What are the best ways to test web applications for cross-site scripting (XSS) vulnerabilities?

1 Understand the types of XSS

XSS attacks can be classified into three types: reflected, stored, and DOM-based. Reflected XSS occurs when the attacker sends a crafted URL or form input that contains malicious code to the web application, and the web application echoes it back to the user's browser. Stored XSS occurs when the attacker stores the malicious code in a persistent location, such as a database, a comment, or a profile, and the web application displays it to the user's browser. DOM-based XSS occurs when the attacker manipulates the document object model (DOM) of the web page using client-side scripts, such as JavaScript, and changes the behavior or content of the page.

2 Use automated tools

One of the easiest and fastest ways to test web applications for XSS vulnerabilities is to use automated tools, such as scanners, fuzzers, or proxies. These tools can generate and send various payloads to the web application and monitor the responses for signs of XSS. Some of the popular tools for XSS testing are OWASP ZAP, Burp Suite, XSSer, and Wapiti. However, automated tools are not perfect and may miss some XSS vulnerabilities or produce false positives. Therefore, you should always verify the results manually and use your own judgment.

3 Perform manual testing

Another way to test web applications for XSS vulnerabilities is to perform manual testing, which involves using your own skills and knowledge to craft and execute XSS payloads. Manual testing can help you discover complex or hidden XSS vulnerabilities that automated tools may not detect. You can use various sources of input, such as URL parameters, form fields, cookies, headers, and DOM elements, to inject XSS payloads. You can also use different types of output, such as HTML, JavaScript, CSS, and JSON, to display XSS payloads. You can use online resources, such as OWASP XSS Cheat Sheet, to find examples of XSS payloads and techniques.

4 Review the code

A third way to test web applications for XSS vulnerabilities is to review the code, which involves examining the source code of the web application and identifying potential XSS flaws. Code review can help you understand the logic and functionality of the web application and spot insecure coding practices, such as improper input validation, output encoding, or sanitization. You can use tools, such as code editors, IDEs, or static analysis tools, to assist you in code review. You can also use guidelines, such as OWASP Code Review Guide, to learn how to perform code review effectively.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?