# Deobfuscating Javascript Malware

> Use when 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. . Use when working with deobfuscating javascript malware.

- Skill: `oyi77/deobfuscating-javascript-malware` (Agent Skill)
- Install (CLI): `npx skillmds add oyi77/deobfuscating-javascript-malware`
- Raw SKILL.md: https://api.skillmd.com/api/skills/oyi77/deobfuscating-javascript-malware/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: Apache-2.0
- Author: oyi77 (https://skillmd.com/u/oyi77)
- Updated: 2026-09-08
- Page: https://skillmd.com/skills/oyi77/deobfuscating-javascript-malware

---


# Deobfuscating Javascript Malware

## Overview

Cybersecurity skill for deobfuscating javascript malware. Follows industry best practices and security standards.

## When to Use
**Trigger phrases:**
- "deobfuscating javascript malware"
- "Deobfuscates malicious JavaScript code used in web-based attacks, phishing pages"


- 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.


## When NOT to Use

- When you lack proper authorization for testing
- For production systems without change management
- When the task requires legal or compliance expertise beyond technical scope
- The target is a legitimate site's request-signing logic, not malware — use
  `js-reverse`; this skill is malware-payload-only.


## 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

```python
# Example: IOC detection
import re

IOC_PATTERNS = {
    "ip": r"\b(?:\d{1,3}\.){3}\d{1,3}\b",
    "domain": r"\b[a-z0-9-]+\.[a-z]{2,}\b",
    "hash_md5": r"\b[a-f0-9]{32}\b",
    "hash_sha256": r"\b[a-f0-9]{64}\b",
}

def extract_iocs(text: str) -> dict:
    return {k: re.findall(v, text) for k, v in IOC_PATTERNS.items()}
```

1. **Define Objectives** — Clarify the goals and scope for javascript malware.
2. **Gather Resources** — Collect tools, data, and access needed for javascript malware.
3. **Execute Process** — Carry out javascript malware operations methodically.
4. **Verify Quality** — Check results against acceptance criteria.
5. **Document Outcomes** — Record findings, decisions, and next steps.

## Tools

- **Analysis Platform** — Data processing and visualization
- **Collaboration Tools** — Team coordination and knowledge sharing


## Process

1. **Reconnaissance** — Gather target information, identify attack surface, enumerate services
1. **Analysis/Exploitation** — Execute the technique, analyze results, document findings
1. **Reporting** — Document IOCs, write findings, provide remediation recommendations

## Verification

- [ ] All javascript malware procedures executed completely and documented
- [ ] Findings validated against multiple data sources
- [ ] False positives identified and filtered
- [ ] Results documented with evidence and timestamps
- [ ] Recommendations provided with risk-based prioritization

## Anti-Rationalization Table

| Rationalization | Reality |
|---|---|
| "We are too small to be targeted" | Automated attacks target everyone. Size does not matter. |
| "Security slows us down" | A breach slows you down 100x more. Build security in from the start. |
| "We will fix it after launch" | Vulnerabilities in production are exploited within hours. Fix before deploy. |