Microsoft releases urgent updates to fix RDP flaws in Windows and Windows Server
Microsoft has released out-of-band updates to resolve issues with Remote Desktop (RDP/RDS), Hyper-V Plan 9 folders and USB audio caused by the September 2026 security patches in specific versions of Windows and Windows Server.
News summary
Between 14 and 15 September 2026, Microsoft released several out-of-band (OOB) updates to resolve issues caused by the September security updates in specific versions of Windows and Windows Server. This is not a universal issue: the impact depends on the installed version, enabled roles, Remote Desktop Services configuration and the use of certain virtualisation features or audio devices.
The issue with the greatest operational impact affects some Remote Desktop Services environments. Microsoft reports connection or login failures via RDP and, on certain servers, freezes during Remote Desktop configuration or when using tools such as the MMC console, RDS Licensing Diagnoser, File Explorer or Windows Update.
The fixes also address a Plan 9 issue that could prevent Linux virtual machines hosted on Hyper-V from mounting shared folders, and errors with multi-channel USB Audio Class 1.0 devices in the versions specified by Microsoft. Each organisation must verify which symptom and which update correspond exactly to their system.
These updates are cumulative and are distributed outside the ordinary cycle. Before installing them in production, it is advisable to verify the system edition and build, have a recoverable copy available, review dependencies, agree on an intervention window and subsequently validate RDP access, sessions, services and the virtual machines involved.
The published packages are: KB5129236 for Windows 10 21H2 and 22H2 (14/09/2026, Remote Desktop Services and associated fixes); KB5129195 for Windows 11 24H2 and 25H2 (14/09/2026, RDP/RDS, Plan 9 on Hyper-V and multi-channel USB Audio Class 1.0 depending on configuration); KB5129194 for Windows 11 26H1 (14/09/2026, RDP/RDS and specific fixes for this version); KB5129238 for Windows Server 2019 and Windows 10 1809 (15/09/2026, Remote Desktop Services issues); KB5129237 for Windows Server 2022 (15/09/2026, Remote Desktop Services issues); and KB5129235 for Windows Server 2025 (15/09/2026, RDP/RDS, Plan 9 on Hyper-V and multi-channel USB Audio Class 1.0 depending on configuration). Microsoft factsheets are the reference for prerequisites, known issues and installation methods: the presence of a version in this list does not in itself confirm that the machine is affected.
In an infrastructure that centralises applications or workstations via RDS, an authentication failure or a non-responsive console can prevent users from accessing their desktops, even if the server remains powered on. The priority is to distinguish an incident introduced by the update from other network, licensing, profile, or capacity issues. The verification must include system logs, the installed build, the timing of the onset of symptoms, and a controlled access test. Uninstalling security updates without analysis may restore operations in the short term, but it also reopens patched vulnerabilities; the corresponding OOB update is the path documented by Microsoft.
Plan 9 allows folder sharing between the host and certain Linux virtual machines in Hyper-V. The fixed bug could prevent the mounting of these folders and affect processes relying on that exchange. Post-validation must check not only that the virtual machine boots, but that shared paths are available again and that applications can access them. Microsoft also documents issues with multichannel USB Audio Class 1.0 devices in specific versions: this is a more localised incident, but it may be relevant in meeting rooms, audiovisual workstations, or equipment using audio interfaces compatible with that class.
A virtual desktop platform concentrates the operations of many users. This concentration simplifies management but necessitates treating patching, backups, monitoring, and access testing as a single continuity process rather than independent tasks. The organisation should know who authorises an urgent update, which backup allows for a rollback, how the service is tested with a pilot group, and what temporary alternative exists if remote access fails. These decisions must be prepared before the next critical update.
Source: Microsoft Support — 18 September 2026
Why this matters to a company operating in Spain
Behind every tech market shift, there is a practical consequence for businesses: price changes, end-of-support for a product, new capabilities, or providers refocusing their strategy. Anticipating these moves avoids rushed purchases and ill-timed renewals.
Our criteria are simple: every innovation must translate into a specific action within the annual technology plan, or it does not deserve management’s time.
Real business impact
Before deciding on an investment, it is advisable to identify what is at stake. In it support projects, we typically review these four areas with management and the IT manager:
- Out-of-support equipment that no longer receives security updates.
- Licence renewals without prior comparison or adjustment to actual usage.
- Technological projects lacking a clear internal lead, causing delays and budget overruns.
- Purchasing decisions made out of urgency rather than planning.
Five-step action plan
A useful plan fits on one page. This is the roadmap we apply with our clients to move from news to measurable improvement, without disrupting daily operations:
- Inventory Windows and Windows Server equipment, including their version, build, and active roles.
- Identify RDS servers, Hyper-V hosts with Plan 9 folders, and equipment with multichannel USB audio that may be within scope.
- Consult official Microsoft documentation and select exclusively the KB corresponding to each version.
- Verify a recoverable backup and agree on a maintenance window before modifying production.
- Apply the update, restart when appropriate, and validate RDP access, sessions, licensing, services, shared storage, and affected peripherals.
- Document the outcome and maintain enhanced monitoring following the change.
Key indicators you should be measuring
What is not measured is not managed. These indicators allow you to verify if the technological investment is yielding results and serve as the basis for the periodic reports we deliver to our clients:
- Percentage of Windows and Windows Server machines with inventoried versions and builds.
- Number of RDS servers and Hyper-V hosts identified within the scope of the advisory.
- Time elapsed from the publication of the corrective KB until its validated application in production.
- Existence of a verified recoverable copy prior to each intervention.
How we approach it at Seintec: IT Support
Uninterrupted 24/7 technical support. We operate from our own datacenter in Spain, with a certified technical team and a single point of contact who knows your infrastructure, so you do not have to explain your environment every time an incident arises.
These are the capabilities we bring to the table in a it support project:
- Hourly packs: Flexible consumption based on actual needs.
- Flat rate: Comprehensive coverage with a fixed monthly cost.
- SLA < 4 h: Response time commitment for incidents.
- Remote support: Immediate resolution without the need for travel.
- On-site support: Technician on-site when the fault requires it.
- Health reports: Periodic KPIs and recommendations.
What you gain by working with a technology partner
Outsourcing does not mean losing control: it means gaining predictability, coverage, and independent technical insight. These are the benefits our clients highlight:
- Ultra-fast Help Desk: Immediate remote support to diagnose and resolve failures without the need for travel.
- On-site visits whenever necessary: A technician travels to your location if the incident requires it, with no additional mileage costs.
- Dedicated Technical Account Manager: A single point of contact who reviews KPIs and delivers periodic health and improvement reports.
- Audit and monitoring included: Initial analysis of the IT infrastructure and continuous surveillance of critical systems.
Frequently Asked Questions
- Are all Windows devices affected?
- No. The impact depends on the version, build, installed updates, and configuration. Each device must be cross-referenced with the official sheet of the corresponding KB and the observed symptoms.
- What is a Microsoft out-of-band update?
- It is an update released outside the regular monthly schedule to resolve an issue that requires an early fix. It must be evaluated and installed with the same version control, backup, and validation as any change in production.
- Is it advisable to uninstall the September security update?
- Not as a general measure. Uninstalling a security update can reopen vulnerabilities. If an incident occurs, the affected system must be identified and the corrective update documented by Microsoft must be applied, with a recovery plan available.
- Is a restart required after installing these KBs?
- Reboot requirements and behaviour must be verified against the official documentation and the specific system. For production servers, it is advisable to schedule a maintenance window and validate services upon completion.
- How often should infrastructure be renewed?
- When maintenance costs and incidents exceed the cost of renewal, usually between four and six years. A staggered renewal plan avoids investment spikes and out-of-support equipment.
- Where should a company wanting to address it support begin?
- With an audit of the current environment. At Seintec, we perform an initial no-cost review that identifies risks, dependencies, and priorities, resulting in a phased plan with fixed deadlines and budgets.
- Is it necessary to halt business operations during the project?
- No. We plan migrations and changes within agreed windows, with prior pilot tests and rollback options, ensuring disruption is minimal or non-existent for users.
- What type of companies do you serve?
- SMEs and mid-market companies in sectors such as industry, automotive, logistics, retail, legal, and healthcare, with both on-premises and hybrid cloud infrastructure.
- What coverage and response times (SLA) do you offer?
- Support from Monday to Friday, 09:00 to 18:00, and 24x7 emergencies 365 days a year, with a committed response SLA and a 99.98% service SLA in 2025.
Concepts mentioned in this article: Virtualisation · Virtual machine · Backup · Monitoring · Virtual desktop
If your company relies on Remote Desktop, Windows servers, or virtualised desktops, an urgent update should not be improvised. At Seintec, we prepare the inventory, backups, and the patching and validation procedure to ensure these alerts are resolved under control. Speak with our team.
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