QWebSocketServer is vulnerable to a DoS attack caused by improper parsing of WebSocket handshake. This happens because QWebSocketServer attempts to read from the client until a newline is encountered. If no newline is sent it will read data from the client until all available memory is consumed.

Addendum: when valid header lines are sent (terminated by CRLF), but when no blank line is sent (double CRLF), then the same behaviour occurs.