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

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