CVE-2026-34830

Rack::Sendfile#map_accel_path interpolates the value of the X-Accel-Mapping request header directly into a regular expression when rewriting file paths for X-Accel-Redirect. Because the header value is not escaped, an attacker who can supply X-Accel-Mapping to the backend can inject regex metacharacters and control the generated X-Accel-Redirect response header.

In deployments using Rack::Sendfile with x-accel-redirect, this can allow an attacker to cause nginx to serve unintended files from configured internal locations.

Details

Rack::Sendfile#map_accel_path processes header-supplied mappings using logic equivalent to:

mapping.split(',').map(&:strip).each do |m|
  internal, external = m.split('=', 2).map(&:strip)
  new_path = path.sub(/\A#{internal}/i, external)
  return new_path unless path == new_path
end

Here, internal comes from the HTTP_X_ACCEL_MAPPING request header and is inserted directly into a regular expression without escaping. This gives the header value regex semantics rather than treating it as a literal prefix.

As a result, an attacker can supply metacharacters such as .* or capture groups to alter how the path substitution is performed. For example, a mapping such as:

X-Accel-Mapping: .*=/protected/secret.txt

causes the entire source path to match and rewrites the redirect target to a clean attacker-chosen internal path.

This differs from the documented behavior of the header-based mapping path, which is described as a simple substitution. While application-supplied mappings may intentionally support regular expressions, header-supplied mappings should be treated as literal path prefixes.

The issue is only exploitable when untrusted X-Accel-Mapping headers can reach Rack. One realistic case is a reverse proxy configuration that intends to set X-Accel-Mapping itself, but fails to do so on some routes, allowing a client-supplied header to pass through unchanged.

Mitigation

• Update to a patched version of Rack that treats header-supplied X-Accel-Mapping values as literal strings rather than regular expressions.
• Strip or overwrite inbound X-Accel-Mapping headers at the reverse proxy so client-supplied values never reach Rack.
• Prefer explicit application-configured sendfile mappings instead of relying on request-header mappings.
• Review proxy sub-locations and inherited header settings to ensure X-Accel-Mapping is consistently set on all backend routes.

Applications using Rack::Sendfile with x-accel-redirect may be affected if the backend accepts attacker-controlled X-Accel-Mapping headers.

In affected deployments, an attacker may be able to control the X-Accel-Redirect response header and cause nginx to serve files from internal locations that were not intended to be reachable through the application. This can lead to unauthorized file disclosure.

The practical impact depends on deployment architecture. If the proxy always strips or overwrites X-Accel-Mapping, or if the application uses explicit configured mappings instead of the request header, exploitability may be eliminated.

CVE-2026-34830 has a CVSS score of 5.9 (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-2026-34830? CVE-2026-34830 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-2026-34830? CVE-2026-34830 has a CVSS score of 5.9 (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-2026-34830? rack (rubygems) versions < 2.2.23 is affected.
4. Is there a fix for CVE-2026-34830? Yes. CVE-2026-34830 is fixed in 2.2.23, 3.1.21, 3.2.6. Upgrade to this version or later.
5. Is CVE-2026-34830 exploitable, and should I be worried? Whether CVE-2026-34830 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-2026-34830 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-2026-34830?
   • Upgrade rack to 2.2.23 or later
   • Upgrade rack to 3.1.21 or later
   • Upgrade rack to 3.2.6 or later