Potential Dynamic IEX Reconstruction via Environment Variables

 1[metadata]
 2creation_date = "2025/04/16"
 3integration = ["windows"]
 4maturity = "production"
 5updated_date = "2025/04/16"
 6
 7[rule]
 8author = ["Elastic"]
 9description = """
10Identifies PowerShell scripts that reconstruct the IEX (Invoke-Expression) command at runtime using indexed slices of
11environment variables. This technique leverages character access and join operations to build execution logic
12dynamically, bypassing static keyword detection and evading defenses such as AMSI.
13"""
14from = "now-9m"
15language = "esql"
16license = "Elastic License v2"
17name = "Potential Dynamic IEX Reconstruction via Environment Variables"
18risk_score = 21
19rule_id = "b0c98cfb-0745-4513-b6f9-08dddb033490"
20setup = """## Setup
21
22The 'PowerShell Script Block Logging' logging policy must be enabled.
23Steps to implement the logging policy with Advanced Audit Configuration:

1
2Steps to implement the logging policy via registry:

 1"""
 2severity = "low"
 3tags = [
 4    "Domain: Endpoint",
 5    "OS: Windows",
 6    "Use Case: Threat Detection",
 7    "Tactic: Defense Evasion",
 8    "Data Source: PowerShell Logs",
 9]
10timestamp_override = "event.ingested"
11type = "esql"
12
13query = '''
14FROM logs-windows.powershell_operational* metadata _id, _version, _index
15| WHERE event.code == "4104"
16
17// Look for scripts with more than 500 chars that contain a related keyword
18| EVAL script_len = LENGTH(powershell.file.script_block_text)
19| WHERE script_len > 500
20
21// Replace string format expressions with 🔥 to enable counting the occurrence of the patterns we are looking for
22// The emoji is used because it's unlikely to appear in scripts and has a consistent character length of 1
23| EVAL replaced_with_fire = REPLACE(powershell.file.script_block_text, """(?i)(\$(?:\w+|\w+\:\w+)\[\d++\]\+\$(?:\w+|\w+\:\w+)\[\d++\]\+['"]x['"]|\$(?:\w+\:\w+)\[\d++,\d++,\d++\]|\.name\[\d++,\d++,\d++\])""", "🔥")
24
25// Count how many patterns were detected by calculating the number of 🔥 characters inserted
26| EVAL count = LENGTH(replaced_with_fire) - LENGTH(REPLACE(replaced_with_fire, "🔥", ""))
27
28// Keep the fields relevant to the query, although this is not needed as the alert is populated using _id
29| KEEP count, replaced_with_fire, powershell.file.script_block_text, powershell.file.script_block_id, file.path, powershell.sequence, powershell.total, _id, _index, host.name, agent.id, user.id
30| WHERE count >= 1
31'''
32
33
34[[rule.threat]]
35framework = "MITRE ATT&CK"
36[[rule.threat.technique]]
37id = "T1027"
38name = "Obfuscated Files or Information"
39reference = "https://attack.mitre.org/techniques/T1027/"
40
41[[rule.threat.technique]]
42id = "T1140"
43name = "Deobfuscate/Decode Files or Information"
44reference = "https://attack.mitre.org/techniques/T1140/"
45
46
47[rule.threat.tactic]
48id = "TA0005"
49name = "Defense Evasion"
50reference = "https://attack.mitre.org/tactics/TA0005/"
51[[rule.threat]]
52framework = "MITRE ATT&CK"
53[[rule.threat.technique]]
54id = "T1059"
55name = "Command and Scripting Interpreter"
56reference = "https://attack.mitre.org/techniques/T1059/"
57[[rule.threat.technique.subtechnique]]
58id = "T1059.001"
59name = "PowerShell"
60reference = "https://attack.mitre.org/techniques/T1059/001/"
61
62
63
64[rule.threat.tactic]
65id = "TA0002"
66name = "Execution"
67reference = "https://attack.mitre.org/tactics/TA0002/"