IRONSMITHINTEL
CRITICAL
|Auth: varies by cve|Reboot: required|Est. 30–60 minutes including reboot|Manual only

KB5055521: Windows Server 2016 Cumulative Update (May 2026)

Windows Server 2016 requires KB5055521 for the April 2026 security fixes — apply before the exploitation window opens.

Published May 7, 2026 · Updated May 10, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

Unpatched Windows Server 2016 systems are exposed to all vulnerabilities addressed in the April 2026 patch cycle.

How the attack works

Microsoft's April 2026 Patch Tuesday includes the monthly cumulative update for Windows Server 2016 (KB5055521). Windows Server 2016 requires cumulative updates to receive all security patches — individual security-only updates are no longer provided as a standalone option.

Am I affected?Quick check

Probably yes if any of these apply:

●All Windows Server 2016 systems
●Running Windows Server 2016 prior to KB5055521

Affected OS versions

Windows Server 2016
Fixed inKB5055521
Real-world incidentsWhat we've seen

Windows Server 2016 remains widely deployed and is a target for attackers who maintain a list of unpatched CVEs and scan for vulnerable systems after Patch Tuesday.

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 CatalogKB5055521

Manual remediation steps

⏱ 30–60 minutes including reboot

Check if KB5055521 Is Installed

Get-HotFix -Id KB5055521

Apply via Windows Update

1
Open Settings → Windows Update
2
Check for updates and install
3
Restart when prompted

Apply Manually

1
Download KB5055521 from Microsoft Update Catalog
2
Run the MSU installer as Administrator
3
Restart the server

Verification

Get-HotFix -Id KB5055521
# Must show an InstalledOn date
PowerShell automationComing soon

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

References

↗ MSRC Advisory↗ KBKB5055521