/deobfuscating-javascript-malware

name: deobfuscating-javascript-malware
description: 'Deobfuscates malicious JavaScript code used in web-based attacks, phishing pages, and dropper scripts by reversing
  encoding layers, eval chains, string manipulation, and control flow obfuscation to reveal the original malicious logic.
  Activates for requests involving JavaScript malware analysis, script deobfuscation, web skimmer analysis, or obfuscated
  dropper investigation.

  '
domain: cybersecurity
subdomain: malware-analysis
tags:
- malware
- JavaScript
- deobfuscation
- web-malware
- script-analysis
version: 1.0.0
author: mahipal
license: Apache-2.0
nist_csf:
- DE.AE-02
- RS.AN-03
- ID.RA-01
- DE.CM-01

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:

# 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:

// 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 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):