IOC Hash Prevalence

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

rule ioc_hash_prevalence {

  meta:
    author = "Google Cloud Security"
    description = "Detect process/file events that match hashes found in our MISP instance and have low rolling max prevalence"
    rule_id = "mr_58c661e3-e609-4df8-ae36-e85480e62a50"
    rule_name = "IOC Hash Prevalence"
    type = "alert"
    tags = "prevalence, threat indicators"
    assumption = "Assumes MISP data has been ingested into entity graph; this rule can be modified to utilize other TI indicators"
    data_source = "microsoft sysmon"
    severity = "High"
    priority = "High"

  events:
    $process.metadata.event_type = "FILE_CREATION" or $process.metadata.event_type = "PROCESS_LAUNCH"
    $process.target.process.file.sha256 = $sha256

    // derived from events ingested by Google SecOps
    $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 = $sha256

    // Correlates with MISP data; can be modified based on your MISP parser or other TI
    $ioc.graph.metadata.entity_type = "FILE"
    $ioc.graph.metadata.source_type = "ENTITY_CONTEXT"
    $ioc.graph.metadata.product_name = "MISP"
    $ioc.graph.entity.file.sha256 = $sha256

  match:
    $sha256 over 5m

  outcome:
    $risk_score = 85
    $event_count = count_distinct($process.metadata.id)
    // added to populate alert graph with additional context
    $principal_hostname = array_distinct($process.principal.hostname)
    $principal_process_pid = array_distinct($process.principal.process.pid)
    $principal_process_command_line = array_distinct($process.principal.process.command_line)
    $principal_process_file_sha256 = array_distinct($process.principal.process.file.sha256)
    $principal_process_file_full_path = array_distinct($process.principal.process.file.full_path)
    $principal_process_product_specific_process_id = array_distinct($process.principal.process.product_specific_process_id)
    $principal_process_parent_process_product_specific_process_id = array_distinct($process.principal.process.parent_process.product_specific_process_id)
    $target_process_pid = array_distinct($process.target.process.pid)
    $target_process_command_line = array_distinct($process.target.process.command_line)
    // Commented out target.process.file.sha256 because it is already represented in graph as match variable. If match changes, can uncomment to add to results
    $target_process_file_sha256 = array_distinct($process.target.process.file.sha256)
    $target_process_file_full_path = array_distinct($process.target.process.file.full_path)
    $target_process_product_specific_process_id = array_distinct($process.target.process.product_specific_process_id)
    $principal_user_userid = array_distinct($process.principal.user.userid)
    $target_file_sha256 = array_distinct($process.target.file.sha256)
    $target_file_full_path = array_distinct($process.target.file.full_path)

  condition:
    $process and $ioc and $prevalence
}