KB5121000 — August 2026 security update
Answer: Released August 11, 2026 for Windows 11 26H1; installs OS build 28000.2704.
Administrator playbook
Decide, verify, and document
Use the curated answer as a starting point, then prove the outcome against the target tenant or device.
What it means
- Applies to Windows 11 26H1
- security update (B release)
- 26H1 is scoped to new devices and is not an in-place update target from 24H2 or 25H2
What to check next
- Confirm the target Windows version and complete build family
- Review the linked Microsoft release record for current known issues and prerequisites
- Validate in a pilot ring before broad deployment
Verify success
- Confirm the complete OS build reaches the expected revision.
- Verify the update appears in Windows Update history or the servicing stack evidence.
- Recheck Microsoft release health for newly published known issues before broad deployment.
Escalate when
- The update repeatedly fails after servicing-health repair and a controlled retry.
- A known issue matches production impact or a safeguard is blocking deployment.
- You have the full build, error, logs, UTC time, and deployment channel.
Copyable admin briefAnswer, next checks, source, and review date
Capture Windows build
Get-ComputerInfo | Select-Object WindowsProductName,WindowsVersion,OsBuildNumber,OsArchitectureRead-only. Match the full revision, not just the base build family.
Check this KB locally
Get-HotFix -Id KB5121000 -ErrorAction SilentlyContinueRead-only where the update is exposed through Win32_QuickFixEngineering. Absence here is not definitive; also check update history and DISM packages.
Evidence to preserve
- Release
- 2026-08-11
- Channel
- B release
- Versions
- Windows 11 26H1
- Builds
- 28000.2704
- Windows version, edition, and architecture
- Complete OS build before and after
- KB installation state and UTC time
- Update ring, policy, and error code