Microsoft to block Excel XLL add-ins – at last!

XLL addins for Excel will be blocked in a coming security measure announced by Microsoft. You can implement a similar block now without waiting for Microsoft’s late change.

Excel can use .XLL files to provide additional code for a workbook. Some Excel extras will include XLL’s but unfortunately hackers and criminals can also use XLL files to infect a computer.

Only now is Microsoft planning to block Internet sourced XLL files by default, starting with Insiders/Preview versions of Excel 365 in March 2023.

There are no further details but presumably Microsoft will be using their “Mark of the Web” system to know if an XLL is from the Internet or not.

Block XLL now

There’s no need to wait for Microsoft to catch up, it’s possible to block Excel add-ins like XLL now in any modern Excel for Windows.  Go to File | Options | Trust Center | File Block Settings then scroll down to “Excel Add-in Files”.

Click the ‘Open’ check box then verify the ‘Open behavior’, the default and best choice is “Open selected file types in Protected View”.

That choice will prompt you if there’s an attempt to install any XLL file, so you can decide if it’s OK or not.  XLL files can be a legitimate and necessary part of an Excel enhancement or extra.

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