Office 365 group modification add member success has exceeded a defined threshold

// Copyright 2025 Google LLC. Licensed under Apache-2.0.

rule o365_group_modification_add_member_success_threshold {

  meta:
    author = "Google Cloud Security"
    description = "Detects when a group GUID in Office 365 has more users added to it by the same user than a defined thresold."
    rule_id = "mr_47e13870-800b-4886-b8b8-8e6b68b203b9"
    rule_name = "Office 365 group modification add member success has exceeded a defined threshold"
    type = "alert"
    platform = "azure"
    data_source = "o365"
    severity = "Low"
    priority = "Low"

  events:
    $group.metadata.event_type = "GROUP_MODIFICATION"
    $group.metadata.product_event_type = "Add member to group."
    $group.metadata.product_name = "Office 365"
    $group.metadata.vendor_name = "Microsoft"
    $group.security_result.action = "ALLOW"
    $group.principal.user.userid = $user
    $group.target.group.product_object_id = $guid

  match:
    $guid, $user over 5m

  outcome:
    $risk_score = 35
    $event_count = count_distinct($group.metadata.id)
    $user_agent = array_distinct($group.network.http.user_agent)
    $security_summary = array_distinct($group.security_result.summary)
    $target_application = array_distinct($group.target.application)
    $principal_user_userid = array_distinct($group.principal.user.userid)
    $group_name = array_distinct($group.target.group.group_display_name)
    $group_guid = array_distinct($group.target.group.product_object_id)
    $user_added = array_distinct($group.target.user.userid)
    $user_count = count_distinct($group.target.user.userid)

  condition:
    //Modify the user_count value to suit your needs
    $group and $user_count > 4
}