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Detecting Malicious PowerShell Scripts With Free Tools

PowerShell is a favorite tool for attackers. It's built into every modern Windows environment, trusted by default, and incredibly powerful for running scripts, downloading payloads, and moving laterally.


But here's the good news: you don't need expensive endpoint detection solutions to start catching malicious PowerShell activity. With a few free tools, you can immediately gain valuable visibility and improve your detection capabilities.


Here's a simple setup I've used in resource-constrained environments like K–12 schools:


Step 1: Install Sysmon for Detailed Logging

Why Sysmon?

Windows Event Logs capture some data, but Sysmon (from Microsoft's Sysinternals suite) provides much richer detail: process creation, command-line arguments, network connections, and more.


How to install Sysmon:

  1. Download from Microsoft Sysinternals: Sysmon.
  2. Run: sysmon -accepteula -i sysmonconfig.xml

(Use a community config like SwiftOnSecurity's sysmon config to get started quickly.)


What to watch for:

  • Event ID 1 (Process creation) with PowerShell command-line arguments.
  • Flags like -enc (encoded command), -nop (no profile), or -w hidden.


Step 2: Forward Logs to a Central Location

Why centralize logs?

You'll miss the bigger picture if you only look at local event logs. Forwarding logs lets you correlate and spot trends across your entire environment.


Options (all free):

  • Windows Event Forwarding (WEF): Built into Windows, no additional cost. Configure clients to forward specific events (like Sysmon Event ID 1).
  • Central server/collector: A simple Windows Server VM can be your log collector.
  • Elastic Stack (free tier) or Wazuh: Both offer more advanced search/alerting if you're comfortable setting them up.


Step 3: Create Simple Detection Rules

You don't need a SIEM to start detecting suspicious PowerShell. Even scheduled tasks or scripts can alert you.

What to look for:

  • PowerShell with encoded commands: powershell.exe -enc <base64 string>
  • Suspicious keywords in command lines:
    • Invoke-Mimikatz
    • DownloadString
    • Invoke-Expression
  • Use regex or simple string matching in your collector/analysis tool.


Example detection with Elastic (Kibana):

Search query:

process.command_line: ("-enc" OR "Invoke-Mimikatz" OR "DownloadString")


Step 4: Test Your Setup

One of the best ways to validate detections is to simulate an attack (safely).


Example test command (non-malicious):

powershell -nop -w hidden -enc SQBFAFgAUABMAE8ASQBU

(This is just base64 for "EXPLOIT" — safe to run for testing.)


Check if your Sysmon logs capture it and whether your alert triggers.


Real-World Example

This setup caught a suspicious script trying to use Invoke-Mimikatz in one of my environments. The attempt was stopped quickly because we had visibility without a commercial EDR product.


Closing Thoughts

Education and other budget-constrained organizations often feel at a disadvantage when it comes to cybersecurity. But visibility doesn't have to cost thousands.


By combining Sysmon, Windows Event Forwarding, and simple rules, you can start detecting malicious PowerShell activity today — at zero cost.


Security isn't always about the fanciest tool. Sometimes, it's about using what you already have in more innovative ways.

Over to you: What free or low-cost tricks have you used to catch suspicious activity?

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