Vulnerability Instance

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:

```html
<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:

```html
<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)