deobfuscating-javascript-malware

What this skill does

# Deobfuscating JavaScript Malware

## When to Use

- Investigating a phishing page with obfuscated JavaScript that performs credential harvesting or redirect
- Analyzing a web skimmer (Magecart-style) injected into an e-commerce site
- Deobfuscating a JavaScript dropper that downloads and executes second-stage malware
- Examining malicious email attachments containing HTML files with embedded obfuscated scripts
- Analyzing browser exploit kits that use heavy JavaScript obfuscation to hide exploit delivery

**Do not use** for obfuscated JavaScript that is merely minified production code; use a standard beautifier instead.

## Prerequisites

- Node.js 18+ installed for executing and debugging JavaScript in a controlled environment
- Python 3.8+ with `jsbeautifier` library for code formatting
- Browser developer tools (Chrome DevTools) for controlled execution in an isolated browser
- CyberChef (https://gchq.github.io/CyberChef/) for encoding/decoding operations
- de4js or JStillery for automated JavaScript deobfuscation
- Isolated analysis VM with no access to production systems or sensitive data

## Workflow

### Step 1: Safely Extract and Examine the Obfuscated Script

Isolate the malicious JavaScript without executing it:

```bash
# Extract JavaScript from HTML file
python3 << 'PYEOF'
from html.parser import HTMLParser

class ScriptExtractor(HTMLParser):
    def __init__(self):
        super().__init__()
        self.in_script = False
        self.scripts = []
        self.current = ""

    def handle_starttag(self, tag, attrs):
        if tag == "script":
            self.in_script = True
            self.current = ""

    def handle_endtag(self, tag):
        if tag == "script":
            self.in_script = False
            if self.current.strip():
                self.scripts.append(self.current)

    def handle_data(self, data):
        if self.in_script:
            self.current += data

with open("malicious_page.html") as f:
    parser = ScriptExtractor()
    parser.feed(f.read())

for i, script in enumerate(parser.scripts):
    with open(f"script_{i}.js", "w") as f:
        f.write(script)
    print(f"Extracted script_{i}.js ({len(script)} bytes)")
PYEOF

# Beautify the extracted JavaScript
npx js-beautify script_0.js -o script_0_pretty.js
```

### Step 2: Identify Obfuscation Techniques

Categorize the obfuscation methods used:

```
Common JavaScript Obfuscation Techniques:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
String Encoding:
  - Hex encoding:          "\x68\x65\x6c\x6c\x6f" -> "hello"
  - Unicode escapes:       "\u0068\u0065\u006c\u006c\u006f" -> "hello"
  - Base64:                atob("aGVsbG8=") -> "hello"
  - charCodeAt/fromCharCode: String.fromCharCode(104,101,108,108,111)
  - Array-based lookup:    var _0x1234 = ["hello","world"]; _0x1234[0]

Eval Chains:
  - eval(atob("..."))
  - eval(unescape("..."))
  - new Function("return " + decoded)()
  - document.write("<script>" + decoded + "</script>")
  - setTimeout(decoded, 0)

Control Flow:
  - Switch-case dispatcher with shuffled case order
  - Opaque predicates (always-true/false conditions)
  - Dead code insertion
  - Variable name mangling (_0x4a3b, _0xab12)

Anti-Analysis:
  - Debugger traps: setInterval(function(){debugger;}, 100)
  - Console detection: overriding console.log
  - Timing checks: performance.now() deltas
  - DevTools detection: window.outerWidth - window.innerWidth > 100
```

### Step 3: Remove Anti-Analysis Protections

Neutralize anti-debugging and anti-analysis traps:

```javascript
// Remove debugger traps before analysis
// Replace in the obfuscated script:

// Before:
setInterval(function() { debugger; }, 100);

// After (neutralized):
setInterval(function() { /* debugger removed */ }, 100);

// Neutralize DevTools detection
// Before:
if (window.outerWidth - window.innerWidth > 160) { window.location = "about:blank"; }

// After:
if (false) { window.location = "about:blank"; }

// Neutralize timing checks
// Override performance.now to return consistent values
const originalNow = performance.now;
performance.now = function() { return 0; };
```

### Step 4: Decode String Obfuscation Layers

Progressively decode encoded strings:

```python
# Python script to decode common JS obfuscation patterns
import re
import base64
import urllib.parse

def decode_hex_strings(code):
    """Replace \\xNN sequences with ASCII characters"""
    def hex_replace(match):
        hex_str = match.group(0)
        try:
            return bytes.fromhex(hex_str.replace("\\x", "")).decode("ascii")
        except:
            return hex_str
    return re.sub(r'(?:\\x[0-9a-fA-F]{2})+', hex_replace, code)

def decode_unicode_escapes(code):
    """Replace \\uNNNN sequences with characters"""
    def unicode_replace(match):
        return chr(int(match.group(1), 16))
    return re.sub(r'\\u([0-9a-fA-F]{4})', unicode_replace, code)

def decode_charcode_arrays(code):
    """Resolve String.fromCharCode calls"""
    def charcode_replace(match):
        codes = [int(c.strip()) for c in match.group(1).split(",")]
        return '"' + "".join(chr(c) for c in codes) + '"'
    return re.sub(r'String\.fromCharCode\(([0-9,\s]+)\)', charcode_replace, code)

def decode_base64_strings(code):
    """Resolve atob() calls with static strings"""
    def atob_replace(match):
        try:
            decoded = base64.b64decode(match.group(1)).decode("utf-8")
            return f'"{decoded}"'
        except:
            return match.group(0)
    return re.sub(r'atob\(["\']([A-Za-z0-9+/=]+)["\']\)', atob_replace, code)

# Apply all decoders
with open("script_0.js") as f:
    code = f.read()

code = decode_hex_strings(code)
code = decode_unicode_escapes(code)
code = decode_charcode_arrays(code)
code = decode_base64_strings(code)

with open("script_0_decoded.js", "w") as f:
    f.write(code)
print("Decoded strings written to script_0_decoded.js")
```

### Step 5: Resolve Eval Chains Safely

Unwrap eval/Function constructor chains without executing:

```javascript
// Node.js script to safely resolve eval chains
// Run in isolated environment: node --experimental-vm-modules deobfuscate.js

const vm = require('vm');

// Create sandboxed context with logging
const sandbox = {
    eval: function(code) {
        console.log("=== EVAL INTERCEPTED ===");
        console.log(code.substring(0, 500));
        console.log("========================");
        return code; // Return the code instead of executing it
    },
    document: {
        write: function(html) {
            console.log("=== DOCUMENT.WRITE INTERCEPTED ===");
            console.log(html.substring(0, 500));
        },
        getElementById: function() { return { innerHTML: "" }; }
    },
    window: { location: { href: "" } },
    atob: function(s) { return Buffer.from(s, 'base64').toString(); },
    unescape: unescape,
    setTimeout: function(fn) { if (typeof fn === 'string') console.log("TIMEOUT CODE:", fn); },
    console: console,
    String: String,
    Array: Array,
    parseInt: parseInt,
    RegExp: RegExp,
};

const context = vm.createContext(sandbox);

// Load and execute the obfuscated script in sandbox
const fs = require('fs');
const code = fs.readFileSync('script_0.js', 'utf8');

try {
    vm.runInContext(code, context, { timeout: 5000 });
} catch(e) {
    console.log("Execution error (expected):", e.message);
}
```

### Step 6: Analyze the Deobfuscated Payload

Examine the revealed malicious logic:

```
Deobfuscated Malware Categories and IOC Extraction:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Credential Harvester:
  - Form action URLs (exfiltration endpoints)
  - XMLHttpRequest/fetch destinations
  - Targeted input field names (username, password, cc_number)

Web Skimmer (Magecart):
  - Payment form overlay injection
  - Card data exfiltration URLs
  - Keylogger event listeners (onkeypress, oninput)

Redirect Script:
  - Destination URLs in location.href assignments
  - Conditional redirects based on user-agent or referrer