Full Report
In this blog, we look at some malicious PowerShell code breadcrumbs that one hacker left behind to unravel how they maintained access during a cyberattack.
Analysis Summary
# Tool/Technique: Weaponized PowerShell Batch Wrapper
## Overview
This technique involves the use of a multi-stage execution chain starting with a Windows Batch script (.bat) that serves as a "trigger" to launch obfuscated PowerShell commands. The primary purpose is to maintain stealth and persistence by leveraging "Living off the Land" (LotL) binaries to download and execute secondary malicious payloads while bypassing traditional security detections.
## Technical Details
- **Type:** Technique / Downloader Utility
- **Platform:** Windows
- **Capabilities:** Environment awareness (32-bit vs 64-bit detection), stealth execution (hidden windows), and process injection/redirection.
- **First Seen:** Analyzed May 4, 2021.
## MITRE ATT&CK Mapping
- **TA0002 - Execution**
- T1059.001 - Command and Scripting Interpreter: PowerShell
- T1059.003 - Command and Scripting Interpreter: Windows Command Shell
- **TA0005 - Defense Evasion**
- T1562.001 - Impair Defenses: Disable or Modify Tools
- T1027 - Obfuscated Files or Information
- T1106 - Native API
## Functionality
### Core Capabilities
- **Architecture Detection:** Uses the `[IntPtr]::Size` check (checking if the value is 4) to determine if the host is a 32-bit or 64-bit system, ensuring the correct version of PowerShell is invoked for subsequent shellcode execution.
- **Stealth Invocation:** Utilizes `cmd.exe /b` and `powershell -w hidden` flags to prevent the creation of visible console windows, hiding the execution from the end-user.
- **Environment Variable Usage:** Leverages `%COMSPEC%` to dynamically resolve the path to the command interpreter, a common evasion tactic to bypass simple path-based triggers.
### Advanced Features
- **Object Manipulation:** Creates a new `System.Diagnostics.Process` object within PowerShell to launch secondary processes with specific arguments (`WindowStyle = 'Hidden'`), further insulating the malicious activity from observation.
- **Payload Staging:** Acts as a delivery mechanism for more advanced implants (such as Cobalt Strike or custom RATs) by preparing the environment for memory-resident execution.
## Indicators of Compromise
- **File Names:** Often disguised as `.bat` or `.cmd` files in temporary directories or user profiles.
- **Network Indicators:** (Based on common patterns for this technique)
- Connections to C2 frameworks via PowerShell: `hxxps[://]shorturl[.]at/[redacted]`
- Use of `bit.ly` or `pastebin.com` for hosted stage-two scripts.
- **Behavioral Indicators:**
- `cmd.exe` spawning `powershell.exe` with `-nop -w hidden -noni -c` arguments.
- PowerShell scripts checking `[IntPtr]::Size`.
- Use of `Start-Process` with `WindowStyle` set to `Hidden`.
## Associated Threat Actors
While this specific instance was a general discovery, these techniques are synonymous with:
- **Commodity Malware Operators**
- **Cobalt Strike Users** (frequently use similar PowerShell stagers)
## Detection Methods
- **Behavioral Detection:** Monitor for child process creation where `cmd.exe` or `powershell.exe` are launched with obfuscation flags (`-enc`, `-w hidden`, `-nop`).
- **Script Block Logging:** Enable PowerShell Script Block Logging (Event ID 4104) to capture the de-obfuscated code as it is executed in memory.
- **SIEM Rules:** Alert on `[IntPtr]::Size` appearing in command-line arguments, as this is a high-fidelity indicator of architecture-aware malicious scripting.
## Mitigation Strategies
- **Execution Policy:** Set PowerShell execution policy to `AllSigned` or `Restricted` via GPO (though bypasses exist).
- **Constrained Language Mode:** Enforce PowerShell Constrained Language Mode to limit the ability to call Win32 APIs and complex .NET objects.
- **Attack Surface Reduction (ASR):** Implement ASR rules to block "executable content from email client and webmail" and "process creations originating from Windows system commands."
## Related Tools/Techniques
- **Cobalt Strike Beacon stagers**
- **Living off the Land Binaries (Lolbins)**
- **Malichus Implant**
- **AsyncRAT Loader**