IRONSMITHINTEL
HIGHCVSS8.8
|
Actively Exploited
|CISA KEV|CVE-2021-42321|Auth: low — authenticated user|Reboot: required|Manual only

KB5007409: Windows Server Security Update (November 2021)

An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.

Published Nov 10, 2021 · Updated May 16, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

A remote attacker, with a low-privilege account, can achieve full data confidentiality loss, arbitrary modification of data, complete denial of service or system unavailability. CISA has confirmed use of this vulnerability in known ransomware campaigns — treat as high priority for remediation. Federal agencies are required to remediate by 2021-12-01 under CISA BOD 22-01.

How the attack worksNo clicks needed

This vulnerability affects Microsoft Exchange. Exploitation requires remote network 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:

●Exchange Administrators
●Messaging Team
●IT Security
●Running exchange server: 2016, 2019
Fixed inKB5007409 (applies to 4 product versions) — build 15.01.2308.020, 15.01.2375.017+
Real-world incidentsWhat we've seen

Used in known ransomware campaigns. Threat-research write-up: http://packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Execution.html

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 CatalogKB5007409

Manual remediation steps

Apply the Microsoft Security Update

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

Required KB Update

    1
    KB5007409 — https://support.microsoft.com/help/5007409

Supersedes: KB5007012

Affected Products

    1
    Microsoft Exchange Server 2016 Cumulative Update 21
    1
    Microsoft Exchange Server 2016 Cumulative Update 22
    1
    Microsoft Exchange Server 2019 Cumulative Update 10
    1
    Microsoft Exchange Server 2019 Cumulative Update 11

Fixed Build Numbers

    1
    15.01.2308.020
    1
    15.01.2375.017
    1
    15.02.0792.019
    1
    15.02.0986.014

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 KB5007409
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 KB5007409 for the affected products in your update management console.

Microsoft Download Center Links

    1
    http://www.microsoft.com/download/details.aspx?familyid=1c42658f-9d60-4afb-a6c6-e35594b17d39
    1
    http://www.microsoft.com/download/details.aspx?familyid=688b79c6-7e43-4332-848d-47e88f60818c
    1
    http://www.microsoft.com/download/details.aspx?familyid=cd28ac6e-eb6f-4747-b9f0-24785b08a012
    1
    http://www.microsoft.com/download/details.aspx?familyid=de4b96e0-8d0e-4830-8354-7ed2128e6f82

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5007409') }

References

    1
    Microsoft Security Response Center: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-42321
    1
    KB article: https://support.microsoft.com/help/5007409
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2021-42321
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-42321

Discovery Credit

zcgonvh @ 360 noah lab, Yuhao Weng & Zhiniang Peng & Feng Dong with Sangfor, Microsoft Threat Intelligence Center (MSTIC), Microsoft Security Response Center

PowerShell automationComing soon

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

References

↗ CVE-2021-42321↗ MSRC Advisory↗ NVD↗ KBKB5007409