Vbs Desobfuscation

VBS Desobfuscation and Analysis

A skill for analyzing, debugging, and deobfuscating VBScript files during forensic investigations.

When to Use This Skill

Use this skill when:

• You need to analyze a suspicious or malicious VBS file
• A VBS script appears obfuscated or encoded
• You're investigating malware that uses VBScript
• You need to understand what a VBS file does before execution
• You're doing incident response involving script-based attacks

Core Analysis Techniques

1. Run with cscript.exe for Debug Output

The most important first step is running the script with cscript.exe instead of wscript.exe to get console output:

# Run and capture output
cscript.exe //nologo <script.vbs>

# Run with echo statements to trace execution
cscript.exe //nologo <script.vbs> 2>&1 | tee output.log

Why this matters: cscript.exe outputs to console while wscript.exe uses GUI dialogs. For analysis, you need the console output to see what the script is doing.

2. Add Debug Echo Statements

Insert Wscript.Echo statements to trace execution flow:

' Add at key points in the script
Wscript.Echo "=== Starting execution ==="
Wscript.Echo "Variable value: " & someVariable
Wscript.Echo "About to execute: " & someFunction

Pattern: Place echo statements before and after suspicious code blocks to understand control flow.

3. Extract and Analyze Comments

VBS comments start with ' (single quote). Extract them to understand the script's intent:

# Extract all comment lines
grep "^'" script.vbs

# Or extract inline comments
grep "'" script.vbs

Why this matters: Comments often reveal the script's purpose, author notes, or even hidden functionality that the obfuscation tries to hide.

4. Write Data to Files for Analysis

When you need to inspect binary data or encoded strings, write them to files:

Function writeBinary(strBinary, strPath)
    Dim oFSO: Set oFSO = CreateObject("Scripting.FileSystemObject")
    Dim oTxtStream
    
    On Error Resume Next
    Set oTxtStream = oFSO.createTextFile(strPath)
    
    If Err.number <> 0 Then 
        Wscript.Echo "Error: " & Err.message
        Exit Function
    End If
    On Error GoTo 0
    
    Set oTxtStream = Nothing
    
    With oFSO.createTextFile(strPath)
        .Write(strBinary)
        .Close
    End With
    
    Wscript.Echo "Written to: " & strPath
End Function

Use case: When a script decodes or decrypts data, use this to capture the decoded output for analysis.

Analysis Workflow

Step 1: Initial Reconnaissance

Before running anything:

1. Check file size and encoding

   file script.vbs
   wc -l script.vbs
   

2. Look for obvious obfuscation patterns

   • Base64 encoded strings
   • Hex-encoded data
   • Eval() or Execute() calls
   • String concatenation chains
   • Chr() or Asc() functions

3. Extract comments to understand intent

Step 2: Safe Execution

Never run suspicious scripts directly on your system. Use:

• A VM with snapshots
• A sandbox environment
• Static analysis first

When you do run:

# In isolated environment
cscript.exe //nologo script.vbs 2>&1 | tee execution.log

Step 3: Trace and Debug

1. Add echo statements at entry points and before suspicious operations
2. Capture all output to a log file
3. Look for:
   • File system operations
   • Registry modifications
   • Network connections
   • Process spawning
   • Data exfiltration

Step 4: Deobfuscate

Common obfuscation patterns and how to handle them:

Common Malicious Patterns

Watch for these indicators:

• PowerShell invocation: CreateObject("WScript.Shell").Run "powershell ..."
• File downloads: CreateObject("MSXML2.XMLHTTP") or CreateObject("WinHttp.WinHttpRequest")
• Registry changes: CreateObject("WScript.Shell").RegWrite
• Persistence: Writing to startup folders or registry run keys
• Anti-analysis: WScript.Sleep, checking for VMs, timing checks

Output Format

When analyzing a VBS file, structure your findings as:

## VBS Analysis Report

### File Information
- Path: <path>
- Size: <size>
- MD5: <hash>

### Static Analysis
- Obfuscation detected: <yes/no>
- Comment count: <number>
- Suspicious patterns: <list>

### Dynamic Analysis
- Execution output: <summary>
- File operations: <list>
- Network activity: <list>
- Registry changes: <list>

### Deobfuscated Code
<key decoded sections>

### Risk Assessment
- Threat level: <low/medium/high>
- Recommended action: <action>

Safety Reminders

1. Always analyze in isolation - Use VMs or sandboxes
2. Network isolation - Disable network access during analysis
3. Take snapshots - Before running anything
4. Document everything - For incident response and reporting
5. Don't trust comments - They can be misleading

Quick Reference

# Run with output
cscript.exe //nologo script.vbs

# Extract comments
grep "'" script.vbs

# Check for base64
grep -E "[A-Za-z0-9+/]{20,}={0,2}" script.vbs

# Check for hex strings
grep -E "&H[0-9A-Fa-f]+" script.vbs

# Find Eval/Execute
grep -iE "(eval|execute)\s*\(" script.vbs

# Find network objects
grep -iE "(xmlhttp|winhttp|socket)" script.vbs

Next Steps

After initial analysis:

1. If malicious: Document indicators of compromise (IOCs)
2. If unclear: Run in deeper sandbox with network monitoring
3. If safe: Consider the script's legitimate purpose
4. Always: Report findings to appropriate security team