Suspicious Encoded PowerShell Command Line

About the rule

Rule Type

Standard

Rule Description

This rule detects PowerShell processes that use base64-encoded commands, often seen in malware campaigns like Emotet. Attackers use encoded strings to obfuscate their intent, hiding malicious scripts from traditional command-line monitoring tools and security software.

Severity

Trouble

Rule journey

Attack chain scenario

Initial access via malicious Office document → Triggers hidden PowerShell with -enc flag → Decodes and executes base64 command to fetch malware → Installs backdoor → Lateral movement

Impact

• Data theft
• Ransomware deployment
• Backdoor creation

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 PowerShell executions (e.g., powershell.exe or pwsh.exe) with -enc or -e flag followed by suspicious base64 strings. This enables you to disable or restrict PowerShell usage where not needed.

Author

Florian Roth (Nextron Systems), Markus Neis, Jonhnathan Ribeiro, Daniil Yugoslavskiy, Anton Kutepov, oscd.community

Future actions

Known False Positives

This rule might be triggered due to some IT automation tools and scripts that use encoding legitimately.

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: Monitor PowerShell invocations with encoded commands.

Mitigation