CVE-2026-34786: Rack: Rack::Static header_rules bypass via URL-encoded paths

Summary

Rack::Static#applicablerules evaluates several headerrules types against the raw URL-encoded PATHINFO, 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 headerrules 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#applicablerules matches rule types such as :fonts, Array, and Regexp directly against the incoming PATHINFO. 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 headerrules.

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 headerrules 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 headerrules 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 headerrules 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 headerrules to a decoded path consistently with static file resolution. Do not rely solely on Rack::Static headerrules 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.