Hunt for Undocumented API - Estimate Access called in Microsoft Graph API

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rule ms_graph_estimate_access {

  meta:
    author = "Google Cloud Security"
    description = "Estimate Access is an undocumented API in MS Graph and is used in the following GraphRunner functions Get-UpdatableGroups, Get-DynamicGroups and Invoke-GraphRecon. Although not inherently malicious, it does submit an action and then determines if the user can perform it"
    rule_id = "mr_381921b8-e935-4720-9ff6-2d966aa00a32"
    rule_name = "Hunt for Undocumented API - Estimate Access called in Microsoft Graph API"
    assumption = "This can be chatty due to other commands calling this single API. Additional tuning of admin consoles and user agent strings may be needed."
    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"
    $api.target.url = "https://graph.microsoft.com/beta/roleManagement/directory/estimateAccess" nocase
    //Can be tuned for specific User Agent strings being used - by default GraphRunner does not forge the UA for this action
    //$api_est.network.http.user_agent = /PowerShell/ nocase
    $api.network.http.method = "POST"
    $api.network.http.response_code = 200
    //Used to filter admin consoles and/or internal Microsoft addresses as needed, the following IP CIDR blocks are part of Azure for instance
    //$api.principal.ip != "10.10.10.10"
    NOT (
        net.ip_in_range_cidr($api.principal.ip, "20.228.0.0/18") or
        net.ip_in_range_cidr($api.principal.ip, "20.228.64.0/18") or
        net.ip_in_range_cidr($api.principal.ip, "20.228.128.0/17")
    )
    /*Used to filter Microsoft IPv6 network ranges, can tune this further as needed;
      additional ranges may need to be added, but this provides an example on how this can be done. */
    NOT net.ip_in_range_cidr($api.principal.ip, "2a01:0111:f100:0000:0000:0000:0000:0000/40")
    $api.principal.ip = $ip
    $api.network.session_id = $session

  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)
    $requesting_ip = array_distinct($api.principal.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)

  condition:
    //Can be modified to add a threshold for how many estimate access events is considered accesptable
    $api // and $event_count > 10
}