Windows 11 KB5121003 Issues and Fixes

Updated August 29, 2026

KB5121003 is the August 11, 2026 cumulative security update for Windows 11 versions 25H2 and 24H2. The headline game-launch problem reported after its release is now resolved, and Microsoft's investigation confirms that Windows updates did not cause it. The affected PCs had certain RGB-lighting peripherals or internal components that installed drivers with names such as inpoutx64. Microsoft documents the builds on the KB5121003 support page and the final cause on its release-health advisory.

What KB5121003 installs

On Windows 11 version 25H2, KB5121003 moves the operating system to build 26200.9168. On version 24H2, it moves the system to build 26100.9168. Microsoft released both on August 11 and identifies the package as a security update for all editions of those versions.

The package includes prior servicing changes from the July 28 preview KB5101684, the July 18 out-of-band KB5121767 update, and the July 14 KB5101650 security update. Microsoft's notes also list added Secure Boot certificate targeting, general security improvements, and removal of the Drop Tray feature. The accompanying servicing stack update is KB5123304, build 26100.9156. All of these details come from the official release notes.

Microsoft says Windows Update downloads and installs KB5121003 automatically. If you are comparing this page with a PC, open Settings, System, About and match both the Windows version and OS build. Do not use a 25H2 build number to diagnose a 24H2 device.

The game launch failures and what actually caused them

Symptoms on systems with inpoutx64

Microsoft received reports involving ARC Raiders, MARVEL Tōkon: Fighting Souls, and THE FINALS. On affected PCs, a game could become unresponsive, close unexpectedly, display EXCEPTION_ACCESS_VIOLATION, or trigger a restart. Microsoft's advisory limits the affected Windows versions to 25H2 and 24H2 and lists no affected Windows Server version.

BleepingComputer reported the game failures and Microsoft's investigation on August 20. Windows Latest reported the same investigation and the suspected inpoutx64 relationship. Both articles reflect the incomplete state at that time. Microsoft's later finding is the authority for the current cause and status.

Microsoft's current status

Microsoft marked the issue resolved on August 26 at 11:30 p.m. Pacific time. Its investigation concluded that Windows updates did not cause the failures. Instead, they were related to RGB-lighting peripherals or internal components that installed drivers or code named inpoutx64 or similar names.

A compatibility block now prevents inpoutx64 from loading on targeted devices where the driver is enabled and ARC Raiders is installed. Microsoft says a message window then explains that the driver has been disabled, after which the game launches as normal. The company is working to expand the block to MARVEL Tōkon: Fighting Souls, while Embark resolved THE FINALS. These per-game details are recorded on Microsoft's resolved-issues page.

Which devices receive the automatic protection

The block propagates automatically to non-managed consumer and business devices. Microsoft says a restart is required and the protection can take up to 24 hours to reach a device from its August 26 rollout at 6:00 p.m. Pacific time. The safeguard will also be included in September 2026 security updates and later updates.

Managed enterprise devices do not receive the compatibility block automatically. Administrators can apply Microsoft's registry workaround through their normal change-control process. Microsoft says it is safe to apply the workaround alongside the resolution, including on a device where the workaround was already applied before the block arrived, and that its ongoing investigation does not indicate that disabling this registry key causes unintended behavior.

Fixes and workarounds that match the current guidance

Allow the automatic block to arrive

1. Save work and restart Windows.
2. Keep the device online so the compatibility safeguard can propagate.
3. Allow up to 24 hours from Microsoft's rollout window, then restart again and test the affected game.
4. Check Windows release health before trying an older unofficial workaround.

This is the preferred path for non-managed consumer and business PCs because it follows Microsoft's deployed resolution. It does not require uninstalling KB5121003 or manually deleting a driver file.

Use the managed-device registry workaround carefully

Microsoft's official workaround disables the inpoutx64 driver through the registry. The sequence below wraps Microsoft's four documented steps in standard administrative practice. Back up the registry before editing it. An incorrect registry change can prevent Windows or an application from working correctly.

1. Open Registry Editor with administrative rights.
2. Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\inpoutx64.
3. Open the Start value and set it to 4.
4. Close Registry Editor and restart the device.
5. Test the affected game and the RGB-lighting software or hardware that installed the driver.

Microsoft says the previous value can be restored to re-enable the driver, so record that original value before changing it.

Handle installation failures separately

A download or installation error is not the same as the inpoutx64 game problem. For error 0x80070643, use the Technobezz 0x80070643 repair guide. If the scan never completes, follow the stuck checking for updates guide.

If repeated attempts still fail, the Windows Update components reset guide provides a broader repair sequence. Apply one bounded change at a time, restart, and retest so the result stays attributable.

Avoid the obsolete file-deletion workaround

Early third-party coverage described a developer workaround that stopped and deleted the inpoutx64 service and removed its driver file. That advice preceded Microsoft's compatibility block and final finding. Do not delete the driver based only on an old report. Use the current automatic resolution or Microsoft's documented registry setting, and get updated instructions from the hardware or software vendor if its lighting controls stop working.

Reports that are not KB5121003 defects

Microsoft currently lists an incorrect notification that says Defender Antivirus is off even when protection remains active. The release-health record does not assign an originating Windows update, so it should not be blamed on KB5121003. Microsoft says it is working on a future Defender update on the Windows 11 25H2 dashboard.

Microsoft also opened a cursor-customization report after the later optional preview KB5120998. The Windows 11 24H2 dashboard ties the report to KB5120998 or later, not to KB5121003. Keeping these records separate prevents a later preview issue from being misdiagnosed as an August security-update problem.

Whether to install KB5121003 now

KB5121003 is the current cumulative security update for Windows 11 25H2 and 24H2, and Microsoft distributes it automatically through Windows Update. The most prominent public concern is now marked resolved, with Microsoft stating that Windows updates did not cause it. A PC that is working normally has no issue-specific reason in Microsoft's current guidance to remove the update.

If a consumer PC with RGB-lighting hardware cannot launch one of the named games, restart and allow the automatic block time to arrive. If the PC is managed by an organization, use the documented enterprise workaround through an administrator. Keep the update, game, device utility, and driver diagnoses separate so a fix for one does not create a new problem in another.

For the broader monthly context, see the sibling Windows 11 Update Tracker. It distinguishes the security baseline from the later optional preview and follows new release-health reports.