Suspicious PowerShell Encoded Command Patterns

About the rule

Rule Type

Standard

Rule Description

Detects the use of encoded PowerShell commands, often found in obfuscated malware execution chains. These patterns indicate attempts to hide malicious logic via base64 or partial encoding switches (-enc, -en, etc.), frequently seen in attacks.

Severity

Trouble

Rule journey

Attack chain scenario

Malicious file upload → Access gained → PowerShell stager dropped → Encoded command executed → C2 connection & data exfiltration

Impact

• In-memory malware execution
• Defense evasion
• Remote command control

Rule Requirement

Prerequisites

Process Creation Auditing

Via Windows Event Viewer (GPO)

1. Open GPMC (gpmc.msc) using a domain admin account.
2. Navigate to:
   Computer Configuration > Policies > Windows Settings > Security Settings > Advanced Audit Policy Configuration > Audit Policies > Detailed Tracking
3. Enable:
   • Audit Process Creation
   • Audit Process Termination
     (Check “Success” box for both)
4. For command-line logging:
   Go to:
   Computer Configuration > Administrative Templates > System > Audit Process Creation
   → Enable “Include command line in process creation events.”
5. Create registry key (if missing):
   Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\EventLog\Microsoft-Windows-Security-Auditing/Operational

Using Sysmon

To set up process creation monitoring with Sysmon:

1.Download and install Sysmon from Microsoft Sysinternals.

2. Open a Command Prompt with administrator privileges.

3. Create or download a Sysmon configuration file that includes process creation monitoring. A basic example for capturing all process creations is:

<Sysmon>
<EventFiltering>
<ProcessCreate onmatch="exclude"/>
</EventFiltering>
</Sysmon>

4. Install Sysmon with your configuration file using the command:

sysmon.exe -i [configfile.xml] (Replace [configfile.xml] with your file's path and name).

5. Ensure a new registry key named "Microsoft-Windows-Sysmon/Operational" exists in the directory Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\EventLog\. If not, create it.

Criteria

Detection

Execution Mode

realtime

Log Sources

Windows

MITRE ATT&CK

Execution: Command and Scripting Interpreter - PowerShell (T1059.001)

Security Standards

Enabling this rule will help you meet the security standard's requirement listed below:

NIST CSF DE.CM-1: Network and physical activities are monitored to detect anomalous events.

When this rule is triggered, you're notified of obfuscated PowerShell scripts using encoded commands with known Base64 pattern. This enables you to enable command-line auditing through process creation events (Event ID 4688).

Author

Florian Roth (Nextron Systems)

Future actions

Known False Positives

This rule might be triggered by other tools that work with encoded scripts in the command line instead of script files.

Next Steps

When this rule is triggered, the following measures can be implemented:

1. Identification: Identify if the flagged event is a new incident or part of an existing incident.
2. Analysis: Analyze the impact and extent of the incident to comprehend the severity of the attack using the Incident Workbench.
3. Response: Respond promptly by initiating an automated workflow to interrupt the network connections and cease the malicious process.
4. Audit PowerShell activities: Use PowerShell logging (Module & Script Block Logging).

Mitigation