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The display of Level 1 attachments must be disabled in Outlook.


Overview

Finding ID Version Rule ID IA Controls Severity
V-223361 O365-OU-000016 SV-223361r508019_rule Medium
Description
This policy setting controls whether Outlook blocks potentially dangerous attachments designated Level 1. Outlook uses two levels of security to restrict users' access to files attached to e-mail messages or other items. Files with specific extensions can be categorized as Level 1 (users cannot view the file) or Level 2 (users can open the file after saving it to disk). Users can freely open files of types that are not categorized as Level 1 or Level 2. If you enable this policy setting, Outlook users can gain access to Level 1 file type attachments by first saving the attachments to disk and then opening them, as with Level 2 attachments. If you disable this policy setting, Level 1 attachments do not display under any circumstances. If you do not configure this policy setting, Outlook completely blocks access to Level 1 files, and requires users to save Level 2 files to disk before opening them.
STIG Date
Microsoft Office 365 ProPlus Security Technical Implementation Guide 2022-06-17

Details

Check Text ( C-25034r442302_chk )
Verify the policy value for User Configuration >> Administrative Templates >> Microsoft Outlook 2016 >> Security >> Security Form Settings >> Attachment Security >> Display Level 1 attachments is set to "Disabled".

Use the Windows Registry Editor to navigate to the following key:

HKCU\Software\Policies\Microsoft\Office\16.0\outlook\security

If the value ShowLevel1Attach is REG_DWORD = 0, this is not a finding.
Fix Text (F-25022r442303_fix)
Set the policy value for User Configuration >> Administrative Templates >> Microsoft Outlook 2016 >> Security >> Security Form Settings >> Attachment Security "Display Level 1 attachments" to "Disabled".