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Microsoft · Windows 11 security update

KB5121003 update details, changes and known issues

Windows 11 24H2/25H2 KB5121003 · Released 2026-08-11

What is this update?

A cumulative update for Windows 11 24H2 and 25H2 containing the August 2026 security and quality fixes, including earlier cumulative changes.

New features, security and compatibility

VENDOR OFFICIAL EVIDENCE

August 2026 security fixes

This cumulative update includes August 2026 security fixes for Windows 11 24H2 and 25H2.

current · Microsoft release documentation · reviewed 2026-08-31

Problems fixed

No separately reviewed item is available for this section yet.

Known issues that remain

VENDOR OFFICIAL EVIDENCE

Game crashes on some RGB-control driver setups

Microsoft is investigating game freezes, exits, and restarts on some PCs where inpoutx64-family drivers are detected.

current · Microsoft release documentation · reviewed 2026-08-31

Manually reviewed public feedback

Updatehey manually reviewed public discussions from 2026-08-08 to 2026-09-07 and retained 3 relevant sources.

MANUALLY REVIEWED · UNCONFIRMED

Unconfirmed question · Possible KB5121003 AC disconnect and black screen

A Lenovo Legion user correlated logged brief AC power transitions and black screens with the day after KB5121003. Causation remains uncertain.

Reported environment: Lenovo Legion

Causal confidence low · posted 2026-08-18 · reviewed 2026-09-07 · Reddit · View original

MANUALLY REVIEWED · UNCONFIRMED

Problem report · KB5121003 Secure Boot failure

An Acer Nitro 17 user reported a Secure Boot failure after KB5121003 and could boot only after disabling Secure Boot.

Reported environment: Acer Nitro 17 AN17-41

Causal confidence medium · posted 2026-08-16 · reviewed 2026-09-07 · Reddit · View original

MANUALLY REVIEWED · UNCONFIRMED

Reported response · The Finals crashes after KB5121003

A Radeon RX 6900 XT user reported The Finals hangs and crashes stopping after KB5121003 was removed.

Reported environment: Windows 11 and Radeon RX 6900 XT

Causal confidence medium · posted 2026-08-14 · reviewed 2026-09-07 · Reddit · View original

These external posts are editorial references, not Updatehey installation reports. They do not automatically change the verdict, and a reported workaround is not a guaranteed fix.

Community-reported issues · automatic repeated signals

COMMUNITY SIGNAL · UNCONFIRMED

Community-reported issues · boot

Public community reports mentioning the same update and symptom are grouped as an early-warning signal.

Observed scope: Shared environment still being verified

1 signals · 1 sources

Not officially confirmed · confidence 55/100 · last seen 2026-09-11 20:17:41

Public Reddit/forum/support-community reports are early-warning signals, not proof of cause or a guaranteed fix. They remain separate from vendor-confirmed issues.

Documented responses

A response is shown with its evidence level. General guidance is not presented as a fix verified for this exact update.

When Windows will not boot after an update

Start with the lowest-risk Windows recovery options when a black screen or restart loop begins after an update.

GENERAL OFFICIAL GUIDANCE

1. Open Windows Recovery Environment

If Windows does not boot normally, open recovery options. A BitLocker recovery key may be required.

Risk: medium · source checked 2026-08-31 · Microsoft release documentation

GENERAL OFFICIAL GUIDANCE

2. Uninstall the latest quality update

If the problem began immediately after updating, consider removing the latest quality update after confirming backups.

Risk: medium · source checked 2026-08-31 · Microsoft release documentation

When a Windows update repeatedly fails to install

Begin with restart and built-in troubleshooting; use a repair reinstall only as a later official step.

GENERAL OFFICIAL GUIDANCE

1. Restart and check Windows Update again

Complete any pending restart and retry from Windows Update while recording a repeated error code.

Risk: low · source checked 2026-08-31 · Microsoft release documentation

GENERAL OFFICIAL GUIDANCE

2. Consider reinstalling the current Windows version

If basic troubleshooting fails, review Microsoft’s current-version reinstall procedure after confirming power and backups.

Risk: high · source checked 2026-08-31 · Microsoft release documentation

General checks before installing

These are general precautions unless a badge above says they were verified for this release.

Before installing

If you play ARC Raiders, MARVEL Tōkon: Fighting Souls or THE FINALS, or use RGB-lighting control software, check Microsoft's investigation status first.

If you are affected

Back up important files and avoid repeated forced reinstalls. Check Windows recovery options and updated drivers from the peripheral vendor first.

Evidence freshness

The official evidence linked to this verdict has not been reviewed recently. Use the review date below when judging freshness.

Last evidence review: 2026-10-09

Evidence timeline

Recent official-document changes and community signals linked to this update. Official and unofficial evidence remain separated.

Official source changes

Community first-seen signals

CURRENT ASSESSMENT
Official issue foundNo Updatehey user sample yet

Read the known issue before installing.

1 open vendor-confirmed issue is registered. The user sample is still too small for a numeric install score.

Verdict basis: Reviewed official issue information · no Updatehey user sample yet · official sources checked 2026-10-09

Content reviewed · official evidence 2 · open known issues 1 · documented responses 2

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User reports

Field reports 0 · stable installs not enough data · confidence low

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Earlier and related updates

Related searches

KB5121003 issues safe to install Windows 11 24H2 25H2 game crash restart RGB inpoutx64 ARC Raiders THE FINALS

Related official and general guides

Pause Windows updates · Uninstall a problematic update · Repair Windows Update

Frequently asked questions

Is KB5121003 safe to install?

Read the known issue before installing. 1 open vendor-confirmed issue is registered. The user sample is still too small for a numeric install score.

Are there known issues with KB5121003?

Game crashes on some RGB-control driver setups. Microsoft is investigating game freezes, exits, and restarts on some PCs where inpoutx64-family drivers are detected.

What should I do before or after installing KB5121003?

Before installing. If you play ARC Raiders, MARVEL Tōkon: Fighting Souls or THE FINALS, or use RGB-lighting control software, check Microsoft's investigation status first.

Update information changes as vendors publish new evidence. Back up important systems before installing.