Stored XSS in Rack::Directory via javascript: filenames rendered into anchor href
Date: 2026-02
Affected: rubygems/rack, rubygems/rack, rubygems/rack
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-25500
CWE: CWE-79
CVSS: 5.4 (CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N)

## 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


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