PhpSpreadsheet HTML writer is vulnerable to Cross-Site Scripting via style information

PHPSpreadsheet is a pure PHP library for reading and writing spreadsheet files. In affected versions `\PhpOffice\PhpSpreadsheet\Writer\Html` doesn't sanitize spreadsheet styling information such as font names, allowing an attacker to inject arbitrary JavaScript on the page. As a result an attacker may used a crafted spreadsheet to fully takeover a session of a user viewing spreadsheet files as HTML. This issue has been addressed in release version 2.1.0. All users are advised to upgrade. There are no known workarounds for this vulnerability.