XWiki Contrib's Syntax Markdown allows importing Markdown content into wiki pages and creating wiki content in Markdown. In versions starting from 8.2 to before 8.9, the Markdown syntax is vulnerable to cross-site scripting (XSS) through HTML. In particular, using Markdown syntax, it's possible for any user to embed Javascript code that will then be executed on the browser of any other user visiting either the document or the comment that contains it. In the instance that this code is executed by a user with admins or programming rights, this issue compromises the confidentiality, integrity and availability of the whole XWiki installation. This issue has been patched in version 8.9.
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.