PhpSpreadsheet is a PHP library for reading and writing spreadsheet files. Versions prior to 3.7.0, 2.3.5, 2.1.6, and 1.29.7 are vulnerable to unauthorized reflected cross-site scripting in the constructor of the `Downloader` class. Using the `/vendor/phpoffice/phpspreadsheet/samples/download.php` script, an attacker can perform a cross-site scripting attack. Versions 3.7.0, 2.3.5, 2.1.6, and 1.29.7 contain a patch for the issue.