The vulnerability stems from improper handling of URL parameters in the Managedinstalls module. The commit diff shows the vulnerable version used rawurldecode() on the parameters and passed them to the view, while the patched version introduced addslashes() and decodeURIComponent() to mitigate XSS. The listing function in managedinstalls_controller.php directly processes these untrusted inputs, making it the entry point for the vulnerability. The JavaScript changes further indicate the parameters were used in DOM contexts requiring proper escaping, which was missing in the vulnerable version.