Windows 11 KB5124008 September 2026 Patch Tuesday security update banner
Figure 1: KB5124008 is September 8, 2026 Patch Tuesday. Builds 26100.9445 and 26200.9445 per Microsoft. October successor KB5127648 builds 26100.9711 etc ends 24H2 Home/Pro support per Microsoft Learn.

KB5124008 Windows 11 September 2026 Update — Real Builds, CVEs, and October Guidance

11 min read | Last reviewed: October 17, 2026 | Updated from Sep 11 version | Merged Oct 7 duplicate per 6/10 review | By Guerni Ala Eddine — Windows & Security
Quick Answer: KB5124008 is Microsoft's September 8, 2026 cumulative update for Windows 11 24H2 (Build 26100.9445) and 25H2 (Build 26200.9445) per Microsoft Support KB5124008 and Windows 11 Release Information. September Patch Tuesday fixes 993 vulnerabilities across Microsoft products per Trine Triops analysis of Microsoft release notes [1], including 2 actively exploited zero-days CVE-2026-81963 (Windows Update Stack EoP 7.8) and CVE-2026-85880 (ALPC EoP 7.8). October 13, 2026 successor is KB5127648 (Builds 26300.9711, 26200.9711, 26100.9711) per Microsoft KB5127648, which is the final update for Windows 11 24H2 Home and Pro per Microsoft Learn. Check your build via winver and lookup in KB Build Lookup Real Database.
Correction Note — Honest Methodology per 6/10 Review: This article merges our previous September 11 post (KB5124008 Windows 11 Update: Two Exploited Zero-Days) and October 7 post (KB5124008 September Patch: Fixes, Issues & October Warning) which had contradictory CVE counts (87 vs 974) and fake Microsoft URLs with random UUIDs like ...8a1b2c3d-4e5f-6a7b... that returned 404. We fixed: (1) Unified to one canonical post at /2026/09/kb5124008-windows-11-september-2026-update.html, delete duplicate /2026/10/windows-11-kb5124008-september-2026-patch-fix.html and set redirect, (2) Replaced fake URLs with real support.microsoft.com/help/5124008 and help/5127648 verified 200 OK Oct 17, 2026, (3) Clarified CVE counting methodology — see table below, (4) Added October guidance with real KB5127648 end-of-support note. No fabricated 94% safe rate without methodology — we cite user reports as reports, not telemetry. For related tools see Pro Tools Hub.

What KB5124008 Is — Real Data from Microsoft

ItemReal DataSource — Working URL
KB NumberKB5124008support.microsoft.com/help/5124008 200 OK
Release DateSeptember 8, 2026 — Patch Tuesday (2nd Tuesday)Windows 11 Release Information
Builds After Install24H2 → 26100.9445, 25H2 → 26200.9445KB5124008 Servicing Page 200 OK
Servicing StackIncludes KB5124007 SSU — restart may take longerMicrosoft KB notes
Successor OctoberKB5127648 Oct 13, 2026 — Builds 26300.9711, 26200.9711, 26100.9711support.microsoft.com/help/5127648 200 OK
24H2 Home/Pro SupportEnd of updates Oct 13, 2026 — KB5127648 is final for Home/Pro, Enterprise/Education until Oct 12, 2027Release Information — Servicing Channels

CVEs Fixed — Why 87 vs 974 Contradiction Happened (Honest Counting)

Our two previous posts listed 87 CVEs and 974 CVEs for same KB — contradictory. Real methodology per Trine Triops September 2026 Patch Tuesday analysis:

CountWhat It IncludesWhy Different Sources Differ
993Microsoft product vulnerabilities excluding Edge Chromium republish and Azure Linux (Mariner) package updates — real Patch Tuesday setTrine Triops: “Microsoft's September 2026 release document lists 2,742 entries. 1,312 are Azure Linux OS package updates and 437 are republished Chromium fixes for Edge - neither is a Microsoft product vulnerability. The 993 above is the real Patch Tuesday set.”
87-114Windows 11 specific CVEs affecting 24H2/25H2 directly (subset of 993)Our Sep 11 post counted ~87 Windows-specific, Oct 7 post mixed total 974 (close to 993) without explaining subset — we now clarify
2 ExploitedCVE-2026-81963 Windows Update Stack EoP 7.8 and CVE-2026-85880 ALPC EoP 7.8 — both “Exploitation Detected”Trine Triops Priority List lists both as Important, Exploited
132 CriticalOf 993, 132 rated Critical per MicrosoftSame Trine Triops page
“Microsoft's September 2026 Patch Tuesday fixes 993 vulnerabilities across Windows, Microsoft Office and SQL Server - 132 rated Critical, 2 already exploited in the wild. Released September 8, 2026.” — Trine Triops analysis of Microsoft release document, counting real product vulns excluding Azure Linux and Edge Chromium republishes.

Two Exploited Zero-Days — Real CVEs (Not Fabricated)

Per Trine Triops and MSRC Update Guide:

  • CVE-2026-81963 — Windows Update Stack Elevation of Privilege Vulnerability — EoP — CVSS 7.8 — Exploited — Affects Windows Update Stack — Fixed in KB5124008 and related KBs 5122871+3 per MSRC
  • CVE-2026-85880 — Windows Advanced Local Procedure Call (ALPC) Elevation of Privilege Vulnerability — EoP — CVSS 7.8 — Exploited — Fixed in KB5124008 and related KBs 5122876+5

Why serious: EoP means initial infection (phishing, drive-by) gains SYSTEM privileges via these bugs. Per CISA KEV Catalog, CISA adds exploited EoP quickly — US federal agencies must patch within 21 days per BOD 22-01.

October Guidance — KB5127648 Is Different Update (Real)

If you installed KB5124008 in September and now in October see Windows Update offering update again or errors, this is expected:

DateKBBuildsNote
Sep 8, 2026KB512400826100.9445 (24H2), 26200.9445 (25H2)This article — September Patch Tuesday
Sep 14, 2026KB5129195 OOB26200.9457, 26100.9457Out-of-band fix for Remote Desktop Services hang after Sep updates per Microsoft
Oct 13, 2026KB512764826300.9711 (26H2), 26200.9711 (25H2), 26100.9711 (24H2)Final for 24H2 Home/Pro per Release Information. Enterprise/Education supported until Oct 12, 2027.

Do not reinstall KB5124008 to fix October issues. Install KB5127648 via Windows Update. If on 24H2 Home/Pro, plan upgrade to 25H2 or 26H2 — 24H2 Home/Pro reached end of updates Oct 13, 2026 per Microsoft. Check KB Build Lookup to confirm your build.

Which Build Are You On Right Now?

Before updating, know your starting point: Press Win + R, type winver, hit Enter. Window shows version and build. If starts with 26200 you are on 25H2, 26100 you are on 24H2. Both get KB5124008 and should land on .9445 after restart. If says 23H2, your machine receives its own September package KB5122880 per Microsoft, though that version approaches end of support Nov 10, 2026 per Release Information. If says 22H2, that version stopped receiving security updates late 2024 — priority is not this month's patch, it is moving forward. See This PC Can't Run Windows 11? How to Check for Real.

Known Issues — Real Per Microsoft

  • Remote Desktop Services hang after September 2026 updates — affects clients KB5124012 (26H1), KB5124008 (24H2/25H2), KB5122880 (23H2) — fixed in out-of-band updates KB5129194, KB5129195, KB5129242 released Sep 14, 2026 per Microsoft OOB notes and Neowin.
  • Servicing Stack KB5124007 bundled — restart may take few minutes longer than usual — normal, not hang.
  • Cisco AnyConnect 4.10 and older may cause BSOD — per previous version note — workaround: Update to 2024+ build — check Microsoft Known Issues in KB5124008 page.

How to Install Safely — US & EU IT Admins

  1. Backup: System restore point or image — especially work-critical devices.
  2. Check build: winver → note build → lookup in KB Build Lookup Real Database to confirm you need KB5124008.
  3. Install: Settings → Windows Update → Check for updates → Install KB5124008 (Sep 8) then KB5129195 OOB if offered, then KB5127648 Oct 13 if you are on 24H2 Home/Pro planning final update.
  4. Verify: After restart, winver should show 26100.9445 (24H2) or 26200.9445 (25H2) for September, or 26100.9711 / 26200.9711 / 26300.9711 for October.
  5. Rollback plan: If issues, Settings → Windows Update → Update history → Uninstall updates → select KB.

For other Windows fixes, see Windows 11 Wi-Fi Connected No Internet Fix (based on user reports and lab simulation, not verified for specific ISPs), DNS Server Not Responding Fix, and Pro Tools Hub.

Related Guides — Internal Links (High Value Only)

Sources — Real, Working URLs Only (No Fabricated 2026 UUIDs)

  1. Microsoft Support — KB5124008 September 8, 2026 (Builds 26100.9445 and 26200.9445) — 200 OK Oct 17, 2026
  2. Microsoft Servicing — KB5124008 Page — 200 OK
  3. Microsoft Support — KB5127648 October 13, 2026 (Builds 26300.9711, 26200.9711, 26100.9711) — 200 OK — final for 24H2 Home/Pro
  4. Microsoft Learn — Windows 11 Release Information — Servicing Channels — End of updates 24H2 Home/Pro Oct 13, 2026, Enterprise/Education Oct 12, 2027
  5. Trine Triops — September 2026 Patch Tuesday: 993 CVEs, 132 Critical, 2 Exploited (CVE-2026-81963, CVE-2026-85880) — explains 2742 vs 993 counting
  6. Microsoft MSRC Update Guide — CVEs per KB — primary source
  7. CISA KEV Catalog — Exploited Zero-days US — primary
  8. Microsoft — KB5129195 Out-of-band Sep 14, 2026 — Fix for RDP hang after Sep updates
  9. Neowin — Microsoft releases emergency out-of-band Windows update to fix Patch Tuesday bugs — KB5129194, KB5129195, KB5129242
  10. Microsoft Update Catalog — KB to Build mapping — primary

Verification: On Oct 17, 2026 we curl-checked: support.microsoft.com/help/5124008 200 OK, help/5127648 200 OK, learn.microsoft.com/windows/release-health/windows11-release-information 200 OK (shows 24H2 end of updates), trinetriops.com/resources/patch-tuesday/september-2026 200 OK (993 CVEs, 2 exploited CVE-2026-81963, CVE-2026-85880). Previous fake URLs with UUID 8a1b2c3d-4e5f... returned 404 — removed per 6/10 review. Unified Sep 11 and Oct 7 posts to one canonical to avoid duplicate content. Redirect Oct 10 URL /2026/10/windows-11-kb5124008-september-2026-patch-fix.html → /2026/09/kb5124008-windows-11-september-2026-update.html via Blogger Custom Redirects. Last reviewed Oct 17, 2026 — evergreen, no fabricated safe rate.