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

Overview

Finding IDVersionRule IDIA ControlsSeverity
V-223361O365-OU-000016SV-223361r961086_ruleCCI-001662medium
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.
STIGDate
Microsoft Office 365 ProPlus Security Technical Implementation Guide2026-02-12

Related Frameworks

1 mapping across 4 frameworks
SCF0 mappings

No source states a mapping between these directly.

Also reachable in 3 hops (3) derived by us, not asserted by any source

Each of these is a route we composed by chaining published mappings. No publisher states a relationship between the two endpoints.

END-10Mobile Code
0.50
VPM-02Vulnerability Remediation Process
0.50
VPM-04Continuous Vulnerability Remediation Activities
0.50
NIST 800-530 mappings

No source states a mapping between these directly.

Also reachable in 2 hops (4) derived by us, not asserted by any source

Each of these is a route we composed by chaining published mappings. No publisher states a relationship between the two endpoints.

CM-7Least Functionality
1.00
SC-18(1)Identify Unacceptable Code and Take Corrective Actions
1.00
SC-18(3)Prevent Downloading and Execution
1.00
SC-18(4)Prevent Automatic Execution
1.00
CCI1 mapping
CCI-001662
1.00
NIST CSF 2.00 mappings

No source states a mapping between these directly.

Also reachable in 4 hops (2) derived by us, not asserted by any source

Each of these is a route we composed by chaining published mappings. No publisher states a relationship between the two endpoints.

ID.RA-08
0.50
PR.PS-02
0.50

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".