Patch tracker / CVE reference / CVE-2025-21307

CVE-2025-21307

CVE-2025-21307 is a critical severity Microsoft vulnerability with a CVSS score of 9.8. Its EPSS score puts the probability of exploitation in the next 30 days at 81%. Microsoft has shipped the fix: deploy one of the 14 updates below from the Microsoft Update Catalog, and see where this CVE sits against the rest on the patch tracker.

Risk summary

Critical

9.8

CVSS

81%

EPSS, 30 days

Critical

Severity

Not in the CISA KEV catalog. EPSS exploit probability 81%. CVSS 9.8 (Critical).

This CVE is among the top 10% of tracked Microsoft CVEs by EPSS exploit probability.

Senserva AI Opinion for CVE-2025-21307

This Critical Severity (CVSS 9.8) Remote Code Execution flaw sits in the Reliable Multicast Transport Driver (RMCAST), and its 81% EPSS deserves a caveat: the driver is only reachable when a program is actually listening on a Windows PGM socket, which most environments never do. Affected platforms are Windows 10 1809 and Windows Server 2019. The risk concentrates in niche setups using multicast messaging middleware.

Do this: Patch broadly per the normal wave, but first check whether any server runs software listening on PGM ports and fast-track those hosts.

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

Senserva AI Opinion and rich prompt for CVE-2025-21307

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I am an IT admin assessing a vulnerability. Facts for CVE-2025-21307 (source: https://senserva.com/cve/CVE-2025-21307.html, data from MSRC, CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2025-21307: Critical severity Microsoft vulnerability, CVSS 9.8
- EPSS 30-day exploitation probability: 81%
- Actively exploited (CISA KEV): no
- Fixed by: KB5049981, KB5049983, KB5049984, KB5049993, KB5050004, KB5050006, KB5050008, KB5050009, KB5050013, KB5050021, KB5050048, KB5050049, KB5050061, KB5050063
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) What to patch or mitigate first. 4) How to check whether my devices are exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Patch this cycle

Not confirmed exploited yet, but the 30-day exploit probability is high (EPSS 81%).

Fixed by KB5049981 and 13 more (by build, listed below). Download from the Microsoft Update Catalog, or deploy via Windows Update, Intune, or WSUS.

Do this now: Patch in this maintenance window; do not let it slip to the next one.

Official patch(es) for CVE-2025-21307

High priority: EPSS puts the odds of near-term exploitation high. In Intune or Defender, confirm whether the fixing updates KB5049981, KB5049983, KB5049984, KB5049993, KB5050004, KB5050006, KB5050008, KB5050009, KB5050013, KB5050021, KB5050048, KB5050049, KB5050061, KB5050063 are installed across the affected products. Senserva does this automatically across your tenant and flags every missing patch.

Apply the Microsoft security updates below to fix CVE-2025-21307. Each KB page carries the Microsoft Update Catalog download, Microsoft's known issues for the update, a copy-paste PowerShell check for whether it is installed, and the Senserva AI read.

KB5049981 KB5049983 KB5049984 KB5049993 KB5050004 KB5050006 KB5050008 KB5050009 KB5050013 KB5050021 KB5050048 KB5050049 KB5050061 KB5050063

Scope and sources

Affected products

Fixed by the same updates

CVE-2025-21333 CVE-2025-21334 CVE-2025-21335 CVE-2025-21189 CVE-2025-21210 CVE-2025-21214 CVE-2025-21219 CVE-2025-21268

Authoritative references

Official CVE record (cve.org)NVD detail Microsoft MSRC FIRST.org EPSS

Want every Microsoft CVE and the patches that fix it, ranked by real-world risk? See the Microsoft Patch Tracker.