A vulnerability in rustls versions 0.23.44, 0.23.45, and certain 0.24.0-dev releases causes buffered application data to be sent to the server before the client sends the mandatory 'ech_required' alert when Encrypted Client Hello (ECH) is rejected. This violates RFC 9849 requirements that no application data be sent before the alert. The issue affects clients using EchMode::Enable that send application data before handshake completion and connect to servers rejecting their ECH config. The flaw was fixed in rustls 0.23.46 and 0.24.0-dev.2.