Full Report
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Analysis Summary
# Tool/Technique: AsyncRAT Loader (VBScript Variant)
## Overview
This tool is a multi-stage malicious loader written in VBScript designed to deliver and execute **AsyncRAT**, a popular Remote Access Trojan. The loader utilizes heavy obfuscation—including decimal encoding, string reversal, and character replacement—to evade signature-based detection and persist on a target system via scheduled tasks or startup directories.
## Technical Details
- **Type:** Malware Loader / Persistence Script
- **Platform:** Windows (VBScript engine)
- **Capabilities:** Obfuscation, Persistence, Payload Decryption, and Execution of AsyncRAT.
- **First Seen:** May 9, 2023 (Date of Huntress report)
## MITRE ATT&CK Mapping
- **TA0002 - Execution**
- T1059.005 - Command and Scripting Interpreter: Visual Basic
- **TA0003 - Persistence**
- T1053.005 - Scheduled Task/Job: Scheduled Task
- T1547.001 - Boot or Logon Autostart Execution: Registry Run Keys / Startup Folder
- **TA0005 - Defense Evasion**
- T1027 - Obfuscated Files or Information
- T1027.013 - Encrypted/Encoded Payload
## Functionality
### Core Capabilities
- **Persistence:** Establishes a foothold by placing a `.vbs` file in the user's startup directory or creating a scheduled task.
- **Decoding Engine:** Uses a combination of `Chr()` functions and decimal values to hide malicious commands.
- **String Manipulation:** Employs `StrReverse` and `Replace` functions to further scramble the script logic, preventing easy static analysis.
### Advanced Features
- **Multi-Stage Loading:** The VBScript acts as the initial stage, which decodes an intermediate layer (often PowerShell or a .NET assembly) to eventually execute the AsyncRAT payload in memory.
- **Dynamic De-obfuscation:** The script reconstructs its primary logic at runtime, making it difficult for traditional AV to flag the file while at rest.
## Indicators of Compromise
- **File Hashes:**
- SHA256: `26c9f29fceaee8b13ba0fe4d7170f50c8046e43e11e461a43ce92b22d8e24bf5`
- **File Names:** Common randomized or masqueraded names in `%AppData%` or Startup folders (e.g., `[RandomName].vbs`).
- **Behavioral Indicators:**
- `wscript.exe` or `cscript.exe` executing scripts from non-standard locations.
- Scheduled tasks pointing to `.vbs` files in user-writable directories.
- Unexpected network connections from `powershell.exe` or `aspnet_compiler.exe` (injected processes).
## Associated Threat Actors
- **AsyncRAT Users:** While AsyncRAT is available to various cybercriminals, it is frequently used by commodity malware actors and occasionally leveraged by APTs for initial access.
## Detection Methods
- **Signature-based detection:** Modern EDRs can flag the specific combination of `Chr`, `StrReverse`, and `Execute` statements within VBScripts.
- **Behavioral detection:** Monitor for `wscript.exe` spawning shells or making network connections.
- **YARA Rule Strategy:**
- Look for high frequency of `Chr(` functions combined with large blocks of decimal integers.
- Identify the use of `Execute` or `Eval` on strings that have undergone `StrReverse`.
## Mitigation Strategies
- **Prevention:** Disable `wscript.exe` and `cscript.exe` via GPO if not required for business operations.
- **Hardening:** Use Attack Surface Reduction (ASR) rules, specifically "Block execution of potentially obfuscated scripts" and "Block JavaScript or VBScript from launching downloaded executable content."
- **Monitoring:** Audit the creation of new scheduled tasks and files appearing in `shell:startup`.
## Related Tools/Techniques
- **AsyncRAT:** The final payload (Remote Access Trojan).
- **CyberChef:** The primary tool used by analysts to de-obfuscate this loader.
- **RegExper:** Used for validating the Regular Expressions required to extract the hidden code.
- **PowerShell Loaders:** Often the second stage following the VBScript execution.