M365EXO-007: Auto-forwarding policy
- Platform
- Entra ID / M365
- Category
- Advanced Threat Protection
- Severity
- Critical
- Zero Trust pillar
- Applications & Workloads (weight 2)
- Golden fixtures
- 3
- Branch coverage
- Observed: fixtures prove the verdicts they exercise
- Provenance
- baseline
What it checks
Automatic email forwarding to external addresses is a common data exfiltration technique used by attackers after compromising a mailbox. An attacker can set up auto-forwarding rules to silently copy all incoming email to an external address, maintaining persistent access to sensitive communications even after their access is revoked. Organizations should block external auto-forwarding by default and audit any existing forwarding rules.
Recommended value
External auto-forwarding blocked via anti-spam outbound policy; existing forwarding rules audited and approved
Remediation
Configure the outbound spam filter policy to set automatic forwarding to 'Automatic - System-controlled' or 'Off' to block external auto-forwarding at the transport level. Audit all existing mailbox forwarding rules and SMTP forwarding configurations to identify any unauthorized external forwarding that may indicate compromise. Remove any unapproved forwarding rules and implement monitoring alerts to detect new forwarding rule creation using the unified audit log.
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 | WARN |
| throttled | Not Assessed |
Framework mappings
- CISA SCuBA
- MS.EXO.1.1v2
- NIST SP 800-53
- AC-4
- CIS M365 Benchmark
- 2.1.6
- MITRE ATT&CK
- T1114.003