How to Fix CVE-2026-21514: Security Vulnerability in Microsoft 365 Apps for Enterprise

By Sai Kiran Pandrala

Patch immediately. CISA added CVE-2026-21514 to the Known Exploited Vulnerabilities catalog on 2026-02-10. Federal civilian agencies must remediate by 2026-03-03. Treat every internet-reachable instance as a priority patch.

What is CVE-2026-21514?

CVE-2026-21514 is a Security Vulnerability flaw in Microsoft 365 Apps for Enterprise. It carries a CVSS base score of 7.8 (high). CISA confirmed real-world exploitation by adding it to the Known Exploited Vulnerabilities catalog on 2026-02-10.

From the source record: Reliance on untrusted inputs in a security decision in Microsoft Office Word allows an unauthorized attacker to bypass a security feature locally.

Why it matters in practice: KEV-listed CVEs draw continuous internet-wide scanning. Any unpatched, internet-reachable installation is on borrowed time. The blast radius depends on how the affected service is exposed. An internet-facing instance with no compensating controls is the highest-risk configuration.

Am I affected?

You are affected if your installation of Microsoft 365 Apps for Enterprise matches a version listed in the Affected row above.

On Windows, check the installed version with PowerShell:

# Generic - list installed product versions
Get-CimInstance Win32_Product | Where-Object Name -like "*Microsoft 365 Apps for Enterprise*" | Select-Object Name, Version
# Windows updates installed (KB lookup)
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 20

How to fix CVE-2026-21514

Apply the vendor patch. Target a build at or above: 16.106.26020821, https://aka.ms/OfficeSecurityReleases. The runnable command set below covers the most common deployment patterns for Microsoft 365 Apps for Enterprise.

Windows (PowerShell, run as administrator)

# 1) Backup affected service config
$stamp = Get-Date -Format yyyyMMdd-HHmm
$backup = "C:\Backup\microsoft-$stamp"
New-Item -ItemType Directory -Path $backup -Force | Out-Null

# 2) Install the latest Windows security updates (the patch for CVE-2026-21514 ships here)
Install-Module PSWindowsUpdate -Force -Scope CurrentUser -ErrorAction SilentlyContinue
Import-Module PSWindowsUpdate
Get-WindowsUpdate -MicrosoftUpdate -AcceptAll -Install -AutoReboot

# 3) Confirm the patch landed
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 10

Vendor-managed app on Windows (winget)

# Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21514
winget upgrade --all --silent --accept-package-agreements --accept-source-agreements
# or target the specific package:
winget search "Microsoft 365 Apps for Enterprise"
winget upgrade --id <PackageId> --silent

Complete PowerShell patch script

# Patch Microsoft 365 Apps for Enterprise for CVE-2026-21514
$log = "C:\Backup\CVE-2026-21514-patch.log"
"[{0}] Starting patch run" -f (Get-Date) | Tee-Object -FilePath $log -Append
try {
    $stamp = Get-Date -Format yyyyMMdd-HHmm
    $backup = "C:\Backup\microsoft-$stamp"
    New-Item -ItemType Directory -Path $backup -Force | Out-Null
    # Backup hook - tailor for the product
    # Copy-Item "C:\Program Files\Microsoft 365 Apps for Enterprise" $backup -Recurse -ErrorAction SilentlyContinue

    if (-not (Get-Module -ListAvailable PSWindowsUpdate)) {
        Install-Module PSWindowsUpdate -Force -Scope CurrentUser
    }
    Import-Module PSWindowsUpdate
    Get-WindowsUpdate -MicrosoftUpdate -AcceptAll -Install -AutoReboot:$false -IgnoreReboot
    "[{0}] Updates installed" -f (Get-Date) | Tee-Object -FilePath $log -Append
    # Verify
    Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5 | Out-File $log -Append
    "[{0}] Patch run complete - reboot at next maintenance window" -f (Get-Date) | Tee-Object -FilePath $log -Append
} catch {
    "[{0}] ERROR: $_" -f (Get-Date) | Tee-Object -FilePath $log -Append
    exit 1
}

After applying the patch

1. Restart the service or device so the patched binary loads.
2. Confirm the running version matches the Fixed in row using the verification command below.
3. Rotate credentials and API keys that the affected service could access if the asset was exposed during the disclosure window.

If you can't patch immediately

Until the patch lands, narrow the attack surface with these runnable controls.

Restrict the affected service via Windows Firewall

# Replace -RemoteAddress with your trusted admin range
New-NetFirewallRule -DisplayName "Block CVE-2026-21514 inbound" -Direction Inbound -Action Block -Protocol TCP -LocalPort 445,3389,5985
New-NetFirewallRule -DisplayName "Allow CVE-2026-21514 admin range" -Direction Inbound -Action Allow -Protocol TCP -LocalPort 445,3389,5985 -RemoteAddress 10.10.10.0/24

Disable the vulnerable feature if the vendor advisory allows it

# Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21514
# Example: stopping a non-essential Windows service
Stop-Service -Name <ServiceName> -Force
Set-Service -Name <ServiceName> -StartupType Disabled

Mitigations are temporary. Apply the vendor patch as soon as a maintenance window opens.

How to verify the fix worked

Confirm the patched build is the one actually running.

# Confirm KB / hotfix is present
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 10
# Confirm product version
Get-CimInstance Win32_Product | Where-Object Name -like "*Microsoft 365 Apps for Enterprise*" | Select-Object Name, Version

Expected: the patched build named in Fixed in (16.106.26020821, https://aka.ms/OfficeSecurityReleases) appears in the version column.

Also worth doing: pull recent log windows for any indicators of compromise listed in the vendor advisory, and re-run an authenticated vulnerability scan with up-to-date signatures.

Frequently asked questions

Related fixes

Other vulnerabilities in the same area that are worth patching alongside this one:

• How to Fix CVE-2026-26110: Microsoft Office Remote Code Execution — Microsoft Office Remote Code Execution
• How to Fix CVE-2026-40411: Input Validation Flaw in Azure Virtual Network Gateway — Input Validation Flaw in Azure Virtual Network Gateway
• How to Fix CVE-2026-21242: Use-After-Free in Windows 10 Version 21H2 , Use-After-Free in Windows 10 Version 21H2
• How to Fix CVE-2026-33844: Improper Input Validation in Azure Managed Instance for Apache Cassandra , Improper Input Validation in Azure Managed Instance for Apache Cassandra
• How to Fix CVE-2026-32084: Information disclosure in Microsoft Windows , Information disclosure in Microsoft Windows

Is CVE-2026-21514 being exploited in the wild?

Yes. CISA added CVE-2026-21514 to the Known Exploited Vulnerabilities catalog on 2026-02-10. KEV listing means at least one confirmed real-world exploitation report exists.

Do I have to take downtime to patch?

For most Microsoft 365 Apps for Enterprise deployments, the patched build needs a service restart or device reboot. HA pairs and clusters can roll the upgrade by patching the standby first, failing over, then patching the former primary.

Will a WAF or IDS rule alone close CVE-2026-21514?

No. Network filters cut down opportunistic scans but they do not remove the flaw. The vendor patch is the only durable fix.

How quickly should I act on CVE-2026-21514?

Within the standard patch cycle if the asset is internal-only. Inside one to two weeks for any internet-facing instance, sooner if compensating controls are not in place.

References

• Official vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21514
• NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2026-21514
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• CISA KEV entry: "Microsoft Office Word Reliance on Untrusted Inputs in a Security Decision Vulnerability" - added 2026-02-10, due 2026-03-03

*Assembled from the official vendor advisory, the NVD record, and the CISA KEV listing on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*