Microsoft shipped its September 2026 update round for Windows 11 this month, and it's a bigger one than usual: close to 1,000 security fixes, several new features, and a handful of regressions that are worth knowing about before you install. Here's what actually changed, and what to watch out for on your own PC.
The update itself
The core release is the September 8, 2026 cumulative update, delivered as KB5124008 for versions 24H2 and 25H2, and KB5124012 for the newer 26H1 branch. Both are full security updates that also fold in last month's preview improvements. Alongside them, Microsoft pushed KB5124010 on September 22 — a non-security preview update that also happens to be the last preview release version 24H2 will ever get, since that version is being wound down.
If you're running Windows 11 Pro or Windows 11 Home, this update reaches your machine automatically through Windows Update unless you've paused updates.
Known issues to check before you update
A few regressions have shown up since release, and they're concentrated on specific hardware:
- HP systems: some users have reported BIOS corruption that leaves the install unrecoverable. HP has since published a new BIOS revision, though it's still not fully clear which side caused it.
- Lenovo systems: reports of the machine black-screening or shutting down unexpectedly, apparently tied to power management.
- AMD graphics: some crash reports tied to this and last month's update.
- File Explorer hang-ups and a handful of broken third-party integrations.
- USB Audio Class 1.0 devices may fail to start or produce no sound after installing KB5124008/KB5124012.
- A Machine Identity Isolation bug can break the secure channel on domain-joined devices; Microsoft's short-term fix is a registry change, detailed in the official KB article.
Microsoft has already pushed an emergency follow-up fix for some of the crash regressions, so if you're on affected hardware it's reasonable to hold off installing until that lands cleanly on your device. Tom's Hardware has a good rundown of the specific crash and hardware reports if you want the full picture.
A date worth marking: October 13, 2026
Separately from this update, Microsoft is retiring support for Windows 11 24H2 Home and Pro on October 13, 2026. After that date those editions stop receiving fixes, security patches, and technical support (Enterprise and Education editions get another year, until October 2027). If your PC is still on 24H2, this is the natural point to move to a newer build via Windows Update, or, if you're setting up a new machine, to start directly on a current edition like Windows 11 Pro or Windows 11 Home rather than one that's about to age out.
Should you install it now?
For most PCs, yes — nearly a thousand security fixes is a meaningful patch, and the vast majority of installs have gone through without issue. The exceptions are HP and Lenovo machines showing the symptoms above, where it's worth waiting for Microsoft's and the OEM's follow-up fixes to fully roll out before updating.
Sources
| KB5124012 — Windows 11 26H1 security update | support.microsoft.com |
| KB5124008 — Windows 11 24H2/25H2 security update | support.microsoft.com |
| KB5124010 — Windows 11 24H2/25H2 preview update | support.microsoft.com |
| Known crash & hardware issue reports | Tom's Hardware |
| Windows 11 version history & support dates | Wikipedia |