deobfuscating-javascript-malware

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.

الانتقال إلى التثبيت

المستودع: AgentFlocks/flocks
آخر نشاط في المصدر: ٥ مايو ٢٠٢٦ في ٠٥:٤٥
لغة SKILL.md المكتشفة: الإنجليزية
النجوم: ٤٤٥
التفرعات: ٨٣

خيارات التثبيت

يُحدَّد Prompt الذي يراجع المصدر أولًا بشكل افتراضي. يمكنك التبديل إلى أمر مباشر أو تنزيل نسخة محلية.

مراجعة ملفات المصدر

اقرأ SKILL.md وأي ملفات مرافقة يعرضها SkillsMP قبل أن تقرر التثبيت.

العودة إلى البحث العودة إلى التثبيت

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, ) as f:
        f.write(script)
    (f)
PYEOF


npx js-beautify script_0.js -o script_0_pretty.js

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

// 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; };

# 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")

// 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);
}

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
  - Cloaking logic (show benign content to bots)

Exploit Kit Landing:
  - Browser/plugin version checks
  - Exploit payload URLs
  - Shellcode embedded as arrays or encoded strings

Term	Definition
Eval Chain	Nested layers of eval(), Function(), or document.write() calls that each decode one layer of obfuscation before passing to the next
String Array Rotation	Obfuscation technique storing all strings in a shuffled array and accessing them by computed index to hide string literals
Dead Code Insertion	Adding non-functional code blocks that never execute to increase analysis complexity and confuse pattern matching
Opaque Predicate	Conditional expression whose outcome is predetermined but difficult to determine statically; used to obscure control flow
Anti-Debugging	JavaScript techniques to detect and thwart browser DevTools or debugger usage including debugger statements and timing checks
Web Skimmer	Malicious JavaScript injected into e-commerce sites to steal payment card data from checkout forms (Magecart attack)

JAVASCRIPT MALWARE DEOBFUSCATION REPORT
=========================================
Source:           checkout.js (injected into example-shop.com)
Obfuscation:      obfuscator.io (string array + rotation + hex encoding)
Layers Removed:   3

OBFUSCATION TECHNIQUES IDENTIFIED
[1] String array with 247 entries, rotated by 0x1a3
[2] Hex-encoded string references (\x68\x65\x6c\x6c\x6f)
[3] Base64-wrapped eval chain (2 layers)
[4] Anti-debugging: setInterval debugger trap

DEOBFUSCATED FUNCTIONALITY
Type:             Magecart Payment Card Skimmer
Target Forms:     input[name*="card"], input[name*="cc_"]
Data Captured:    Card number, expiration, CVV, cardholder name
Exfil Method:     POST via XMLHttpRequest
Exfil URL:        hxxps://analytics-cdn[.]com/collect
Exfil Format:     JSON { "cn": card_number, "exp": expiry, "cv": cvv }
Trigger:          Form submit event on checkout page

EXTRACTED IOCs
Domains:          analytics-cdn[.]com
IPs:              185.220.101[.]42
URLs:             hxxps://analytics-cdn[.]com/collect
                  hxxps://analytics-cdn[.]com/gate.js