analyzing-powershell-script-block-logging

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.

Instructions

Install dependencies: pip install python-evtx lxml
Collect PowerShell Operational logs: Microsoft-Windows-PowerShell%4Operational.evtx
Parse Event ID 4104 entries using python-evtx to extract ScriptBlockText, ScriptBlockId, and MessageNumber/MessageTotal for multi-part script reconstruction.
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)
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.