PT-2026-29919 · Rubygems · Rack

Published

2026-04-02

·

Updated

2026-04-02

CVSS v3.1

5.3

Medium

VectorAV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Summary

Rack::Static#applicable rules evaluates several header rules types against the raw URL-encoded PATH INFO, while the underlying file-serving path is decoded before the file is served. As a result, a request for a URL-encoded variant of a static path can serve the same file without the headers that header rules were intended to apply.
In deployments that rely on Rack::Static to attach security-relevant response headers to static content, this can allow an attacker to bypass those headers by requesting an encoded form of the path.

Details

Rack::Static#applicable rules matches rule types such as :fonts, Array, and Regexp directly against the incoming PATH INFO. For example:
ruby
when :fonts
 /.(?:ttf|otf|eot|woff2|woff|svg)z/.match?(path)
when Array
 /.(#{rule.join('|')})z/.match?(path)
when Regexp
 rule.match?(path)
These checks operate on the raw request path. If the request contains encoded characters such as %2E in place of ., the rule may fail to match even though the file path is later decoded and served successfully by the static file server.
For example, both of the following requests may resolve to the same file on disk:
text
/fonts/test.woff
/fonts/test%2Ewoff
but only the unencoded form may receive the headers configured through header rules.
This creates a canonicalization mismatch between the path used for header policy decisions and the path ultimately used for file serving.

Impact

Applications that rely on Rack::Static header rules to apply security-relevant headers to static files may be affected.
In affected deployments, an attacker can request an encoded variant of a static file path and receive the same file without the intended headers. Depending on how header rules are used, this may bypass protections such as clickjacking defenses, content restrictions, or other response policies applied to static content.
The practical impact depends on the configured rules and the types of files being served. If header rules are only used for non-security purposes such as caching, the issue may have limited security significance.

Mitigation

  • Update to a patched version of Rack that applies header rules to a decoded path consistently with static file resolution.
  • Do not rely solely on Rack::Static header rules for security-critical headers where encoded path variants may reach the application.
  • Prefer setting security headers at the reverse proxy or web server layer so they apply consistently to both encoded and unencoded path forms.
  • Normalize or reject encoded path variants for static content at the edge, where feasible.

Exploit

Fix

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Weakness Enumeration

CWE-180

Related Identifiers

GHSA-Q4QF-9J86-F5MH

Affected Products

Rack