Elastic medium stable eql

M365 Exchange Inbox Forwarding Rule Created

Identifies when a new Inbox forwarding rule is created in Microsoft 365. Inbox rules process messages in the Inbox based on conditions and take actions. In this case, the rules will forward the emails to a defined address. Attackers can abuse Inbox Rules to intercept and exfiltrate email data without making organization-wide configuration changes or having the corresponding privileges.

View Source

Detection Logic

web where
    event.provider == "Exchange" and
    event.action in (
        "New-InboxRule",
        "Set-InboxRule",
        "Set-Mailbox",
        "Set-TransportRule",
        "New-TransportRule"
    ) and event.outcome == "success" and
    (
        (?o365.audit.Parameters.ForwardTo != null and not endsWith~(?o365.audit.Parameters.ForwardTo, user.domain)) or
        (?o365.audit.Parameters.ForwardAsAttachmentTo != null and not endsWith~(?o365.audit.Parameters.ForwardAsAttachmentTo, user.domain)) or
        (?o365.audit.Parameters.ForwardingAddress != null and not endsWith~(?o365.audit.Parameters.ForwardingAddress, user.domain)) or
        (?o365.audit.Parameters.ForwardingSmtpAddress != null and not endsWith~(?o365.audit.Parameters.ForwardingSmtpAddress, user.domain)) or
        (?o365.audit.Parameters.RedirectTo != null and not endsWith~(?o365.audit.Parameters.RedirectTo, user.domain)) or
        (?o365.audit.Parameters.RedirectToRecipients != null and not endsWith~(?o365.audit.Parameters.RedirectToRecipients, user.domain)) or
        (?o365.audit.Parameters.RedirectMessageTo != null and not endsWith~(?o365.audit.Parameters.RedirectMessageTo, user.domain)) or
        (?o365.audit.Parameters.BlindCopyTo != null and not endsWith~(?o365.audit.Parameters.BlindCopyTo, user.domain))
    )

False Positives

  • ⚠ Users and Administrators can create inbox rules for legitimate purposes. Verify if it complies with the company policy and done with the user's consent. Exceptions can be added to this rule to filter expected behavior.

Field Validations

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