 Edit controls. Surrogate pairs appear as empty boxes when text fields reach their length limit. support.microsoft.com
  3. 3. Fixed: An issue that affects PowerShell Remoting and WinRM in which commands time out after 600 seconds. support.microsoft.com
  4. 4. Fixed: This update addresses an issue observed in rare cases after installing the May 2025 security update and subsequent updates causing devices to experience stability issues. support.microsoft.com
  5. 5. This update enforces a security hardening improvement by requiring use of Key Storage Provider (KSP) instead of Cryptographic Service Provider {CSP) for RSA-based smart card certificates. support.microsoft.com
  6. 6. After installing this update, File Explorer automatically disables the preview feature for files downloaded from the internet. This change is designed to enhance security by preventing a vulnerability when users preview potentially unsafe files. support.microsoft.com
  7. 7. This update makes quality improvements to the servicing stack, which is the component that installs Windows updates. support.microsoft.com
  8. 8. This update removes the ltmdm64.sys driver. Fax modem hardware dependent on this specific driver will no longer work in Windows. support.microsoft.com
Checked automatically: 8 of 8 checks passed, 8 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 4/5, 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