O365 Email Send Attachments Excessive Volume
The following analytic identifies when an O365 email account sends an excessive number of email attachments to external recipients within a short period (within 1 hour). This behavior may indicate a compromised…
Description
The following analytic identifies when an O365 email account sends an excessive number of email attachments to external recipients within a short period (within 1 hour). This behavior may indicate a compromised account where the threat actor is attempting to exfiltrate data from the mailbox. Threat actors may attempt to transfer data through email as a simple means of exfiltration from the compromised mailbox. Some account owner legitimate behaviors can trigger this alert, however these actions may not be aligned with organizational expectations / best practice behaviors.
Detection logic
Detection requirements
- Platform
- ContainersESXiIaaSLinuxmacOSOffice SuiteWindows
The rule states no platform. This is derived from the 2 ATT&CK techniques it maps to.
- Log source category
- office_365_universal_audit_log
Known benign triggers
- Users or processes that are send a large number of attachments may trigger this alert, adjust thresholds accordingly.
MITRE ATT&CK mappings
0 exclusive techniques.This is coverage no other published rule has; it is not this rule's total technique count.
References
| Reference | Cited by |
|---|---|
| attack.mitre.org/techniques/T1114/ | 9 |
| intelligence.abnormalsecurity.com/attack-library/threat-actor-convincingly-impersonates-employee-requesting-direct-deposit-update-in-likely-ai-generated-attack | 7 |
| www.hhs.gov/sites/default/files/help-desk-social-engineering-sector-alert-tlpclear.pdf | 7 |
From the source
- At source
- Open at source
- Upstream identifier
- 70a050a2-8537-488a-a628-b60a9558d96a
- Tagged by the source as
- Office 365 Account TakeoverSuspicious EmailsO365 TenantcloudSplunk CloudSplunk EnterpriseSplunk Enterprise Securitythreat
Licence
- Published under
- Apache License 2.0Read the licence
- Attribution
- Required
- Obtained under
- Apache-2.0Read the origin licence
Authorship
- Written by
- Published
- Sep 30, 2026
- Version
- 2