KB5066586: Windows Server 2019 Security Update (December 2025)

Update summary

Senserva AI Opinion for KB5066586

KB5066586 is the December 2025 rollup for Windows 10 1809 and Windows Server 2019, fixing 98 CVEs with a top rating of Critical at CVSS 9.9. One of the included CVEs is already listed in CISA KEV, meaning it is being exploited in the wild, though none of these are currently tied to ransomware campaigns. Anyone still running the aging 1809 branch or Server 2019 fleets is exposed to that confirmed active exploitation.

Do this: Deploy KB5066586 to all Windows Server 2019 and Windows 10 1809 systems now, prioritizing the CISA KEV-listed CVE given its confirmed exploitation.

AI-generated from public data, 2026-07-21. Verify against the vendor advisory before acting.

Full tracking and change history for KB5066586

Senserva began recording day-to-day change history on 2026-07-07, and refreshes it several times a day. Release dates are historical where Microsoft publishes them; revision history is tracked from that date forward, so it does not include changes made before then.

Exploitation and severity

Fixes 98 CVEs. At least one is actively exploited (CISA KEV). Most severe CVSS 9.9 (Critical). EPSS exploit probability up to 6.0%.

Among the top half of tracked Microsoft updates by EPSS exploit probability.

What to do

Treat as urgent: it is being exploited in the wild (CISA KEV), so patch ahead of items that are not. Senserva flags whether KB5066586 is missing on your devices.

Check if KB5066586 is installed

Copy-paste PowerShell: one machine, or your whole estate in one pass. No output means the update is not installed.

Get-HotFix covers Windows updates. Office, SharePoint, and Exchange server updates show under installed updates in Programs and Features, or check the Update Catalog history.

CVEs fixed by this update

Sorted exploited-first, then by CVSS. Every CVE links to its Senserva page with the full risk facts, references, and a ready-to-paste AI prompt.

Affected products

• Windows 10 Version 1809 for 32-bit Systems
• Windows 10 Version 1809 for x64-based Systems
• Windows Server 2019

Also released in December 2025

The highest-risk updates from the same release:

The full month, summarized: Microsoft Patch Tuesday.

Authoritative references