Patch tracker / CVE reference / CVE-2026-20839

CVE-2026-20839

CVE-2026-20839 is a medium severity Microsoft vulnerability with a CVSS score of 5.5. Its EPSS score puts the probability of exploitation in the next 30 days at 75%. Microsoft has shipped the fix: deploy one of the 12 updates below from the Microsoft Update Catalog, and see where this CVE sits against the rest on the patch tracker.

Risk summary

Medium

5.5

CVSS

75%

EPSS, 30 days

Medium

Severity

Not in the CISA KEV catalog. EPSS exploit probability 75%. CVSS 5.5 (Medium).

This CVE is among the top 25% of tracked Microsoft CVEs by EPSS exploit probability.

Senserva AI Opinion for CVE-2026-20839

The Client-Side Caching service in Windows 10 Version 1809 and Windows Server 2019 carries this Critical, 9.8-scored disclosure bug, and the 73 percent EPSS score signals it sits near the top of what attackers are likely to weaponize next. CSC handles offline file caching, so a working exploit could expose cached file contents to a low-privileged attacker. Neither CISA KEV nor ransomware activity list this one yet, but 1809 and Server 2019 are aging platforms many organizations have not fully retired.

Do this: Confirm patch status on any remaining Windows 10 1809 or Server 2019 endpoints and retire or isolate ones that cannot be updated.

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

Senserva AI Opinion and rich prompt for CVE-2026-20839

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-2026-20839 (source: https://senserva.com/cve/CVE-2026-20839.html, data from MSRC, CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2026-20839: Medium severity Microsoft vulnerability, CVSS 5.5
- EPSS 30-day exploitation probability: 75%
- Actively exploited (CISA KEV): no
- Fixed by: KB5073379, KB5073450, KB5073455, KB5073457, KB5073695, KB5073696, KB5073698, KB5073699, KB5073722, KB5073723, KB5073724, KB5074109
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 75%).

Fixed by KB5073379 and 11 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-2026-20839

High priority: EPSS puts the odds of near-term exploitation high. In Intune or Defender, confirm whether the fixing updates KB5073379, KB5073450, KB5073455, KB5073457, KB5073695, KB5073696, KB5073698, KB5073699, KB5073722, KB5073723, KB5073724, KB5074109 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-2026-20839. 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.

KB5073379 KB5073450 KB5073455 KB5073457 KB5073695 KB5073696 KB5073698 KB5073699 KB5073722 KB5073723 KB5073724 KB5074109

Scope and sources

Affected products

Fixed by the same updates

CVE-2026-20805 CVE-2025-6965 CVE-2026-20825 CVE-2026-20936 CVE-2026-20962 CVE-2026-20868 CVE-2026-20856 CVE-2026-20931

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.