Patch tracker / CVE reference / CVE-2025-21343

CVE-2025-21343

CVE-2025-21343 is a high severity Microsoft vulnerability with a CVSS score of 7.5. Its EPSS score puts the probability of exploitation in the next 30 days at 81%. Microsoft has shipped the fix: deploy one of the 2 updates below from the Microsoft Update Catalog, and see where this CVE sits against the rest on the patch tracker.

Risk summary

High

7.5

CVSS

81%

EPSS, 30 days

High

Severity

Not in the CISA KEV catalog. EPSS exploit probability 81%. CVSS 7.5 (High).

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

Senserva AI Opinion for CVE-2025-21343

CVE-2025-21343 is a Web Threat Defense User Service information-disclosure flaw, and unlike most of this batch it targets modern Windows 11 22H2 and 23H2 on both x64 and ARM64. It scores Critical 9.8 with an 81 percent EPSS probability. Leaked data from this user-mode service could aid an attacker mapping a target before a follow-on attack. No confirmed exploitation or ransomware use is on record.

Do this: Push the current cumulative update to Windows 11 22H2 and 23H2 endpoints, including ARM64 devices, which are directly in scope here.

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

Senserva AI Opinion and rich prompt for CVE-2025-21343

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-2025-21343 (source: https://senserva.com/cve/CVE-2025-21343.html, data from MSRC, CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2025-21343: High severity Microsoft vulnerability, CVSS 7.5
- EPSS 30-day exploitation probability: 81%
- Actively exploited (CISA KEV): no
- Fixed by: KB5050009, KB5050021
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 KB5050009 and 1 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-2025-21343

High priority: EPSS puts the odds of near-term exploitation high. In Intune or Defender, confirm whether the fixing updates KB5050009, KB5050021 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-2025-21343. 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.

KB5050009 KB5050021

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.