What it does. Parse Windows PowerShell Script Block Logs (Event ID 4104) from EVTX Part of mukul975/Anthropic-Cybersecurity-Skills (817 security skills) (mukul975/Anthropic-Cybersecurity-Skills).

Install

• npx skills add mukul975/Anthropic-Cybersecurity-Skills --skill analyzing-powershell-script-block-logging, or copy the skill folder into ~/.claude/skills/analyzing-powershell-script-block-logging/.
• Raw file: curl -sL https://raw.githubusercontent.com/mukul975/Anthropic-Cybersecurity-Skills/HEAD/skills/analyzing-powershell-script-block-logging/SKILL.md

SKILL.md (verbatim)

name: analyzing-powershell-script-block-logging
description: Parse Windows PowerShell Script Block Logs (Event ID 4104) from EVTX
  files to detect obfuscated commands, encoded payloads, and living-off-the-land techniques.
  Uses python-evtx to extract and reconstruct multi-block scripts, applies entropy
  analysis and pattern matching for Base64-encoded commands, Invoke-Expression abuse,
  download cradles, and AMSI bypass attempts.
domain: cybersecurity
subdomain: security-operations
tags:
- powershell
- script-block-logging
- event-id-4104
- obfuscation-detection
- windows-forensics
- endpoint-security
version: '1.0'
author: mahipal
license: Apache-2.0
nist_csf:
- DE.CM-01
- RS.MA-01
- GV.OV-01
- DE.AE-02
mitre_attack:
- T1059.001
- T1027.010
- T1140
- T1105

Analyzing PowerShell Script Block Logging

When to Use

• When investigating security incidents that require analyzing powershell script block logging
• When building detection rules or threat hunting queries for this domain
• When SOC analysts need structured procedures for this analysis type
• When validating security monitoring coverage for related attack techniques

Prerequisites

• Familiarity with security operations concepts and tools
• Access to a test or lab environment for safe execution
• Python 3.8+ with required dependencies installed
• Appropriate authorization for any testing activities

Instructions

1. Install dependencies: pip install python-evtx lxml
2. Collect PowerShell Operational logs: Microsoft-Windows-PowerShell%4Operational.evtx
3. Parse Event ID 4104 entries using python-evtx to extract ScriptBlockText, ScriptBlockId, and MessageNumber/MessageTotal for multi-part script reconstruction.
4. Apply detection heuristics:
   • Base64-encoded commands (-EncodedCommand, FromBase64String)
   • Download cradles (DownloadString, DownloadFile, Invoke-WebRequest, Net.WebClient)
   • AMSI bypass patterns (AmsiUtils, amsiInitFailed)
   • Obfuscation indicators (high entropy, tick-mark insertion, string concatenation)
5. Generate a report with reconstructed scripts, risk scores, and MITRE ATT&CK mappings.

python scripts/agent.py --evtx-file /path/to/PowerShell-Operational.evtx --output ps_analysis.json

Examples

Detect Encoded Command Execution

import base64
if "-encodedcommand" in script_text.lower():
    encoded = script_text.split()[-1]
    decoded = base64.b64decode(encoded).decode("utf-16-le")

Reconstruct Multi-Block Script

Scripts split across multiple 4104 events share a ScriptBlockId. Concatenate blocks ordered by MessageNumber to recover the full script.

Other files in this skill

• LICENSE
• references/api-reference.md
• scripts/agent.py

references/api-reference.md (verbatim)

API Reference: PowerShell Script Block Logging Analysis

python-evtx Library

FileHeader

from Evtx.Evtx import FileHeader
with open(evtx_path, "rb") as f:
    fh = FileHeader(f)
    for record in fh.records():
        xml_string = record.xml()  # Returns XML string of the event

Event XML Structure (Event ID 4104)

<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <EventID>4104</EventID>
    <TimeCreated SystemTime="2024-01-15T10:30:00.000Z"/>
  </System>
  <EventData>
    <Data Name="MessageNumber">1</Data>
    <Data Name="MessageTotal">3</Data>
    <Data Name="ScriptBlockText">...powershell code...</Data>
    <Data Name="ScriptBlockId">guid-string</Data>
    <Data Name="Path">C:\script.ps1</Data>
  </EventData>
</Event>

lxml etree Parsing

from lxml import etree
NS = {"evt": "http://schemas.microsoft.com/win/2004/08/events/event"}
root = etree.fromstring(xml_bytes)
event_id = root.find(".//evt:System/evt:EventID", NS).text
data_elems = root.findall(".//evt:EventData/evt:Data", NS)
for elem in data_elems:
    name = elem.get("Name")
    value = elem.text

Script Block Reconstruction

Large PowerShell scripts are split across multiple Event 4104 entries:

• ScriptBlockId: Unique GUID shared across all parts
• MessageNumber: Part index (1-based)
• MessageTotal: Total number of parts
• Reconstruct: concatenate parts ordered by MessageNumber

Key Detection Patterns

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