M365EXO-027: Attachment filter assesses true file type (MS.EXO.9.2)
- Platform
- Entra ID / M365
- Category
- Advanced Threat Protection
- Severity
- Medium
- Zero Trust pillar
- Applications & Workloads (weight 1)
- Golden fixtures
- 3
- Branch coverage
- Observed: fixtures prove the verdicts they exercise
- Provenance
- baseline
What it checks
SCuBA MS.EXO.9.2 recommends that the attachment filter attempt to determine the true file type and assess the file extension. Attackers rename files to disguise dangerous types (for example, renaming an executable to a .txt). True-type detection catches mismatches where the extension does not match the actual file content, closing a common evasion technique.
Recommended value
Attachment filtering configured to inspect true file type rather than relying on the file extension alone
Remediation
Ensure the attachment filtering solution inspects the actual file content to determine the true type rather than trusting the file extension. In the native solution this is provided by the common attachment filter's type detection; verify it is enabled. For third-party gateways, confirm true-type or content-based inspection is configured so renamed dangerous files are still caught.
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
- T1036.008