PowerShell Obfuscation via Character Array Reconstruction

This detection identifies PowerShell scripts employing character array reconstruction to obfuscate their contents. This technique involves building strings from char[] arrays, index lookups, or repeated ([char]NN)+ concatenation/join operations. Threat actors leverage this method to conceal malicious commands, URLs, or payloads, making them difficult to detect through static analysis and AMSI (Anti-Malware Scan Interface). The rule focuses on identifying scripts containing these character array manipulation patterns, enabling security teams to uncover potentially malicious PowerShell activity that would otherwise be missed. This technique is especially useful for attackers to evade detection in environments where PowerShell logging is enabled but not actively monitored for obfuscated code.

Attack Chain

1. Initial Access: The attacker gains initial access through various means, such as phishing emails, compromised credentials, or exploiting software vulnerabilities.
2. Payload Delivery: The attacker delivers a PowerShell script containing obfuscated code using character array reconstruction.
3. Obfuscation: The PowerShell script utilizes character array manipulation to construct malicious commands, URLs, or payloads dynamically.
4. Defense Evasion: The character array reconstruction technique bypasses static analysis and AMSI, hindering traditional security measures.
5. Execution: The script executes the reconstructed commands, potentially downloading and executing additional payloads or performing malicious actions on the system.
6. Persistence: The attacker may establish persistence by creating scheduled tasks or modifying registry keys to ensure the script runs automatically.
7. Command and Control: The script communicates with a command and control (C2) server to receive further instructions and exfiltrate sensitive data.
8. Impact: The attacker achieves their objective, which could include data theft, system compromise, or ransomware deployment.

Impact

Successful exploitation allows attackers to execute arbitrary code on the compromised system, potentially leading to data theft, system compromise, or ransomware deployment. The use of character array reconstruction significantly increases the likelihood of bypassing traditional security measures and successfully executing malicious actions. The severity of the impact depends on the attacker’s objectives and the level of access they gain on the compromised system.

Recommendation

• Enable PowerShell script block logging to capture the necessary events for detection. Refer to the setup instructions in the rule details.
• Deploy the provided Sigma rule to your SIEM and tune it for your environment to minimize false positives.
• Investigate alerts generated by the Sigma rule to identify potentially malicious PowerShell scripts using character array reconstruction. Focus on analyzing the reconstructed strings and the script’s overall behavior.
• Implement strict PowerShell execution policies to restrict the execution of unsigned or untrusted scripts.
• Monitor for suspicious process creations originating from PowerShell, such as spawning command-line interpreters or executing system utilities.
• Block known malicious domains and IP addresses associated with command and control servers.