PHPSpreadsheet is a pure PHP library for reading and writing spreadsheet files. `\PhpOffice\PhpSpreadsheet\Writer\Html` does not sanitize "javascript:" URLs from hyperlink `href` attributes, resulting in a Cross-Site Scripting vulnerability. This issue has been addressed in release versions 1.29.2, 2.1.1, and 2.3.0. All users are advised to upgrade. There are no known workarounds for this vulnerability.