Patch tracker / KB5066366
KB5066366: Microsoft Exchange Server Subscription Security Update (October 2025)
KB5066366 is a High security update for Microsoft Exchange Server Subscription, released 2025-10-14. It closes 3 CVEs with a maximum CVSS of 8.8. It is the 19th largest of the 60 updates Microsoft shipped that month, by CVEs closed.
Update summary
Fixes 3 CVEs. Most severe CVSS 8.8 (High). EPSS exploit probability up to <1%. Among the lower half of tracked Microsoft updates by EPSS exploit probability.
A newer security update for Microsoft Exchange Server 2019 Cumulative Update 15 has shipped since this one: KB5121573 (2026-08-11). For cumulative update families the newer package includes these fixes, so installing that one is usually the shorter path. Verify against your own update rings before you skip this package.
KB5066366 is the October 14, 2025 security update for Exchange Server 2019 (CU14 and CU15) and Exchange Server Subscription Edition RTM, addressing three CVEs including CVE-2025-53782, CVE-2025-59249, and CVE-2025-59248. The top issue rates High at CVSS 8.8, so any organization still running on-premises Exchange is exposed. Exchange remains a high-value target for attackers, and keeping cumulative updates current is required before you can even apply these fixes.
Do this: Confirm your Exchange 2019 servers are on CU14 or CU15 (or SE RTM) and deploy KB5066366 to all mailbox and edge roles this patch cycle.
AI-generated from public data, 2026-07-21. Verify against the vendor advisory before acting.
Senserva AI Opinion and rich prompt for KB5066366
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5066366
- Released
- 2025-10-14
- Last changed
- 2025-10-14
- Changes tracked
- 1
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.
Check if KB5066366 is installed
Elevated PowerShell. No output means it is not installed.
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.
| CVE | Severity | CVSS | EPSS | Exploited |
|---|---|---|---|---|
| CVE-2025-59249 | High | 8.8 | <1% | No |
| CVE-2025-53782 | High | 8.4 | <1% | No |
| CVE-2025-59248 | High | 7.5 | <1% | No |
Scope and sources
- Affected products
- Also released in October 2025
KB5070879
1 CVE · Critical · exploitedKB5070881
1 CVE · Critical · exploitedKB5070882
1 CVE · Critical · exploitedKB5070883
1 CVE · Critical · exploitedKB5070884
1 CVE · Critical · exploitedKB5070886
1 CVE · Critical · exploitedKB5070887
1 CVE · Critical · exploitedKB5070892
1 CVE · Critical · exploitedThe full month, summarized: Microsoft Patch Tuesday.
- Authoritative references
See this and every Microsoft update ranked by real-world risk on the Microsoft Patch Tracker.