rules/linux/persistence_unusual_exim4_child_process.toml

[metadata]
creation_date = "2025/04/30"
integration = ["endpoint"]
maturity = "production"
updated_date = "2025/04/30"

[rule]
author = ["Elastic"]
description = """
This rule detects the execution of unusual commands via a descendant process of exim4. Attackers may use descendant
processes of exim4 to evade detection and establish persistence or execute post-exploitation commands on a target system.
"""
from = "now-9m"
index = ["logs-endpoint.events.process*"]
language = "kuery"
license = "Elastic License v2"
name = "Unusual Exim4 Child Process"
references = [
  "https://www.qualys.com/2021/05/04/21nails/21nails.txt",
  "https://blog.qualys.com/vulnerabilities-threat-research/2021/05/04/21nails-multiple-vulnerabilities-in-exim-mail-server"
]
risk_score = 21
rule_id = "6eb862bb-013d-4d4f-a14b-341433ca1a1f"
severity = "low"
tags = [
    "Domain: Endpoint",
    "OS: Linux",
    "Use Case: Threat Detection",
    "Tactic: Persistence",
    "Data Source: Elastic Defend",
]
timestamp_override = "event.ingested"
type = "new_terms"
query = '''
host.os.type:linux and event.type:start and event.action:exec and process.parent.name:exim4 and
not process.name:(
  exim4 or start-stop-daemon or run-parts or systemctl or update-exim4.conf or install or plymouth or
  readlink or grep or stat or cmake or gcc or cppcheck or sort or sshd
)
'''

[[rule.threat]]
framework = "MITRE ATT&CK"

[[rule.threat.technique]]
id = "T1037"
name = "Boot or Logon Initialization Scripts"
reference = "https://attack.mitre.org/techniques/T1037/"

[[rule.threat.technique]]
id = "T1554"
name = "Compromise Host Software Binary"
reference = "https://attack.mitre.org/techniques/T1554/"

[rule.threat.tactic]
id = "TA0003"
name = "Persistence"
reference = "https://attack.mitre.org/tactics/TA0003/"

[rule.new_terms]
field = "new_terms_fields"
value = ["process.executable"]

[[rule.new_terms.history_window_start]]
field = "history_window_start"
value = "now-14d"