How to Detect Malicious PowerShell Activity Using Log Analysis

PowerShell is a powerful tool often exploited by attackers to execute malicious scripts, download payloads, or control systems remotely. Detecting its misuse requires monitoring specific logs and analyzing suspicious behavior.

---

1. Enable PowerShell Script Block Logging

What to Do:

• Enable Script Block Logging:

  • Ensure that PowerShell Script Block Logging is activated to capture script content in logs (Event ID 4104).

• Analyze Script Content:

  • Look for:

    • Obfuscated commands (e.g., Base64-encoded scripts).

    • Known malicious commands or behaviors, such as downloading files or disabling security tools.

Red Flags: Scripts containing obfuscation techniques, suspicious commands, or references to known malicious domains.

---

2. Monitor for PowerShell Executions with High Privileges

What to Look For:

• Process Creation Events:

  • Event ID 4688: Tracks process creation. Focus on:

    • Instances where powershell.exe is executed with administrative privileges.

    • PowerShell executions from non-standard locations or by unauthorized accounts.

Red Flags: PowerShell processes initiated with elevated privileges or spawned by non-administrative accounts.

---

3. Detect Encoded Commands

What to Look For:

• Encoded Commands:

  • Examine command lines for the -EncodedCommand parameter. This is often used to obfuscate malicious scripts.

• Decoding Activity:

  • Decode Base64-encoded commands to reveal their intent and content.

Red Flags: Encoded commands containing malicious operations like downloading files, disabling security, or lateral movement.

---

4. Check for Download and Execution

What to Look For:

• Download Commands:

  • Look for PowerShell using commands such as:

    • Invoke-WebRequest

    • New-Object System.Net.WebClient

    • Invoke-Expression

• Command Context:

  • Monitor for remote URLs or IPs embedded within the command lines.

Red Flags: PowerShell commands downloading files from suspicious or unrecognized external sources.

---

5. Review for PowerShell Remoting Activity

What to Look For:

• Remoting Commands:

  • Event ID 4104: Logs the use of remoting commands like:

    • Enter-PSSession

    • Invoke-Command

  • These may indicate an attacker attempting to control remote systems.

• Execution Context:

  • Correlate with authentication logs to validate whether the remoting activity is legitimate.

Red Flags: Unexpected remoting commands initiated by non-administrative accounts or targeting critical systems.

---

6. Immediate Mitigation

What to Do:

• Terminate Malicious Processes:

  • Stop identified malicious PowerShell processes immediately to prevent further execution.

• Isolate Affected Systems:

  • Disconnect compromised systems from the network to contain potential lateral movement or data exfiltration.

• Analyze the Impact:

  • Investigate the script’s purpose and impact on the system, such as file modifications or service disruptions.

• Strengthen PowerShell Security:

  • Implement stricter execution policies, such as:

    • Restricting to signed scripts only.

    • Enforcing Constrained Language Mode for non-administrative users.

• Audit for Unauthorized Changes:

  • Review logs for additional signs of compromise or persistence mechanisms.

Post-Incident Steps:

• Update Monitoring Rules:

  • Add detection rules for similar patterns in SIEM or endpoint protection tools.

• Educate Users and IT Staff:

  • Train teams on secure PowerShell usage and recognizing malicious behavior.

• Deploy Advanced Security Tools:

  • Use solutions like Microsoft Defender for Endpoint to monitor PowerShell activity in real time.

---

Conclusion

By systematically analyzing PowerShell execution logs, process creation events, and script block logs, malicious activity can be detected early. Implementing proactive measures, such as script restrictions and logging, minimizes the risk of PowerShell-based attacks.

---