Patch tracker / KB5123066
KB5123066: Windows Server 2012 R2 Security Update (September 2026)
KB5123066 is a Critical security update for Windows Server 2012 R2, released 2026-09-08. It closes 423 CVEs with a maximum CVSS of 9.8. At least one of them is actively exploited (CISA KEV), so it is urgent, and Microsoft lists 2 known issues: test it before broad deployment. It is the ninth largest of the 54 updates Microsoft shipped that month, by CVEs closed.
Before you deploy: Microsoft lists 2 known issues for this update.
Our data comes from Microsoft, CISA and other public sources, and it can be wrong or out of date. If you are unsure, check the linked source before you act, and tell us when we are wrong so we can fix it.
Update summary
Fixes 423 CVEs. At least one is actively exploited (CISA KEV). Most severe CVSS 9.8 (Critical). EPSS exploit probability up to 3.6%. Among the top half of tracked Microsoft updates by EPSS exploit probability.
A newer security update for Windows Server 2012 R2 has shipped since this one: KB5126836 (2026-10-13). For cumulative update families the newer package includes these fixes, so installing that one is usually the shorter path. Verify against your own update rings before you skip this package.
Microsoft documents 2 known issues with this update.
- USB Audio Class 1.0 devices with error Code 10 or no output
- Remote Desktop Services might stop responding after September 2026 security update
Senserva AI Opinion and rich prompt for KB5123066
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5123066
- Released
- 2026-09-08
- Last changed
- 2026-10-16
- Changes tracked
- 7
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.
Microsoft revised KB5123066 on 2026-10-13: CVEs fixed 418 to 425.
- 2026-09-12 CVEs fixed 417 to 418
- 2026-10-13 CVEs fixed 418 to 425
Recorded by Senserva on the dates shown, from the MSRC release feed. This is what moved and when, which neither the vendor advisory nor NVD publishes.
Check if KB5123066 is installed
Elevated PowerShell. No output means it is not installed.
Known issues, from Microsoft
Quoted from Microsoft's support article for KB5123066 (checked 2026-10-16). Microsoft lists 2 known issues. Verify against the article before acting.
1. USB Audio Class 1.0 devices with error Code 10 or no output
Symptoms
After installing the September 8, 2026, Windows security update, some USB Audio Class 1.0 devices might fail to start or produce audio. Affected devices might experience one or more of the following symptoms:
The device displays an error in Device Manager: "This device cannot start (Code 10).”
No audio output.
Volume controls are unresponsive or remain at zero.
Sound settings are unresponsive or unavailable.
Some devices might function in standard stereo configurations but fail when using multichannel audio features, including 8-channel or 3D audio modes. Some customers have reported that they're able to restore audio in these cases by switching to 2-channel mode.
This issue is limited to USB Audio Class 1.0 devices.
Resolution
This issue is partially resolved in the out-of-band (OOB) update released on September 14, 2026. This OOB update resolves the symptoms experienced on devices using 8-channel or 3D audio modes. Microsoft is working to address the other symptoms and will provide more information when it is available.
IT administrators who need an immediate workaround for the symptoms not addressed yet by the OOB update should contact Microsoft Support for Business for assistance.
2. Remote Desktop Services might stop responding after September 2026 security update
Symptoms
After installing the September 2026 Windows security update, some organizations might experience issues with Remote Desktop Services (RDS).
In some environments, RDS might become unstable, resulting in RDP connections failing after several minutes, sign-in issues, or servers hanging at "Please wait for the Remote Desktop Configuration". Related tools, including Microsoft Management Console (MMC), RDS Licensing Diagnoser, and File Explorer might also become unresponsive. Additionally, the Windows Update page might stop responding and continuously display a loading indicator.
Note
This issue does not affect Windows 365 or Azure Virtual Desktop.
Resolution
This issue is resolved in Windows updates released on and after September 14, 2026 such as KB5129243 . We recommend you install the latest Windows update for your device as it contains important improvements and issue resolutions, including this one.
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.
Scope and sources
- Affected products
- Also released in September 2026
KB5122871
684 CVEs · Critical · exploitedKB5122876
620 CVEs · Critical · exploitedKB5122878
572 CVEs · Critical · exploitedKB5122880
608 CVEs · Critical · exploitedKB5122882
645 CVEs · Critical · exploitedKB5123065
398 CVEs · Critical · exploitedKB5123099
559 CVEs · Critical · exploitedKB5124008
635 CVEs · Critical · exploitedThe full month, summarized: Microsoft Patch Tuesday.
- Authoritative references
See this and every Microsoft update ranked by real-world risk on the Microsoft Patch Tracker.