Ensure 'Set default file block behavior' is set to 'Enabled: Blocked files are not opened'

HighLevel 1Automatedoffice

Description

This policy setting determines if users can open, view, or edit Excel files that are by default blocked by Microsoft Office. The recommended state for this setting is: Enabled: Blocked files are not opened. Rationale: By default, users can open a wide range of legacy file formats in Office that were not designed with modern security protections in mind. Several of these formats have been associated with vulnerabilities that attackers have exploited. User setting: the audit checks HKEY_USERS\<SID>\SOFTWARE\Policies\Microsoft\office\16.0\excel\security\fileblock:openinprotectedview (REG_DWORD) in every loaded user hive (S-1-5-21-*) and is compliant when each holds 0. It prints 'no-user-hive-loaded' (FAIL) when no user profile is loaded, so scan while users are logged on.

How it is checked

Audit method: Windows registry

registry

$sids = @(Get-ChildItem -Path 'Registry::HKEY_USERS' -ErrorAction SilentlyContinue | Where-Object { $_.PSChildName -match '^S-1-5-21-\d+-\d+-\d+-\d+$' } | ForEach-Object { $_.PSChildName }); if ($sids.Count -eq 0) { 'no-user-hive-loaded' } else { $bad = @(); foreach ($s in $sids) { $v = (Get-ItemProperty -Path ('Registry::HKEY_USERS\' + $s + '\SOFTWARE\Policies\Microsoft\office\16.0\excel\security\fileblock') -Name 'openinprotectedview' -ErrorAction SilentlyContinue).'openinprotectedview'; if (-not ($null -ne $v -and @(0) -contains [int64]$v)) { $bad += $s + ':openinprotectedview=' + $v } }; if ($bad.Count -eq 0) { 'compliant' } else { 'non-compliant: ' + ($bad -join ', ') } }

Expected output: compliant

How it is fixed

Fix method: Guidance or infrastructure patch

Configure through Group Policy (User Configuration): User Configuration\Administrative Templates\Microsoft Excel 2016\Excel Options\Security\Trust Center\File Block Settings\Set Default File Block Behavior = Enabled: Blocked files are not opened

Properties

Operating systems: Any
Tags: officeproductivitycis-l1
Reference: CIS Microsoft Office Enterprise Benchmark v2.0.0 - 2.2.4.7.2.2.13 Ensure 'Set default file block behavior' is set to 'Enabled: Blocked files are not opened'

Check this host

Run this on a machine you are allowed to audit. The scan is read-only.

shell

tmaegis scan --target office