September Office security update KB5002914 breaks Excel copy-paste and autofill

Microsoft's KB5002914 Office security update, released as part of the September 2026 Patch Tuesday cycle, is breaking copy-and-paste operations and formula dragging for some Excel users, according to reports on Reddit and Microsoft's Q&A forums. Users say the failures span Office 2016 through 2024, MSI and Click-to-Run installations, and Office LTSC Standard 2021. Affected users report that uninstalling or rolling back the update restores functionality, though that also removes the month's security fixes. Microsoft told BleepingComputer it is aware of the reports and is investigating. BleepingComputer tested Excel 2019 Version 2508 and could not reproduce the problem.