[metadata]
creation_date = "2023/07/06"
integration = ["endpoint"]
maturity = "production"
updated_date = "2026/03/24"

[rule]
author = ["Elastic"]
building_block_type = "default"
description = """
Identifies the image load of a compression DLL. Adversaries will often compress and encrypt data in preparation for
exfiltration.
"""
from = "now-119m"
index = ["logs-endpoint.events.library-*"]
interval = "60m"
language = "eql"
license = "Elastic License v2"
name = "Compression DLL Loaded by Unusual Process"
risk_score = 21
rule_id = "d197478e-39f0-4347-a22f-ba654718b148"
severity = "low"
tags = [
    "Domain: Endpoint",
    "OS: Windows",
    "Use Case: Threat Detection",
    "Tactic: Collection",
    "Data Source: Elastic Defend",
    "Rule Type: BBR",
]
timestamp_override = "event.ingested"
type = "eql"

query = '''
library where host.os.type == "windows" and event.action == "load" and
  dll.name : ("System.IO.Compression.FileSystem.ni.dll", "System.IO.Compression.ni.dll") and
  not 
  (
    (
      process.executable : (
        "?:\\Program Files\\*",
        "?:\\Program Files (x86)\\*",
        "?:\\Windows\\Microsoft.NET\\Framework*\\mscorsvw.exe",
        "?:\\Windows\\System32\\sdiagnhost.exe",
        "?:\\Windows\\System32\\inetsrv\\w3wp.exe",
        "?:\\Windows\\SysWOW64\\inetsrv\\w3wp.exe",
        "?:\\ProgramData\\Microsoft\\Windows Defender Advanced Threat Protection\\DataCollection\\*\\OpenHandleCollector.exe"
      ) and process.code_signature.trusted == true
    ) or
    (
      process.name : "NuGet.exe" and process.code_signature.trusted == true and user.id : ("S-1-5-18", "S-1-5-20")
    )
  )
'''


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

[[rule.threat.technique]]
id = "T1560"
name = "Archive Collected Data"
reference = "https://attack.mitre.org/techniques/T1560/"

[[rule.threat.technique.subtechnique]]
id = "T1560.002"
name = "Archive via Library"
reference = "https://attack.mitre.org/techniques/T1560/002/"

[rule.threat.tactic]
id = "TA0009"
name = "Collection"
reference = "https://attack.mitre.org/tactics/TA0009/"