YesWiki is a wiki system written in PHP. In versions up to and including 4.4.5, it is possible for an authenticated user with rights to edit/create a page or comment to trigger a stored XSS which will be reflected on any page where the resource is loaded. The vulnerability makes use of the content edition feature and more specifically of the "{{attach}}" component allowing users to attach files/medias to a page. When a file is attached using the "{{attach}}" component, if the resource contained in the "file" attribute doesn't exist, then the server will generate a file upload button containing the filename. This vulnerability allows any malicious authenticated user that has the right to create a comment or edit a page to be able to steal accounts and therefore modify pages, comments, permissions, extract user data (emails), thus impacting the integrity, availability and confidentiality of a YesWiki instance. Version 4.5.0 contains a patch for the issue.