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Detection rules › YARA-L

Google Safebrowsing File Contacts Tor Exit Node

Severity
critical
Type
alert
Time window
1h
Match by
hostname
Author
Google Cloud Security
Source
github.com/chronicle/detection-rules

A malicious file contacting a known Tor Exit Node.

MITRE ATT&CK coverage

TacticTechniques
Command & Control

Telemetry coverage

ProviderRecord / event type
SysmonEvent ID 3: Network connection
Security-AuditingEvent ID 5156: The Windows Filtering Platform has permitted a connection.

Rule body

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

rule google_safebrowsing_file_contacts_tor_exit_node {

  meta:
    author = "Google Cloud Security"
    description = "A malicious file contacting a known Tor Exit Node."
    rule_id = "mr_f6733319-50b3-477e-bad6-96bb4c14c632"
    rule_name = "Google Safebrowsing File Contacts Tor Exit Node"
    tactic = "TA0011"
    technique = "T1090.003"
    type = "alert"
    data_source = "microsoft defender atp, crowdstrike, microsoft sysmon"
    severity = "Critical"
    priority = "High"

  events:
    $network.metadata.event_type = "NETWORK_CONNECTION"
    $network.principal.process.file.sha256 != ""
    $network.principal.process.file.sha256 = $file_hash
    $network.principal.hostname = $hostname
    $network.target.ip = $ip

    // Safe Browsing file hashes provided by GCTI Feed
    $safebrowse.graph.metadata.product_name = "Google Safe Browsing"
    $safebrowse.graph.metadata.entity_type = "FILE"
    $safebrowse.graph.metadata.source_type = "GLOBAL_CONTEXT"
    $safebrowse.graph.metadata.threat.category = "SOFTWARE_MALICIOUS"
    $safebrowse.graph.entity.file.sha256 = $file_hash

    // Tor IP listing provided by GCTI Feed
    $gcti.graph.entity.artifact.ip = $ip
    $gcti.graph.metadata.entity_type = "IP_ADDRESS"
    $gcti.graph.metadata.source_type = "GLOBAL_CONTEXT"
    $gcti.graph.metadata.threat.threat_feed_name = "Tor Exit Nodes"
    $gcti.graph.metadata.vendor_name = "Google Cloud Threat Intelligence"
    $gcti.graph.metadata.product_name = "GCTI Feed"

  match:
    $hostname over 1h

  outcome:
    $risk_score = 95
    $event_count = count_distinct($network.metadata.id)
    //added to populate alert graph with additional context
    $principal_ip = array_distinct($network.principal.ip)
    $target_ip = array_distinct($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 $gcti and $safebrowse
}

YARA-L rule structure

Condition

Fires when at least one $network event in the 1h window and $gcti matches entity context and $safebrowse matches entity context.

Events

$gcti entity context (IP_ADDRESS)

  • graph.metadata.entity_type = "IP_ADDRESS"
  • graph.metadata.source_type = "GLOBAL_CONTEXT"
  • graph.metadata.threat.threat_feed_name = "Tor Exit Nodes"
  • graph.metadata.vendor_name = "Google Cloud Threat Intelligence"
  • graph.metadata.product_name = "GCTI Feed"

$network NETWORK_CONNECTION (3, 5156)

  • principal.process.file.sha256 is not null

$safebrowse entity context (FILE)

  • graph.metadata.product_name = "Google Safe Browsing"
  • graph.metadata.entity_type = "FILE"
  • graph.metadata.source_type = "GLOBAL_CONTEXT"
  • graph.metadata.threat.category = "SOFTWARE_MALICIOUS"

Match and correlation

Match key
$hostname (principal.hostname)
Within
1h

Outcome

Outcome variables add context to a detection. When the rule generates an alert, $risk_score is displayed with it.

FieldExpression
risk_score95
event_countcount_distinct($network.metadata.id)
principal_iparray_distinct($network.principal.ip)
target_iparray_distinct($network.target.ip)
principal_process_pidarray_distinct($network.principal.process.pid)
principal_process_command_linearray_distinct($network.principal.process.command_line)
principal_process_file_sha256array_distinct($network.principal.process.file.sha256)
principal_process_file_full_patharray_distinct($network.principal.process.file.full_path)
principal_process_product_specific_process_idarray_distinct($network.principal.process.product_specific_process_id)
principal_process_parent_process_product_specific_process_idarray_distinct($network.principal.process.parent_process.product_specific_process_id)
target_process_command_linearray_distinct($network.target.process.command_line)
target_process_file_sha256array_distinct($network.target.process.file.sha256)
target_process_file_full_patharray_distinct($network.target.process.file.full_path)
target_process_product_specific_process_idarray_distinct($network.target.process.product_specific_process_id)
target_process_parent_process_product_specific_process_idarray_distinct($network.target.process.parent_process.product_specific_process_id)
principal_user_useridarray_distinct($network.principal.user.userid)
target_user_useridarray_distinct($network.target.user.userid)