Self XSS when pasting HTML into Text app with Ctrl+Shift+V

How it was found

Summary:

ctrl-shift-v is meant to paste plaintext as is. However it will paste it into a dom elements innerHtml and can thus be used to inject malicious html.

Steps To Reproduce:

1. copy "<h1>html</h1>"
2. use ctrl-shift-v to paste it into a .md file
3. See the heading getting added.

Supporting Material/References:

https://github.com/nextcloud/text/blob/main/src/extensions/Markdown.js#L97

• [attachment / reference]

Impact

If you can trick someone into using ctrl-shift-v to paste content you control you can insert html into the page leading to a possible xss attack.

The html will be inserted into the editors schema - but before that happens it's already pasted into the innerHtml of a dom element.