CVE-2026-34763

Rack::Directory interpolates the configured root path directly into a regular expression when deriving the displayed directory path. If root contains regex metacharacters such as +, *, or ., the prefix stripping can fail and the generated directory listing may expose the full filesystem path in the HTML output.

Details

Rack::Directory::DirectoryBody#each computes the visible path using code equivalent to:

show_path = Utils.escape_html(path.sub(/\A#{root}/, ''))

Here, root is a developer-configured filesystem path. It is normalized earlier with File.expand_path(root) and then inserted directly into a regular expression without escaping.

Because the value is treated as regex syntax rather than as a literal string, metacharacters in the configured path can change how the prefix match behaves. When that happens, the expected root prefix is not removed from path, and the absolute filesystem path is rendered into the HTML directory listing.

Mitigation

• Update to a patched version of Rack in which the root prefix is removed using an escaped regular expression.
• Avoid using Rack::Directory with a root path that contains regular expression metacharacters.

If Rack::Directory is configured to serve a directory whose absolute path contains regex metacharacters, the generated directory listing may disclose the full server filesystem path instead of only the request-relative path.

This can expose internal deployment details such as directory layout, usernames, mount points, or naming conventions that would otherwise not be visible to clients.

CVE-2026-34763 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. 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 (2.2.23, 3.1.21, 3.2.6); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-34763? CVE-2026-34763 is a medium-severity security vulnerability in rack (rubygems), affecting versions < 2.2.23. It is fixed in 2.2.23, 3.1.21, 3.2.6.
2. How severe is CVE-2026-34763? CVE-2026-34763 has a CVSS score of 5.3 (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-2026-34763? rack (rubygems) versions < 2.2.23 is affected.
4. Is there a fix for CVE-2026-34763? Yes. CVE-2026-34763 is fixed in 2.2.23, 3.1.21, 3.2.6. Upgrade to this version or later.
5. Is CVE-2026-34763 exploitable, and should I be worried? Whether CVE-2026-34763 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-2026-34763 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-2026-34763?
   • Upgrade rack to 2.2.23 or later
   • Upgrade rack to 3.1.21 or later
   • Upgrade rack to 3.2.6 or later