Potential POWERTRASH Script Execution

About the rule

Rule Type

Standard

Rule Description

POWERTRASH is a type of malware written in PowerShell and leveraged by the FIN7 APT group. It has been used by the group as a Trojan dropper to deliver and execute embedded payloads on target systems. This rule detects the execution of such malicious POWERTRASH scripts, indicating the potential deployment of malware in the system.

Severity

Trouble

Rule journey

Attack chain scenario

Initial access → Execution → Command and Scripting Interpreter → PowerShell script execution → Impact

Impact

• System compromise
• Malware execution
• Persistence

Rule Requirement

Prerequisites

Logon to Group Policy Management Console with administrative privileges and enable Module Logging for Windows PowerShell in the Group Policy Management Editor. Ensure to enter * in the Module Names window to record all modules. Similarly enable PowerShell Script Block Logging for Windows PowerShell. Finally, create a new registry key "Microsoft-Windows-Powershell/Operational" in the directory "Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\EventLog\".

(((( SCRIPTEXECUTED CONTAINS ""io.compression.deflatestream"" ) AND( SCRIPTEXECUTED CONTAINS ""io.memorystream"" ) AND( SCRIPTEXECUTED CONTAINS ""::frombase64string"" ) AND( SCRIPTEXECUTED CONTAINS ""getdelegateforfunctionpointer"" ) AND( SCRIPTEXECUTED CONTAINS "".invoke()"" ) AND( SCRIPTEXECUTED CONTAINS ""globalassemblycache"" ) )))

This rule is triggered when the executed script contains the following suspicious elements:

• io.compression.deflatestream: A class used to extract compressed data, typically malware files.
• io.memorystream: A class often used to load and manipulate payloads.
• ::frombase64string: A method used to turn binary data into text, used to hide malware scripts.
• getdelegateforfunctionpointer: A method often used for dynamic code executions.
• .invoke(): A method to invoke code execution.
• globalassemblycache: Refers to the .NET Global Assembly Cache which is used to load malicious .NET components.

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:

DE.CM-09: Computing hardware and software, runtime environments, and their data are monitored to find potentially adverse events.

When this rule is triggered, you're notified of a potential POWERTRASH script execution. This enables you to monitor runtime environments like PowerShell, identify POWERTRASH script activity, and detect the deployment of payloads and execution of malware.

Author

Nasreddine Bencherchali (Nextron Systems)

Future actions

Known False Positives

This rule might be triggered when admins execute automated PowerShell scripts for legitimate software deployment.

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: Continuously monitor PowerShell executions, restrict script execution privileges to administrators only, and disable PowerShell on devices where it is not required.

Mitigation