IRONSMITHINTEL
HIGHCVSS7.8
|
Actively Exploited
|CISA KEV|CVE-2026-21509|Auth: none — unauthenticated|Reboot: required|Manual only

KB5002713: Windows Server Security Update (January 2026)

Microsoft Office contains a security feature bypass vulnerability in which reliance on untrusted inputs in a security decision in Microsoft Office could allow an unauthorized attacker to bypass a security feature locally. Some of the impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.

Published Jan 26, 2026 · Updated May 16, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

A local 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 2026-02-16 under CISA BOD 22-01.

How the attack worksNo clicks needed

This is a Software Vulnerability (CWE-807) (CWE-807) vulnerability in Microsoft Office. Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally. Exploitation requires local access, low attack complexity, no authentication required, and user interaction required.

Am I affected?Quick check

Probably yes if any of these apply:

IT Security
Running 365 apps: -; office: 2016, 2019; office long term servicing channel: 2021, 2024
Fixed inKB5002713 (applies to 10 product versions) — build 16.0.10417.20095, 16.0.5539.1001+
Real-world incidentsWhat we've seen

CISA added this CVE to the Known Exploited Vulnerabilities catalog on 2026-01-26 based on evidence of active exploitation in the wild. Federal agencies required to remediate by 2026-02-16.

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 CatalogKB5002713

Manual remediation steps

Apply the Microsoft Security Update

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

Required KB Update

    1
    KB5002713 — https://support.microsoft.com/help/5002713

Supersedes: KB5002573

Affected Products

    1
    Microsoft 365 Apps for Enterprise for 32-bit Systems
    1
    Microsoft 365 Apps for Enterprise for 64-bit Systems
    1
    Microsoft Office 2016 (32-bit edition)
    1
    Microsoft Office 2016 (64-bit edition)
    1
    Microsoft Office 2019 for 32-bit editions
    1
    Microsoft Office 2019 for 64-bit editions
    1
    Microsoft Office LTSC 2021 for 32-bit editions
    1
    Microsoft Office LTSC 2021 for 64-bit editions
    1
    Microsoft Office LTSC 2024 for 32-bit editions
    1
    Microsoft Office LTSC 2024 for 64-bit editions

Fixed Build Numbers

    1
    16.0.10417.20095
    1
    16.0.5539.1001
    1
    https://aka.ms/OfficeSecurityReleases

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 (may or may not be required for this update)

Microsoft Update Catalog (manual download)

1
Open https://catalog.update.microsoft.com
2
Search for KB5002713
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 KB5002713 for the affected products in your update management console.

Microsoft Download Center Links

    1
    https://www.microsoft.com/en-us/download/details.aspx?id=108535

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5002713') }

References

    1
    Microsoft Security Response Center: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509
    1
    KB article: https://support.microsoft.com/help/5002713
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2026-21509
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509

Discovery Credit

Microsoft Threat Intelligence Center (MSTIC), Microsoft Security Response Center (MSRC), and Office Product Group Security Team, Google Threat Intelligence Group

PowerShell automationComing soon

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