Microsoft 365 Exchange inbox rule set up to automatically forward email

microsoft-365

Classification:

attack

Tactic:

TA0009-collection

Technique:

T1114-email-collection

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Goal

Detect when a user configures an inbox forwarding rule.

Strategy

Monitor Microsoft 365 Exchange audit logs to look for the operation New-InboxRule or Set-InboxRule. Inbox rules automatically perform specific actions on emails that arrive in an inbox. Forwarding rules allow users to send email messages to another inbox inside or outside of the organization. Attackers can use inbox rules to collect and exfiltrate email data to an external account.

Triage and response

  1. Inspect the inbox rule for any indicators:
    • Email addresses that are external to your organization.
  2. Determine if there is a legitimate use case for the inbox rule by contacting the user {{@usr.email}}.
  3. If the user {{@usr.email}} is not aware of the inbox rule:
    • Investigate other activities performed by the user {{@usr.email}} using the Cloud SIEM - User Investigation dashboard.
    • Begin your organization’s incident response process and investigate.

Changelog

  • 17 August 2023 - Updated query to replace attribute @threat_intel.results.subcategory:tor with @threat_intel.results.category:tor.