IRONSMITHINTEL
CRITICALCVSS9.8
|
Actively Exploited
|CISA KEV|CVE-2021-31166|Auth: none — unauthenticated|Reboot: required|Manual only

KB5003173: Windows Server Security Update (May 2021)

Microsoft HTTP Protocol Stack contains a vulnerability in http.sys that allows for remote code execution.

Published May 11, 2021 · Updated May 16, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

A remote attacker, without authentication, can achieve full data confidentiality loss, arbitrary modification of data, complete denial of service or system unavailability. Federal agencies are required to remediate by 2022-04-27 under CISA BOD 22-01.

How the attack worksNo clicks needed

This is a Use After Free (CWE-416) vulnerability in Microsoft HTTP Protocol Stack. HTTP Protocol Stack Remote Code Execution Vulnerability Exploitation requires remote network access, low attack complexity, no authentication required, and no user interaction required.

📧

Phishing link

→

🖼

Malicious file

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Server compromised

Am I affected?Quick check

Probably yes if any of these apply:

●IT Security
●Running windows 10 2004: v < 10.0.19041.982; windows 10 20h2: v < 10.0.19042.982; windows server 2004: v < 10.0.19041.982; windows server 20h2: v < 10.0.19042.982
Fixed inKB5003173 (applies to 7 product versions) — build 10.0.19041.982, 10.0.19042.982
Real-world incidentsWhat we've seen

CISA added this CVE to the Known Exploited Vulnerabilities catalog on 2022-04-06 based on evidence of active exploitation in the wild. Federal agencies required to remediate by 2022-04-27.

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 CatalogKB5003173

Manual remediation steps

Apply the Microsoft Security Update

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

Required KB Update

    1
    KB5003173 — https://support.microsoft.com/help/5003173

Supersedes: KB5001330

Affected Products

    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 Server, version 2004 (Server Core installation)
    1
    Windows Server, version 20H2 (Server Core Installation)

Fixed Build Numbers

    1
    10.0.19041.982
    1
    10.0.19042.982

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 KB5003173
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 KB5003173 for the affected products in your update management console.

Microsoft Download Center Links

    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5003173

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5003173') }

References

    1
    Microsoft Security Response Center: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166
    1
    KB article: https://support.microsoft.com/help/5003173
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2021-31166
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31166

Discovery Credit

Microsoft Platform Security & Vulnerability Research

PowerShell automationComing soon

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

References

↗ CVE-2021-31166↗ MSRC Advisory↗ NVD↗ KBKB5003173