If members of your plone site are allowed to insert html, your site may be vulnurable to XSS attack. By default Plone register Kupu as default editor for members and kupu allows html inserting. Just click on the 'html' picture. To filter javascripts Plone rely on safe_html transform which can be easily bypassed.

Take this example:

- Create a new document
- Disable Javascript and CSS from your browser
- write in the Text field:
<img src="j ava sc   ript:alert(document.cookie)" />

- save and see your document in IE6

safe_html transform cannot filter the example above. IE6 will execute it without problem.