CISA.MS.EXO.9.1 - Emails SHALL be filtered by attachment file types.

Overview

Emails SHALL be filtered by attachment file types.

Rationale: Malicious attachments often take the form of click-to-run files. Sharing high risk file types, when necessary, is better left to a means other than email; the dangers of allowing them to be sent over email outweigh any potential benefits. Filtering email attachments based on file types can prevent spread of malware distributed via click-to-run email attachments.

Remediation action:

1. Sign in to Microsoft 365 Defender.
2. In the left-hand menu, go to Email & Collaboration > Policies & Rules.
3. Select Threat Policies.
4. From the Templated policies section, select Preset Security Policies.
5. Under Standard protection, slide the toggle switch to the right so the text next to the toggle reads Standard protection is on.
6. Under Strict protection, slide the toggle switch to the right so the text next to the toggle reads Strict protection is on.

Note: If the toggle slider in step 5 is grayed out, click on Manage protection settings instead and configure the policy settings according to Use the Microsoft 365 Defender portal to assign Standard and Strict preset security policies to users | Microsoft Learn.

Related links

• Defender admin center - Preset security policies
• Defender admin center - Order and precedence of email protection
• CISA 9 Attachment File Type - MS.EXO.9.1v2
• CISA ScubaGear Rego Reference
• Microsoft Learn - Common attachments filter in anti-malware policies

Test Metadata

Field	Value
Test ID	CISA.MS.EXO.9.1
Severity	Medium
Suite	CISA
Category	exchange
PowerShell test	Test-MtCisaAttachmentFilter
Tags	CISA, CISA.MS.EXO.9.1, MS.EXO, MS.EXO.9.1

Source

• Pester test: tests/cisa/exchange/Test-MtCisaAttachmentFilter.Tests.ps1
• PowerShell source: powershell/public/cisa/exchange/Test-MtCisaAttachmentFilter.ps1