WebSocket destination paths are not properly validated against the configured rewrite prefix. The `WebSocketProxy.findUpstream()` function resolves destinations using the WHATWG URL constructor, which collapses dot segments. This allows a crafted upgrade request containing path traversal sequences to bypass the rewrite prefix and access upstream endpoints that should not be exposed. This issue is exploitable via non-normalizing WebSocket clients, such as raw HTTP clients or downstream proxies that forward the request target without modification, as standard browsers and the `ws` package normalize paths before transmission.