Network Connection First Seen In Past Day

// Copyright 2023 Google LLC. Licensed under Apache-2.0.

rule network_connection_first_seen_in_past_day {

  meta:
    author = "Google Cloud Security"
    description = "Detect network connection to an ip address that was only seen for the first time in the past 24 hours"
    rule_id = "mr_c3c9e4d3-eaa5-4f5b-9c64-70ce93247c51"
    rule_name = "Network Connection First Seen In Past Day"
    type = "alert"
    tags = "first last seen"
    data_source = "microsoft windows , gcp firewall, microsoft sysmon, crowdstrike, gcp scc, zeek"
    severity = "Low"
    priority = "Low"

  events:
    $network.metadata.event_type = "NETWORK_CONNECTION"
    $network.target.ip = $ip
    //Target IP is not part of RFC1918 space, can modify as needed
    not net.ip_in_range_cidr ($network.target.ip, "10.0.0.0/24")
    not net.ip_in_range_cidr ($network.target.ip, "172.16.0.0/20")
    not net.ip_in_range_cidr ($network.target.ip, "192.168.0.0/16")

    //derived from events ingested by Chronicle
    $entity.graph.entity.artifact.ip  = $ip
    $entity.graph.metadata.entity_type = "IP_ADDRESS"
    $entity.graph.metadata.source_type = "DERIVED_CONTEXT"
    $entity.graph.entity.artifact.first_seen_time.seconds > 0

    //calculate the different between current time and the first time the ip was listed as seen in the entity graph
    86400 > timestamp.current_seconds() - $entity.graph.entity.artifact.first_seen_time.seconds

  match:
    $ip over 24h

  outcome:
    $risk_score = 35
    $event_count = count_distinct($network.metadata.id)
    //added to populate alert graph with additional context
    $principal_ip = array_distinct($network.principal.ip)
    // Commented out target.ip because it is already represented in graph as match variable. If match changes, can uncomment to add to results
    //$target_ip = array_distinct($network.target.ip)
    $principal_process_pid = array_distinct($network.principal.process.pid)
    $principal_process_command_line = array_distinct($network.principal.process.command_line)
    $principal_process_file_sha256 = array_distinct($network.principal.process.file.sha256)
    $principal_process_file_full_path = array_distinct($network.principal.process.file.full_path)
    $principal_process_product_specific_process_id = array_distinct($network.principal.process.product_specific_process_id)
    $principal_process_parent_process_product_specific_process_id = array_distinct($network.principal.process.parent_process.product_specific_process_id)
    $target_process_pid = array_distinct($network.target.process.pid)
    $target_process_command_line = array_distinct($network.target.process.command_line)
    $target_process_file_sha256 = array_distinct($network.target.process.file.sha256)
    $target_process_file_full_path = array_distinct($network.target.process.file.full_path)
    $target_process_product_specific_process_id = array_distinct($network.target.process.product_specific_process_id)
    $target_process_parent_process_product_specific_process_id = array_distinct($network.target.process.parent_process.product_specific_process_id)
    $principal_user_userid = array_distinct($network.principal.user.userid)
    $target_user_userid = array_distinct($network.target.user.userid)

  condition:
    $network and $entity
}