Rules: First Seen Client Generating MailIItemsAccessed Event from User
This alert looks at a First Seen client accessing an Office 365/Exchange mail box item. The MailItemsAccessed may not always be enabled within an Entra/Azure/Office 365 tenant and is dependent on Microsoft licensing requirements. See the following guide from CISA for additional information on this event type and investigation steps: https://www.cisa.gov/news-events/cybersecurity-advisories/aa23-193a
| Detail | Value |
|---|---|
| Type | First Seen |
| Category | Collection |
| Apply Risk to Entities | user_username |
| Signal Name | First Seen Client Generating MailIItemsAccessed Event from {{user_username}} |
| Summary Expression | A First Seen client was observed as accessing an Office 365 mail item since the baseline period. |
| Retention Window | 7776000000 |
| Baseline Window | 1209600000 |
| Baseline Type | PER_ENTITY |
| Score/Severity | Static: 4 |
| Enabled by Default | True |
| Prototype | False |
| Tags | _mitreAttackTactic:TA0009, _mitreAttackTechnique:T1114 |
| Origin | Field |
|---|---|
| Normalized Schema | action |
| Normalized Schema | metadata_product |
| Normalized Schema | user_username |