Analyzing Powershell Script Block Logging

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

Detection Gaps & Validation

• If 4104 is disabled, you have nothing: Script Block Logging must be enabled via GPO (Administrative Templates > Windows PowerShell > Turn on PowerShell Script Block Logging) or registry HKLM\...\PowerShell\ScriptBlockLogging\EnableScriptBlockLogging=1. Attackers running powershell -version 2 downgrade to an engine with no 4104/AMSI at all. Hunt for 400/600 engine-start events showing v2 and for the absence of expected 4104 volume on a host.
• AMSI bypass precedes the malicious block: amsiInitFailed, [Ref].Assembly.GetType('...AmsiUtils'), and reflection/obfuscated variants neutralize scanning. Detect the bypass string itself in 4104, since the subsequent payload may log as benign or not at all.
• Obfuscation defeats naive regex: tick marks (I+EX), string -join/-f format ops, [char] arrays, gzip+base64, and SecureString hide IEX/DownloadString. Score on Shannon entropy and decoded content, not literal keywords. Reconstruct multi-part scripts by ScriptBlockId ordered by MessageNumber before scoring — a split payload evades per-event matching.
• Watch 4103 too: Module/pipeline logging (EID 4103) captures invocation detail 4104 misses; correlate both.
• Validate the rule fires: in a lab, run an -EncodedCommand download cradle and an AMSI bypass, then confirm your parser flags both EID 4104 entries and that base64 decodes as UTF-16LE. FP tuning: legitimate admin tooling, Chocolatey, SCCM, and installers emit large/encoded blocks — baseline by signing cert and parent process before alerting.

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.