Impact

An editor with write access to a page, using the Markdown editor, could inject an XSS payload into the content. If another editor (with write access as well) load the same page into the Markdown editor, the XSS payload will be executed as part of the preview panel.

The rendered result does not contain the XSS payload as it is stripped by the HTML Sanitization security module. This vulnerability only impacts editors loading the malicious page in the Markdown editor.

Patches

Commit 05e8a71 fixes this vulnerability by sanitizing the preview HTML from any malicious code.

Thanks to @JoranHonig for reporting this vulnerability.