M365EXO-013: Automatic forwarding to external domains disabled (MS.EXO.1.1)
- Platform
- Entra ID / M365
- Category
- Advanced Threat Protection
- Severity
- High
- Zero Trust pillar
- Data (weight 2)
- Golden fixtures
- 3
- Branch coverage
- Observed: fixtures prove the verdicts they exercise
- Provenance
- baseline
What it checks
SCuBA MS.EXO.1.1 requires that automatic forwarding to external domains be disabled. Remote domain configuration in Exchange Online governs whether mail can be auto-forwarded out of the tenant. When the default remote domain (*) permits auto-forwarding, a compromised mailbox can silently exfiltrate all inbound mail to an attacker-controlled external address, persisting access even after credentials are reset.
Recommended value
AutoForwardEnabled set to False on the default remote domain (*) and on every custom remote domain unless a specific partner integration is documented
Remediation
In the Exchange admin center under Mail flow, open Remote domains and edit the Default (*) entry so that automatic forwarding is not allowed (AutoForwardEnabled = False). Repeat for every custom remote domain that does not have a documented business need for cross-tenant auto-forwarding. Confirm the outbound spam policy auto-forward setting is consistent so that both control layers enforce the same restriction.
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
- CISA SCuBA
- MS.EXO.1.1v2
- NIST SP 800-53
- AC-4
- MITRE ATT&CK
- T1114.003