Copy-and-paste or drag-and-drop into designMode document allows XSS — Mozilla

Security researcher Paul Stone reported that when an HTML selection containing JavaScript is copy-and-pasted or dropped onto a document with designMode enabled the JavaScript will be executed within the context of the site where the code was dropped. A malicious site could leverage this issue in an XSS attack by persuading a user into taking such an action and in the process running malicious JavaScript within the context of another site.