M365EXO-028: Disallowed attachment file types set (MS.EXO.9.3)
- Platform
- Entra ID / M365
- Category
- Advanced Threat Protection
- Severity
- High
- Zero Trust pillar
- Applications & Workloads (weight 2)
- Golden fixtures
- 3
- Branch coverage
- Observed: fixtures prove the verdicts they exercise
- Provenance
- baseline
What it checks
SCuBA MS.EXO.9.3 requires that disallowed file types be determined and set, blocking at minimum click-to-run files such as .exe, .cmd, and .vbe. An attachment filter that is enabled but has an empty or insufficient block list provides no real protection. The organization must define and maintain the list of blocked extensions in line with its risk tolerance.
Recommended value
Attachment filter block list populated with executable/click-to-run types including at least exe, cmd, vbe, vbs, js, ps1, bat
Remediation
Populate the attachment filter block list with the file types the organization will not accept over email, ensuring click-to-run executables such as exe, cmd, vbe, vbs, js, ps1, and bat are included at minimum. Review the list against current threat trends and the organization's risk tolerance. Verify the list is non-empty and applied to all recipients.
Fixture-proven verdicts
Every verdict below is proven by a golden fixture in the module's gating test suite. This table derives from the last green run; it cannot be edited by hand.
| Scenario | Expected verdict |
|---|---|
| clean | PASS |
| known-bad | FAIL |
| throttled | Not Assessed |
Framework mappings
- NIST SP 800-53
- SI-3
- MITRE ATT&CK
- T1204.002