Cross-site Scripting (or XSS) is a common web application vulnerability with varying levels of severity. Generally the capabilities of a XSS are limited to the locations of vulnerable inputs and outputs, and crafting complex XSS payloads can be a time-consuming process.
XSS-Track (cached) helps simplify cross-site scripting by allowing the attacker to silently track the user across the entire site, using a single embedded XSS. It does this by cleverly creating a full-window invisible iFrame, and maintaining control of that window as the user browses the site. This also allows the attacker to look for valuable pieces of information, such as passwords or credit card numbers.
Combining XSS-Track with the older XSS-Shell script, which turns the browser into a zombie of sorts, could give an attacker a significant amount of power over infected sites and their users.