Stored XSS in Rack::Directory via javascript: filenames rendered into anchor href

Summary

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 begins with the javascript: scheme (e.g. javascript:alert(1)), the generated index includes an anchor whose href attribute is exactly javascript:alert(1). Clicking this entry executes arbitrary JavaScript in the context of the hosting application.

This results in a client-side XSS condition in directory listings generated by Rack::Directory.

Details

Rack::Directory renders directory entries using an HTML row template similar to:

<a href='%s'>%s</a>

The %s placeholder is populated directly with the file’s basename. If the basename begins with javascript:, the resulting HTML contains an executable JavaScript URL:

<a href='javascript:alert(1)'>javascript:alert(1)</a>

Because the value is inserted directly into the href attribute without scheme validation or normalization, browsers interpret it as a JavaScript URI. When a user clicks the link, the JavaScript executes in the origin of the Rack application.

Impact

If Rack::Directory is used to expose filesystem contents over HTTP, an attacker who can create or upload files within that directory may introduce a malicious filename beginning with javascript:.

When a user visits the directory listing and clicks the entry, arbitrary JavaScript executes in the application's origin. Exploitation requires user interaction (clicking the malicious entry).

Mitigation

• Update to a patched version of Rack in which Rack::Directory prefixes generated anchors with a relative path indicator (e.g. ./filename).
• Avoid exposing user-controlled directories via Rack::Directory.
• Apply a strict Content Security Policy (CSP) to reduce impact of potential client-side execution issues.
• Where feasible, restrict or sanitize uploaded filenames to disallow dangerous URI scheme prefixes.

HackerOne profile: https://hackerone.com/thesmartshadow

GitHub account owner: Ali Firas (@thesmartshadow)