Patch tracker / KB5071873
KB5071873: Microsoft Exchange Server Subscription Security Update (December 2025)
KB5071873 is a High security update for Microsoft Exchange Server Subscription, released 2025-12-09. It closes 2 CVEs with a maximum CVSS of 7.5. It is the 47th largest of the 59 updates Microsoft shipped that month, by CVEs closed.
Update summary
Fixes 2 CVEs. Most severe CVSS 7.5 (High). EPSS exploit probability up to 1.0%. Among the lower half of tracked Microsoft updates by EPSS exploit probability.
A newer security update for Microsoft Exchange Server Subscription Edition RTM 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.
KB5071873 is a December 2025 Exchange Server patch covering the Subscription Edition RTM, 2016 CU23, and 2019 CU15, fixing CVE-2025-64667 and CVE-2025-64666 at a highest Severity of High (CVSS 7.5). On-premises Exchange organizations still running these builds are exposed, and because Exchange is an internet-facing mail platform it remains a high-value target even without current KEV or ransomware association. EPSS sits at 1 percent, so there is no evidence of active exploitation yet, but Exchange flaws tend to draw attacker attention quickly once details circulate.
Do this: Apply KB5071873 to all on-premises Exchange Subscription Edition RTM, 2016 CU23, and 2019 CU15 servers, confirming each server is on a supported CU baseline first.
AI-generated from public data, 2026-07-21. Verify against the vendor advisory before acting.
Senserva AI Opinion and rich prompt for KB5071873
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5071873
- Released
- 2025-12-09
- Last changed
- 2026-07-08
- Changes tracked
- 2
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 KB5071873 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-2025-64666 | High | 7.5 | 1.0% | No |
| CVE-2025-64667 | Medium | 5.3 | 1.0% | No |
Scope and sources
- Affected products
- Also released in December 2025
KB5066586
98 CVEs · Critical · exploitedKB5066780
114 CVEs · Critical · exploitedKB5066782
103 CVEs · Critical · exploitedKB5066791
98 CVEs · Critical · exploitedKB5066793
108 CVEs · Critical · exploitedKB5066835
134 CVEs · Critical · exploitedKB5068779
32 CVEs · Critical · exploitedKB5068781
33 CVEs · 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.