Rack is a modular Ruby web server interface. Prior to versions 2.2.22, 3.1.20, and 3.2.5, Rack::Directory generates an HTML directory index where each file entry is rendered as a clickable link. If a file exists on disk whose basename starts with the javascript: scheme (e.g. javascript:alert(1)), the generated index contains an anchor whose href is exactly javascript:alert(1).

Clicking the entry executes JavaScript in the browser (demonstrated with alert(1)). Versions 2.2.22, 3.1.20, and 3.2.5 fix the issue.