Windows 11 Patch Tuesday KB5065426: Install Failures and Workarounds

Microsoft's September cumulative for Windows 11 (KB5065426) landed as a routine Patch Tuesday release but quickly became a headache for many users: installers fail with a slew of cryptic error codes, Microsoft Update Catalog packages sometimes abort mid‑install, and, in a number of reports, network file sharing and SMB credentials broke after the patch applied. This piece unpacks what is known, verifies the key technical facts, explains practical workarounds that are already helping people, and lays out sensible mitigation and remediation steps for home users and IT teams.

Background: what KB5065426 is and why it matters​

KB5065426 is the September cumulative update for Windows 11, version 24H2 (OS Build 26100.6584). It was released through Microsoft's standard channels and includes a combined Servicing Stack Update (SSU) and Latest Cumulative Update (LCU), which is the common distribution method Microsoft uses to ensure the servicing stack itself is up to date before applying the LCU. The update bundles security fixes (including high‑impact patches), quality improvements, and feature rollouts that Microsoft is enabling incrementally on devices.
Because the package combines an SSU with the LCU, it is larger than a typical monthly patch and is installed automatically by Windows Update on consumer systems unless users have configured deferrals or paused updates. Microsoft’s release notes confirm the release date and build number and explicitly document the SSU+LCU packaging and the presence of at least one known issue affecting certain virtualized scenarios.

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What users are reporting — installation failures and error codes​

Across community threads and independent reporting, multiple error codes have been associated with failed installs of KB5065426. The commonly reported codes include:

• 0x800F0991
• 0x800F0922
• 0x80071A2D
• 0x800F081F
• 0x80070302
• 0x80070306
• 0x8000FFFF
• Variants such as 0x800700C1 or 0x800f0805 during download/install phases

Symptoms described by users fall into a few patterns:

• Windows Update repeatedly downloads and attempts to install KB5065426 but ends with “did not install” or a generic install error and presents a Retry loop.
• Manual installation via the Microsoft Update Catalog/standalone installer begins, progresses to roughly the halfway or two‑thirds point, and then fails—sometimes with no clear additional diagnostic message.
• Some systems that successfully installed the patch subsequently experienced network file‑sharing problems (SMB), including “System error 86” credential failures or broken access to shares between Windows 11 machines.
• A smaller number of reports describe broader network or connectivity regressions.

These reports are consistent across several independent outlets and community discussion boards and align with a known, documented edge‑case in Microsoft’s own update notes (see “known issues” regarding certain hotpatched or mismatched host/guest scenarios).

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What Microsoft has documented (and what is verified)​

Microsoft’s official update page for the September release confirms these important points:

• The update is published as OS Build 26100.6584 and was made available in early September.
• The package includes a servicing stack update (SSU) and LCU combined into a single installable package — an important detail because combined packages change how uninstalls and rollbacks behave.
• Microsoft lists at least one documented known issue relating to PowerShell Direct (PSDirect) connectivity for hotpatched devices where host/guest update mismatches can cause socket cleanup failures and random connection errors.
• Microsoft notes guidance for removing LCUs via DISM (because the combined SSU+LCU packaging prevents wusa.exe from removing the SSU portion).

Those Microsoft points were cross‑checked against the update metadata and published support notes to verify the build number, the SSU+LCU packaging, and the documented known issue. Independent reporting corroborates the presence of install failures and SMB/file‑sharing regressions in the field. Where claims are anecdotal (for example, precise frequency of failure, or an assertion that every attempt to uninstall will fail), these are flagged as community reports and not universal facts — because large‑scale telemetry from Microsoft confirming a broad regression beyond the published known issue has not been released publicly at this time.

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Why this update can cause more friction than usual​

Several technical reasons explain why a cumulative update like KB5065426 may trigger problems that are harder to fix than ordinary patches:

• SSU+LCU combined packaging: When updates include the servicing stack, uninstall semantics change. Running wusa.exe /uninstall will not remove the SSU, and in some configurations the package is not removable via the normal GUI paths. That makes rollback trickier for casual users and forces more involved DISM commands for removal.
• Size and scope: The package is larger and contains a wider range of fixes and components. Larger installers have more surface area for partial failure, and network or disk I/O hiccups can turn a straightforward install into a half‑completed state.
• Different failure points: The error codes reported range from servicing‑stack/manifest errors (0x800F0991, 0x800F081F) to filesystem/installer errors (0x80070302, 0x80070306) to general catastrophic failures (0x8000FFFF). A single KB invoking multiple subsystems increases the variety of failure modes.
• Environmental variability: Custom device drivers, third‑party security tools, or unusual network configurations (including SMB credential setups) may interact with the update differently, producing inconsistent results across systems.

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Practical workarounds and fixes that are working now​

Multiple community reports and hands‑on threads indicate the following approaches have helped affected machines. Each technique is listed with the expected effect and important cautions.

• Use the Media Creation Tool to perform an in‑place upgrade/repair install and keep personal files. This procedure re‑applies a clean copy of Windows while preserving apps and settings if the “Keep personal files and apps” option is selected. Many users report that running the Media Creation Tool to upgrade this PC now completes successfully and resolves the KB5065426 install loop. Caution: always back up important data before an in‑place upgrade.
• Use the Windows Update Assistant as an alternative — it performs an upgrade path that can bypass the failing Windows Update path.
• If the update partially installed or corrupted update components, run the standard repair commands:
• sfc /scannow
• DISM /Online /Cleanup‑Image /RestoreHealth
  These commands check and repair system file corruption and the component store, which can resolve some installation errors.
• Reset Windows Update components manually (stop services, rename SoftwareDistribution and catroot2 folders, restart services). This clears the local cache used by Windows Update and can fix repeated download/verification loops.
• If a full rollback is necessary, administrators can use DISM /Online /Get‑Packages to list installed packages and DISM /Online /Remove‑Package with the specific package name to remove the cumulative update — this is the supported method for removing combined SSU+LCU packages when wusa.exe cannot.
• For systems where the Microsoft Update Catalog installer fails, the Media Creation Tool/In‑place upgrade is the most reliable alternative reported in the field.
• Pause updates temporarily (Settings > Windows Update > Pause updates) if the device is stable and avoiding the update is acceptable short‑term. Pausing prevents repeated download attempts while awaiting a fix.

These options have been validated by multiple independent community reports and confirmed by the presence of Microsoft guidance on combined SSU+LCU packaging and DISM removal semantics.

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Step‑by‑step: two safe remediation paths​

Below are two reproducible, step‑by‑step methods for home users and IT staff. Both require administrative privileges.
1) Quick repair path: SFC/DISM + Windows Update reset

• Open an elevated Command Prompt (Run as Administrator).
• Run: sfc /scannow and wait for completion.
• Run: DISM /Online /Cleanup‑Image /RestoreHealth and wait for completion.
• Stop update services:
• net stop wuauserv
• net stop cryptSvc
• net stop bits
• net stop msiserver
• Rename cache folders:
• rename C:\Windows\SoftwareDistribution SoftwareDistribution.old
• rename C:\Windows\System32\catroot2 catroot2.old
• Restart services:
• net start wuauserv
• net start cryptSvc
• net start bits
• net start msiserver
• Reboot and check Windows Update.
  Expected result: clears corrupt cache and allows a fresh download/install. If the build continues to fail, proceed to the in‑place upgrade option.

2) Robust fallback: In‑place upgrade using Media Creation Tool (keeps apps and files)

• Create a current backup of important files (image or file backup).
• Obtain the Media Creation Tool from Microsoft’s download portal and run as admin.
• Choose “Upgrade this PC now”.
• When prompted, select the option to keep personal files and apps.
• Follow the on‑screen steps and allow the tool to complete the upgrade.
• After reboot, check Windows Update to ensure the latest build is present.
  Expected result: a fresh install path that typically bypasses a broken Windows Update pipeline and completes application of cumulative updates.

Always confirm all steps in a controlled environment for business deployments before broad rollout.

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Enterprise considerations: testing, deployment, and blocking​

For managed environments, the update behavior and the difficulty of rollback raise specific operational issues:

• Test before broad deployment: place the update into a pilot ring for a sample of representative devices (hardware, apps, drivers) to validate real‑world impact.
• Use Windows Update for Business / WSUS / Configuration Manager: defer or approve the update from a central console until the issue is resolved or mitigations are validated.
• Avoid mass uninstall attempts: because the package contains an SSU, uninstall semantics are not identical to a pure LCU — use DISM as Microsoft documents for removal if necessary.
• Document restores and backups: Imaging and backup strategies are essential in case an in‑place or remediation step requires full OS recovery.
• Monitor Microsoft Release Health: subscribe to Microsoft’s release channels and the Windows Release Health dashboard to track any service‑side fixes or advisories.

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Risks and tradeoffs: security vs. stability​

There are two competing risks when an important cumulative update fails widely:

• Installing the update may resolve critical security vulnerabilities (including some high‑severity fixes flagged as “Exploitation More Likely”) and therefore reduce exposure to active exploits.
• Skipping or uninstalling the update increases exposure to those same vulnerabilities, which may be unacceptable in certain environments, especially where SMB or remote access is used.

Given those tradeoffs, the proper posture depends on threat tolerance and environment:

• For home users and low‑risk setups, the Media Creation Tool or Update Assistant is likely the fastest way to get the security fixes without waiting for a server‑side rollback.
• For enterprises, delaying broad deployment until a tested fix is available and using compensating controls (network segmentation, disabling exposed services, firewall rules) is prudent.

Always balance the urgency of security patches against the operational impact of applying a patch that can break productivity.

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What to watch for — Microsoft’s next steps and how to respond​

Microsoft typically responds to wide‑scale installation regressions in one of several ways:

• Publish an explicit “Known issues” entry and roll out an updated package or hotpatch to address the failure mode.
• Implement a server‑side change to the Windows Update distribution to halt or modify rollout for affected device classes.
• Release a follow‑up cumulative or hotpatch (often within days) that corrects the problem.

Until Microsoft issues a confirmed fix, recommended actions are:

• Back up critical data.
• If affected and unable to install via the usual Windows Update path, use the Media Creation Tool or Update Assistant as a primary workaround.
• If the device is currently stable and the update is failing repeatedly, pause updates and monitor Microsoft’s release health and support notes.
• For administrators, hold the update in a pilot and use patches only after validation. Prepare DISM removal procedures and create post‑install verification scripts (event logs, SMB tests, credential tests) for fast triage if deployments proceed.

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Hard lessons and longer‑term best practices​

This incident highlights a few recurring themes in modern OS servicing:

• Large combined packages (SSU+LCU) complicate rollback. Plan for that by documenting removal steps and making recovery images available.
• Automatic rollout systems are efficient but put a premium on robust pre‑release testing across diverse hardware and third‑party software ecosystems.
• A single cumulative update can affect unrelated subsystems (for example, security fixes for core networking code may interact with older SMB configurations or legacy credentials).
• Maintain immutable backups and system images for fast restoration. Don’t rely solely on Windows’ built‑in recovery for business continuity.
• Use staged rollouts in enterprise environments and keep a secure offline image for emergency recovery.

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Conclusion — measured steps for users and IT teams​

KB5065426 includes meaningful security and quality fixes for Windows 11, but the rollout has produced a non‑trivial number of installation failures and some post‑install regressions for affected users. Verified facts show the build number and SSU+LCU packaging, Microsoft’s documented known issue regarding PowerShell Direct in hotpatched environments, and repeated community reports of install errors and SMB credential problems. Practical remediation methods that are already working for many people include the Media Creation Tool in‑place upgrade and the Update Assistant; standard repair tools (SFC/DISM) and Windows Update cache resets also help in many cases.
Recommended immediate actions:

• Back up important data.
• If upgrades fail via Windows Update, run SFC/DISM, reset update components, then try the Media Creation Tool or Update Assistant.
• For enterprise deployments, stage the update, test widely, and hold final approval until a fix or official guidance is published.
• If stability is paramount and fixes are not yet available, pause updates and apply compensating security controls.

This is an operationally significant Patch Tuesday for Windows 11: patch promptly where possible, proceed cautiously where not, and document every remediation step so recovery stays fast and reliable.

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Source: PCWorld Windows 11's September update is borked, won't install for some users