CVE-2026-34785

Rack::Static determines whether a request should be served as a static file using a simple string prefix check. When configured with URL prefixes such as "/css", it matches any request path that begins with that string, including unrelated paths such as "/css-config.env" or "/css-backup.sql".

As a result, files under the static root whose names merely share the configured prefix may be served unintentionally, leading to information disclosure.

Details

Rack::Static#route_file performs static-route matching using logic equivalent to:

@urls.any? { |url| path.index(url) == 0 }

This checks only whether the request path starts with the configured prefix string. It does not require a path segment boundary after the prefix.

For example, with:

use Rack::Static, urls: ["/css", "/js"], root: "public"

the following path is matched as intended:

/css/style.css

but these paths are also matched:

/css-config.env
/css-backup.sql
/csssecrets.yml

If such files exist under the configured static root, Rack forwards the request to the file server and serves them as static content.

This means a configuration intended to expose only directory trees such as /css/... and /js/... may also expose sibling files whose names begin with those same strings.

Mitigation

• Update to a patched version of Rack that enforces a path boundary when matching configured static URL prefixes.
• Match only paths that are either exactly equal to the configured prefix or begin with prefix + "/".
• Avoid placing sensitive files under the Rack::Static root directory.
• Prefer static URL mappings that cannot overlap with sensitive filenames.

An attacker can request files under the configured static root whose names share a configured URL prefix and obtain their contents.

In affected deployments, this may expose configuration files, secrets, backups, environment files, or other unintended static content located under the same root directory.

CVE-2026-34785 has a CVSS score of 7.5 (High). 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-2026-34785? CVE-2026-34785 is a high-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-2026-34785? CVE-2026-34785 has a CVSS score of 7.5 (High). 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-2026-34785? rack (rubygems) versions < 2.2.23 is affected.
4. Is there a fix for CVE-2026-34785? Yes. CVE-2026-34785 is fixed in 2.2.23, 3.1.21, 3.2.6. Upgrade to this version or later.
5. Is CVE-2026-34785 exploitable, and should I be worried? Whether CVE-2026-34785 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-2026-34785 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-2026-34785?
   • Upgrade rack to 2.2.23 or later
   • Upgrade rack to 3.1.21 or later
   • Upgrade rack to 3.2.6 or later