Microsoft has confirmed that the optional update KB5124010, released in September 2026, causes crashes in applications and games relying on the built-in AC-3 audio decoder. The issue affects Windows 11 versions 24H2, 25H2, and 26H2.
The bug stems from the msmpeg2ac3dec.dll component. Devices upgraded from Windows 11 22H2 retain this file and are susceptible, while clean installations of 24H2 or 25H2 do not contain the component and remain unaffected.
Symptoms vary from applications failing to launch to sudden crashes during audio playback. Microsoft is currently developing a fix. Users requiring immediate stability are advised to uninstall KB5124010 via the Windows Update history settings.
The September 2026 update cycle has faced multiple stability issues. The KB5124008 update, which addressed 974 security vulnerabilities, caused black screens for AMD Radeon users, Remote Desktop service failures, and issues with Linux folder sharing.
A subsequent emergency patch, KB5129195, failed to fully resolve audio problems and introduced new bugs in the File History backup feature. The KB5124010 update attempted to address these backup issues, but testing indicates that file versioning remains non-functional.
