Potential PowerShell Obfuscation Via Reversed Commands

About the rule

Rule Type

Standard

Rule Description

This rule detects the use of reversed strings in PowerShell command lines, such as hctac (catch) or dnammoc (command). Attackers often reverse commands as a simple obfuscation technique to evade detection by signature-based security tools and make scripts harder to analyze. By reversing common keywords and functions, malicious code can bypass basic pattern-matching defenses while still executing normally once reversed in memory.

Severity

Trouble

Rule journey

Impact

• Execution of hidden or obfuscated code
• Bypass security standards
• Data theft

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

Defense Evasion: Obfuscated Files or Information (T1027), 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 execution of PowerShell processes containing reversed keywords and functions. This enables you to regularly audit PowerShell execution logs for uncommon patterns like reversed strings.

Author

Teymur Kheirkhabarov (idea), Vasiliy Burov (rule), oscd.community, Tim Shelton

Future actions

Known False Positives

This rule might be triggered by legitimate administrative scripts using string manipulation or testing obfuscation techniques.

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: Enable PowerShell Script Block Logging to capture all executed commands.

Mitigation