IRONSMITHINTEL
MEDIUMCVSS5.5
|
Actively Exploited
|CISA KEV|CVE-2021-31955|Auth: low — authenticated user|Reboot: required|Manual only

KB5003635: Windows Server Security Update (June 2021)

Microsoft Windows Kernel contains an unspecified vulnerability that allows for information disclosure. Successful exploitation allows attackers to read the contents of kernel memory from a user-mode process.

Published Jun 8, 2021 · Updated May 16, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

A local attacker, with a low-privilege account, can achieve full data confidentiality loss. Federal agencies are required to remediate by 2021-11-17 under CISA BOD 22-01.

How the attack worksNo clicks needed

This is a Software Vulnerability (CWE-497) (CWE-497) vulnerability in Microsoft Windows. Windows Kernel Information Disclosure Vulnerability Exploitation requires local access, low attack complexity, a low-privilege authenticated account, and no user interaction required.

Am I affected?Quick check

Probably yes if any of these apply:

●Windows Administrators
●Systems Engineers
●IT Security
●Running windows 10 1809: v < 10.0.17763.1999; windows 10 1909: v < 10.0.18363.1621; windows 10 2004: v < 10.0.19041.1052; windows 10 20h2: v < 10.0.19042.1052; windows 10 21h1: v < 10.0.19043.1052; windows server 2004: v < 10.0.19041.1052; windows server 2019: v < 10.0.17763.1999; windows server 20h2: v < 10.0.19042.1052
Fixed inKB5003635, KB5003637, KB5003646 (applies to 18 product versions) — build 10.0.17763.1999, 10.0.18363.1621+
Real-world incidentsWhat we've seen

CISA added this CVE to the Known Exploited Vulnerabilities catalog on 2021-11-03 based on evidence of active exploitation in the wild. Federal agencies required to remediate by 2021-11-17.

How to patch

Manual download

For air-gapped servers or out-of-band deployment. Microsoft Update Catalog returns every OS-version variant of this update.

↗ Microsoft Update CatalogKB5003635

Manual remediation steps

Apply the Microsoft Security Update

Microsoft has released an official security update that fixes this vulnerability.

Required KB Updates

    1
    KB5003635 — https://support.microsoft.com/help/5003635
    1
    KB5003637 — https://support.microsoft.com/help/5003637
    1
    KB5003646 — https://support.microsoft.com/help/5003646

Supersedes: KB5003169, KB5003171, KB5003173

Affected Products

    1
    Windows 10 Version 1809 for 32-bit Systems
    1
    Windows 10 Version 1809 for ARM64-based Systems
    1
    Windows 10 Version 1809 for x64-based Systems
    1
    Windows 10 Version 1909 for 32-bit Systems
    1
    Windows 10 Version 1909 for ARM64-based Systems
    1
    Windows 10 Version 1909 for x64-based Systems
    1
    Windows 10 Version 2004 for 32-bit Systems
    1
    Windows 10 Version 2004 for ARM64-based Systems
    1
    Windows 10 Version 2004 for x64-based Systems
    1
    Windows 10 Version 20H2 for 32-bit Systems
    1
    Windows 10 Version 20H2 for ARM64-based Systems
    1
    Windows 10 Version 21H1 for 32-bit Systems
    1
    Windows 10 Version 21H1 for ARM64-based Systems
    1
    Windows 10 Version 21H1 for x64-based Systems
    1
    Windows Server 2019
    1
    Windows Server 2019 (Server Core installation)
    1
    Windows Server, version 2004 (Server Core installation)
    1
    Windows Server, version 20H2 (Server Core Installation)

Fixed Build Numbers

    1
    10.0.17763.1999
    1
    10.0.18363.1621
    1
    10.0.19041.1052
    1
    10.0.19042.1052
    1
    10.0.19043.1052

Installation Methods

Windows Update (recommended)

1
Settings → Windows Update → Check for updates
2
The security update is offered if your system is in scope
3
Restart when prompted — a reboot IS required to complete the install

Microsoft Update Catalog (manual download)

1
Open https://catalog.update.microsoft.com
2
Search for KB5003635
3
Download the package matching your OS architecture and Windows build
4
Run the .msu installer with administrator privileges
5
Restart when prompted

WSUS / SCCM / Intune

Approve KB5003635 for the affected products in your update management console.

Microsoft Download Center Links

    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5003635
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5003637
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5003646

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5003635','KB5003637','KB5003646') }

References

    1
    Microsoft Security Response Center: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31955
    1
    KB article: https://support.microsoft.com/help/5003635
    1
    KB article: https://support.microsoft.com/help/5003637
    1
    KB article: https://support.microsoft.com/help/5003646
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2021-31955
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31955

Discovery Credit

Boris Larin (oct0xor) of Kaspersky Lab

PowerShell automationComing soon

No tested PowerShell script for this entry yet. We’re prioritising automation based on user demand.

References

↗ CVE-2021-31955↗ MSRC Advisory↗ NVD↗ KBKB5003635