IRONSMITHINTEL
HIGHCVSS7.3
|
Actively Exploited
|CISA KEV|CVE-2021-33766|Auth: none — unauthenticated|Reboot: required|Manual only

KB5001779: Windows Server Security Update (July 2021)

Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.

Published Jul 14, 2021 · Updated May 16, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

A remote attacker, without authentication, can achieve partial data exposure, partial data tampering, partial service disruption. Federal agencies are required to remediate by 2022-02-01 under CISA BOD 22-01.

How the attack worksNo clicks needed

This vulnerability affects Microsoft Exchange Server. Exploitation requires remote network access, low attack complexity, no authentication required, and no user interaction required.

📧

Phishing link

→

🖼

Malicious file

🔓

Server compromised

Am I affected?Quick check

Probably yes if any of these apply:

●Exchange Administrators
●Messaging Team
●IT Security
●Running exchange server: 2013, 2016, 2019
Fixed inKB5001779 (applies to 5 product versions) — build 15.00.1497.015, 15.01.2176.012+
Real-world incidentsWhat we've seen

CISA added this CVE to the Known Exploited Vulnerabilities catalog on 2022-01-18 based on evidence of active exploitation in the wild. Federal agencies required to remediate by 2022-02-01.

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 CatalogKB5001779

Manual remediation steps

Apply the Microsoft Security Update

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

Required KB Update

    1
    KB5001779 — https://support.microsoft.com/help/5001779

Affected Products

    1
    Microsoft Exchange Server 2013 Cumulative Update 23
    1
    Microsoft Exchange Server 2016 Cumulative Update 19
    1
    Microsoft Exchange Server 2016 Cumulative Update 20
    1
    Microsoft Exchange Server 2019 Cumulative Update 8
    1
    Microsoft Exchange Server 2019 Cumulative Update 9

Fixed Build Numbers

    1
    15.00.1497.015
    1
    15.01.2176.012
    1
    15.01.2242.008
    1
    15.02.0792.013
    1
    15.02.0858.010

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 KB5001779
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 KB5001779 for the affected products in your update management console.

Microsoft Download Center Links

    1
    http://www.microsoft.com/download/details.aspx?familyid=52da6d67-e0c4-4af0-a133-1e47217b6309
    1
    http://www.microsoft.com/download/details.aspx?familyid=5aa2aaf7-860d-4977-acd4-82096c83c5f0
    1
    http://www.microsoft.com/download/details.aspx?familyid=93809dc0-0265-4116-bc51-510ce641008b
    1
    http://www.microsoft.com/download/details.aspx?familyid=b13f23a9-5603-4b13-8e16-6d35b5b33524
    1
    http://www.microsoft.com/download/details.aspx?familyid=f827ff3b-194c-4470-aa8f-6cedc0d95d07

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5001779') }

References

    1
    Microsoft Security Response Center: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766
    1
    KB article: https://support.microsoft.com/help/5001779
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2021-33766
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33766

Discovery Credit

LE XUAN TUYEN - VNPT ISC working with Trend Micro Zero Day Initiative

PowerShell automationComing soon

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

References

↗ CVE-2021-33766↗ MSRC Advisory↗ NVD↗ KBKB5001779