CVE-2026-29045

Summary

When using serveStatic together with route-based middleware protections (e.g. app.use('/admin/*', ...)), inconsistent URL decoding allowed protected static resources to be accessed without authorization.

The router used decodeURI, while serveStatic used decodeURIComponent. This mismatch allowed paths containing encoded slashes (%2F) to bypass middleware protections while still resolving to the intended filesystem path.

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Details

The routing layer preserved %2F as a literal string, while serveStatic decoded it into / before resolving the file path.

Example:

Request: /admin%2Fsecret.html

• Router sees: /admin%2Fsecret.html → does not match /admin/*
• Static handler resolves: /admin/secret.html

As a result, static files under the configured static root could be served without triggering route-based protections.

This only affects applications that both:

• Protect subpaths using route-based middleware, and
• Serve files from the same static root using serveStatic.

This does not allow access outside the static root and is not a path traversal vulnerability.

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Impact

An unauthenticated attacker could bypass route-based authorization for protected static resources by supplying paths containing encoded slashes.

Applications relying solely on route-based middleware to protect static subpaths may have exposed those resources.