Ensure 'Signature Warning' is set to 'Enabled: Always warn about invalid signatures'

HighLevel 1Automatedoffice

Description

This policy setting controls how Outlook warns users about messages with invalid digital signatures. The recommended state for this setting is: Enabled: Always warn about invalid signatures. Rationale: If users are not notified about invalid signatures, it might prevent the user from detecting a fraudulent signature sent by a malicious user. User setting: the audit checks HKEY_USERS\<SID>\SOFTWARE\Policies\Microsoft\office\16.0\outlook\security:warnaboutinvalid (REG_DWORD) in every loaded user hive (S-1-5-21-*) and is compliant when each holds 1. 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\' + $s + '\SOFTWARE\Policies\Microsoft\office\16.0\outlook\security') -Name 'warnaboutinvalid' -ErrorAction SilentlyContinue).'warnaboutinvalid'; if (-not ($null -ne $v -and @(1) -contains [int64]$v)) { $bad += $s + ':warnaboutinvalid=' + $v } }; if ($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 Outlook 2016\Security\Cryptography\Signature Warning = Enabled: Always warn about invalid signatures

Properties

Operating systems: Any
Tags: officeproductivitycis-l1
Reference: CIS Microsoft Office Enterprise Benchmark v2.0.0 - 2.5.14.2.7 Ensure 'Signature Warning' is set to 'Enabled: Always warn about invalid signatures'

Check this host

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

shell

tmaegis scan --target office