Microsoft has released a security enhancement for Excel in the August 2022 update. This security enhancement blocks Excel XLL add-ins with an incorrect or missing file extension.
Excel XLL add-in File Extension Requirements
Valid file extensions for Excel XLL add-in files are .xll and .dll – Excel XLL add-in files with other file extensions or no file extension are invalid and after the August update is applied these invalid Excel XLL add-in files will be blocked.
When an Excel XLL add-in is blocked, users will see the following alert:
Temporarily restoring old behavior
There is a way to temporarily disable this security enhancement for users and organizations that rely on Excel XLL add-ins with missing or incorrect file extensions. This is provided to assist users while their Excel XLL add-ins and related solutions are updated to use a correct file extension. This mechanism will be removed in the January 2023 update, at which point users must use a valid file extension for Excel XLL add-ins.
To restore the old behavior and allow Excel XLL add-ins with missing or incorrect file extensions:
Open the Registry Editor
Caution: Incorrectly editing the registry might severely damage your system. Before making changes to the registry, you should back up any valued data on the computer.
Add the following registry value as a DWORD:
Office 2013: Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Excel\Security\EnforceXllExtension
Office 2016, 2019, 2021, 365: Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Security\EnforceXllExtension
Set the value to 0
To re-enable the security fix, delete the registry value created in step 2.