Echo's WebSocket proxy middleware in versions 4.15.0 through 4.16.0 and 5.0.0 through 5.4.0 can retain connections and handlers after a WebSocket client disconnects if the upstream server keeps its connection open. This resource retention can accumulate with repeated connections, potentially causing denial of service. The issue does not affect confidentiality or integrity. Fixed in versions 4.16.1 and 5.4.1.