copyparty renders unsanitized filenames as HTML when user uploads empty files

Summary

A DOM-Based XSS was discovered in copyparty, a portable fileserver. The vulnerability is considered low-risk.

Details

By handing someone a maliciously-named file, and then tricking them into dragging the file into copyparty's Web-UI, an attacker could execute arbitrary javascript with the same privileges as that user. For example, this could give unintended read-access to files owned by that user. The bug is triggered by the drag-drop action itself; it is not necessary to actually initiate the upload. The file must be empty (zero bytes).

Note: As a general-purpose webserver, it is intentionally possible to upload HTML-files with arbitrary javascript in <script> tags, which will execute when the file is opened. The difference is that this vulnerability would trigger execution of javascript during the act of uploading, and not when the uploaded file was opened.

Proof of Concept (POC)

1. Create an empty file named <img src=x onerror="alert(1)">
2. Drag-and-drop the file into the browser to initiate an upload
3. The alert(1) is executed