PowerShell Base64 Encoded Reflective Assembly Load

About the rule

Rule Type

Standard

Rule Description

Reflective Assembly Loading is a fileless attack method where a .NET assembly is loaded directly into memory without touching disk. When combined with Base64 encoding in PowerShell, attackers can bypass AV and EDR detection, execute payloads covertly, and avoid leaving artifacts on the system.

Severity

Trouble

Rule journey

Attack chain scenario

Malicious update uploaded → Encoded PowerShell executed → Base64 script loads .NET assembly in memory → Payload runs in RAM → Persistence established

Impact

• Deploy in-memory malware
• Privilege escalation
• Execution of additional payloads

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)
, Defense Evasion: Reflective Code Loading (T1620)

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 encoded PowerShell commands containing the term Invoke-. This enables you to monitor for suspicious Base64 encoded PowerShell patterns in process command lines.

Author

Christian Burkard (Nextron Systems), pH-T (Nextron Systems)

Future actions

Known False Positives

This rule might be triggered by legitimate security testing tools using in-memory .NET assemblies.

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 and Constrained Language Mode.

Mitigation