Block Excel XLL Add-ins that come from an untrusted source: registry value blockxllfrominternetUserHKCU\software\policies\microsoft\office\16.0\excel\security

Policy ID: L_BlockXLLFromInternet
Scope: User Configuration
Supported on: At least Windows Server 2016, Windows 10
Source: Microsoft 365 Apps / Office Administrative Templates (version 5568.1000)
Category: Microsoft Excel 2016 › Excel Options › Security › Trust Center

Registry

blockxllfrominternet

REG_DWORDUser

HKCU\software\policies\microsoft\office\16.0\excel\security Current key

Enabled value: Depends on the option value
Element: -

Options
Item	Stored value
Block	1
Show Additional Warning	0
Allow	2

Apply with .reg · CMD · PowerShell

.reg

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\software\policies\microsoft\office\16.0\excel\security]

; Options (use only one)
; Block
"blockxllfrominternet"=dword:00000001
; Show Additional Warning
"blockxllfrominternet"=dword:00000000
; Allow
"blockxllfrominternet"=dword:00000002

CMD

:: Options (use only one)

:: Block
reg add "HKCU\software\policies\microsoft\office\16.0\excel\security" /v "blockxllfrominternet" /t REG_DWORD /d 1 /f
:: Show Additional Warning
reg add "HKCU\software\policies\microsoft\office\16.0\excel\security" /v "blockxllfrominternet" /t REG_DWORD /d 0 /f
:: Allow
reg add "HKCU\software\policies\microsoft\office\16.0\excel\security" /v "blockxllfrominternet" /t REG_DWORD /d 2 /f

PowerShell

# Common: create the key
New-Item -Path 'HKCU:\software\policies\microsoft\office\16.0\excel\security' -Force | Out-Null

# Options (use only one)
# Block
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\excel\security' -Name 'blockxllfrominternet' -PropertyType DWord -Value 1 -Force | Out-Null
# Show Additional Warning
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\excel\security' -Name 'blockxllfrominternet' -PropertyType DWord -Value 0 -Force | Out-Null
# Allow
New-ItemProperty -Path 'HKCU:\software\policies\microsoft\office\16.0\excel\security' -Name 'blockxllfrominternet' -PropertyType DWord -Value 2 -Force | Out-Null