Google Safebrowsing With Prevalence

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

 rule google_safebrowsing_with_prevalence {

  meta:
    author = "Google Cloud Security"
    description = "Detect process launch events that correlate to Safe Browsing hash data and have low prevalence"
    rule_id = "mr_4b77947d-b7a5-4308-b54c-a87b2b1cb80f"
    rule_name = "Google Safebrowsing With Prevalence"
    type = "alert"
    tags = "safe browsing, prevalence"
    data_source = "microsoft sysmon"
    severity = "Critical"
    priority = "High"

  events:
    $execution.metadata.event_type = "PROCESS_LAUNCH"
    $execution.principal.hostname = $hostname
    $execution.target.process.file.sha256 = $hash

    // Safe Browsing file hashes provided by GCTI Feed
    $safebrowsing.graph.metadata.entity_type = "FILE"
    $safebrowsing.graph.metadata.threat.severity = "CRITICAL"
    $safebrowsing.graph.metadata.product_name = "Google Safe Browsing"
    $safebrowsing.graph.metadata.source_type = "GLOBAL_CONTEXT"
    $safebrowsing.graph.entity.file.sha256 = $hash

    //derived from events ingested by Chronicle
    $prevalence.graph.metadata.entity_type = "FILE"
    $prevalence.graph.metadata.source_type = "DERIVED_CONTEXT"
    $prevalence.graph.entity.file.prevalence.day_count = 10
    $prevalence.graph.entity.file.prevalence.rolling_max <= 3
    $prevalence.graph.entity.file.prevalence.rolling_max > 0
    $prevalence.graph.entity.file.sha256 = $hash

  match:
    $hostname, $hash over 5m

   outcome:
    $risk_score = 95
    $event_count = count_distinct($execution.metadata.id)
    // added to populate alert graph with additional context
    // Commented out principal.hostname because it is already represented in graph as match variable. If match changes, can uncomment to add to results
    //$principal_hostname = array_distinct($execution.principal.hostname)
    $principal_process_pid = array_distinct($execution.principal.process.pid)
    $principal_process_command_line = array_distinct($execution.principal.process.command_line)
    $principal_process_file_sha256 = array_distinct($execution.principal.process.file.sha256)
    $principal_process_file_full_path = array_distinct($execution.principal.process.file.full_path)
    $principal_process_product_specific_process_id = array_distinct($execution.principal.process.product_specific_process_id)
    $principal_process_parent_process_product_specific_process_id = array_distinct($execution.principal.process.parent_process.product_specific_process_id)
    $target_process_pid = array_distinct($execution.target.process.pid)
    $target_process_command_line = array_distinct($execution.target.process.command_line)
    $target_process_file_sha256 = array_distinct($execution.target.process.file.sha256)
    $target_process_file_full_path = array_distinct($execution.target.process.file.full_path)
    $target_process_product_specific_process_id = array_distinct($execution.target.process.product_specific_process_id)
    $principal_user_userid = array_distinct($execution.principal.user.userid)

  condition:
    $execution and $safebrowsing and $prevalence
}