ProjeQtor versions 7.0 through 12.4.3 contain a stored cross-site scripting vulnerability in the checkValidHtmlText() function within Security.php that fails to properly sanitize user input by only detecting specific patterns while returning unsanitized strings without output encoding. Attackers can inject malicious payloads that bypass the filter using alternative syntax such as img tags with event handlers, which are stored and executed in the browsers of users viewing the affected content.