solspace/craft-freeform Vulnerable to XSS in `PhpSpreadsheet` HTML Writer Due to Unsanitized Styling Data

Summary Short summary of the problem. Make the impact and severity as clear as possible. For example: An unsafe deserialization vulnerability allows any unauthenticated user to execute arbitrary code on the server. \PhpOffice\PhpSpreadsheet\Writer\Html doesn't sanitize spreadsheet styling...