M365EXO-011: OAuth/SMTP AUTH per-mailbox audit
- Platform
- Entra ID / M365
- Category
- Advanced Threat Protection
- Severity
- High
- Zero Trust pillar
- Applications & Workloads (weight 2)
- Golden fixtures
- 3
- Branch coverage
- Observed: fixtures prove the verdicts they exercise
- Provenance
- baseline
What it checks
Legacy authentication protocols such as SMTP AUTH allow mailbox authentication using only username and password, bypassing multi-factor authentication and Conditional Access controls. Attackers who obtain mailbox credentials through phishing or password spraying can use SMTP AUTH to access email without triggering MFA challenges. Disabling SMTP AUTH and legacy OAuth flows on mailboxes that do not require them closes a significant authentication bypass vector.
Recommended value
SMTP AUTH disabled organization-wide with per-mailbox exceptions only for documented service accounts; legacy OAuth disabled
Remediation
Disable SMTP AUTH at the organization level using Set-TransportConfig and then selectively enable it only for specific service account mailboxes that require it for application integration. Audit all mailboxes with SMTP AUTH enabled to verify there is a documented business justification and that the credentials are managed securely. Monitor sign-in logs for SMTP AUTH usage to detect potential credential abuse and plan migration of legacy applications to modern authentication methods.
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.5.1v1
- NIST SP 800-53
- IA-2
- CIS M365 Benchmark
- 1.1.16
- MITRE ATT&CK
- T1078