/analyzing-powershell-script-block-logging

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

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.