CVE-2026-25500

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.

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)

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).

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-25500 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.2.22, 3.1.20, 3.2.5); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-25500? CVE-2026-25500 is a medium-severity cross-site scripting (XSS) vulnerability in rack (rubygems), affecting versions < 2.2.22. It is fixed in 2.2.22, 3.1.20, 3.2.5. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-25500? CVE-2026-25500 has a CVSS score of 5.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of rack are affected by CVE-2026-25500? rack (rubygems) versions < 2.2.22 is affected.
4. Is there a fix for CVE-2026-25500? Yes. CVE-2026-25500 is fixed in 2.2.22, 3.1.20, 3.2.5. Upgrade to this version or later.
5. Is CVE-2026-25500 exploitable, and should I be worried? Whether CVE-2026-25500 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-25500 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-25500?
   • Upgrade rack to 2.2.22 or later
   • Upgrade rack to 3.1.20 or later
   • Upgrade rack to 3.2.5 or later