M365EXO-021: Contact folder sharing not open to all domains (MS.EXO.6.1)
- Platform
- Entra ID / M365
- Category
- Advanced Threat Protection
- Severity
- Medium
- Zero Trust pillar
- Data (weight 1)
- Golden fixtures
- 3
- Branch coverage
- Observed: fixtures prove the verdicts they exercise
- Provenance
- baseline
What it checks
SCuBA MS.EXO.6.1 requires that contact folders not be shared with all domains. Exchange Online sharing policies can relax the default restriction on outbound contact sharing. A policy that shares contacts with all domains (a '*' domain entry) exposes directory and contact data broadly, creating a data exfiltration avenue and aiding reconnaissance for social engineering.
Recommended value
No sharing policy rule grants contact sharing to the wildcard domain (*); sharing limited to specific approved domains only
Remediation
Review every Exchange Online sharing policy and remove any rule that shares contact folders with the wildcard domain (*). Where external contact sharing is genuinely required, scope it to specific named partner domains rather than all domains. Validate that the default sharing policy does not silently re-enable all-domain contact sharing.
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.6.1v1
- NIST SP 800-53
- AC-4, AC-21
- MITRE ATT&CK
- T1087