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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.

Known benign triggers

  • Users or processes that are send a large number of attachments may trigger this alert, adjust thresholds accordingly.

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