analyzing-powershell-script-block-logging

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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: [analyzing, powershell, script, block] version: "1.0" author: mahipal license: Apache-2.0

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.