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. Get it from the Microsoft Update Catalog below, and see where it sits against the rest of the month on the patch tracker.
Microsoft security update released 2025-12-09. Fixes 2 CVEs.
No exploitation is confirmed for the CVEs in this update and the EPSS exploitation probability is currently 1.0%. That can change: CVEs from past updates regularly get exploited months later, which is why deploying on your normal cadence, without exceptions that linger, is what keeps this one boring.
Exchange Server updates are their own discipline: they install on the servers themselves (Windows Update, Microsoft Update, or the downloaded package run as administrator), not through Intune device policies. The install guidance below covers the Exchange path. For the bigger picture: how the Microsoft patching tools fit together, this month's Patch Tuesday summary, and every update ranked by real-world risk.
Updated July 26, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.
Download KB5071873
Get the official update for KB5071873. The download and file size for each supported product are on the Microsoft Update Catalog page.
Update summary
Senserva AI Opinion for KB5071873
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
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 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.
What to do
Moderate priority: schedule it in your normal patch cycle. Senserva flags whether KB5071873 is missing on your devices.
Check if KB5071873 is installed
Copy-paste PowerShell: one machine, or your whole fleet 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.
Writing about KB5071873? Cite this page
Copy-paste for a blog post, forum answer, or internal doc. The link always carries the current risk data.
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 | High | 7.5 | 1.0% | No |
Affected products
Also released in December 2025
The highest-risk updates from the same release:
The full month, summarized: Microsoft Patch Tuesday.
How to install KB5071873
This is an Exchange Server security update. It installs on each Exchange server, not through client update rings:
- Windows Update / Microsoft Update on the server offers Exchange security updates when enabled.
- Manual install: download from the Update Catalog above and run the package from an elevated prompt (running it without elevation is the classic cause of a broken install).
- Verify afterward with Microsoft's Exchange Server Health Checker script, and confirm the build number matches the update.
Common questions about KB5071873
Is KB5071873 urgent?
KB5071873 is rated High. Include it in your normal patch cycle unless your environment raises its priority.
How do I download KB5071873?
From the Microsoft Update Catalog (linked on this page), or let Intune, Windows Update for Business, Windows Autopatch, or WSUS deliver it. The catalog page lists the download for each supported product.
How do I deploy KB5071873 with Intune?
Assign a Windows Update ring (quality update policy) to the device groups and the update flows on the ring's deferral schedule. For urgent updates, Intune expedite policies can bypass the ring deferrals. Verify the rollout with update compliance reporting rather than assuming delivery.
Can I ask my own AI about KB5071873?
Yes. This page includes a free, ready-to-paste AI prompt containing KB5071873's key facts: severity, CVE count, exploitation status, and the download link. Copy it into Claude, ChatGPT, or Copilot for a rollout plan; the data is refreshed twice a day (5 AM and 3 PM US Central).
Authoritative references
See this and every Microsoft update ranked by real-world risk on the Microsoft Patch Tracker.
