CVE-2026-29045: Hono: Arbitrary file access via serveStatic vulnerability

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.

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.

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.