How do we detect XSS bugs?

Most of the vulnerabilities described in this book and that you can find in web applications, the basic tool to detect, analyze, and exploit them, is an HTTP proxy. In order to detect XSS bugs, we are going to use the HTTP proxy to analyze each HTTP request made by the application that we are assessing. And field by field, we are going to be modifying the content with some basic testing strings.

The most basic string to use is this one:

<script>alert(1)<script>

This string launches a pop-up message in the browser showing the number 1; it is less useful, but perfect for finding XSS vulnerabilities. It is important to note that in easy cases, the use of the HTTP Proxy may not be necessary, and you can inject the testing string directly into the fields on the website. But nowadays, basically all applications have controls implemented in the frontend to avoid basic injections. These controls encode the strings entered with special characters, into formats...