CISA.MS.EXO.9.3 - Disallowed file types SHALL be determined and enforced.
Overview
The attachment filter SHOULD attempt to determine the true file type and assess the file extension.
Rationale: Users can change a file extension at the end of a file name (e.g., notepad.exe to notepad.txt) to obscure the actual file type. Verifying the file type and checking that this matches the designated file extension can help detect instances where the file extension was changed.
Remediation action
- Sign in to Microsoft 365 Defender.
- In the left-hand menu, go to Email & Collaboration > Policies & Rules.
- Select Threat Policies.
- From the Templated policies section, select Preset Security Policies.
- Under Standard protection, slide the toggle switch to the right so the text next to the toggle reads Standard protection is on.
- 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
- CISA 9 Attachment File Type - MS.EXO.9.2v1
- CISA ScubaGear Rego Reference
- Microsoft Learn - True type matching in the common attachments filter
Test Metadata
| Field | Value |
|---|---|
| Test ID | CISA.MS.EXO.9.3 |
| Severity | High |
| Suite | CISA |
| Product | Exchange Online |
| Related products | Defender |
| Feature | Anti-malware |
| Area | Email and collaboration protection |
| Resource type | Tenant |
| User impact | Low |
| Implementation cost | Low |
| MITRE ATT&CK | T1204.002 Malicious File, T1566.001 Spearphishing Attachment |
| Supports benchmark | CISA SCuBA Secure Configuration Baselines for Microsoft 365 (version ScubaGear main, 2026-10-11), requirement MS.EXO.9.3 |
| Criticality | SHALL |
| Upstream requirement | Moved to MS.SECURITYSUITE.1.1v1 (was MS.EXO.9.3v1) |
| Supports controls | cisa-scuba:MS.EXO.9.3, nist-800-53-r5:SI-3 |
| PowerShell test | Test-MtCheckCISAMSEXO93 |
| Services | ExchangeOnline, SecurityCompliance |
Source
- Test:
tests/cisa/exchange/Test.CISA.MS.EXO.9.3.ps1 - Documentation:
tests/cisa/exchange/Test.CISA.MS.EXO.9.3.md