@hono/node-server: Backslash in the request path is parsed as a path separator, bypassing path-based access control at a reverse proxy

Summary

@hono/node-server passes the raw request-target to the WHATWG URL parser, which treats a backslash in the path as / and then resolves .. segments. A request for /admin\secret or /public\..\admin/secret reaches the application as /admin/secret. A reverse proxy that follows RFC 3986 forwards the backslash unchanged and applies its path-based rules to the original path, so the request reaches a route the proxy is meant to block.

Details

A raw backslash is not allowed in a request-target (RFC 3986, RFC 9112 Section 3.2.1), and RFC 9112 Section 3.2 says a recipient should reject such a request rather than correct it. Proxies such as nginx on Linux, HAProxy and Envoy keep the backslash as an ordinary character, so they and the application see different paths.

The fix responds with 400 Bad Request to a request whose path contains a raw backslash, for HTTP/1.1, HTTP/2, absolute-form request-targets and WebSocket upgrades. A percent-encoded backslash and a backslash in the query string are still accepted.

Impact

An unauthenticated client can reach a route that a reverse proxy, load balancer or WAF in front of the application blocks by path.

This affects applications served by @hono/node-server behind a front component that makes path-based decisions and forwards a backslash unchanged. Access control inside the Hono application, such as middleware on /admin/*, runs on the rewritten path and is not affected.