Hunt for group members enumeration in the Microsoft Graph API

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 * Licensed under the Apache License, Version 2.0 (the "License");
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 *     https://www.apache.org/licenses/LICENSE-2.0
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 rule ms_graph_group_members_enumeration {

  meta:
    author = "Google Cloud Security"
    description = "Identify the enumeration of members within groups in Entra ID. This is based on GraphRunner Get-SecurityGroups function behavior so modifications may be needed for other scenarios"
    rule_id = "mr_a7af3e4e-19b0-4102-a794-ed2ca151df8c"
    rule_name = "Hunt for group members enumeration in the Microsoft Graph API"
    reference = "https://github.com/dafthack/GraphRunner/blob/main/GraphRunner.ps1"
    type = "Hunt"
    data_source = "MS Graph Activity Logs"
    platform = "Azure"
    severity = "Low"
    priority = "Low"

  events:
    $api.metadata.event_type = "NETWORK_HTTP"
    $api.metadata.product_event_type = "Microsoft Graph Activity"
    re.regex($api.target.url, `^https://graph.microsoft.com/v1.0/groups/.*/members$`) nocase
    //Can be tuned for specific User Agent strings being used
    //$api.network.http.user_agent = /PowerShell/ nocase
    $api.network.http.method = "GET"
    $api.network.http.response_code = 200
    $api.principal.ip[0] = $ip
    $api.network.session_id = $session
    re.capture($api.target.url, `^https://graph.microsoft.com/v1.0/groups/(.*)/members$`) = $group_guid

  match:
    $ip, $session over 5m

  outcome:
    $risk_score = 35
    $event_count = count_distinct($api.metadata.id)
    $requesting_user_guid = array_distinct($api.principal.user.userid)
    //Using $ip placeholder variable since we specify first value in repeated field only, if repeated field is used, it can cause duplicate counts in sums below
    $requesting_ip = array_distinct($ip)
    $user_agent = array_distinct($api.network.http.user_agent)
    $location = array_distinct($api.principal.location.name)
    $target_application_id_guid = array_distinct($api.target.resource.product_object_id)
    $session_id = array_distinct($api.network.session_id)
    $group_count = count_distinct($group_guid)
    $group_list = array_distinct($group_guid) //limited to 25 groups outputted to screen

  condition:
    //option to add additional threshold here for number of groups enumerated if desired
    $api //and $group_count > 10
}