M365EXO-049: Executable attachment types blocked (MS.EXO.9.5)
- Platform
- Entra ID / M365
- Category
- Advanced Threat Protection
- Severity
- Medium
- Zero Trust pillar
- Applications & Workloads (weight 2)
- Golden fixtures
- 5
- Branch coverage
- Observed: fixtures prove the verdicts they exercise
- Provenance
- baseline
What it checks
SCuBA MS.EXO.9.5 requires that click-to-run executable file types (for example .exe, .cmd, and .vbe) be blocked at the mail gateway. Microsoft Defender for Office 365 enforces this through the Common Attachment Filter on the anti-malware policy, which rejects messages carrying the listed file types outright regardless of content scanning. Executable attachments are a primary malware-delivery vector; blocking the high-risk types by extension stops the most common payloads before any detonation is required. This check inspects every anti-malware policy and confirms the Common Attachment Filter is enabled and covers at minimum .exe, .cmd, and .vbe.
Recommended value
Common Attachment Filter enabled on all anti-malware policies, blocking at minimum .exe, .cmd, and .vbe
Remediation
In the Microsoft Defender portal under Email & collaboration > Policies & rules > Threat policies > Anti-malware, enable the Common Attachment Filter on each policy and ensure the blocked file-type list includes the click-to-run executables (.exe, .cmd, .vbe and related types). Prefer the Standard or Strict preset security policies, which enable this by default. Confirm the policy applies 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 |
| filter-off | FAIL |
| gap | FAIL |
| no-policies | WARN |
| not-assessed | Not Assessed |
Framework mappings
- NIST SP 800-53
- SI-3, SC-7