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Windows 10 21H2 / 22H2 Windows 10 21H2 / 22H2 Security update
Patch Tuesday Zero-day

KB5066791

KB5066791: Cumulative update for Windows 10, versions 21H2 and 22H2, October 2025. Exploited modem and Remote Access flaws patched

In short

The update fixes 99 vulnerabilities, including 2 critical ones and 3 exploited flaws affecting the modem driver, IGEL OS Secure Boot, and Remote Access Connection Manager. Windows 10 support ended on October 14, 2025.

Does it affect you? Applies to: Windows 10, version 22H2, all editions, and version 21H2 Enterprise LTSC 2021 and IoT Enterprise LTSC 2021. Windows 10 support ended on October 14, 2025; the listed LTSC editions continue to receive security updates according to their lifecycles.

Builds
19044.6456 · 19045.6456
Security
99 fixed · 2 critical · 3 exploited
Restart
Required

What changes

Fixes 3 changes

Input and Composition

2 items
  • Private Unicode characters in the Chinese input method display correctly and meet the GB18030 standard.1
    Corrects private Unicode character handling in the Chinese Input Method Editor (IME).
  • Long text fields no longer show paired Unicode characters as empty boxes when they reach their length limit.2
    Fixes surrogate pairs in USER32 Edit controls when text fields reach their length limit.

Windows Remote Management (WinRM)

For: People using PowerShell Remoting or WinRM

  • PowerShell Remoting and WinRM commands no longer time out after 600 seconds.3

Improvements 2 changes

Cryptography

  • RSA-based smart card certificates now use Key Storage Provider instead of Cryptographic Service Provider. Smart card authentication might have problems as a result.4
    Enforces KSP rather than CSP for RSA-based smart card certificates.

File Explorer

  • File Explorer automatically disables previews for files downloaded from the internet, helping prevent a vulnerability when previewing potentially unsafe files.5
    Files downloaded from the internet have the preview feature disabled automatically; the page links to steps for unblocking files.

New feature 1 change

Licensing

  • Azure-hosted devices get improved environment validation from an updated certificate chain in the servicing stack.6
    The SSU includes enhanced logic to verify whether a device is hosted on Azure, using an updated certificate chain.

Removal 1 change

Fax modem driver

For: Devices using fax modem hardware that depends on this driver

  • Fax modem hardware that depends on the ltmdm64.sys driver no longer works in Windows.7
    The update removes the ltmdm64.sys driver.

Known issues

  1. Resolved Affects: Organizations and IT administrators Resolved by KB5068781

    An incorrect "End of Support" message might appear in Settings

    On Windows 10, version 22H2 Pro, Education, or Enterprise editions enrolled in ESU with an ESU product key, and on Enterprise LTSC 2021 and IoT Enterprise LTSC 2021, Settings might incorrectly say that Windows has reached end of support. The issue affects only the displayed message and is resolved by KB5068781.8

    Resolution Fixed in KB5068781

    Install KB5068781 (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
2025-10-14
Classification
Security Updates
Type
Cumulative update
Restart
Required
Replaces
KB5066198, KB5065429, KB5063842, KB5066188
+24 earlier KB5063709, KB5062649, KB5062554, KB5061087, KB5063159, KB5060533, KB5058481, KB5061979, KB5061768, KB5055612, KB5053643, KB5052077, KB5050081, KB5046714, KB5045594, KB5043131, KB5041582, KB5040525, KB5039299, KB5037849, KB5036979, KB5035941, KB5034843, KB5034203
Replaced by
KB5068781
VersionBuildChannel
21H2 19044.6456 General availability
22H2 19045.6456 General availability

Security

99 vulnerabilities fixed
3 exploited 2 critical 97 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 99 vulnerabilities →

Updated components

1,421 files changed since the previous update for the same versions.

Top changed componentsFiles
Graphics and display36
Networking30
Secure Boot19
Security and sign-in18
File Explorer and shell15
Search15
Storage and file systems14
Kernel12
Remote Desktop11
Printing10
Bluetooth9
Hyper-V and virtualization8

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. 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
  5. 5. After installing this update, File Explorer automatically disables the preview feature for files downloaded from the internet. support.microsoft.com
  6. 6. This update introduces a servicing stack update (SSU) which incorporates an updated certificate chain to improve Azure environment validation. support.microsoft.com
  7. 7. This update removes the ltmdm64.sys driver. Fax modem hardware dependent on this specific driver will no longer work in Windows. support.microsoft.com
  8. 8. This issue is resolved in Windows updates released on and after November 11, 2025 (KB5068781). 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 5/5, clarity 4/5, completeness 4/5. 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