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M365 Exchange Mailbox Accessed by Unusual Client

Identifies suspicious Microsoft 365 mail access by ClientAppId. This rule detects when a user accesses their mailbox using a client application that is not typically used by the user, which may indicate potential…

Description

Identifies suspicious Microsoft 365 mail access by ClientAppId. This rule detects when a user accesses their mailbox using a client application that is not typically used by the user, which may indicate potential compromise or unauthorized access attempts. Adversaries may use custom or third-party applications to access mailboxes, bypassing standard security controls. First-party Microsoft applications are also abused after OAuth tokens are compromised, allowing adversaries to access mailboxes without raising suspicion.

Detection logic

Its licence does not clear it for publishing here

Sunturai publishes a detection's own text where the licence it arrived under has been reviewed and permits it, and Elastic License 2.0 has not. The query as its source wrote it, its canonical form and the hash that pins this revision are in the workspace record.

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Detection requirements

Platform
LinuxmacOSOffice SuiteWindows

The rule states no platform. This is derived from the ATT&CK technique it maps to.

Log source category
event_index

Known benign triggers

  • User using a new mail client or mobile application to access their mailbox.
  • Users accessing their mailboxes using custom or third-party applications that are authorized by the organization, such as CRM or ERP systems.
  • Users accessing their mailboxes using first-party Microsoft applications that are not typically used by the user, such as Outlook on the web or Outlook mobile app.

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