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-223361r961086_rule | CCI-001662 | 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 | 2025-03-05 | |||
Details
Check Text (C-223361r961086_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".