Configure the 'Block at First Sight' feature: registry value DisableBlockAtFirstSeenComputerHKLM\Software\Policies\Microsoft\Windows Defender\Spynet

Policy ID: DisableBlockAtFirstSeen
Scope: Computer Configuration
Supported on: At least Windows Server 2016, Windows 10
Source: Administrative Templates (.admx) for Windows 11 2026 Update (26H2) (version 1.0)
Category: Windows Components › Microsoft Defender Antivirus › MAPS

This feature ensures the device checks in real time with the Microsoft Active Protection Service (MAPS) before allowing certain content to be run or accessed. If this feature is disabled, the check will not occur, which will lower the protection state of the device.
Enabled – The Block at First Sight setting is turned on.
Disabled – The Block at First Sight setting is turned off.

This feature requires these Group Policy settings to be set as follows:
MAPS -> The “Join Microsoft MAPS” must be enabled or the “Block at First Sight” feature will not function.
MAPS -> The “Send file samples when further analysis is required” should be set to 1 (Send safe samples) or 3 (Send all samples). Setting to 0 (Always Prompt) will lower the protection state of the device. Setting to 2 (Never send) means the “Block at First Sight” feature will not function.
Real-time Protection -> The “Scan all downloaded files and attachments” policy must be enabled or the “Block at First Sight” feature will not function.
Real-time Protection -> Do not enable the “Turn off real-time protection” policy or the “Block at First Sight” feature will not function.

See the full description on the Group Policy page ›

Registry

DisableBlockAtFirstSeen

REG_DWORDComputer

HKLM\Software\Policies\Microsoft\Windows Defender\Spynet Current key

Enabled value: 0
Disabled value: 1

Apply with .reg · CMD · PowerShell

.reg

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows Defender\Spynet]

; Options (use only one)
; Enabled
"DisableBlockAtFirstSeen"=dword:00000000
; Disabled
"DisableBlockAtFirstSeen"=dword:00000001

CMD

:: Options (use only one)

:: Enabled
reg add "HKLM\Software\Policies\Microsoft\Windows Defender\Spynet" /v "DisableBlockAtFirstSeen" /t REG_DWORD /d 0 /f
:: Disabled
reg add "HKLM\Software\Policies\Microsoft\Windows Defender\Spynet" /v "DisableBlockAtFirstSeen" /t REG_DWORD /d 1 /f

PowerShell

# Common: create the key
New-Item -Path 'HKLM:\Software\Policies\Microsoft\Windows Defender\Spynet' -Force | Out-Null

# Options (use only one)
# Enabled
New-ItemProperty -Path 'HKLM:\Software\Policies\Microsoft\Windows Defender\Spynet' -Name 'DisableBlockAtFirstSeen' -PropertyType DWord -Value 0 -Force | Out-Null
# Disabled
New-ItemProperty -Path 'HKLM:\Software\Policies\Microsoft\Windows Defender\Spynet' -Name 'DisableBlockAtFirstSeen' -PropertyType DWord -Value 1 -Force | Out-Null