As part of Patch Tuesday for September and among the 76 vulnerabilities addressed, Microsoft is fixing a security vulnerability in Microsoft Excel 2016. However, patchers should be aware that this security fix actually introduces an immediate display issue that is only resolved by installing a second update.
From KB4011050 (the security KB):
After this update is installed, black borders may appear around rows or cells in Excel spreadsheets when you enter text. To fix this issue, install September 12, 2017, update for Excel 2016 (KB4011165).
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