Trust Bar notification must be enabled for unsigned application add-ins in Excel and blocked.
This policy setting controls whether the specified Office 2016 applications notify users when unsigned application add-ins are loaded or silently disable such add-ins without notification. This policy setting only applies if you enable the ''Require that application add-ins are signed by Trusted Publisher'' policy setting, which prevents users from changing this policy setting. If you enable this policy setting, applications automatically disable unsigned add-ins without informing users. If you disable this policy setting, if an application is configured to require that all add-ins be signed by a trusted publisher, any unsigned add-ins the application loads will be disabled and the application will display the Trust Bar at the top of the active window. The Trust Bar contains a message that informs users about the unsigned add-in. If you do not configure this policy setting, the disable behavior applies, and in addition, users can configure this requirement themselves in the ''Add-ins'' category of the Trust Center for the application.
Set policy value for User Configuration >> Administrative Templates >> Microsoft Excel 2016 >> Excel Options >> Security >> Trust Center >> Disable Trust Bar Notification for unsigned application add-ins and block them to "Enabled".
Verify the policy value for User Configuration >> Administrative Templates >> Microsoft Excel 2016 >> Excel Options >> Security >> Trust Center >> Disable Trust Bar Notification for unsigned application add-ins and block them is set to "Enabled". Use the Windows Registry Editor to navigate to the following key: HKCU\software\policies\microsoft\office\16.0\excel\security If the value for notbpromptunsignedaddin is REG_DWORD = 1, this is not a finding.